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Ultra-broadband Photonic Patch AntennaE. R. Lopez, L. A. Bui, K. Ghorbani, W. S. T. Rowe, A. Mitchell

School of Electrical and Computer Engineering. RMIT University, GPO Box 2476V,Melbourne, VIC 3001, Australia. Tel +613-99251722, Fax +613-99252007.

s3008525@student.rmit.edu.au

Abstract This paper presents a demonstration an ultra-broadband photonic antenna. The antenna utilizes externaloptical intensity modulators and optic fiber to photonicallyremotes the Aperture Stacked Patch antennas and replacesmicrowave diplexers with simple photonic combination. Theresulting photonic antenna is demonstrated to provide abandwidth of 110% with gain of 7 dBi.

Index Terms- Microstrip antennas, Microwave Photonics,Optical Fiber Antenna Remoting.

I. INTRODUCTION

T HE demand for wireless services of ever increasingbandwidth continues to drive antenna engineering

research. Emerging and next generation wirelesscommunications applications, such as broadband peer topeer networking and particularly the exchange of videoinformation, will require greater bandwidths than arepossible with present day wireless infrastructure.

The extraordinary expense of deploying new physicalinfrastructure is prohibitive, thus the next generation oftechnology must be flexible such that it can accommodatecontinuous development in communications technologiesand applications. Software defined radio (SDR) has beenconceived as a method by which the same physical hardwarecan be used for multiple formats with new protocols beingsubstantially possible with a software upgrade [1]. Newinfrastructure should thus be designed to accommodate asbroad a bandwidth as could be conceived useful. Similar toSDR, Ultra wide-band (UWB) technology applicationsrequire different design and optimization requirements suchas power, throughput, distance, cost [2]. Extremely broadbandwidth antennas will be key components in such versatilewireless communications systems.

Printed antennas offer the advantage of low-costmanufacture and, specifically, can be engineered withcompact, low-profile and even conformal geometries.Traditionally, printed antennas had only offered relativelynarrow bandwidths, insufficient for the SDR or UWBapplications. Further, photonic remoting of broadbandantennas has also attracted much attention in recent years[3]. Replacing traditional coaxial cable with optic fiber cangreatly increase the bandwidth of the antenna feeding systemand remote placement of high-frequency antennas [4]. Opticfiber can also significantly reduce bulk and weight.

In this paper we demonstrate a technique by which twoadjacent band optic fiber remoted Aperture Stacked Patch(ASP) antennas can be photonically combined to realize asingle broadband radiating element.

This paper is organized in five main sections Section IIintroduces and verifies the operation of the ASP antennasoriginally used in [5]. Section III details the proposed ultra-broadband photonic patch antenna system. It explains theconfiguration and setup, and describes the measurementprocess undertaken. Section IV presents the characterizationof the photonically combined system including gainbandwidth across the useable band. Section V discusses thefindings of this investigation and suggests some avenues forfurther research.

II. VERIFICATION OF ASP ANTENNA PERFORMANCE

The investigation presented in [5] aimed to develop andenlarge the effective bandwidth of base-station antennas. Anultra-broadband patch antenna was realised using twobroadband ASP antennas combined into a single radiatingelement using a microwave diplexer and the assembly wascharacterised as a single antenna.To conduct the current investigation, it was proposed that

the same ASP antennas used in [5] be reused to implementan ultra-broadband antenna unit, but with the passivediplexer replaced with photonic combination. Before thephotonic combination experiments could be conducted, itwas necessary to verify that the antennas still operated asreported in [5].

Fig. 1 and 2 show the gain response of ASPI and ASP2,respectively. The results show that the average gain is 7 dBifor both patches.

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01 1.5 2 2.5 3 3.5 4

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Fig. 1. Electrical Gain of ASP1.

0-7803-9542-5/06/$20.00 C2006 IEEE

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In order to prove the validity of our photonic approach, itis necessary to demonstrate that our proposed system obtainsessentially the same bandwidth and gain as when usingmicrowave diplexer. Therefore, we attempt to demonstratewhether or not the system is valid, undertaking an analysis ofthe gain performance of the system created using thephotonic configuration.

