Omnidirectional Antennas · global positioning system (GPS), access point of wireless local area...

Omnidirectional Antennas

Xianming Qinga* and Zhi Ning Chena,baInstitute for Infocomm Research (I2R), Singapore, SingaporebNational University of Singapore, Singapore, Singapore

Abstract

An omnidirectional antenna which radiates electromagnetic wave uniformly in a specific plane (often inthe azimuth plane) is one of the most popularly used antennas in wireless applications. This chapterillustrates the basic principles and recent development of the omnidirectional antennas. The discussion iscarried out based on a category of polarization of the omnidirectional antennas. The polarizations of theomnidirectional antennas include linear polarizations of vertical polarization and horizontal polarization,dual linear polarization, and circular polarization. A brief literature review about omnidirectional antennasis also presented along with state-of-the-art designs.

Keywords

Omnidirectional antenna; Vertically polarized omnidirectional antenna; Horizontally polarized omnidi-rectional antenna; Dual polarized omnidirectional antenna; Circularly polarized omnidirectional antenna;Dipole antenna; Monopole antenna; Loop antenna; Slot antenna; Dielectric resonator antenna

Introduction

Antennas are essential components of all wireless systems. An antenna consisting of an arrangement ofmetallic conductors (elements) is used with either a wireless transmitter or a wireless receiver. Anoscillating current of electrons forced through the antenna by a transmitter will create an oscillatingmagnetic field around the antenna elements, while the charge of the electrons also creates an oscillatingelectric field along the elements. These time-varying fields radiate away from the antenna into space as amoving transverse electromagnetic field wave. Conversely, during reception, the oscillating electric andmagnetic fields of an incoming electromagnetic wave exert the force on the electrons in the antennaelements, causing them to move back and forth, creating oscillating currents in the antenna.

The tremendous numbers of antennas have been designed and applied in various systems since 1888when Heinrich Hertz carried out his pioneering experiments to prove the existence of electromagneticwaves. Generally, the antennas can be classified on the basis of:

• Frequency: very-low-frequency (VLF) antenna, low-frequency (LF) antenna, high-frequency(HF) antenna, very-high-frequency (VHF) antenna, ultrahigh-frequency (UHF) antenna, microwaveantenna, millimeter/submillimeter (mmW) antenna, Terahertz antenna, and optical antenna.

• Aperture: wire antenna, reflector antenna, microstrip patch antenna, slot antenna, and so on.• Polarization: linearly (vertically/horizontally) polarized antenna, dual linearly polarized antenna, and

circularly polarized antenna.

*Email: [email protected]

Handbook of Antenna TechnologiesDOI 10.1007/978-981-4560-75-7_52-1# Springer Science+Business Media Singapore 2015

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• Radiation: isotropic antenna, omnidirectional antenna, and directional antenna.

As shown in Fig. 1, an omnidirectional antenna is able to radiate radio wave uniformly in all directionsand offers a 360� coverage in a specific plane (normally in Azimuth plane), which make it very suitable tocommunicate multiple users. Omnidirectional antennas are widely used for radio broadcasting, satellite,global positioning system (GPS), access point of wireless local area network (WALN), and mobiledevices such as cell phone, cordless phone, FM radio, laptop, and so on (Riblet 1947; Croswell andCockrell 1969).

The first omnidirectional antenna dates back to very early days, a simple dipole or monopole antennagenerates vertically polarized omnidirectional radiation in a specific plane. Generally, the omnidirectionalradiation is achieved either by the circularly symmetrical current distribution on the radiating elementsuch as monopole antennas and dipole antennas or by combining the radiation of cylindrically positionedmultiple directional radiating elements such as microstrip patches and slot antennas. This chapter isorganized to concisely summarize the fundamentals of omnidirectional antennas first. Various types ofomnidirectional antennas are then reviewed; some of recently developed technologies such asmetamaterial-based omnidirectional antennas are also discussed.

Omnidirectional Radiation PatternBased on the IEEE Standard Definitions of Terms for Antennas (IEEE Standard 145–1983), the relevantdefinitions regarding the antenna radiation are as follows:

Isotropic antenna: a hypothetical, lossless antenna having equal radiation intensity in all directionsOmnidirectional antenna: an antenna having an essentially nondirectional pattern in a given plane of the

antenna and a directional pattern in any orthogonal planeDirectional antenna: an antenna having the property of radiating or receiving electromagnetic waves

more effectively in some directions than others

An isotropic antenna radiates its energy equally in all directions with a spherical radiation pattern,namely, it exhibits omnidirectional radiation pattern in any plane. However, an isotropic antenna neverphysically exits; it is used as the reference antenna to compare the characteristics of actual antennas. Anomnidirectional antenna radiates radio wave power uniformly in all directions in one plane, with the

Fig. 1 Omnidirectional antenna versus directional antenna; (a) radiation from omnidirectional antenna and (b) radiation fromdirectional antenna


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radiated power decreasing against elevation angles above or below the plane, dropping to zero on theantenna’s axis, and producing a “doughnut-shaped” radiation pattern, as shown in Fig. 2.

Directivity of Omnidirectional AntennaThe directivity of an isotropic antenna is unity since its power is radiated equally in all directions. For allother antennas, the maximum directivity will always be greater than unity, and it is a relative “figure ofmerit” which gives an indication of the directional properties of the antenna as compared with those of anisotropic antenna. It is known that the directivity of a directional antenna can be estimated accuratelyusing the half-power beamwidth (HFBW) of the antenna in the twomain planes (Balanis 1997). However,such a formula is not valid for the omnidirectional antenna because the HFBW definition is not applicablein the plane with omnidirectional radiation.

The radiation pattern of an omnidirectional antenna in the elevation plane can be approximated by

U ¼ sin n yð Þj j, 0 � y � p, 0f � 2p (1)

where n can be integer or non-integer values. The directivity of an antenna with pattern represented byEq. 1 can be determined in a closed form using the definition with known n. However, an approximateformula for directivity calculation using HFBW will be more convenient for practical design.

McDonald (1978) derived a formula of antenna directivity based on the array factor of a broadsidecollinear array; it is given by

Fig. 2 Typical radiation patterns of an omnidirectional antenna, (a) three-dimensional pattern, (b) omnidirectional pattern inthe azimuth plane (x-y plane), and (c) pattern in the elevation plane (x-z plane)


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D0 ffi 101

HPBW degreesð Þ � 0:0027 HPBW degreesð Þ½ �2 (2)

Pozar (1993) derived the formula of antenna directivity based on the exact values obtained using Eq. 1 andthen represented the data in a closed form using curve fitting, and it is expressed as

D0 ffi �172:4þ 191ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi0:818þ 1=HPBW degreesð Þ

p(3)

In general, the formula Eq. 3 should be more accurate for omnidirectional patterns with minor side lobesof very low intensity (ideally no minor lobes), while Eq. 2 should be more accurate for omnidirectionalpatterns without minor side lobes, as shown in Fig. 3a, b respectively.

OmnidirectionalityAn omnidirectional radiation pattern of a circle can never be achieved for practical antenna designs. Asshown in Fig. 4, the omnidirectional radiation pattern of a practical antenna generally exhibits asymmetryor ripples which caused by the asymmetrical radiating structure, the fabrication tolerance, the undesiredantenna assembly, the measurement error, and so on.

Omnidirectionality, or roundness, an engineering term, is not defined by the IEEE Standard Definitionsof Terms for Antennas but widely used in practical applications. It is a parameter to characterize theuniformity of the omnidirectional radiation in each direction and quantified by the ratio of the maximumto minimum gain of the antenna. An ideal omnidirectional antenna radiation pattern should be a circlewith consistent amplitude at any direction so that the omnidirectionality or the roundness is zero dB sincethe gain of the antenna keeps unchanged along the angles. The engineering requirement of omnidirec-tionality for an omnidirectional antenna is typically from 6 dB (or �3 dB) to 2 dB (or �1.0 dB) or even1 dB (or �0.5 dB) for specific applications. Figure 4 demonstrates an omnidirectional pattern where anomnidirectionality of 5 dB is exhibited.

Fig. 3 Omnidirectional patterns with/without minor lobes; (a) with minor lobes and (b) without minor lobes


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Configurations of Omnidirectional AntennasFigure 5 shows the typical omnidirectional antenna configurations. The simplest way to realize omnidi-rectional radiation is to utilize a radiator with circularly symmetrically distributed current. A straightdipole is with such a current distribution and able to generate the omnidirectional radiation in the planeperpendicular to the radiator. An electrically small loop antenna exhibits similar performance wherein thecurrent flows along the circular loop and produces omnidirectional radiation in the plane accordance withthe loop.

Alternatively, the omnidirectional radiation can be realized by using multiple directional radiators.These directional radiating elements are normally positioned on the surface of a cylindrical structure oralong a circle in a planar manner to generate a combined omnidirectional pattern (Knudsen 1956; Chu1959; Jayakumar et al. 1986; Herscovici et al. 2001; Li et al. 2003; Wang et al. 2012). For multipleantenna element configurations, the positioning of the elements is the key factor for achieving a desiredomnidirectional radiation pattern. In general, the omnidirectional antennas with one circularly symmet-rical radiating element have better omnidirectionality than those with multiple radiating elements.

Design Considerations for Omnidirectional AntennasThe same as any other antennas, a desired omnidirectional antenna should be with specific radiatingcharacteristics with size and cost constraints. Among the factors, the issues as following should be moreimportant for an omnidirectional antenna design.

Polarization: The polarization is the first factor to be considered for omnidirectional antenna design,which is the starting point of the antenna configuration. A vertically positioned dipole is able to generatevertically polarized but horizontally polarized omnidirectional radiation, namely, a dipole can onlyproduce the omnidirectional radiation pattern in H-planes instead of E-planes. To achieve horizontallypolarized omnidirectional radiation, the electrically small loop antenna or multiple radiators have to beapplied with proper arrangements. For dual linearly polarized or circularly polarized omnidirectionalantenna, the configuration is more complicated so that orthogonal field components as well as phase delayshould be considered.

