Published in IET Microwaves, Antennas & PropagationReceived on 13th March 2012Revised on 24th January 2013Accepted on 16th March 2013doi: 10.1049/iet-map.2012.0151

ISSN 1751-8725

Wideband log-periodic dielectric resonator array withoverlaid microstrip feed lineRuna Kumari, Santanu Kumar Behera

Department of Electronics and Communication Engineering, National Institute of Technology, Rourkela, Odisha, India

E-mail: runakumari15@gmail.com

Abstract: A log-periodic dielectric resonator antenna for possible wideband operation is considered. The proposed designconsists of seven dielectric resonators (DRs) of rectangular cross-sections using a low-cost Teflon-based dielectric materialwith low permittivity. It has a multilayer configuration, where the DRs and feeding line are located at different layers. Forease of fabrication, the radiators are excited by an overlaid microstrip feed line on the first layer. Different characteristics suchas S-parameters, gain, input impedance and radiation patterns of the proposed design are studied. The fabricated prototypeexhibits a gain of 6 dBi (or better) over a wideband of 6.5–11.3 GHz with |S11| less than − 10 dB. The results from thesimulation are found to closely follow those of the fabricated prototype.

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1 Introduction

Over the last few decades, a tremendous growth has been seenin the wideband high-frequency communication systems. Inaddition to other necessary requirements, a compact size,high-efficiency antenna design with low conductor loss ismandatory for enabling high data rate communication. Inrecent years, dielectric resonator antennas (DRAs) haveproven to be of exceptional low profile, light weightantennas, which provide wide bandwidth with lowdissipation loss compared with microstrip antennas [1, 2].DRAs offer a high degree of flexibility and versatility overa wide frequency range. As compared with microstripantennas, the DRA has a much wider impedancebandwidth. DRAs have negligible metallic loss makingthem highly efficient when operated at millimetre wavefrequencies. The available basic shapes of dielectricresonators are cylindrical, rectangular and hemispherical,whereas different modified shapes are also possible, whichinclude ring, disc, sectored cylindrical, half-split cylindrical,triangular, notched rectangular, conical and elliptical [3].Different feed mechanisms such as probe, microstrip lines,slots, coplanar waveguide and dielectric image guide feedmay be used.In practice, there are innumerable techniques to achieve

wideband applications, but the most common techniquesadopted for wideband DRAs are changing the shapes of thedielectric resonator, use of modified feed geometries,optimising the feeding mechanism and DRA parameters,stacked DRAs, embedded DRAs and DRA arrays [4–8].A DRA design using one- or two-step stair geometries ofdielectric resonators has been proposed to achieve wideband,where the obtained impedance bandwidth is more than 20%for one-step or more than 40% for a two-step DRA design[9]. A wideband rectangular-shaped dielectric resonator with

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a horizontal tunnel provides an impedance bandwidth of20% for WLAN applications [10]. A DRA design fed by anL-shaped microstrip monopole yields more than 25%bandwidth, which is greater than an ordinary DRA [11]. Ithas been reported that by using a circularly polarisedtrapezoidal DRA excited by a single rectangular slot, a widebandwidth over 20% can be obtained [12]. A widebandfour-element cylindrical DRA array gives 29% bandwidthwith monopole radiation patterns [13]. A circularly polarisedwideband rectangular DRA excited by a concentric openhalf-loop has been presented with 20% impedance matchingbandwidth [14]. In addition, a wideband rectangular-shapedDRA is designed to provide 30.9% of impedance bandwidth,which is possible with the strip-fed excitation method [15].All these DRA designs provide wide bandwidth with goodradiation patterns; however, the bandwidth is limited to only40%. Recently, an ultra-wideband rectangular DRA designachieved a 60–110% bandwidth with an average gain of 6dBi by employing a low permittivity dielectric insert offull-length between the DR and the ground plane [16].In the recent era, the log-periodic technique has been

employed to achieve wideband characteristics with anegligible variation in electrical parameters such as inputimpedance and radiation patterns of the antenna. Variouslog-periodic antenna designs have also been reported toachieve modest levels of gain with wide bandwidth forhigh-frequency applications. In the 1960s, some theoreticalas well as experimental works on log-periodic dipole arraydesign were carried out by Isbel and Carrel, whodemonstrated that log-periodic antennas have reasonablegain with broad bandwidth [17–19]. Therefore a number ofdifferent log-periodic antennas have been investigated bydifferent researchers. A log-periodic Koch-dipole array wasdesigned as a miniaturised wideband antenna for 2–3 GHzimpedance bandwidth [20], whereas a 94 GHz log-periodic