Fig. 3 depicts the photonic configuration proposed by this1 1.5 2 2.5 3 3.5 4 investigation that substitutes the diplexer analyzed in [5]. A

Frequency (GHz) single optical source is divided by a 3 dB coupler anddirected through Polarization Controllers (PC) into Mach-

Fig. 2. Electrical Gain of ASP2 Zehnder Modulators (MZM). The PCs are used to adjust the

polarization received by the modulators, which are used toaparing these gain responses to the reported values in modulate the optical carrier with the electrical signalmilar results are obtained with a slight decrease of in- provided by each antenna. Once converted into the optical;ain and a small shift in the radiating frequency band domain, the signals of the antennas are propagated usingTed. Differences in the patches performance are Polarization Maintaining fibers (PM), through another set ofted to the absence of the large ground plane used in PCs, up to a Polarization Beam Splitter (PBS). Thefter analyzing these results, it can be concluded that polarization rotating characteristic of the PBS makesitennas still operate sufficiently well to conduct the possible to use this device to combine two optical carrierst experiment without an additional ground plane. without coherent interference [3], [6]. The PCs ensure that

the signals arrive at the PBS, at the correct polarization.After the PBS, the single optical carrier contains the

1. PHOTONIC APPROACH FOR ULTRA-BROADBAND information of both antennas propagating at differentANTENNA - PHOTONIC COMBINATION polarizations (ASP 1, TE mode; ASP2 TM mode). This

optical carrier is amplified using an erbium-doped fiberpite the small changes presented in the amplifier (EDFA) before being converted by a Phototerization of the antennas, the result obtained in Detector (PD) back into the electrical domain. Finally, then II provides enough evidence to verify that both PD directs the electrical carrier into the Vector Networkias works as stated in [5]. Analyzer (VNA) through a broadband amplifier.

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PBS PD

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Fig. 3. Proposed Photonic System Configuration.

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Fig. 4. Proposed System (a) Photonic Combination, (b) ASP Antennas and Modulators Setup Inside of an Anechoic Chamber.

Fig. 4a) shows a photograph of the photonic combinationsetup, and Fig. 4b) shows the configuration of the antennasinside of an anechoic chamber, the electro-opticalmodulators and the microwave trombone phase shifters.

In order to make a correct measurement of the photonicconfiguration, all the components were adjusted according tofollowing specifications: to ensure each MZM received thecorrect polarization (TE), the PCs were adjusted formaximum RF signal transmission; the electro-optical lengthdifference of the paths (ASP1 link and ASP2 link) wascompensated using the microwave Trombone Lines (TL) toa zero phase difference at 2 GHz (cross-over region); and,the modulated signals have to be identically polarized usingthe PCs before to reach the PBS.

IV. PHOTONIC UBP ANTENNA RESULTS

This Section presents the gain of the measurement usingthe procedure and configuration detailed in Section III. It isanticipated that the major difference between the gainresponse of the photonic configuration and the one reportedin [5] will be a signal interference effect that will be presentdue to the overlapping region of the operational bandwidthsof the two ASP antennas in the absence of the diplexerfilters.

Fig. 5 displays the gain response of the combined ultra-broadband photonic patch antenna. The response show thatthe amplitude of the gain varies from 6 dBi to 9.5 dBi andthe higher point of this variation is situated around cross-over region at 2 GHz. For comparison, the individual gainresponses measured for each antenna, as shown in Fig. 1 and2, have been numerically summed in-phase and arepresented in Fig. 6 The phase difference between the two

ASP responses was adjusted to achieve a good matchbetween the measured and numerically summed responses.Both responses follow a similar trend along the entire band.

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8

.t 4

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0 I

1 1.5 2 2.5 3 3.5 4

Frequency (GHz)

Fig. 5. UBP Antenna Gain, Photonic Response.

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Fig. 6. UBP Antenna Gain, Electrical Response.