Omnidirectionality: The omnidirectionality is somehow the most critical parameter for engineeringapplications. From a design point of view, a single radiator with circular symmetry is able to achievedesired omnidirectionality more easily than the multiple elements with combined omnidirectionalradiation.

Fig. 4 The definition of the omnidirectionality


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Bandwidth: The bandwidth is another key factor for omnidirectional antennas, which can be extremelylarge for modern wireless systems for ultra-wideband or multiband operation. For example, an omnidi-rectional antenna for ultra-wideband radios is required to cover the frequency range of 3.1–10.6 GHz(109.5 %). The antenna for Long-Term Evolution (LTE) indoor access point is required to operate at afrequency band of 670 MHz to 2,690 MHz with a fractal bandwidth of 122.7 %. It is a big challenge todesign such broadband antennas with constraints of omnidirectional radiation and polarization. Usually, abroadband vertically polarized omnidirectional antenna can be realized by enlarging the volume of aradiator. For a horizontally polarized omnidirectional antenna, the broadband operation can be achievedusing specially configured radiator or multiple broadband elements such as tapered slots and log-periodantennas.

Gain: Compared to a directional antenna, an omnidirectional antenna is normally with lower gain. Tohave omnidirectional antenna with higher gain, an antenna array is the most direct solution. For example,a dipole antenna exhibits gain of 2.15 dBi, while a 1 � 4 dipole antenna array is able to achieve gain of6�7 dBi. In addition, using some specific configurations is able to achieve high gain as well; theomnidirectional antenna with a dual-shaped reflector exhibits the gain of more than 10 dBi.

Fig. 5 Omnidirectional antenna configurations; (a) single radiator with circular symmetrical current distribution and(b) multiple radiators for omnidirectional radiation


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Vertically Polarized Omnidirectional Antenna

This section discusses a variety of vertically polarized omnidirectional antennas. When the antenna ispositioned to generate vertically polarized omnidirectional radiation in a horizontal plane, orH-plane, thevertically polarized omnidirectional antenna is also named as an H-plane omnidirectional antenna.

Dipole-Based Omnidirectional Antennas

Straight DipoleDipoles are one of the fundamental classes of antennas that are evolved through many years. In fact, Hertzemployed a dipole antenna on his spark-gap generator during the historical experiment in 1888. Being oneof the fundamental types of radiators, it generates omnidirectional radiation because of the inherentcircular symmetrical structure. The analysis and design principle of the dipole antenna have been fullycarried out (Kraus andMarhefka 2008) and are not repeated here. It is worth to note that the shorter dipoleantenna (overall length of the dipole is less than one operating wavelength) is with a main lobe only andmaximum radiation in the horizontal plane (y= 90�), while a longer dipole (overall length of the dipole islarger than one operating wavelength) is with multiple beams, as shown in Fig. 6.

Coaxial Collinear Dipole Antenna (CoCo Antenna)The coaxial collinear (CoCo) antenna, introduced by H. A. Wheeler (1956), is a kind of high-gainomnidirectional antennas (Balsley and Ecklund 1972; Judasz and Balsley 1989). It radiates as a collineararray of wire dipoles driven in phase and provides a narrow broadside beam and an omnidirectionalpattern in the plane perpendicular to the antenna axis. It is used both as an isolated antenna element and inlarge arrays. For its simple structure and easy fabrication, it has been used over the past few decadesmostly in radar and communication systems.

As illustrated in Fig. 7, the CoCo antenna consists of a sequence of collinear sections of a coaxial cablethat are half-wave long (in terms of the guided wavelength, lg), where the inner conductor of one coaxialcable section is electrically connected to the outer conductor of the adjacent one. The intent is to have theexcitation voltages across all the slots in phase and equal in magnitude, so that a strong in-phase currentdistribution can be driven on the outer surface of the coaxial cable to obtain a high gain in the broadsidedirection. Therefore, every coaxial cable section acts as not only a transmission line but also a radiator.

Besides the coaxial line configuration, a microstrip version of the collinear dipole antenna wasintroduced by R. Bancroft and B. Bateman (2004). The geometry of the antenna is shown in Fig. 8, itconsists of two metallic strips on the opposite sides of a printed circuit board (PCB). The strip on the toplayer starts with a wide trace of width (W1) and follows a narrow trace (W2); the strips are with the samelength, L. The strips alternate between a narrow section and a wide section until a wide section terminatesthe antenna on the bottom of the PCB. The strip on the bottom layer begins with a narrow strip andalternates from narrow to wide for complementing the wide to narrow variation of the strip on the toplayer. The first and last strips on the bottom are shorted to the center of the corresponding wide strips onthe top layer. The width (W2) of the narrow strip is chosen such that it forms a 50-Omicrostrip line with thewide strip on the opposite side viewed as a ground plane. The width of the wide strip sections (W1) isapproximately five times of the narrow strip. The length L of each section is selected to be approximatelylg/2, which is determined by the operating frequency, the dielectric constant, and the thickness of thePCB. The four-section antenna design example at 2.4 GHz band exhibits a simulated maximum gain of6.4 dBi (measured gain of 4.6 dBi), side lobe levels of 11 dB, and VSWR � 2 over a frequency band of15.5 %.


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Biconical AntennaBiconical antennas feature dipole-like characteristics, e.g., omnidirectional radiation in the H-plane,“eight figure”-shaped patterns in the E-plane, fixed phase center, and comparable gain, with an enormouswide bandwidth achieved by double cone elements. During the last decades, many biconical antennashave been developed with the frequency range from MHz to GHz and above with bandwidths of three ormore octaves.

As shown in Fig. 9, a biconical antenna is formed by placing two metallic cones that extend oppositeone another. It is typically hourglass shaped, as both the cones have a common axis. It can be thought to bea uniformly tapered transmission line (Barrow et al. 1939; Nagasawa and Matsuzuka 1988).

Fig. 6 Current distributions of the dipole antennas and three-dimensional radiation patterns; (a) half-wave dipole, (b) full-wave dipole, and (c) two-wave dipole


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The input impedance of the infinite biconical antenna, Zin, can be written as

Z in ¼ 120ln cota4

� �h i(4)

which is a pure resistance where a is a cone angle. For small cone angles a, Zin can be expressed as

Z in ¼ �

pln cot

a4

� �h i¼ �

pln

1

tan a=4ð Þ� �

ffi �

pln

4

a

� �(5)

The input impedance of the biconical antenna is reasonably constant over a wide frequency range. Thebiconical antennas with small angles are not very practical (small a offers large Zin so that the antenna maynot matched with feed line easily), but wide-angle configurations (30� < a/2 < 60�) are often used.Besides the cone angle, another key factor in practical antenna design is the spacing between the cones, inparticular at higher frequencies. At microwave frequencies, it is a common practice to support the conesusing a spacer; the property of the spacer may affect the impedance of the antenna as well. Fortunately, forpractical antenna design, we do not need to calculate the impedance by all these formula; with aid ofpowerful electromagnetic simulation software, designers will optimize a reasonable cone angle forantennas.

Fig. 7 Coaxial collinear dipole antennas: (a) bottom feed and (b) central feed


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Fig. 8 Microstrip collinear dipole antenna

Fig. 9 Biconical antenna geometry and radiated spherical waves


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The biconical antenna can be fed by a coaxial line directly or with a balun for bandwidth enhancement.Some practical biconical antennas are shown in Fig. 10; the cones can be formed by wires, the shape of thecone can be pyramid shaped, and the length of the cones can be different as well.

Discone AntennaThe discone antenna is a vertically polarized omnidirectional antenna with multi-octave bandwidth(Nagasawa and Matsuzuka 1988; Bergmann 2003; Kim et al. 2005; Qing et al. 2005; Chen et al. 2011).As illustrated in Fig. 11, a discone antenna is formed by a disk and a cone – that is where the name“discone” comes from – where the disk is on the top of the cone and perpendicular to the cone’s axis. Thediscone antenna is easy to be fed by a coaxial line, the disk is attached to the inner conductor of the coaxialcable, and the cone is connected at its apex to the outer shield of the coaxial cable. The input impedance ofthe discone antenna is strongly dependent on the apex angle, as long as the cone is longer than about aquarter wavelength and the apex angle is relatively large. In general, the performance of discone antennaas a function of frequency is similar to a high-pass filter. Below a cutoff frequency, it becomes inefficient,and it produces severe standing waves in the feed line. At cutoff frequency, the slant height of the cone isapproximately one quarter wavelength.

A discone antenna for ultra-wideband (UWB) applications is shown in Fig. 12 (Qing et al. 2005), andthe performance of the antenna is exhibited in Fig. 13. The |S11| of less than �10 dB and gain of about2 dBi (at y = 90�, f = 0�) are achieved over the frequency range of 3.0–11.0 GHz. The measuredradiation patterns are shown in Fig. 13c; good omnidirectional patterns are observed in the x-y plane withthe omnidirectionality of 3 dB.

Fig. 10 Different biconical antenna configurations

Fig. 11 Discone antenna; (a) antenna geometry and (b) practical discone antennas


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Monopole-Based Omnidirectional AntennaA conventional monopole antenna consists of a straight rod-shaped conductor, almost always mountedperpendicularly above some sort of ground planes. The monopole antenna was invented in 1895 byGuglielmo Marconi, who discovered if he attached one terminal of his transmitter to a long wiresuspended in the air and the other to the Earth, he could transmit radio waves for longer distances. Forthis reason, it is sometimes called aMarconi antenna, although Alexander Popov independently inventedit at nearly the same time.

