2OO5IAPBOOK(OEMO A tB RMO tSPROWfflIK5December 20-21, 2005, Johor Bahru, Johor, MALAYSIA

Simulation of Wideband Inverted Suspended Patch Antenna

Yusnita Rahayu, Razali Ngah, and Tharek A. Rahman

Wireless Communication CentreFaculty of Electrical EngineeringUniversiti Teknologi Malaysia

81310 Johor Bahru, Johor, Malaysia

Abstract - The radio spectrum is finite, but bandwidthutilization for wireless communication is increasingexponentially due to the emergence of new standardsdriven by the rapid growth in information servicesprovided by the Intemet. Therefore, there is a need fornew technology that can open a new door to wirelesscommunication. Ultra wideband (UWB) technologybased on the use of very narrow pulses on the order ofnanoseconds, covering a very wide bandwidth in thefrequency domain, could be a possible solution to thisproblem. UWB has had a substantial effect on antennadesign. This paper presents a wideband invertedsuspended patch antenna design for UWB applications.The designed antenna is simulated using AdvancedDesign System (ADS). For initial result, this antennaobtains 5 GHz of bandwidth at frequency 4.5 GHz to9.5 GHz with maximum VSWR of 2.

1. Introduction

Ultra wideband is a wireless technologydeveloped to transfer data at high rates over very shortdistances at very low power densities and an area ofimmense current interest, with numerous potentialapplications. A UWB frequency allocation has beenmade by the US Federal Communication Committee(FCC) between 3.1 and 10.6 GHz at a limited transmitpower of -413 dBm/MHz, and work is underway byregulatory bodies to achieve the same in Europe andAsia [1].

UWB has had a substantial effect on antennadesign. UWB antennas are specifically designed totransmit and receive very short time durations ofelectromagnetic energy. Currently modemtelecommunication systems require antennas withwider bandwidth and smaller dimensions thanconventionally possible. This has initiated antennaresearch in various directions, one ofwhich is by usingUWB antennas. Some research projects on UWBantenna have been reported in recent years [2]-[6].

Among the most important advantages of UWBtechnology are low system complexity and low cost.UWB systems can be made nearly 'all-digital", withminimal RF or microwave electronics. Because of the

inherent RF simplicity ofUWB designs, these systemsare highly frequency adaptive, enabling them to bepositioned anywhere within the RF spectrum. Thisfeature avoids interference to existing services, whilefully utilizing the available spectrum.

This paper presents a wideband invertedsuspended patch antenna design for UWB applications.The designed antenna is simulated using AdvancedDesign System (ADS) simulation software. The basicstructure of this antenna consists of a rectangularconductive plate and foam with inverted suspendedconfiguration as shown in Figure 1. This configurationis chosen in order to obtain broad bandwidth.

Probe position

Polypropelene

Foam Ground plane

Figure 1: Inverted suspended configuration ofUWB antenna(3D View)

2. UWB Antenna Requirements

There are two vital design considerations in UWBradio systems. Firstly, radiated power density spectrumshaping must comply with certain emission limit maskfor coexistence with other electronic systems [7].Another consideration is that the design source pulsesand transmitting/receiving antennas should be optimalfor performance of overall systems [8]. Emissionlimits will be crucial considerations for the design ofsource pulses and antennas in UWB systems.

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For many years, the idea of an UWB antenna thatwould allow stable pattern control over manyfrequency decades seemed elusive at the best. UWBantennas require the phase centre and voltage standingwave ratio (VSWR) to be constant across the wholebandwidth of operation. A change in phase centre maycause distortion on the transmitted pulse and worseperformance at the receiver. The antennas aresignificant pulse-shaping filters. Any distortion of thesignal in the frequency domain (filtering) causesdistortion of the transmitted pulse shape, therebyincreasing the complexity of the detection mechanismat the receiver [9].

Other consideration that must be taken intoaccount is group delay. Group delay is given by thederivative ofthe unwrapped phase of an antenna. Ifthephase is linear throughout the frequency range, thegroup delay will be constant for the frequency range.This is an important characteristic because it helps toindicate how well a UWB pulse will be transmittedand to what degree it may be distorted or dispersed. Itis also a parameter that is not typically considered fornarrowband antenna design because linear phase isnaturally achieved for narrowband resonance.

A nearly omnidirectional radiation pattern isdesirable in that it enables freedom in the receiver andtransmitter location. This implies maximizing the halfpower beamwidth and minimizing directivity and gain.

Conductor and dielectric losses should beminimized in order to maximize radiation efficiency.High radiation efficiency is imperative for an UWBantenna because the transmit power spectral density isexcessively low. Therefore, any excessive lossesincurred by the antenna could potentially compromisethe functionality of the system. The physicalconstraints require compatibility with portableelectronic devices. As such, a small and compactantenna is required.

3. Wideband Inverted Suspended PatchAntenna

Extremely wideband antennas intended for UWBapplications have to be designed very carefully with allthe antenna parameters to be taken into consideration.The design of the UWB antenna involves calculatingthe dimension of rectangular antenna, simulating andoptimising the design using ADS Simulation Software.Other geometries such as triangular, circular andelliptical are considered for the next simulation.

3.1 Numerical Analysis

There are many methods for broadening thebandwidth of antennas. One of methods is byincreasing the substrate thickness, which is applied inthis preliminary antenna design.

From (10], the bandwidth increases with anincrease in substrate thickness h and decrease ineffective dielectric £,. The effect of the increase in hand the decrease in yr can also be realized using thesuspended-microstrip configuration or invertedsuspended-microstrip. The suspended configurationconsisting oftwo dielectric layers can be replaced by asingle layer of equivalent dielectric constant Cq ofthickness a+ h, where A is gap spacing.

