Design and Realization of a Distributed VectorSensor for Polarization Diversity Applications

L. Lo Monte, B. Elnour, D. ErricoloUniversity of Illinois at Chicago

Department of Electrical and Computer EngineeringChicago, IL 60607-7053

Email: lomonte@IEEE.orgbelnou2@uic.edu

erricolo@ece.uic.edu

A. NehoraiWashington University in St. Louis

Department of Electrical and Systems EngineeringSt. Louis, MO 63130

Email: nehorai@ese.wustl.edu

Abstract— Polarization is one type of waveform diversity thatmay be exploited to improve both radar and communicationsystems performance. Analytical results show that in order toobtain the best performance improvements, based upon the use ofpolarization diversity, knowledge of the full electric and magneticfield components is required. Vector sensor antennas are able tomeasure these components and thus they enable the exploitationof polarization diversity. This article describes a distributedapproach to design a 6D vector antenna in a distributed fashionusing both electric dipole and magnetic loops as constitutiveelements.

I. INTRODUCTION

Polarization is one type of waveform diversity that maybe exploited to improve both radar [1] and communicationsystems performance [2], [3].

Polarization diversity was considered by Nehorai and Paldiin [4], where it was shown that full knowledge of the sixelectric and magnetic field components at one point leads toimproved estimation accuracy of the direction of arrival of thesignal from either one or two sources.

As in the case considered by Nehorai and Paldi, applicationsbased on the full use of polarization diversity require theknowledge of the six components of the electromagnetic field.The optimal receiver antenna to measure the components ofthe electric and magnetic field is a 6D vector antenna, whichis considered in this article.

In the case of direction of arrival (DoA) applications, themain reason for exploiting polarization diversity and usingvector antennas is explained in the following. It is well knownthat the far-field behavior of an electromagnetic wave istransverse electromagnetic (TEM), where the electric

−→E and

magnetic−→H fields are both perpendicular to each other and

to the direction of propagation−→k . Therefore, if the DoA is

known, one could direct the receiving antenna towards thedirection of the incoming signal. Then, the polarization of theelectric field may be determined by measuring

−→E along two

perpendicular directions. In real applications, the DoA maynot be known, so that one needs to measure

−→E and

−→H along

three mutually perpendicular directions to estimate it.

Additionally, a vector antenna may be required if two sig-nals from uncorrelated sources have small angular separation,they may be distinguished if their polarization state is different.

This article describes an approach to design and implementa 6D vector sensor antenna, which is necessary for all appli-cations based on the full use of polarization diversity.

II. PRELIMINARY RESULTS FOR A SYNTHESIZED VECTOR

ANTENNA

Fig. 1. Dipole antenna

Fig. 2. Loop antenna

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Fig. 3. MUSIC spatial spectrum: vector sensor (red line) and array antenna(blue line) results

We performed some preliminary measurements on a syn-thesized 6D co-located vector antenna [5], [6]. The vectorantenna was synthesized because the components of the elec-tromagnetic field were obtained by repeating three times themeasurements using a dipole, shown in Fig. 1, and three timesthe measurements using a loop, shown in Fig. 2. The loop anddipole antennas were designed and manufactured based uponthe description provided by [7], [8] and operate at 2.65 GHz.

Figure 3 shows the results of the MUSIC algorithm appliedto the measured data. This figure shows the spatial spectrumfor the synthesized vector antenna (red line) and for an array oftwo identical elements (blue line). This diagram shows a betterperformance of the vector sensor because the curve obtainedwith the vector antenna only has one maximum, while thecurve obtained with the array antenna shows an ambiguousbehavior.

III. DESIGN OF A DISTRIBUTED ELEMENT VECTOR

ANTENNA

We are developing a 6D vector antenna for radar applica-tions with carrier frequency at 3 GHz. At this frequency, oneof the major challenges is the coupling among the antennaelements. Coupling becomes a particular difficult issue withco-located antennas. Furthermore, it is not trivial to feed6 elements at the same geometrical point. Hence, we arecurrently considering a different design based on a distributedelement approach. The starting point for a distributed designis shown in Fig. 4, where it is assumed that the distributedantenna is located in the far-field of the source so that thefield incident on each element may be locally approximatedas a plane wave.

A PCB, oriented along the x axis in the xz plane, supportsthe six elements that are intended to perform the followingfunction:

• (1) Monopole : measures Ey

Fig. 4. Scheme of a 6D Distributed Vector Sensor

• (2) Half loop : measures Hz

• (3) Half loop : measures Hx

• (4) Printed Half dipole : measures Ex

• (5) Printed Half dipole : measures Ez

• (6) Printed Full loop : measures Hy

Elements no. 4, 5, and 6 do not have a ground plane in orderto obtain the radiation pattern of ideal loops and dipoles,whereas elements 1, 2, and 3 have a ground plane due toimage theorem. Half wavelength dipoles and loops are chosento increase the received power. The spacing among consecutiveelements will be chosen to be the minimum distance thatminimizes mutual coupling effects and other factors affectingthe performance of the vector antenna.

The basic elements that are employed in both configurationsare described as follows.

• Dipole : designing of printed dipoles has been made usingstandard techniques and procedures. With an accuratechoice of the width and the length, we were able to obtaina radiation resistance close to 50 ohm. Therefore nofurther impedance matching is required. The problem ofbalancing the current distribution cannot be solved usingcommon printed circuit techniques because they generateasymmetries in the radiation pattern. Since our vectorantennas are intended to be narrowband, commercialbazooka baluns may be a good solution to balance thetransition coax-to-dipole.