As shown in Fig. 14, a monopole antenna can be visualized as being formed by replacing the bottomhalf of a vertical dipole antenna with a ground plane at right angle to the remaining half. From imagetheory, it is known that the radiation pattern of a monopole antenna with a perfectly conducting, infiniteground plane is identical to the top half of the pattern of a dipole with double length. Monopole antennasup to quarter wavelength long have a single “lobe,” with field strength declining monotonically from amaximum in the horizontal direction, but longer monopoles have more complicated patterns with severalconical “lobes” (radiation maxima) directed at angles into the sky. Because it radiates only into the spaceabove the ground plane, or half the space of a dipole antenna, a monopole antenna will have a directivityof twice (3 dB over) the directivity of a full dipole antenna. In addition, the input impedance of themonopole antenna is one half of that of a full dipole antenna. For example, for a quarter-wave monopole

Fig. 12 Discone antenna for UWB applications; (a) detailed dimensions of the antenna and (b) antenna prototypes


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antenna (L = 0.25l) with an ideal infinite ground plane, its directivity will be 5.15 dBi and inputimpedance Zin = 36.5 + j21.25 Ohms.

The ground plane used with a monopole may be the actual earth; in this case, the antenna is mounted onthe ground, and one side of the feed line is connected to an earth ground at the base of the antenna. Such adesign is used for the mast radiator antennas employed in radio broadcasting at low frequencies. At VHFand UHF frequencies, the physical size of the ground plane needed is smaller, so metallic plane is used to

Fig. 13 Measured results of the UWB discone antenna; (a) reflection coefficient and gain, (b) normalized radiation pattern inthe x-z plane, and (c) normalized radiation pattern in the x-y plane


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allow the antenna to be mounted above it; the examples of such surfaces are the roof of a car orairplane body.

The general effect of the electrically small ground plane as well as imperfectly conducting earth groundon the performance of the monopole antenna is to tilt the direction of maximum radiation up to higherelevation angles. Figure 15 compares the radiation patterns of a quarter-wave monopole with differentground sizes.

Three-Dimensional Monopole AntennasUnlike a dipole antenna, the monopole antenna is an unbalanced antenna so that it is very convenient to befed using a coaxial cable. As shown in Fig. 16, the inner conductor of the coaxial connector such assubminiature version A (SMA) is connected to the lower end of the monopole and the other conductor ofthe SMA attached to the ground plane. Besides the basic rod shape, the radiator of the monopole antennacan be with different configurations, for example, conical cone, tapered cone, cross plate, parallel plate,and rolled plate. The radiator with larger volume exhibits the broader bandwidth over the rod-shaped one,wherein the tapered structure will provide better impedance matching (Taniguchi and Kobayashi 2002;Wong et al. 2005; Wong and Su 2005; Chen 2005). Note that all the configurations are able to offerbroadband impedance matching, while only those antennas with circular symmetrical radiator cangenerate the desired omnidirectional radiation.

Printed Planar Monopole AntennasA number of printed omnidirectional antennas have been reported, wherein the planar monopole and theground are etched on a PCB. As exhibited in Fig. 17, such an antenna is composed of a radiator and a smallground plane and fed by a microstrip line or a coplanar waveguide (CPW). For microstrip-fed antenna, theradiator and the microstrip feed line are positioned on one side of the PCBwhile the ground plane is on theother side. For CPW-fed configuration, both the radiator and the CPW are on the same side of thePCB. The antennas are able to provide broad impedance and gain bandwidth while desired omnidirec-tional radiation, in particular, at high frequencies, wherein a bidirectional instead of an omnidirectionalpattern is achieved. Changing the shape of the radiator is helpful to adjust the bandwidth of the antennaand realize some specific characteristic such as band-notch performance.

Slot Array AntennaThe vertically polarized omnidirectional radiation can be achieved by cutting transverse slot on thecircular waveguide or coaxial line. Normally, a slot array instead of a single slot is applied for higher gain,where the slot elements are positioned with a spacing of guided wavelength (lg) to achieve the in-phase

Fig. 14 Monopole analysis using image theory


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excitation. A circular waveguide carrying the TM01 mode is suitable for implementing the slot array foromnidirectional radiation because of the circular symmetrical current distribution along the wall of thewaveguide. One point to be noted is that the lg of a hollow circular waveguide carrying TM01 mode islarger than the free space wavelength (l0); the slot antenna array will have grating lobes. To avoid thegrating lobes, lg should be less than l0. Filling dielectric into the hollow circular waveguide or utilizationof the slow wave structure is suggested.

Fig. 15 Radiation patterns of a quarter-wave monopole with different sizes of ground planes; (a) antenna configuration,(b) D = 0.25l, (c) D = 1l, and (d) D = 2l


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Coaxial line is also suitable for omnidirectional slot array implementation. Baumer and Landstorfer(1990) demonstrated a slot array antenna as shown in Fig. 18. A slow wave structure is used as the innerconductor of the coaxial line; the diameter of the outer conductor is about a quarter of the free spacewavelength at the center frequency. For implementation convenience, the nonresonant transverse slots area little bit less than 360�. The eight-element slot array antenna is able to achieve desired omnidirectionalradiation patterns in azimuth planes with the measured directivity of 10 dBi.

Chen et al. proposed a planar omnidirectional slot array antenna as shown in Fig. 19 (Chen et al. 2011).The antenna consists of a central conductor and two metal ground planes. The central conductor isembedded in a PCB and sandwiched in between two parallel ground planes to form a strip line, and oneend of the central conductor is connected to the feeding coaxial cable and the other end open. A series ofrectangular loop slots are etched on the top and bottom metal grounds. It is found that a nonuniform arraywith uneven loop slots having unequal distances between them is able to provide greater flexibility inantenna design and might achieve better performance compared to one with equal spaced slots withuniform size. The antenna is easily implemented using low-cost and precise planar printed technique. Anantenna with eight back-to-back slots exhibits impedance bandwidth of 4.6 % (|S11| < �10 dB) with

Fig. 16 Coaxial line fed monopole antenna and different three-dimensional radiator configurations; (a) rod-shape, (b) cone-shape, (c) taper-shape, (d) crossed plates, (e) parallel plates, and (f) rolled plates


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center frequency of 5.8 GHz; it has a well-formed omnidirectional pattern with omnidirectionality of0.5 dB in the horizontal plane, a gain of 10 dBi, side lobe levels of 10 dB below the main lobe, and nobeam squint.

Dual-Reflector Omnidirectional AntennaThe coming generation of communication systems is expected to provide services through high-data-ratewireless channels. They will operate at millimeter-wave band up to 60 GHz, which requests the antennaoffering the absolute operating frequency bandwidth of several gigahertz. The conventional rod-shapeddipole antenna and monopole antenna usually suffer from narrow operating bandwidth. The biconical

Fig. 17 Printed omnidirectional antennas; (a) microstrip-fed printed monopole and (b) coplanar waveguide-fed printedmonopole antennas

Fig. 18 Slot antenna array on coaxial line for vertically polarized omnidirectional radiation


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antenna and discone antenna can offer broader operating bandwidths but lower gain and difficultimplementation at millimeter frequencies.

The dual-reflector antenna (Willoughby and Heider 1959; Orefice and Pirinoli 1993) is a desiredcandidate for vertically polarized omnidirectional coverage at higher frequencies up to millimeter-wavebands. As shown in Fig. 20, an omnidirectional dual-reflector antenna is composed of two circularlysymmetric reflectors with a common symmetry axis and fed by coaxial horn. The feed is located in thefocus of the sub-reflector, a paraboloid. The field is firstly reflected by the paraboloid and then directedtowards the main conical reflector; the plane wave is therefore eventually reflected in all the directions ofthe horizontal plane. The conical reflector can be shaped in order to point the maximum radiation to aspecific elevation angle.

Fig. 20 Schematic diagram of dual-reflector omnidirectional antenna

Fig. 19 Slot antenna array on strip line for vertically polarized omnidirectional radiation


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A key feature of the antenna is the use of an unconventional feed for obtaining the vertical polarization.The feed is requested to radiate a field with only a y-directed component and a null on the axis. A pureradial-mode aperture such as a coaxial horn or a TM01 excited conical horn is suitable for the antenna feed.

The configuration of the reflectors can be with different shapes as illustrated in Fig. 21, where theazimuth omnidirectional radiation is kept unchanged while the shape of the E-elevation radiation patternas well as the title angle of the maximum radiation can be controlled by the profile of the reflectors (Pinoet al. 2000; Bergmann and Moreira 2004; Silva and Bergmann 2005; Zang and Bergmann 2013).

Low-Profile Omnidirectional AntennaA conventional monopole antenna with a quarter wavelength height is undesirable for today’s wirelesssystems such as LTE and TVwhite space radios, where the operating frequency starts from a few hundredmegahertz. Meanwhile, a low-profile antenna is always preferable, in particular, for indoor systems. Thevertical height of a monopole antenna can also be reduced through antenna loading techniques usingdielectric, inductive loading, and capacitive loading (Delaveaud et al. 1998; Foltz et al. 1998; Mcleanet al. 1999; Liu et al. 2004; Heydari et al. 2009). Apart from the miniaturized monopole-type antennas, theother low-profile vertically polarized antennas with monopole-like omnidirectional radiation patternsinclude Goubau antenna (Goubau et al. 1982), a multielement antenna consisting of four electrically smallvertical conductors, each one terminated in a conductive plate; quadripod kettle antenna (QKA)(Tokumaru 1976), comprising of multiple metallic plates; the spiral-mode microstrip (SMM) antenna(Wang et al. 2011), a traveling wave-based broadband antenna; and so on. These antennas are with a smallantenna height of a twentieth operating wavelength.