Parameters value for this proposed UWB antennais summarized in Table 1.

Table 1: Parameters value for wideband inverted suspendedpatch antenna

Parameters Value

A 0.7cmFoam E 1.07

Tan 6 0.0009

h 0.167Polypropelene 2.18

Tan 6 0.003

Antenna dimension calculation can be derivedfrom equation I to 4 as below (10]:

Le=L+2AL (I)

M=hAL = ~

L = 15e

2fo t(r + 1)2of

(2)

(3)

(4)

Where L, AL is effective length and extensionlength respectively in cm, fo is in gigahertz. Bysubstituting the given parameters from Table I into theequations thus results antenna dimensions such widthWof 2.05 cm with length L of 1.583 cm.

As shown in Figure 1, the patch is fabricated onone side of the dielectric substrate, and it is suspendedin foam gap. An inverted suspended configuration, thetop dielectric substrate acts as a protective layer withthe current plot. The preciseness on the construction ofthe feedpoint is a major concern, as it will affect theperfornance of the antenna. Antenna dimensions andfeed position are optimised using optimisation ADSsoftware to obtain a broader bandwidth.

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3.2 Simulation Results

The aim of this design is to obtain the bandwidthof 7.5 GHz. The effect of feed point location to centrefrequency and gap spacing (A) was also studied. Forfeed location of x = 0.417 cm and y = -0.268 cm fromthe centre of the patch was found to be optimum forminimal effects on the input bandwidth. For initialresult, this antenna obtains 5 GHz of bandwidth atfrequency 4.5 GHz to 9.5 GHz as shown in Figure 2with maximum VSWR of 2.

21E-a 1E4

IE7

P adabdPo rr1R~dP~ ir

nff23- A.:S; k .:. 0 :a. 0 ) -Wi_r: ol: " iTH7A=OO_ J W ~~~~fI* | I i I I

00

8 ° ETA

6 ___ _1 mi 1 rr2

5 freq=4.500GHz freq=9500GVSVR=1.369 VSWR=1.895

12'

40 4.5 5.0 55 60 65 70 75 ao 8.5 9.0 9.5 10.C

freo, GHz

Figure 2: Simulated result ofVSWR for initial antennadesign

Gain, directivity and radiated power are describedin Figure 3 and Figure 4, respectively. Both graphs arein 2D plot. For the planar cut, the angle phi, which isrelative to the x-axis, is kept constant. The angle theta,which is relative to the z-axis, is swept to create aplanar cut. Theta is swept from 100 to -100 degree. Asshown in Figure 3, the gain is 9.124 dB with radiatedpower of0.002 watts (see Figure 4).

Figure 4: Radiated power simulation result

4. Summary

This paper presents the wideband invertedsuspended patch antenna design for UWB applicationusing ADS simulation software. Some parameters suchbandwidth, gain, directivity, and radiated power aredetermined in order to ensure the antenna'sperformance meet the international standards (FCCand ETSI). From simulation result, the bandwidth of 5GHz is achieved. This bandwidth is smaller than therequired bandwidth of 7.5 GHz defined by FCC. Gainand directivity are 9.12 dB with radiated power of0.002 watts. These results are still higher thannormally UWB gain value of 4.2 dB [I1].

Since the bandwidth doesn't meet the FCCrequirement. Optimisation bandwidth performance andantenna development are proposed for future work.

References

Power mlTHETA=0.000101OglO(real(Gain))=9.124

HllA

Figure 3: Gain and directivity simulation result

[I] Kazimierz Siwiak and Debra Mc. Keown, UltraWideband Radio Technology, John Wiley & Sons,Ltd., 2004.

[2] Kamya Y.Y. and Ryuji Kohno, "Ultra WidebandAntenna," IEEE Radio Communications, June 04.

[3] Hans Gregory Schantz, "A Brief History ofUWBAntennas," IEEEA&E System Mag., April 04.

[4] Albert K.Y. Lai, et al, "A Novel Antenna for UltraWideband Application," IEEE Transactions onAntennas and Propagation, vol. 40, no.7, July 92.

[5] Anatoliy Boryssenko, "Time Domain Studies ofUltra Wideband Antennas,"in Proceedings of the1999 IEEE Canadian Conference on Electricaland Computer Engineering Conference Centre,Edmonton, Alberta, Canada, May 1999.

[6] Debatosh Guha, "Broadband Design of MicrostripAntenna: Recent Trends and Developments,"Facta Universitatis, series: Mechanics, AutomaticControl and Robotics, vol. 3, pp. 1083-1088, 03.

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[7] Federal Communications Commission (FCC),"First Report and Order, ET Docket 98-153, FCC0248,7 Revision of Parn 15 of the Commission'sRules Regarding Ultra-Ifideband TransmissionSystems. April 2002. http://www.fcc.gov.

[8] Zhi Ning Chen, et al, "Considerations for SourcePulses and Antennas in UWB Radio Systems,"IEEE Transactions on Antennas and Propagation,vol. 52, no. 7, July 2004.

[91 Siwiak, 'Impact of UWB Transmission onGeneric Receiver,"' in Proceedings of the 2001IEEE Vehicular Tech. Conference (VTC), May2001.

[10] Girish Kumar and K.P. Ray, BroadbandMicrostrip Antenna, Artech House Inc., 2003.

[1 l]Hyungkuk Yoon, et al, "A Study on The UWBAntenna with Band Rejection Characteristic,"'IEEE Radio Communications, 2004.

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