A New Planar Dual-Band GPS Antenna Designed for Reduced Susceptibility to Low-Angle Multipath

2358 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 55, NO. 8, AUGUST 2007

A New Planar Dual-Band GPS Antenna Designed forReduced Susceptibility to Low-Angle Multipath

Lorena I. Basilio, Member, IEEE, Richard L. Chen, Member, IEEE, Jeffery T. Williams, Senior Member, IEEE, andDavid R. Jackson, Fellow, IEEE

Dr. Richard L. Chen passed away on May 4, 2006,after bravely struggling with brain cancer

for almost two years.

AbstractA new Global Positioning System (GPS) microstrippatch antenna designed for dual-band (L1 L2) operation is intro-duced. The antenna design is based on the reduced-surface-wave(RSW) concept and, as a result, is much less susceptible to low-angle multipath interference effects than some of the more com-monly-used high-precision GPS antennas. In this paper, the radia-tion characteristics of this new design will be compared to a dual-band choke-ring and a dual-band pinwheel antenna. In addition tohaving the advantages typically associated with microstrip patchantennas, this planar dual-band antenna lacks the design compli-cations associated with the frequently-used stacked-patch methodfor realizing dual-band microstrip antenna performance. Thus, thesimplicity of the design, together with the reduced horizon andbackside radiation levels and excellent circular polarization char-acteristics indicate that this new antenna design is a promising can-didate for dual-band, high-precision applications.

Index TermsChoke ring antennas, dual-band antennas, GlobalPositioning System (GPS), microstrip antennas, multipath inter-ference.

I. INTRODUCTION

AGLOBAL Positioning System (GPS) receiver measuresthe apparent transit time of a signal from the GPS satelliteto the receiver and, from this time, computes the satellite-to-user range [1]. This measurement can be corrupted by timedelays associated with the complicated nature of the propaga-tion medium, receiver noise, phase-center variation in the re-ceiving antenna, and multipath interference. Although a numberof these errors, to some extent, can be reduced with signal-pro-cessing techniques, in the case of high-precision GPS applica-tions where accuracies on the order of centimeters are required,more stringent demands on the GPS receiving antenna are alsonecessary. More specifically, to realize extremely accurate posi-tional outputs, the antenna must be designed so that it has stable

Manuscript received January 10, 2005; revised February 4, 2007. This workwas supported in part by NASA under Grant NAG9-1293 and in part by the Stateof Texas (Advanced Technology Program) under Grant 003652-0306-2001.

L. I. Basilio was with the Department of Electrical and Computer En-gineering, University of Houston, Houston, TX 77204 USA. She is nowwith the Electromagnetics and Plasma Physics Department, Sandia NationalLaboratories, Albuquerque, NM 87185-1152 USA.

R. L. Chen, deceased, was with the Department of Electrical and ComputerEngineering, University of Houston, Houston, TX 77204 USA.

J. T. Williams and D. R. Jackson are with the Department of Electrical andComputer Engineering, University of Houston, Houston, TX 77204 USA.

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TAP.2007.901818

phase-center characteristics [2] and a reduced susceptibility tomultipath interference. In this study, a dual-band antenna designthat reduces the amount of multipath error introduced into thereceiver at both the L1 (1.575 GHz) and L2 (1.227 GHz) GPSfrequencies is presented.

Multipath occurs when a signal reaches the receiving antennavia more than one path. In a GPS system, an antenna will typ-ically receive the direct (line-of-sight) signal from the satelliteand one or more reflections from the ground and structures in thevicinity of the antenna (indirect signals). Since the GPS receivercomputes a position based on the sum of the received signals,a satellite-to-user range measurement can be corrupted by thismultipath interference, where the degree of positional error de-pends on the strength of the reflected signal (relative to the directsignal) and the time delay between the reflected and direct sig-nals. For high-precision applications, it is the multipath eventsassociated with reflections occurring near the antenna that tendto be the most problematic. Since these signals are relativelystrong and are typically not significantly separated in time fromthe direct signal, signal-processing methods are typically unsuc-cessful at filtering out these signal components.