Multielement Low-Profile Omnidirectional AntennaAmulti-plate omnidirectional antenna, namely, quadripod kettle antenna (QKA) as shown in Fig. 22, waspresented by Shinobu Tokumaru in 1976 (Tokumaru 1976). The QKA is a self-standing purely multi-platemetallic structure that consists of a ground plane, a fed lower plate, and an upper plate. The lower metallicplate, in the form of cross with a central small square or circular patch, is fed by a probe at the center andshorted to the ground at the ends. The upper plate in the shape of a square or octagon/circle is connected to

Fig. 21 Variations of the shape of the reflectors


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the lower plate by means of another four pods at the edge. The antenna can be fed by a coaxial cableconveniently by connecting the external conductor of the coaxial cable to the ground plane and extendingthe inner conductor of the coaxial cable to the lower plate. The length of cross arms of the lower platedetermines the lowest operating frequency, while the shape of the upper plate determines the impedancematching characteristic over the bandwidth. The QKA is an axially symmetrical antenna which is able togenerate vertically polarized radiation similar to a monopole antenna with antenna height of less than 0.05wavelength and bandwidth of more than 50 %.

Besides the quadripod configuration, the multi-plate antenna can be configured with tripod, hexapod,or octopod. Zurcher proposed a tripod kettle antenna (TKA) (Zurcher 2013), as shown in Fig. 23.The antenna designed at center frequency of 7.5 GHz exhibits an impedance bandwidth of 94 %(|S11| < �10 dB) with an antenna height of 5.9 mm (0.079l at lower edge frequency).

Another multielement low-profile vertically polarized omnidirectional antenna is shown in Fig. 24. Theantenna features less than l/20 in height and l/5 or smaller in lateral dimension. Similar to the QTAantenna, the antenna is composed of a ground plane, a lower metallic structure with four ground ends andan upper metallic structure. Different from the QTA, the metallic plates are replaced by strips, and theupper plate is center fed by the probe together with the lower strips. Furthermore, the upper strips are openended. The lateral dimension of the antenna is able to be miniaturized by meandering and turning thestrips into form of a multi-arm spiral. The antenna is able to be matched to a 50-O coaxial line without the

Fig. 22 Schematic diagram of quadripod kettle antenna (QKA), (a) octagon structure and (b) circular structure


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need for external matching. The meandering between the short-circuited end and the feed point alsofacilitates the impedance matching.

Metamaterial-Based Low-Profile Omnidirectional AntennaThe artificial metamaterial structures can also be applied for omnidirectional antenna design. A low-profileomnidirectional zeroth-order resonator (ZOR) antenna using mushroom structures is exhibited in Fig. 25.The mushroom structure as a transmission line has negative, zero, and positive propagation constantdepending on its operation frequency. Zeroth-order resonance, of which length is independent of thephysical length, is naturally presented. In particular, mushroom ZOR antenna generates a uniform verticalelectric field against a ground plane. The vertical electric field of mushroom ZOR is similar to that of shortmonopole antenna on a flat metal ground plane, so that it is able to generate the vertically polarizedomnidirectional radiation in a horizontal plane. The resonant frequency of the mushroom ZOR and thenumber of unit cell can be determined from its dispersion curve.

Lee (Lee and Lee 2007) demonstrated an omnidirectional ZOR antenna at 7.8 GHz. The antenna isdesigned on a 1.57-mm thick RT/duroid 5880 PCB with dielectric constant of 2.2. The 3 � 2 mushroomarray is with a unit cell size of 4.8 � 4.8 mm (patch), gap of 0.2 mm, and diameter of via of 0.3 mm. Thelow-profile antenna with a height of 0.04l achieves the gain of 3.1 dBi and reflection coefficient of�15 dB at 7.8 GHz. Desired omnidirectional radiation pattern is achieved with the omnidirectionality of2 dB.

Fig. 23 Tripod kettle antenna

Fig. 24 Schematic diagram of miniaturized multielement monopole antennas (Hong and Sarabandi 2009)


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Horizontally Polarized Omnidirectional Antenna

In urban or indoor wireless environments, the polarization of propagating radio waves may changesignificantly because of the complicated multiple reflections or scatterings. Although vertically polarizedantennas are used in many current wireless systems, it has been reported that using horizontally polarizedantennas with both the transmitter and receiver can achieve a 10-dB improvement in terms of system gainas compared to vertically polarized antennas at both the ends of the link (Chizhik et al. 1998). In a third-generation (3G) network, the horizontally polarized omnidirectional antennas are often used together withvertically polarized omnidirectional antennas to form a multiple-input–multiple-output (MIMO) antennasystem that provides polarization diversity in substitution for space diversity. Polarization diversity can beused to improve the reliability of a communication link, where the orthogonal polarization allows thefrequency to be reused and isolation to be increased between the independent local area networks.

In contrast to the vertically polarized omnidirectional antenna which radiates equally in the planealigning the H-field, the design of horizontally polarized omnidirectional antenna is more challengingsince it is required to radiate equally in the plane aligning the E-field. The key consideration of such anantenna design is to form a structure with uniform and in-phase current distribution along a circle in theazimuth plane, either by using single loop or multielement configurations.

Loop-Based Omnidirectional Antenna

Electrically Small Loop AntennaAn electrically small solid-line loop is able to generate horizontally polarized omnidirectional radiationbecause of the current flowing along it features single direction and uniform distribution. However, suchelectrically small solid-line loop antenna has a very small radiation resistance and a large reactance, whichmakes the antenna very difficult to match with the excitation source. A solid-line loop antenna withperimeter comparable to one wavelength has a reasonable radiation resistance and reactance for imped-ance matching, while there is no longer omnidirectional radiation since the current flowing along the loopfeatures phase inversion. In general, the small loop antenna having a circumference of less than a tenthwavelength is with uniform and in-phase current distribution. Figure 26 exhibits the radiation patterns of

Fig. 25 ZOR antenna with coaxial probe feed


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the loop antenna with different circumferences. It is observed that the loop antenna is able to achievedesired omnidirectional radiation when the circumference of the loop is less than 0.1l. The loop antennaexhibits a bidirectional radiation when its circumference becomes electrically larger.

Alford Loop AntennaThe Alford loop antenna was presented by Alford and Kandoian in 1940 (Alford and Kandoian 1940). Asexhibited in Fig. 27, it is composed of two Z-shaped metallic wires/strips, which are crossly positioned toform a square outline. The antenna is fed at the central portion of (K, K’); due to the symmetric structure,the current flowing on the wire sections AB and CD will have the same magnitude but opposite direction,which is the same as wire sections BC and AD. Furthermore, since the wire sections BB’, DD’, and AC

Fig. 26 Three-dimensional radiation patterns of the loop antenna with different circumference, (a) loop antenna withcoordinate, (b–d) pattern with circumference of 0.1l, 0.5l, and 1l, respectively


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are positioned very close to each other, the radiation from the opposite-directed current flowing alongthese sections cancel out each other. Therefore, the currents along the outline of the Alford loop form asquare “loop”-type current distribution and achieve horizontally polarized omnidirectional radiation. Inpractical design, a circular instead of a square loop configuration is applied for better omnidirectionalradiation performance. The circumference of the outline can be about one wavelength so that it is suitablefor applications up to gigahertz with convenient implementation. A number of variations have beenreported for different applications; some design examples are discussed in the following sections.

Printed Alford Loop AntennaA printed Alford loop antenna as shown in Fig. 28a (Lin et al. 2006) was proposed for the integration withan external interface card, such as the personal computer memory card international association(PCMCIA) card and WLAN card for a laptop PC. The antenna consists of two Z-shaped strips printedon the top and bottom sides of a PCB. The bottom strip is arranged in such a manner that the “arm” ismapped to that of the top strip through the PCB substrate. A coaxial connector is used to connect thecentral feed point of the top and bottom strips. The “wing” length of the Alford loop is of the order of aquarter wavelength. Due to structure symmetry, the antenna current distribution on the two strips will havethe same magnitude but opposite flowing directions. Since the thickness of the PCB substrate is verysmall, the radiation of the antenna current along the “arm”will cancel out each other. The antenna currentson the two “wings” of each Z-shaped strip establish a square “loop”-type current distribution and generatehorizontally polarized omnidirectional radiation.

Awindmill-shaped loop antenna (Kim et al. 2007) is exhibited in Fig. 28b. The antenna consists of twocrossed Z-shaped strip lines etched on the opposite sides of a PCB. Each strip line has four arms witharc-shaped outer section which is of about one eighth wavelength. The arc-shaped outer sections on theupper side of the PCB are the mirror images of those on the bottom sides of the PCB or vice versa. Such anarrangement configures a combined loop antenna which consists of eight arc-shaped sections with acircumference of one wavelength. Feeding from the center of the antenna, the currents flowing on cross-shaped parallel strips are with opposite directions, while the currents flowing along the combined loop arewith the same magnitude and single direction. Therefore, omnidirectional radiation is generated in theE-plane (x-y plane). An antenna using RT/duroid 5880 PCB with the dielectric constant of 2.2 mm and

Fig. 27 Alford loop


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height of 1.6 mm operates at 2.6 GHz and achieves an impedance bandwidth of 6 % (|S11|<�10 dB) andantenna gain of 1.5 dBi.

A dual-band windmill-shaped Alford loop antenna (Ahn et al. 2009) can be configured by adding onemore set arc-shaped strips, as shown in Fig. 28d. The antenna is composed of two combined loops withdifferent diameters, which resonant at different frequencies. The dual-frequency operation is achievedwithout any extra matching circuits or parasitic components. An antenna design has been exemplified atdual bands with the center frequencies of 2.45 and 3.9 GHz, respectively.

Fig. 28 Printed Alford loops, (a) PCB configuration, (b) square-shaped loop, (c) windmill-shaped loop, and (d) dual-bandwindmill-shaped loop


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Broadband Alford Loop AntennaThe traditional wire-type Alford loop antenna or printed strip-type Alford loop antenna is with limitedbandwidth of less than 10 %, which is unable to meet the 4G LTE system requirements of high datathroughput and long link range, where a peak speed of 100 Mb/s for high-mobility communication and1 Gb/s for low-mobility communication is required.

Figure 29 shows a broadband Alford loop antenna (Yu et al. 2013). Similar to a conventional printedAlford loop antenna, the multiple arc-shaped strips on the opposite sides of a PCB are used to form thecombined loop for horizontally polarized omnidirectional radiation. For bandwidth enhancement, taperedstrip lines, parasitic strips, and a balun are applied.