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V. DISCUSSION

The gain response of the ultra-broadband photonic patchantenna, shown in Fig. 5, exhibits more ripple than thesystem presented using microwave diplexers [5]. Referringto Fig. 1 and 2, ASPI and ASP2 show simultaneous positivegain at several frequency bands (1.6-2.2 GHz, 2.5-3.1 GHzand 3.5-3.9 GHz). This causes the gain ripple seen in Fig. 5and 6 at these frequency bands. It is encouraging to observethat the photonic response measured corresponds almostexactly with the numerically summed gain responses of theindividual antennas (shown in Fig. 6). This suggests that thephotonic combination can be considered as a simple signalsummation.A number of options exist to ensure that the antennas do

not interact outside of the band. Firstly, the ASP antennas of[5] were not designed to ensure radiation only within theband; instead emphasis was placed on obtaining the broadestpossible bandwidth. It should be possible to redesign theantennas such that there is no significant gain outside of theoperational bandwidth of each patch. This redesign wouldremove much of the ripple seen from 2.5 GHz to 3.1 GHzand from 3.5 GHz to 3.9 GHz in Fig. 5. Even withredesigned ASP antennas, significant interaction would stillbe expected at the cross-over region at 2 GHz.To minimize the interaction of the two antennas at the

cross-over region it would be necessary to implement lowand high-pass filters to isolate the two antenna bands. Thedesign and implementation of these filters would be fareasier than the diplexer of [5] as the two filters are notphysically connected (each terminates in the broadband 50Ohm load of the optical modulator) and thus they can bedesigned and implemented independently. An investigationis currently underway to verify that filtered photoniccombination can achieve similar results to those presented in[5], but with far simpler filter designs.

It is also anticipated that the current approach could beextended, through the use of wavelength divisionmultiplexing (WDM) to support three, four or even morebroadband patches. Using this approach it may be possibleto cover the entire spectral mask for operation of UWBdevices approved by FCC, which major part lies between 3.1and 10.6 GHz [7].

bandwidth of any type of antennas and expand the currentdefinition of ultra-broadband antennas. The secondinvestigation will attempt to expand the technique to enablemultiple antennas to be combined to cover a full rangerequired for many UWB wireless applications.

REFERENCES[1] J. Mitola, "Software radio architecture evolution: Foundations,

technology tradeoffs and architecture implications," IEICE Trans.Commun., vol. E83-B, no. 6, 2000.

[2] J. Foerster et al., "Ultra-Wideband Technology for Short or MediumRage Wireless Communications," Intel Technology Journal, Quarter2 (2001).

[3] A. Mitchell, L. A. Bui, K. Ghorbani, S. Mansoori, W. Rowe, E.Lopez, and T. H. Chio, "Demonstration of a wideband photonicphased array using integrated optical RF phase shifter based on thevector sum approach," presented at Phased Array Systems andTechnology, 2003. IEEE International Symposium on, 2003.

[4] A. Nirmalathas, D. Novak, C. Lim, R. B. Waterhouse, and D.Castleford, "Fiber networks for wireless applications," presented atLasers and Electro-Optics Society 2000 Annual Meeting. LEOS2000. 13th Annual Meeting. IEEE, 2000.

[5] K. Ghorbani and R. B. Waterhouse, "Ultrabroadband printed (UBP)antenna," Antennas and Propagation, IEEE Transactions on, vol. 50,pp. 1697-1705, 2002.

[6] H. C. Kandpal, S. Anand, and J. S. Vaishya, "Experimentalobservation of the phenomenon of spectral switching for a class ofpartially coherent light," Quantum Electronics, IEEE Journal of, vol.38, pp. 336-339, 2002.

[7] S. A. Hanna, "UWB implications for spectrum management,"Aerospace and Electronic Systems Magazine, IEEE, vol. 19, pp. 17-20, 2004.

VI. CONCLUSION

An Ultra-Broadband Photonic Patch antenna has beendemonstrated. This antenna has been characterized with aradiation bandwidth from 1.1 to 3.9 GHz which represents abandwidth of approximately 110%. This demonstrationexhibited approximately 2 dB ripple in gain over theoperation bandwidth. The 2 dB ripple, though tolerable, washigher than a previous demonstration using a microwavediplexer. Two ongoing investigations aim to improve theobtained results. The first aims to show that photoniccombination and simple microwave filters can be used toachieve the same results obtained previously using asophisticated microwave diplexer. These would eliminatethe interactions occurring outside of the designated

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