In this paper, a new GPS antenna with reduced suscep-tibility to low-angle multipath (ground reflections or reflectionsfrom the antenna supporting structure) is proposed for high-pre-cision applications. The antenna is a planar design consisting oftwo concentric annular-ring microstrip patch elements, each be-longing to the class of reduced-surface-wave (RSW) Antennasdeveloped at the University of Houston [3]. A RSW microstrippatch antenna produces only a small amount of surface-wave ra-diation and, if printed on an electrically-thin substrate, also pro-duces only a small amount of lateral radiation (the space wavethat propagates horizontally along the air-substrate interface).As a result of minimizing surface waves and lateral radiation,diffraction at the edge of the substrate or ground plane is re-duced, and hence, the patch is characterized by a radiation pat-tern that has less scalloping in the forward region (above thehorizon) and less back-scattered energy (less radiation below thehorizon). Unlike the commonly-used choke-ring designs whichrely on a corrugated ground plane (an electromagnetically softsurface) to suppress the surface waves excited by a conventionalmicrostrip antenna element, a RSW microstrip patch antenna isdesigned specifically so that the surface-wave and lateral radi-ation produced by the patch is significantly reduced. In min-imizing the radiation from edge diffraction due to these twomechanisms, the radiation characteristics of the RSW antennabecome less dependent on the supporting structure. While theintent of both the choke-ring and RSW designs is to reducethe amount of back-scattered radiation, a RSW design is a less

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BASILIO et al.: A NEW PLANAR DUAL-BAND GPS ANTENNA 2359

costly and more compact antenna solution to the problem ofmultipath interference than its choke-ring counterpart. In [4]a single-band (L1) GPS-RSW antenna design was presentedand shown to provide comparable multipath performance to thecommonly-used choke-ring design. While a GPS antenna basedon the reduced-surface-wave concept has also been consideredin a study by Boccia et al. in [5], [6], in this paper the focusis to compare the radiation performance of a new planar dual-band GPS-RSW antenna to the more commonly-used GPS an-tenna designs. (A stacked dual-band GPS antenna based on theRSW concept is presented in [7].) With the properties of reducedhorizon and backside radiation levels, excellent circular polar-ization characteristics, and similar performance at both the L1and L2 frequencies, this new antenna design shows promise fordual-band, high-precision GPS applications.

II. HIGH-PRECISION GPS ANTENNASFor high-precision GPS applications, the receiving antenna

should ideally be characterized by a radiation pattern that isvery broad above the horizon and capable of rejecting all sig-nals arriving from below the horizon. An antenna with these ra-diation characteristics provides nearly hemispherical coveragewhile being less susceptible to multipath interference. The an-tenna should also be characterized by a stable phase center (oreffective point of radiation as referred to in [2]) and nearly pureright-handed circularly polarized radiation (RHCP). In addition,dual-frequency operation (L1 and L2) requires that the antennahave similar radiation patterns, similar polarization characteris-tics, and a common phase center at each frequency.

To date the most common antenna configuration that is usedin high-precision GPS systems is a microstrip patch antennamounted on a choke-ring ground plane [8]. A choke-ring groundplane is a corrugated surface (comprised of short-circuitedquarter-wavelength radial waveguide baffles) that presents asoft boundary condition along the horizon where and

are zero. While a microstrip patch element is employedbecause of its relatively broad gain pattern, low cost, and designsimplicity, the choke-ring ground plane is used to suppressthe edge diffraction effects from the surface-wave excitationand the patch lateral radiation and thus, taper the radiationpattern of the antenna so that indirect signals are not as easilyreceived. The patch antenna is designed to receive a right-handcircularly-polarized wave at one or both of the GPS frequencies(L1 and L2). It is important to note that, by using the inherentpolarization difference between the direct and reflected GPSsignals, most properly designed GPS antennas adequately elim-inate the perturbing effects of high-elevation angle multipathevents. In addition, signal-processing techniques can be usedto eliminate the effects of spatially-removed multipath events(reflections occurring further from the antenna).