IET Microw. Antennas Propag., 2013, Vol. 7, Iss. 7, pp. 582–587doi: 10.1049/iet-map.2012.0151

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planar on-chip antenna on GaAs was successfully fabricatedby using millimeter-wave monolithic integrated circuitprocess technology [21]. A single-layer printed log-periodicdipole array design fed by substrate-integrated waveguidewas presented to provide low profile, light weight andwideband characteristics [22]. Recently, a technique hasbeen demonstrated to reduce the size of log-periodic dipolearray antennas, which employs inductive loads on theelements of antenna [23]. For UWB applications, a printedlog-periodic dipole antenna with multiple notched bandshas been introduced [24]. Nevertheless, it was found that allthe above-described dipole or planar log-periodic antennassuffer from metallic losses, which limit their performance.One advanced method to achieve wide bandwidth with

high-efficiency and less metallic losses is possible byapplying the log-periodic technique to a DRA array. In thepresent work, a seven-element frequency-independentdielectric resonator array with an overlaid microstrip linefeeding has been designed, simulated and experimentallytested. The dielectric resonators are the resonating portionof array that offers low loss and moderate gain with goodefficiency for high-frequency applications. In a log-periodicdielectric resonator antenna (LPDRA) array, the resonator’slength, width, height and spacing between resonatingelements along the array are varied with a scaling factor, t.The scaling factor chosen for this design is 0.96. From thesimulated and measured results, it has been found that theproposed antenna is suitable for wideband applications,which cover the 6.5–11.3 GHz frequency band.

2 Design and analysis of LPDRA array

The proposed frequency-independent LPDRA array consistsof seven dielectric resonators as shown in Fig. 1. In thepresent DRA design, we have used rectangular-shapedresonators since the rectangular shape is more flexibleamong all other basic shapes. The main objective of thisDRA array is to achieve a wide bandwidth by applying thelog-periodic technique. The geometry of the LPDRA arrayentails two sheets of substrate with the same width, sameheight but different length. The dielectric resonators ofdifferent heights, widths and lengths are mounted on theupper side of overlaid double substrates and a partialground plane is fabricated on the rear side of the lowersubstrate with a microstrip line feeding in between the twosubstrates as shown in Fig. 1.The array is fed at the small end of the structure, and the

maximum radiation is towards this end, whereas the far endof the microstrip line is terminated with an open circuit.The alternating resonators of this array are arranged in sucha way that they are 180° apart from one another in phase.The length, width, height and spacing of the array elementsare graduated logarithmically from one end to the other insuch a way that certain dimensions of adjacent elementsallow a constant ratio with each other. If the design ratio isdenoted by t, the length (L), width (W ), height (H ) andspacing (S) between the DRA elements are given by (1)

t = LmLm+1

= Wm

Wm+1= Hm

Hm+1= Sm

Sm+1(1)

where t is a scaling factor. From equation (1), when thedimensions of larger resonators are multiplied by t, it scalesinto itself with the larger resonator m + 1 becoming m, and

IET Microw. Antennas Propag., 2013, Vol. 7, Iss. 7, pp. 582–587doi: 10.1049/iet-map.2012.0151

resonator m becoming m− 1, which implies that the arraywill have the same electrical characteristics at allfrequencies that are related by t.An analytical study has been carried out on a LPDRA array

design. The design of the proposed antenna initialises withthe t and σ (relative spacing) values depending on ourdesired gain, which has been chosen from the Carrel’stable. According to the Carrel’s table, the maximum valueof t should be 1 [18]. For the proposed antenna, the valueof t is chosen as 0.96 and the related σ value can beobtained by using (2) and (3), where σi is the ideal value ofrelative spacing [25]

si = 0.258t− 0.066 (2)

0.05 ≤ s ≤ si (3)