The antenna consists of two pairs of arc-shaped radiators and a circular patch on the top and bottomsides of a PCB, respectively, as well as four parasitical strips on the top side. The four arc-shaped strips onthe top layer are connected to a small circular conducting patch located at the center through four taperedstrip lines, respectively. The end of the arc-shaped strip is notched to create two resonances that are closeto each other. Four parasitical stripes are also printed on the top layer of the substrate to suppress thereactance of the antenna for bandwidth enhancement. The arrangement of the radiators on the bottomlayer is similar to those on the top side, while the arc-shaped strips are positioned in a counter clockwisemanner and connected to a larger circular conducting patch located at the center. The circular patchesalong with the tapered feeding lines constitute a balun for the coax feeding to the antenna. When excited,the quasi-TEM waves between two circular patches are guided to four pairs of arc-shaped radiatorsthrough the tapered strip lines. Due to the opposite-directed currents flowing on the straight strips on theopposite sides, the currents on the arc-shaped strips flow synchronously in clockwise or counterclockwiseand form a circular combined loop and generate omnidirectional radiation. A practical antenna is able tooperate from 1.76 to 2.68 GHz with a return loss greater than or equal to 10 dB, peak gain of 3.6–4.2 dBi,an average radiation efficiency of 83 %, and very good omnidirectional radiation pattern in E-plane overthe entire impedance bandwidth.

Metamaterial-Based Zero-Phase-Shift Line Loop AntennaThe metamaterial-based zero-phase-shift line loop antennas have been attracted more attention recentlyfor horizontally polarized omnidirectional antenna design. A transmission line with zero-phase-shift isideal to configure an electrically large loop antenna for horizontally polarized omnidirectional since auniform current distribution without phase inversion can be easily achieved. Two types of metamaterial-based transmission line, namely, the composite right-/left-handed transmission line (CRLH-TL)

Fig. 29 Wideband Alford loop antenna for 4G LTE applications


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[16]–[18] and mu-negative transmission line (MNG-TL), have been utilized for designing the loop-typeantennas for omnidirectional radiation in E-plane.

CRLH Line Loop AntennaCRLH lines have been widely used for improving the performance of microwave devices (Sanadaet al. 2003; Caloz et al. 2004) and leak wave antenna (Liu et al. 2002). These metamaterial-basedtransmission lines are characterized by the existence of three different spectral regions: the left-handedregion at lower frequencies supporting a backward wave wherein a negative propagation constant b isobserved (b < 0), right-handed region at higher frequencies supporting a forward wave wherein thepropagation constant b is positive (b > 0), and a transition point wherein b = 0. The unique property ofthe zero propagation constant with nonzero group velocity at the zeroth-order resonance makes the CRLHlines suitable to form a loop antenna with omnidirectional radiation. Figure 30a exhibits a CRLH line loopantenna where surface-mounted lumped inductors and capacitors are used (Borja et al. 2007). The antennaexhibits E-plane omnidirectional radiation patterns but a low gain of 0.3 dBi at 500 MHz. The low gainattributes to the utilization of the lossy lumped components for CLRH line implementation. Figure 30bshows a revised design (Locatelli et al. 2012), where all the lumped components have been replaced byprinted ones. The antenna demonstrates an omnidirectionality of 0.4 dB for E-plane pattern and maximumgain of 1.35 dBi at 2.4 GHz.

Segmented Line Loop AntennaSegmented loop antennas have been investigated firstly for UHF radio-frequency identification (RFID)applications, where the reader antenna is required to generate strong and even magnetic field over a largeinterrogation zone while the conventional solid-line loop antenna cannot make it. Dobkin et al. firstlypresented the segmented magnetic antenna consisting of a number of segments, and each segment iscomposed of a metal line and a series lumped capacitor (Dobkin et al. 2007). In this structure, segmentingand combining the parasitic inductance of each section with a lumped capacitor make the electrically largeloop keeping the uniform current flowing in the same direction and generating strong magnetic field.Based on the method, several segmented loop antennas (Qing et al. 2009; Ong et al. 2010) have beenpresented; the lumped capacitor is replaced by either couple lines or distributed capacitors as shown inFig. 31.

Since the electrically large segmented line loop supports the uniform and uni-directed current distri-bution, it has been applied to design the horizontally polarized omnidirectional antenna for 2.4 GHz and

Fig. 30 CRLH line loop antennas, (a) antenna design with lumped components and (b) printed version


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5 GHz WLANs (Qing and Chen 2012; Hasse et al. 2012; Wei et al. 2012). Wei et al. propose a circuitmodel to analyze the segmented line with periodically loaded parallel-plate lines (Wei et al. 2012). It isfound that the effective permeability of the segmented line-based unit cell is negative, zero, and positive.When the transmission line operates at the mu-zero frequency, it has a unique property that supports a zeropropagation constant with nonzero group velocity. At the zeroth-order resonance, there is no phase shiftacross the resonator so that the current distribution along the loop remains in phase and could yield adesired horizontally polarized omnidirectional pattern. Note that the general theoretical analysis of thesegmented line is still a challenge till today since no valid transmission line mode can be applied to asingle line, so that such an antenna design is mainly carried out with the help of simulation tools.

Compared to Alford loop antennas which must be fed from the center of the antenna, the side fedsegmented line loop antenna is more convent to configure an antenna array for higher gain. Figure 32shows some horizontally polarized omnidirectional segmented line loop arrays for the WLAN applica-tions at 2.4 GHz and 5 GHz bands with series feeding or corporate feeding network.

Figures 33 and 34 demonstrate the detailed configuration and performance of the antenna array shownin Fig. 32c. The four-element antenna array is designed for 5-GHz WALN applications. The segmentedline loop is printed onto a piece of FR4 PCB slab (er = 4.4, tand = 0.02) with thickness of 0.5 mm. Thefour elements are connected to the outputs of the parallel-line feeding network, respectively. The parallel-line feeding network is etched on the opposite sides of a 0.8-mm thick RO4003 PCB (er = 3.38,tand = 0.0023). Two open-circuited stubs are used to enhance the impedance matching of the antennaarray. The antenna array exhibits wideband characteristics: reflection coefficient of less than�10 dB, gain

Fig. 32 Segmented line loop antenna arrays; (a) 2.4-GHz four-element series-fed MNT-TL loop antenna array (Weiet al. 2012), (b) 2.4-GHz four-element corporate-fed segmented line loop antenna array (Qing and Chen 2013), and (c) 5-GHz four-element corporate-fed segmented line loop antenna array (Qing and Chen 2014)

Fig. 31 Configurations of the segmented lines


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of greater than 5 dBi, omnidirectional E-plane radiation patterns with omnidirectionality of less than 5 dBover the frequency range of 5.05–5.9 GHz, which is desired for WALN mesh networks.

Waveguide Slot Array AntennaSimilar to vertically polarized omnidirectional slot antennas, the waveguide slot array is able to achievethe horizontally polarized omnidirectional radiation as well. A circular waveguide carrying rotationallysymmetric mode TM01 or coaxial line with TEM mode is suitable to configure such antennas. Figure 35shows the basic configuration of the horizontally polarized omnidirectional antenna using circularwaveguide and rectangular waveguide (Sangster and Wang 1995; Grabherr and Huder 1999; Kyouichiand Tanaka 2000).

Generally, the antenna contains a number of rows of slots in its axial direction. Each row consists ofseveral equally spaced identical half-wave slots arranged on the circumference of the waveguide. It hasbeen found that eight slots are adequate to achieve the desired omnidirectional radiation in the azimuthplanes. Because the currents on the wall of the circular waveguide carrying TM01 mode are axiallydirected, the slots must be inclined with a certain angle for efficient coupling from the waveguide to freespace. The radiation pattern in the elevation plane is determined by the number and distance of the rows ofslots as well as by the shape and the inclination angle of the slots themselves. Succeeding rows aregenerally positioned with a distance of half wavelength of the TM01 mode that propagates in the circularwaveguide. To realize a stable broadside omnidirectional radiation, the waveguide is short circuited atquarter wavelength behind the last row of slots, which makes the antenna to be a resonant array. The innerdiameter of the circular waveguide has to be chosen sufficiently small in order to allow the propagation ofthe TM01 mode, but below the cutoff for the next higher TM11 mode. On the other hand, the wavelength of

Fig. 33 Segmented line loop antenna array for WLAN applications, (a) array configuration, (b) segmented line loop element,and (c) feeding network


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the TM01 mode in the waveguide and the distance of successive rows of slots increase with decreasing itsinner diameter. To avoid grating lobes in the radiation pattern of the elevation plane, the distance of therows must be smaller than one free space wavelength. In general, the minimum inner diameter of thecircular waveguide can be selected to be one wavelength in free space at the central operating frequency.For the slot array on the circular waveguide carrying TE11 mode, the slots are not needed to be inclinedbecause the currents on the wall of the circular waveguide are f directed.

Fig. 34 Measured results of the 5-GHz segmented line loop array; (a) |S11| and gain, (b) normalized radiation pattern inE-plane, and (c) normalized radiation pattern in H-plane


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The rectangular waveguide carrying TE10 mode can be used to configure the slot array for horizontallypolarized radiation as well. Furthermore, the longitudinal slot array features very low cross-polarizationlevels. As shown in Fig. 35c, the longitudinal slots are cut on both the broad walls of the waveguide toachieve desired omnidirectional radiation. The resonant slots are positioned at the opposite side of thecentral line of the waveguide alternatively with half a guided wavelength of the TE10 mode in thewaveguide. Such an arrangement counteracts the 180� phase shift of the waveguide mode at each half-guided wavelength and ensures the slots with in-phase excitation.