III. REDUCED-SURFACE-WAVE ANTENNA DESIGNS

A. Reduced-Surface-Wave ConceptIt is well known that a resonant circular patch antenna oper-

ating in the mode can be approximately mod-eled by an equivalent edge magnetic current given by

. A RSW antenna design relies on the principle that this

ring of magnetic current will not excite a surface wave (orlateral radiation in the case of a thin substrate) provided that theradius of the circular patch is given by

(1)

where is the first zero of (for the smallest possiblepatch size) and is the propagation wavenumber for the

surface wave [3].In order to make an RSW antenna resonant at the same

frequency at which the surface-wave mode is not excited,the patch cavity is made resonant by introducing a circularshort-circuit boundary condition concentric to the radiatingedge, forming a shorted-annular-ring antenna [4]. A short-cir-cuit boundary condition is used so as not to introduce anadditional radiating edge into the antenna design.

B. Single-Band Linearly-Polarized RSW DesignsIn order to realize the smallest-size RSW antenna for a given

permittivity substrate and a given frequency, the short-circuitboundary is chosen to lie within the interior of the magnetic cur-rent ring (a in [4]). From the governing boundary conditionsfor the patch cavity [3], a transverse-resonance equation can bederived and the radius of the short-circuit boundary can beshown to satisfy the transcendental equation

(2)

where is given by (1) and is the wavenumber in the dielec-tric substrate. For this design, it is important to recognize thatthe solution of (2) is selected such that is always less than

. A reduced-surface-wave patch antenna that is designed ac-cording to the specifications given in (1) and (2) is referred toas the shorted-annular-ring reduced-surface-wave (SAR-RSW)design. The schematic for a SAR-RSW design can be found in[4]. Unlike the RSW designs presented in [5][7], for the de-signs presented in this paper (as in [4]) the short-circuited innerboundary condition is implemented using a circular array ofvias. For high-precision, narrow-band applications such as GPS,this method of realizing a short-circuit wall at the inner radiushas proven to be the most effective in providing repeatable an-tenna performance. (It is important to note that the measuredresonant frequency of the RSW antenna has been found to be es-pecially sensitive to the radius of the short-circuited boundary.)Although the discrete spacing between each of the vias is knownto result in a reactive surface impedance at the inner boundary(rather than a precise short circuit), the effect is assumed to benegligible for the small via-spacing considered in this study.

A resonant patch cavity can also be realized by placing theshort-circuit boundary outside of the circular ring of magneticcurrent. In this case, the radius of the short-circuited wallsatisfies the transcendental equation given by

(3)


Fig. 1. A probe-fed inverted shorted-annular-ring reduced-surface-wave patchantenna. (a) Side view and (b) top view.

where is given by (1) and [3]. A microstrip patchantenna that is designed with these dimensions is referred to asthe inverted shorted-annular-ring reduced-surface-wave (ISAR-RSW) design.

The ISAR-RSW antenna used in this study is shown in Fig. 1.As with the SAR-RSW design, the shorted-inner wall boundarycondition is realized using a closely-spaced circular array ofvias. For convenience, the top metal surface in this design isextended beyond the via fence and is terminated at the edges ofthe substrate. It is significant to note that in this situation sur-face waves will inherently not be excited, since surface-waveexcitation requires that the top metal surface surrounding theantenna is removed (as in the SAR-RSW design). However, forthe antenna shown in Fig. 2, the design of the magnetic cur-rent ring radius according to (1) is still valuable since it dictatesthat lateral radiation will be significantly reduced (again, for thestructure shown in Fig. 2, surface-wave excitation is a non-issue)and, consequently, that a narrower radiation pattern comparedto a conventional patch pattern will still result. The effect of theparticular radiation mechanisms associated with the SAR-RSWand ISAR-RSW antennas on the characteristic radiation patternsis discussed in detail later on in this section. (It is worth men-tioning that by removing the top metal surface surrounding thevia fence, the ISAR-RSW antenna in Fig. 2 becomes an exactcomplement to the SAR-RSW antenna and both surface-waveexcitation and lateral radiation are reduced by the design equa-tion in (1).