Fig. 1 Proposed seven elements LPDRA array

a Top viewb Schematic viewc Front view of fabricated arrayd Rear view of fabricated array

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The elements of a typical log-periodic antenna are arranged inascending order starting from the feeding end towards the farend of the antenna, which makes an angle 2α as shown in Fig. 2.The design parameter α can be realised by taking the values

of t and σ

a = tan−1 1− t

4s

[ ](4)

Generally, 100≤ α≤ 450. The value of t starts to decreasewith an increasing α.In a log-periodic array, the designed bandwidth (Bd) should

be greater than the desired bandwidth (B0), whereas thedesired bandwidth of the array (B0) is the ratio of thehighest ( fH) to the lowest ( fL) range of frequency given by

B0 =fHfL

(5)

Thus, the designed bandwidth can be expressed in terms of B0

(desired bandwidth) and Br (active region bandwidth) as

Bd = B0Br (6)

Bd = B0 1.1+ 7.7(1− t)2cota[ ]

(7)

The number of resonating elements (NR) required for designof LPDRA array can be obtained as given below

NR = 1+ ln Bd

ln (1/t)

[ ](8)

According to (1), the dimensions (L, W, H and S) of eachdielectric resonator vary log periodically from one end tothe other. The dimensions of the largest element are alwaysassociated with the lowest frequency, whereas the smallestelement dimensions are related to the highest frequency.If Lm + 1 is the length of largest element (m + 1) and λmax isthe maximum wavelength associated with lowest frequency( fmin), and then Lm + 1 can be realised by

Lm+1 =lmax

4(9)

where

lmax =c

fmin���1r

√

c is the speed of light and ɛr is the relative permittivity of thedielectric resonator.

Fig. 2 Typical log-periodic antenna configuration

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After resolving the value of length, the other dimensionsassociated with the largest element such as width (Wm + 1),height (Hm + 1) and spacing between elements m + 1 and m(Sm + 1↔m) can be achieved as given below

Wm+1 = 0.8×Lm+1 (10)

Sm+1↔m = t× Lm+1 (11)

Lm+1

4≤ Hm+1 ≤

Lm+1

3(12)

According to the self-scaling property reported by Carrel, thedimensions (L, W, H and S) of each log-periodic element willbe scaled into dimensions of the next element using (1). Thebasic design of a LPDRA array is similar to that of a normallog-periodic array.The resonators of a LPDRA array are made up of Teflon

material with dielectric constant ɛr = 2.1. From the point ofease of fabrication, Teflon is best suited for very small sizedielectric resonators since the material is not prone tochipping and much easier to machine, which is mostdesired for the proposed array design [1].The resonators of the array can be directly coupled on the

microstrip line without using a second layer of substrate. TheLPDRA designed with a single layer of substrate is not able toprovide any proper matching as the inductive effect is higher.One additional layer of substrate is added to DRA in order tocompensate the inductive effect. This will exhibit betterflexibility of coupling as well as proper matching of the

Fig. 3 S11 against frequency plot for LPDRA array

Fig. 4 Input impedance against frequency plot of LPDRA array

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Fig. 5 Gain and radiation pattern measurement of LPDRA array in an anechoic chamber

a Front viewb Side view

Fig. 6 Measured radiation patterns of LPDRA array

a H-plane at 7.5 GHzb H-plane at 10.5 GHzc E-plane at 7.5 GHzd E-plane at 10.5 GHz

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DRA elements with feeding [26]. In this proposed arraydesign, all the dielectric resonators are supported by thedouble layer, inexpensive FR4 substrate having dielectricconstant (ɛs) 4.4 with the thickness of each sheet being 1.6mm. The upper substrate is of 74 mm length (Lus) with 30mm width (Wus), whereas the lower substrate has the sameheight and width, but with 80 mm length. A partial groundplane dimensioned as 74 mm × 30 mm is printed on the rearside of the lower substrate. The array is excited by anoverlaid microstrip line feeding, where the centre-alignedfeed line length (Lf) is 80 mm with 2.5 mm width (Wf). Thelargest dielectric resonator is of length (L) 12.3 mm, width(W ) 9.84 mm and height (H ) 3.4 mm with 11.8 mmcentre-to-centre spacing (S) between two resonators,whereas the dimensions of the other dielectric resonatorsare scaled by t as shown in Figs. 1a and b. The overalllength of the array is 80 mm × 30 mm. Since the resonantfrequency and the radiation resistance depend primarily onthe dielectric resonator’s dimension and are slightlyinfluenced by the substrate thickness, the height of both thesubstrate layer and the feed line is kept constant. Thisantenna design is used where wideband applications withmoderate gain and more directionality is required. Figs. 1cand d show the front and rear views of fabricated prototypeLPDRA array.