• Monopole: realization of a classical monopole has beenaccomplished using standard techniques and with minormodifications we were able to obtain a 50 ohm radiationresistance.

• Loop : theoretical loop has a very high radiation re-sistance that is not compatible with our requirements.Therefore, a reshaping and a tapering of such antennahave been necessary to meet our impedance constraint.The reshaping has been performed using Ansoft HFSS,and we have obtained acceptable values of radiationresistance. The new shape of this printed loop can beseen in Fig. 5.

• Half Loop : Half loops were designed and resulted inelements with an initial radiation resistance above 80

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ohm. Therefore a matching network is required. Forour purposes, a simple LC matching network can beeasily implemented. Details about the half loop designare shown in Fig. 6

Fig. 5. Printed loop design.

IV. CHARACTERIZATION OF THE ANTENNA AND

EXPERIMENTAL SETUP

The characterization and validation of the design of thedistributed vector antenna occurs in three phases.

When antennas are in close proximity, they affect each other.For this reason, during the first phase, we have conducted CADsimulations of the performance of the antenna structure ofFig 4 using Ansoft HFSS. We have computed parameters suchas input impedance, radiation pattern, and mutual coupling forvarious values of the spacing among the antenna elements.

During the second phase, these simulations are being vali-dated in the anechoic chamber of the Andrew ElectromagneticLaboratory at the University of Illinois at Chicago, as depictedin the schematic of Fig. 7. Using a vector network analyzerHP 8510, we are currently measuring the input impedance,the mutual coupling among the elements and the radiationpattern of the antenna elements. Since our instrumentation onlyallows for using one element at a time, the antenna is operatedby connecting each element to the vector network analyzerthrough a system of switches that are remotely controlled bya computer. The antenna elements that are not used will beconnected either to an open circuit, a short circuit or a matchedtermination depending upon the type of measurement.

The third phase is the validation of the distributed vectorantenna. During this phase, we will use the distributed vectorantenna to measure the direction of arrival of signals usingthe setup shown in Fig. 8. The transmitter is an AgilentTechnology Inc. arbitrary waveform generator E8267C-520,which can directly modulate an RF signal with a bandwidthup to 80 MHz. The receiver is an Agilent technology spectrumanalyzer E4440A, equipped with an 80 MHz digitizer module.

Our experiments will consist of placing the distributedvector antenna on a steering platform and measure all the six

Fig. 6. Half loop design: the capacitance Cp encompasses capacitive effectsdue to microstrip-pin transition.

components of the electromagnetic field in both free spaceand multipath scenarios. Then, we will estimate the directionof arrival by processing the recorded data and compare the es-timated value with the true value used during the experiments.

V. CONCLUSIONS

We are designing, developing and manufacturing a full 6Ddistributed vector antenna that will be used to enable measure-ments that exploit polarization diversity. We are measuringthe antenna parameters and its performance for direction ofarrival estimation. In the future, we will also investigate novelstrategies to co-locate the six elements and to distribute theseelements in different ways.

ACKNOWLEDGMENTS

The authors are thankful to Mr. Stefano M. Canta for hishelp in carrying out the measurements.

REFERENCES

[1] D. Giuli, “Polarization diversity in radars,” Proc. IEEE, vol. 74, no. 2,pp. 245–269, Feb. 1986.

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Fig. 7. Experimental setup to measure the mutual coupling and the radiationpattern of the distributed vector antenna.

Fig. 8. Setup for the experiment to validate the measurements of the directionof arrival using a distributed vector antenna.

[2] M. R. Andrews, P. Mitra, and R. deCarvalho, “Tripling the capacityof wireless communications using electromagnetic polarization,” Nature,vol. 409, pp. 316–318, 2001.

[3] D. D. Stancil, A. Berson, J. P. V. Hof, R. Negi, S. Sheth, and P. Patel,“Doubling wireless capacity using co-polarized, colocated electric andmagnetic dipoles,” Electron. Lett., vol. 38, no. 14, pp. 746–747, Jul. 2002.

[4] A. Nehorai and E. Paldi, “Vector-sensor array processing for electromag-netic source localization,” IEEE Trans. Signal Process., vol. 42, no. 2,pp. 376–398, Feb. 1994.

[5] B. Elnour and D. Erricolo, “Experiments to measure the direction ofarrival using a vector antenna,” in IEEE AP-S International Symposiumon Antennas and Propagation and USNC/URSI National Radio ScienceMeeting and AMEREM, Albuquerque, NM, July 9-15, 2006.

[6] B. Elnour, D. Erricolo, M. Hurtado, and A. Nehorai, “Experiments forthe direction of arrival using a vector antenna,” in Fourth Tri-ServiceWaveform Diversity Workshop, Fourth Tri-Service Waveform DiversityWorkshop, Nov. 14-15, 2006.

[7] A. Konanur, K. Gosalia, S. Krishnamurthy, B. Hughes, and G.Lazzi, “In-creasing Wireless Channel Capacity Through MIMO Systems EmployingCo-Located Antennas,” IEEE Trans. Microw. Theory Tech., vol. 53, no. 6,pp. 1837–1844, June 2005.

[8] A. Rajagopalan and G. Lazzi, “A novel prototype for UWB MIMO vectorantenna,” in Digest of the National Radio Science Meeting, University ofColorado at Boulder, Boulder, Co, Jan. 4-7 2006, p. 99.

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