Fig. 35 Waveguide slot arrays, (a) slot array on circular waveguide carrying TM01 mode, (b) slot array on circular waveguidecarrying TE11 mode, and (c) slot array on rectangular waveguide carrying TE10 mode


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The conventional metallic rectangular waveguide slot array antennas have been widely used in radar,navigation, and the communication systems for years. However, the volume, weight, and cost of theseantennas are often puzzled for some applications. With the development of the substrate integratedwaveguide (SIW) technology, it is possible to design a fully printed slot array antenna with horizontallypolarized omnidirectional radiation, which is desired for WALN applications owing to the merits of smallsize, light weight, low manufacture cost. The SIW is a planar-guided wave structure which supports thesimilar TE-modes as in a conventional metallic rectangular waveguide. It can easily be integrated intomicrowave and millimeter-wave integrated circuits and has been extensively investigated for designingfilters, couplers, power dividers, antennas, and even a passive front end (Takenoshita and Fujii 1998;Deslandes and Wu 2001; Yan et al. 2004; Hao et al. 2005).

A horizontally polarized omnidirectional SIW slot array is exemplified in Fig. 36 (Hua et al. 2008). Theantenna is designed using a normal PCB (er= 2.2, tand= 0.003, thickness= 1.5 mm) and fabricated witha standard PCB process, wherein longitudinal slots are etched on the upper and bottom grounds of theSIW. The antenna designed at 6GHz shows a bandwidth (VSWR � 1.5) of 3 % and very good E-planeomnidirectional radiation patterns.

Apart from the waveguide slot antenna array, the horizontally polarized omnidirectional antennas basedon microstrip slot configuration have been reported as well (Qing et al. 2012).

Planar Antenna ArrayConsider a group of antenna elements arranged in a ring formation as shown in Fig. 37a, where N antennaelements are uniformly positioned along a circle with a radius of r. Assuming the antenna elements areequally excited with dominant electric current components in the direction of increasing f, horizontallypolarized omnidirectional radiation is expected to be generated by selecting suitable N and r (McEwanet al. 2003). The antenna configuration features the null radiation at zenith while varied maximumradiation at a specific elevation angle. For example, the maximum radiation will occur at y of 90� ifdipoles are used as the element; the maximum radiation will be directed to a certain elevation angle whenthe antenna elements are with the ground plane (such as a microstrip antenna), wherein the omnidirec-tional radiation with a conical beam is generated.

One antenna design is demonstrated in Fig. 38, where four broadband T-dipole elements (Sabatier2003) are used to achieve E-plane omnidirectional radiation. The antenna is implemented on a circularPCB such as RF4; the T-dipoles are etched on the upper side of the PCB and are interconnected by ametallic square, which is also the ground plane for the microstrip line on the opposite side. The feed islocated at the center of the antenna. The ground of the coaxial cable is connected to the ground plane of theantenna, and the inner conductor of the coaxial cable is connected to the microstrip line through thesubstrate. A proper impedance matching can be achieved using two stubs: a parallel stub located between

Fig. 36 SIW slot array antenna


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the two horizontal aims of the T-dipole and a serial stub at the end of the microstrip line. The four-elementT-dipole antenna array (Ma and Wu 2006) achieves an impedance bandwidth of 80 % (3–7 GHz) for thereflection coefficient of less than�9 dB and gain of about 0 dBi. Good E-plane omnidirectional radiationpatterns are achieved from 3 to 5.4 GHz with omnidirectionality of 3 dB.

Using more antenna elements may benefit the enhancement of the omnidirectionality while making theantenna configuration more complicated (Wu et al. 2007). In addition, applying broadband antenna

Fig. 37 Planar antenna array, (a) basic configuration, (b) antenna array with four dipole antennas, and (c) antenna array withfour microstrip patch antennas


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elements such as log-periodic antenna or tapered slot is helpful to improve the bandwidth of the antennaarray (Fu and Hu 2008).

For the ceiling mounted indoor WLAN base station antennas, omnidirectional radiation with conicalbeam is preferable, where the antenna offers omnidirectional pattern in azimuth plane with a null at zenithbut peak radiation at specific elevation angle (such as 60�). Monopole-type antennas are widely used forsuch purpose while they radiate vertically polarized wave and normally with high profile. Figure 39 showsa broadband horizontally polarized omnidirectional antenna with conical beam (Wu and Luk 2009). Thefour magnetoelectric dipoles are excited by an in-phase tapered power divider located below the groundplane. The configuration of each magnetoelectric dipole is a combination of an electric dipole and amagnetic dipole. The upper part of the antenna, which is an electric dipole, is constructed by a pair ofsectorial-shape horizontal plates. The electric dipole is connected to the lower part of antenna, which isequivalent to a folded magnetic dipole. The magnetic dipole is due to a pair of vertically oriented shortedpatch antennas, coupled by a G-shaped strip feed to produce a magnetic current along the edge of the twovertical walls. The low-profile antenna (height of 0.1867l0) achieves an impedance bandwidth of 38 %(1.61–2.38 GHz) and average antenna gain of 5 dBi. Stable conical beam patterns are achieved across theoperating band.

Fig. 39 Broadband omnidirectional antenna with conical beam using magnetoelectric dipoles

Fig. 38 Horizontally polarized omnidirectional antennas with four T-dipoles


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Dielectric Resonator AntennaBesides the patch array and magnetoelectric dipole array, a dielectric resonator antenna is able to generatehorizontally polarized omnidirectional radiation with a conical beam as well. Regarding the first threefundamental modes in a cylindrical dielectric resonator (DR), TM01d, HEM01d, and TE01d, the first twomodes have been extensively studied for the vertically polarized omnidirectional radiation with broadsideradiation patterns, respectively. In contrast, the TE01d mode is usually used as a filter because of its high-quality (Q) factor. Recently, it was reported that the TE01dmode in a cylindrical DR is able to be utilized toproduce horizontally polarized omnidirectional radiation with conical beam patterns.

A design example is illustrated in Fig. 40 (Zuo and Fumeaux 2011). A cylindrical DR with er of10, diameter of 40 mm, and height of 6.62 mm is mounted on a Rogers Ultralam PCB (er of 2.5 andthickness of 1.524 mm) with metalized bottom side. The TE01d mode is excited by two curved microstriplines with identical sizes printed onto the top side of the substrate. Two SMA connectors, located at theback of the substrate, are utilized to connect the twomicrostrip lines to a splitter. The measured impedancebandwidth (|S11|< �10 dB) covers a frequency range from 3.84 to 4.06 GHz. The antenna gain is above4 dBi with a maximum of 5.2 dBi. Compared to other types of E-plane omnidirectional antennas, the DRantenna is attractive for its simplicity, low profile, and relatively high gain.

Dual Linearly Polarized Omnidirectional Antenna

Antennas with dual polarization are widely studied and applied in wireless communication systemsbecause of the improved channel capacity and mitigated multipath fading. The polarization diversity thatcombines pairs of antennas with orthogonal polarizations has been widely used in mobile communica-tions. For the polarization diversity scheme with a 360� full coverage, dual-polarized (vertically/horizon-tally) omnidirectional antennas are needed for base stations. Therefore, the design of dual-polarizedomnidirectional antennas has become an interesting topic in wireless communications nowadays.

It is well known that the antenna configurations with vertically polarized omnidirectional radiation aredistinct from those with horizontally polarized omnidirectional radiation. It is almost impossible togenerate the orthogonal polarized omnidirectional radiation using one radiator; the most applicablesolution is to combine the vertically and the horizontally polarized omnidirectional antenna elementstogether for achieving the dual-polarized omnidirectional radiation. An exception is the dual-polarizedomnidirectional DR antenna, where different modes in one radiator are used to generate the orthogonalpolarized omnidirectional radiation.

Fig. 40 Dielectric resonator antenna with TE01d mode for horizontally polarized omnidirectional radiation


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Figure 41 illustrates some dual linearly polarized omnidirectional antennas. In Fig. 41a (Li et al. 2012,2014), the dual polarizations are achieved by positioning two orthogonal slots cut onto the walls of aslender columnar cuboid. The antenna design at the WLAN band of 2.4–2.48 GHz achieves gains ofgreater than 3.17 dBi and 1.19 dBi for vertical and horizontal polarizations, respectively, and portisolation of higher than 33.5 dB with an antenna size of 0.664l0 � 0.088l0 � 0.088l0. A compactdesign with reduced size was reported (Li et al. 2014), where a cavity-backed notch is applied for thehorizontal omnidirectional polarization and a folded slot for the vertical omnidirectional polarization.With the antenna size of 0.336l0 � 0.096l0 � 0.096l0, the antenna achieves isolation of higher than32.5 dB and a maximum gain of 2.78 dBi and 1.35 dBi for vertical and horizontal polarization,respectively, from 2.4 to 2.48 GHz. The radiation patterns at 2.44 GHz show an omnidirectionality of4.5 dB for vertical polarization and 2.4 dB for horizontal polarization.

Figure 41b exhibits a multiband/wideband dual linearly polarized antenna (Dai et al. 2013). Theantenna is able to cover the frequency ranges of 806–960 MHz (17.4 %) and 1,880–2,700 MHz (35 %)for vertical polarization and 1,880–2,700MHz (35 %) for horizontal polarization, which makes it suitablefor GSM900, CDMA800, GSM1800, GSM 1990, PCS, UMTS, and LTE communication systems. Theantenna is a combination of a modified asymmetric biconical antenna for the vertical polarization and sixprinted dipoles with concentric placement for the horizontal polarization. The element for verticalpolarization consists of two modified cones with different diameters, designed to work in the multiband.The printed dipoles for horizontal polarization are fed by a six-way power splitter printed on a PCB. Thehorizontal polarized element is positioned between two cones of the vertical polarized element, forming acompact structure. The antenna exhibits desired performance with gain of 1.4–2.2 dBi for verticalpolarization at lower band, and about 4.5 dBi for both polarizations at higher band; an isolation of greaterthan 25 dB; cross-polarization levels of lower than �15 dB in the azimuth plane for both polarizations;and desired omnidirectional patterns with omnidirectionality of 1.5 dB.