3 Results and discussions

A LPDRA array for 6.5–11.3 GHz bandwidth has beendesigned, fabricated and measured. The results of the sevenelement array are discussed in terms of bandwidth response,input impedance, gain and radiation pattern characteristics.The S-parameter measurement of the fabricated DRA arraywas performed using an 8720B Agilent Network Analyzer.The gain and radiation pattern measurements were carriedout in an anechoic chamber.

3.1 Bandwidth response and input impedancecharacteristics

The proposed log-periodic array with seven resonatorsprovides a wide bandwidth from 6.5 to –11.3 GHz. Thesimulated as well as measured S-parameter curves of theseven element frequency-independent LPDRA array withscaling factor 0.96 are plotted against frequency in Fig. 3.The simulated S-parameter curve approaches the measuredcurve. From the figure, we note that the proposed antennaprovides a multi-resonant wide bandwidth in comparisonwith other antennas seen in the literature. The bandwidth ofthe proposed antenna is found to be 54%.The results of the LPDRA array also show very good input

impedance values over the entire frequency range. The inputimpedance against frequency curves of the proposed antennaare presented in Fig. 4. The input resistance at resonantfrequencies is found to be nearly 50 Ω, whereas theimaginary part of the input impedance is zero. Thisprovides a very good impedance match to 50 Ω microstripline feed.

3.2 Gain and radiation pattern characteristics

The photographs of radiation patterns and gain measurementof LPDRA array are shown in Fig. 5. The measured co-polaralong with the cross-polar radiation patterns of the proposedLPDRA array are given in Fig. 6. The E-plane as well as

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H-plane at different frequencies (7.5 and 10.5 GHz) hasbeen studied. The H-plane radiation patterns are almostomnidirectional, whereas the E-plane radiation patterns arealmost in the broadside direction for high frequencies. Themeasured cross-polar rejection is found to be below − 20 dB.The measured gain of the antenna is shown in Fig. 7. The

overall gain is better than 6 dBi within the operating band,whereas the measured peak gain of the array is 8.6 dBi at9.8 GHz. From the far-field simulation, the estimatedantenna efficiency is found to be better than 83% within theoverall band.

4 Conclusions

A multi-resonant wideband LPDRA array has beenintensively studied. The proposed antenna is based onrecent developments of wideband DRA arrays andmulti-frequency log-periodic techniques. In this arraydesign, multiple rectangular shaped resonators of differentsizes are integrated in a log-periodic fashion. Thedimensions of adjacent resonators maintain a constantdesign ratio (t) with each other. The value of t for theproposed array is 0.96. An overlaid microstrip line is usedas a feeding to excite the log-periodic array. The prototypeof the designed antenna has been fabricated as well asmeasured. The S-parameter, input impedance, gains andradiation patterns have been reported. The log-periodicarray offers 54% bandwidth with continuous operation from6.5 to 11.3 GHz. It provides a peak gain of 8.6 dBi at 9.8GHz. The obtained radiation characteristics at differentfrequencies are in the broadside direction with a goodomnidirectional radiation in the H-plane of the antenna.There is good agreement between the simulated andexperimental results.

5 Acknowledgments

Authors thank Mr. Rajeev Jyoti, Group Head AntennaSystems Group, Space Applications Center (SAC) ISRO,Ahmedabad, India for providing simulation facilities in hisantenna design Laboratory. The authors also thankProfessor Amalendu Patnaik, Department of Electronics andComputer Engineering, IIT, Roorkee, India for providingmeasurement facilities in his Microwave Laboratory.Thanks are because of Professor R. K. Mishra, Departmentof Electronic Science, Berhampur University, India for hisvaluable suggestions during preparation of the paper.

Fig. 7 Measured gain of the LPDRA array

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