Fig. 41 Dual linearly polarized omnidirectional antennas; (a) compact slot antennas in slender column (Li et al. 2012, 2014),(b) multiband/wideband antenna (Dai et al. 2013), (c) low-profile antenna (Deng et al. 2012), and (d) dielectric resonatorantenna (Zou et al. 2012)


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A low-profile broadband dual linearly polarized antenna as shown in Fig. 41c was designed to operateat 2-GHz band for the based-station applications (Deng et al. 2012). The antenna is a combination of amodified low-profile monopole for the vertical polarization and a circular planar loop for the horizontalpolarization. The modified low-profile monopole is a circular folded patch shorted by four tubes, while thecircular loop consists of four half-wavelength arc dipoles. The antenna achieves a bandwidth of 25 %(1.7–2.2 GHz) with an isolation of around 40 dB and omnidirectionality of 2.5 dB and 1.5 dB for thevertical and horizontal polarizations, respectively.

In contrast to the conventional dual linearly polarized antenna designs employing different radiatingelements to generate the orthogonal polarized radiations, a DR antenna is able to generate dual linearlypolarized radiation with single radiator. Figure 41d shows an omnidirectional cylindrical DR antennasimultaneously realizing the horizontally and vertically polarized omnidirectional radiation with lowcross coupling (Zou et al. 2012). The horizontally and vertically polarized radiations are achieved byexciting the orthogonal TE01d and TM01d modes in a single cylindrical dielectric resonator. The TE01d

mode corresponding to horizontally polarized radiation is with higher Q-factor so that the bandwidth ofhorizontal polarization is smaller than that of the vertically polarized radiation generated by the TM01d

mode. To increase the bandwidth of the TE01d mode and suppress the influence of higher-order modes,two groups of four radially arranged microstrip feeding lines with two different lengths are utilized, and adual-polarized omnidirectional radiation is realized at the overlapping operating band of the modes with abandwidth of 7.4 % (3.78–4.07 GHz).

The other dual linearly polarized omnidirectional antenna designs are reported with different config-urations; the combinations include notched disk antenna and wire antenna, planar conical cone antennaand circular loop antenna, collinear array antenna and slotted cylinder antenna, and so on. For conve-nience, a comparison of some antennas is tabulated in Table 1.

Circularly Polarized Omnidirectional Antenna

A linearly polarized antenna radiates wholly in one plane containing the direction of propagation. Anantenna is vertically or horizontally polarized when its electric field is perpendicular or parallel to theEarth’s surface. For a circularly polarized antenna, the plane of the polarization rotates in a corkscrewpattern, making one complete revolution in each wavelength. The electric field of the circularly polarizedantenna lies in both the vertical and horizontal planes as well as all the planes in between in a rotatingmanner. Compared to linearly polarized antenna, a circularly polarized antenna features the uniqueadvantage as follows:

• Able to establish a reliable signal link regardless of the orientation of the antennas.• More effective to suppress the multipath interferences.• More resistant to signal degradation due to inclement weather conditions.• The circularly polarized wave is more effective be propagated through wall and can achieve an overall

better reach throughout the building.

Circularly polarized omnidirectional antennas have been widely used in television broadcasts; mobilesatellites; space vehicles such as airplanes, missiles, rockets, and spacecraft; mobile communication; andWLANs.

A circularly polarized radiation can be obtained by generating two orthogonal field components with90� phase difference. To design a circularly polarized omnidirectional antenna is more complicated thanthat of the linearly polarized one since the designer has to consider the spatial orthogonal field components


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Tab

le1

Com

parisonof

theduallin

earlypolarizedom

nidirectionalantennas

Reference

Antenna

confi

guratio

nFrequency

range(G

Hz)

Bandw

idth

(%)Return

loss

(dB)

Gain

(dBi)

Isolation

(dB)

Cross-

polarizatio

n(dB)

Omnidirectionality

(dB)

Rem

ark

Kuga

etal.(1998

)VP:three

verticalwireloop

1.688–

1.838

8.5

10

0*>14

N.A.

5*A

verage

gain

HP:n

otched

disk

1.648–

1.878

131

00

N.A.

3

Ando

etal.(2008

)VP:p

lanardiscone

2.4–

2.5

4.1

10

�2.5

>20

�14.4

1.6

Dual-band

design

4.9–

6.0

20.2

0.9

�13.8

3.8

HP:twocircular

dipole

2.4–

2.5

4.1

10

0�8

.65.4

4.9–

6.0

20.2

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and the proper phase delay of the field components over a specific bandwidth at desired directions. Thestudies of circularly polarized omnidirectional antenna can be dated back to 1940s (Wheeler 1947;Lindenblad 1941; Brown and Woodward 1947). Over the last few decades, a number of circularlypolarized omnidirectional antennas have been developed; the configurations can be categorized asfollows:

• Single radiator• Conformal array with circularly polarized radiator• Combination of linear polarized radiator

Circularly Polarized Omnidirectional Antennas with Single Radiator

Helical AntennaHelical antenna is well known for its circularly polarized radiation characteristics. When the circumfer-ence of a helical antenna is approximately one wavelength, the axial mode of radiation is dominant; whenthe circumference is much smaller the one wavelength, the normal mode is dominant, as shown in Fig. 42.

As illustrated in Fig. 43, the far field of a helix may be described by two components of the electric field,Ef and Ey, which are contributed by the small loop and the short dipole, respectively. For a small helix, thecurrent is assumed to be uniform in magnitude and in phase over the entire length of the helix. The far fieldof the small loop has only an Ef component as below (Kraus and Marhefka 2008):

Ef ¼ 120p2 I½ � sin yr

A

l2(6)

where A is the area of the loop, A ¼ prD2=4The far field of the short dipole has only an Ey; it can be expressed as

Ey ¼ j60p I½ � sin y

r

S

l(7)

Fig. 42 Field patterns of the axial and normal radiation modes of the helical antenna; (a) normal mode and (b) axial mode


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Comparing Eqs. 6 and 7, the j operator in Eq. 7 and its absence in Eq. 6 indicate that Ef andEy are in-phasequadrature. The ratio of the magnitudes of Eqs. 6 and 7 then gives the axial ratio of the polarization ellipseof the far field

AR ¼ Eyj jEf�� ¼ Sl

2pA¼ 2Sl

p2D2 ¼2SlC2

l

(8)

where Sl and Cl are the pitch and circumference of the helix in wavelength, respectively.A circularly polarized radiation can be achieved when AR is a unit; from Eq. 8, we have

pD ¼ffiffiffiffiffiffiffiffi2Sl

por Cl ¼

ffiffiffiffiffiffiffi2Sl

por A ¼ l

2pS (9)

The relation was first presented by Wheeler (1947) so that such antenna is also called “Wheeler coil.”A monofilar normal-mode helix is a resonant and narrowband circularly polarized omnidirectionalantenna. To enhance the bandwidth, the bifilar configuration can be applied (Amin et al. 2013).

Dipole-/Monopole-Type Antenna with PolarizerIt is known that a pair of 45� tilted dipole antennas with a l/4 spacing, in orthogonal crossed disposition,radiates circularly polarized wave in both directions of the array axe if current distribution on the dipoles isidentical. Figure 44 exhibits the schematic view of a dipole antenna with a polarizer for circularlypolarized omnidirectional radiation; where the polarizer is formed by a concentric array of 45� tiltedparasitic elements, the spacing between the dipole and the parasitic elements is set to R = l/4. Theradiation field from the vertical dipole can be divided into two orthogonally crossed electric fields (paralleland perpendicular components to the parasitic strip elements); each tilted element is excited by the parallelcomponent of the field accordingly. Because of the l/4 separation between the dipole and the parasiticelements, the reflection of the perpendicular component is canceled. The 90� phase difference between thetwo radiated field components is achieved by the interaction between the parasitic elements and theparallel component.

Generally, the parasitic elements are uniformly spaced approximately l/2 apart. The length of theelements is around 0.5l and the width of each element about l/10. In practical design, the parasiticelements are normally implemented with a supporting material such as a piece of foam or PCB; thedimensions as well as the spacing between the dipole antenna and the parasitic elements are required to beadjusted for optimal performance.

Fig. 43 Small helix for normal mode calculation; (a) helix, (b) small loop, and (c) short dipole


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Based on the basic configuration, a number of variation of the polarizer have been reported forgenerating circularly polarized omnidirectional radiation, where biconical cones, sleeve dipole, and aconical skirt monopole are used as the radiators while multilayer parasitic elements are applied (Kelleherand Morrow 1955; Goatley and Green 1956; Sakaguchi and Hasebe 1993; Fernandez et al. 2007).

Circularly Polarized Omnidirectional Antennas with Multiple Circularly PolarizedRadiators

Conformal Cylindrical ArrayThe arbitrarily oriented satellites and space vehicles such as missiles and rockets require a circularlypolarized omnidirectional antenna to reliably maintain a communication link with a central receiving link.A low-profile conformal cylindrical omnidirectional antenna array is desirable for such applications(Galindo and Green 1965; Wu 1995). To configure such antenna arrays, the most convenient way is to

Fig. 44 Dipole/sleeve antenna with a conformal array of 45� tilted parasitic elements; (a) dipole configuration and (b) sleevedipole configuration


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position a certain number of circularly polarized radiators with equidistant intervals on the surface of acylindrical surface. Desired omnidirectional radiation is able to be achieved by arranging adequatenumber of radiators on the cylindrical surface properly. A variety of low-profile radiators such as crossdipoles, truncated microstrip patch, spiral, slots, and so on can be utilized as the array elements (Dubostet al. 1979; Nakayama et al. 1999) as exhibited in Fig. 45.

Cutting slots directly on the surface of a circular waveguide is able to achieve circularly polarizedomnidirectional radiation as well. Such an antenna configuration may be more preferable from a systempoint of view because of the merits of simplicity, robustness, and lightweight. Figure 46 shows a slot arraydesign (Masa-Campos et al. 2007), where the conformal slot array is designed at the millimeter-waveband of 36.7–37 GHz; the eight slot pairs are symmetrically positioned on the wall of a circular waveguidecarrying TM01 mode. Each slot pair consists of two resonant slots with a length of l0/2 and a separation oflg/4 (lg is the guided wavelength of the propagated TM01 mode).

The conformal cylindrical antenna arrays with traditional circularly polarized radiator are with limitedbandwidth for impedance matching, in particular, the circular polarization characteristic of axial ratio,which makes the antennas not suitable for modern mobile communications. For example, the second- andthird-generation mobile communication systems require a base-station antenna covering the frequencyrange from 1.71 to 2.17 GHz; a bandwidth of at least 25 % is needed. Recently, Quan et al. (2013)presented a broadband conformal circularly polarized omnidirectional antenna as shown in Fig. 47. Thefour rectangular loop elements are first printed on a flexible thin dielectric substrate and then rolled into ahollow cylinder. A conducting cylinder is added inside the hollow cylinder for achieving desiredomnidirectional circularly polarized performance. The antenna exhibits a bandwidth of 41 %(1.65–2.5 GHz) for axial ratio of less than 3 dB and omnidirectionality of 1 dB and 45 %(1.58–2.5 GHz) for return loss greater than 10 dB.

Back-to-Back PatchesTo simplify the complicated feeding network of the conformal cylindrical array, the back-to-back coupledpatch antennas are investigated for circularly polarized omnidirectional radiation; H. lwasaki and Chibareported a coplanar waveguide (CPW) fed back-to-back patch antenna (lwasaki and Chiba 1999) as

Fig. 45 Conformal cylindrical array; (a) basic configuration and (b) CP elements


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Fig. 46 Conformal circularly polarized omnidirectional antenna with slot radiators on a circular waveguide

Fig. 47 A broadband omnidirectional circularly polarized antenna


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shown in Fig. 48a, where two truncated microstrip patch elements are positioned on the opposite sides of aPCB slab and a CPW feed line is sandwiched in between. The antenna exhibits narrow impedancebandwidth of 13 % (VSWR < 2) and axial ratio of less than 4 dB in the azimuth plane.

Narbudowicz et al. (2013) proposed a dual-band circularly polarized omnidirectional antenna usingback-to-back slotted patch configuration. As illustrated in Fig. 48b, the antenna consists of two layers ofsubstrate with three layers of metallization. The inner metallization forms the ground plane and comprisesthe coplanar waveguide (CPW) feeding structure, whereas the outer ones form two patches. The patchesare additionally connected together by a thin copper strip. The radiators are electromagnetically coupledto the 50-OCPW located along the diagonal of the patches. The two slots of slightly different lengths witha lumped capacitor connected across the center are utilized to provide dual-band circularly polarizedradiation. The conducting strip connected the patches, and a reduced ground plane size allows betterimpedance matching and reduced overall size. The dual-band antenna radiates right-handed circularpolarization at both the lower and upper frequencies of 1.329 and 1.565 GHz with bandwidth of about20 MHz, respectively. The axial ratio in the omnidirectional plane varies from 0.4 to 3.8 dB for the lowerband and from 0.6 to 4.8 dB for the upper band. The omnidirectionality in the omnidirectional plane is3.3 dB for the lower band and 3.8 dB for the upper band.

Fig. 48 Circularly polarized omnidirectional antennas with back-to-back patches, (a) truncated patches and (b) slottedpatched


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In short, the back-to-back patch design features simple configuration with limited bandwidth andradiation performance. Furthermore, it is difficult to achieve desired omnidirectionality and axial ratio, inparticular, at the directions apart from the boresight.

Circularly Polarized Omnidirectional Antennas with Combined Linearly PolarizedRadiators

Lindenblad AntennaThe circularly polarized omnidirectional radiation can be realized by positioning multiple linearlypolarized radiators properly. The Lindenblad antenna might be the first circularly polarized omnidirec-tional antenna with such a configuration (Lindenblad 1941). As shown in Fig. 49, a Lindenblad antennauses four dipole elements that are fed in phase to create an omnidirectional circularly polarized radiationpattern. The dipoles are canted at 30o from horizontal and positioned equally around a circle of about l/3diameter. For easy impedance matching with a 50-O feed line, folded dipole instead of straight dipole ismost used in practical designs. Seventy years after the invention, this antenna is still widely used inbroadcasting stations, satellite communications, and amateur radios.

Monopole and Loop CombinationA dipole or monopole antenna is desired for vertically polarized omnidirectional radiation, and a loopantenna with a uniform current flowing in a single direction is able to generate horizontally polarizedomnidirectional radiation. The combination of the dipole and loop antennas with an adequate arrangementto provide a 90� phase difference between the currents on the orthogonal radiators features the possibilityto produce a circularly polarized omnidirectional radiation.

As shown in Fig. 50, considering the combination of the short dipole of length L and a loop with aradius of a, the dipole is positioned coincidently with the z-axis and with its center at the origin. The farfield generated by the short dipole is expressed as (Kraus and Marhefka 2008)

E ¼ jI½ �Lom04pr

sin yy (10)

where [I] is the current on the short dipole, o is the angular frequency, m0 is the permeability of the freespace, and r is the distance between the origin and observation point.

The loop is placed in the x-y plane with its center at the origin; its axis is along the z-axis. The current onthe loop is assumed to be the same with the dipole, which is uniform and in phase along the loop. The farfield of the loop can be written as [16]

Fig. 49 Lindenblad antenna, (a) antenna configuration and (b) practical antenna design


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E ¼ I½ �aom02r

J 1 ba sin yð Þf (11)

where J1 is the Bessel function of the first order and of argument (basiny).By superposing the far fields of the dipole and the loop, the far field of the combining dipole/loop

antenna is given by

E ¼ jI½ �Lom04pr

sin yyþ I½ �aom02r

J 1 ba sin yð Þf (12)

Then, the axial ratio of the combining antenna can be given by

AR ¼ 20log10L

2pasin y

J 1 ba sin yð Þ��

��

�� (13)

The circularly polarized omnidirectional antenna is required to achieve good axial ratio in the azimuthplane (x-y plane). Considering y = 90�, Eq. 13 can be written as

AR ¼ 20log10L

2pa1

J 1 bað Þ��

�� (14)

The AR equals 0 dB when

L

2pa1

J 1 bað Þ��

�� ¼ 1 (15)

From the above equation, it can be found that, by choosing a suitable value of L and a in Eq. 15, the axialratio can equal 0 dB, and thus the antenna can achieve a perfect circularly polarized radiation.

Fig. 50 Configuration of an ideal omnidirectional circularly polarized antenna combining a short dipole with the height ofL and a small loop with the radius of a


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One of the antenna examples as shown in Fig. 51a is designed for 2.4-GHzWLAN applications, wherea top-loaded cylindrical monopole generates vertically polarized omnidirectional radiation; four printedidentical arc-shaped dipoles form a virtual loop to produce horizontally polarized omnidirectionalradiation (Hisao and Wong 2005). The 90� phase difference between the two orthogonal components isachieved through tuning the length of the feed lines for the arc-shaped dipoles, thereby resulting incircularly polarized omnidirectional radiation. Figure 51b exhibits another example, where a metal sleeveacts as a monopole and the printed spoke-like metal strips fabricated onto two pieces of substrate function

Fig. 51 CP antenna designs with combining radiators, (a) top-loaded monopole plus arc-shaped dipoles and (b) sleeveantenna plus printed spoke-like strips, (c) zeroth- and first-order resonance modes epsilon-negative transmission line pluscurved strips, and (d) dielectric resonator antenna plus modified Alford loop


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as a loop (Li et al. 2013). Figure 51c demonstrates a metamaterial-based dual-band circularly polarizedomnidirectional antenna comprising a circular mushroom structure with curved branches. The dual-bandvertically polarized radiation is achieved by using the zeroth- and the first-order resonance modes of theepsilon-negative (ENG) transmission line, and the horizontally polarized omnidirectional radiation isrealized by the curved branches (Park and Lee 2012).

A compact omnidirectional circularly polarized antenna combining cylindrical DR antenna and atop-loaded modified Alford loop is exhibited in Fig. 51d (Li and Leung 2013). Fed by an axial probe,the DR antenna operates in its TM01d mode, which radiates like a vertically polarized electric monopole.The modified Alford loop comprising a central circular patch and four curved branches is placed on thetop of the dielectric resonator antenna and provides an equivalent horizontally polarized magnetic dipolemode. Omnidirectional circularly polarized radiation can be obtained when the two orthogonally polar-ized fields are equal in amplitude but 90� different in phase.

Conclusion

Omnidirectional antennas are essential for modern wireless communication systems, in particular forbase-station applications for providing a wide coverage. Over the last several decades, many omnidirec-tional antennas have been developed for various applications. In general, the omnidirectional radiationcan be achieved by using single radiator with circularly symmetrically distributed current or multipleradiators with a combined circular current distribution. This chapter has outlined the classic omnidirec-tional antenna configurations based on the antenna polarizations and discussed a number of antennadesign examples for each topic. The relevant literature review is believed to benefit the antennaresearchers, engineers, and students to further understand the omnidirectional antennas.

Cross-References

▶Antenna Systems for Cellular Base Stations▶Antennas in Access Points of WLAN/WiFi▶Broadband and Multiband Planar Antennas▶Circularly Polarized Antennas▶Conformal Array Antennas▶Dielectric Resonator Antennas▶Linear Wire Antennas▶Loop Antennas▶Low-profile Antennas▶Metamaterials and Antennas▶ Spiral, Helical and Rod Antennas▶ Substrate-integrated Waveguide Antennas▶Ultra-wideband Antennas▶Waveguide Slot Array Antennas


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