Since both the SAR-RSW and ISAR-RSW antennas (alsoreferred to as the SAR and ISAR, respectively) are based uponthe magnetic current ring radius given by (1), then for the samefrequency and the same permittivity substrate, the magnetic cur-rent ring radius will be the same in both designs .For the same conditions however, the radius of the short-circuit

Fig. 2. A comparison between the E-plane linearly-polarized radiation patterns(normalized to 0 dB) of a conventional circular, SAR, and ISAR patch antennaat 1.575 GHz on a 1 m diameter circular ground plane.

boundary in the ISAR-RSW design will always be larger thanthe corresponding radius in the SAR-RSW design .Although the disadvantages of the larger patch antenna may insome cases hinder the practicality of the ISAR over its SARcounterpart for linearly-polarized single-band applications,there are certain advantages associated with the ISAR geometryfor both circularly-polarized single-band (CP) and dual-bandapplications (in both linear and CP operation). While thebenefits of the larger patch for single-band CP applications arediscussed in [9], the benefits of the ISAR patch geometry fordual-band CP operation are discussed in the following section.

In order to assess the performance of the two RSW designs,the measured linearly-polarized radiation patterns for these an-tennas are compared to those of a conventional circular patchdesign. All patches were fabricated on a Rogers RT/Duroid 6002substrate with a dielectric constant of 2.94, thickness of 0.1524cm, and a loss tangent of 0.0012. Using these parameters and adesign frequency of 1.575 GHz (L1), the radii of the SAR andISAR antennas were calculated as cm,cm, and cm, using (1)(3) [note that for this fre-quency cm]. (Although a full cavity-modeanalysis accounting for losses is eventually used for more pre-cisely determining the resonant dimensions of the dual-bandGPS-RSW patch presented later, these convenient expressionsare used here to simply demonstrate the RSW radiation charac-teristics. Using (1)(3)it is expected that there will be a slightshift in the measured patch resonant frequency.) For the L1 fre-quency, a conventional circular patch antenna on the same sub-strate has a radius equal to 3.17 cm [10]. Thus, for the samefrequency, the RSW antennas are larger than the conventionalcircular patch design. Each antenna was probe-fed at its ap-proximate 50 match point corresponding to cmfor the SAR antenna, cm for the ISAR design, and

cm for the conventional patch. The probe feeds arealong the -axis in each case (Fig. 1). The vias used


for the short-circuit boundary in the both RSW antenna designswere spaced 10 mils apart (from edge to edge) and were each of25 mils diameter (this via arrangement was used throughout thestudy). A complete electric characterization of the ISAR-RSWantenna is presented in [9] (theoretical and measured results in-clude the input impedance characterization of the antenna).

The measured far-field radiation patterns in the E-planefor the SAR, ISAR, and conventional linearly-polar-

ized designs are compared in Fig. 2. (A one-meter diametercircular ground plane was used for each these measurements.)For the conventional circular patch antenna, the scalloping inthe front-side pattern and the considerable amount of radiationon the backside clearly indicate that there is a significantamount of diffraction off the edges of the ground plane due tothe surface-wave and lateral radiation.

In contrast to the radiation patterns measured for the con-ventional designs, the radiation patterns for both the SAR andISAR designs are relatively smooth in the forward region anddemonstrate significantly reduced back-scattered radiation.More quantitatively, the gains for the conventional, SAR, andISAR design are approximately 6, 9, and 8 dB, respectively[9]. Based on these more directive linearly-polarized radiationpatterns (relative to the conventional design), the SAR andISAR antennas appear to be promising candidates for GPS ap-plications. It is worth noting that the discrepancies (scallopingand below-the-horizon radiation) between the conventionaland RSW radiation patterns is most evident in the E-planepattern because surface-wave excitation and lateral radiationoccur mainly in this plane. A comparison between the H-planepatterns can be found in [11], where the primary differences inthe conventional and RSW patterns occur in the regions near0 and 180 .

Given the similarity between the SAR and ISAR patterns inFig. 2, it can be concluded that it is primarily the reduction in thelateral radiation [dictated by (1)] that is responsible for the dif-ference in pattern shape for these designs compared to the con-ventional pattern. As previously discussed, with the extensionof the top conductor on the ISAR antenna as shown in Fig. 1,surface-wave excitation is inherently not present in this design.However, since this top metal surface does not affect the lateralradiation, it is the reduction in this component of the antenna ra-diation that can be said to be the key to both the SAR and ISARdesigns. A significant decrease in the horizon-level radiation forthese designs is clearly demonstrated in Fig. 2.

From Fig. 2, it is also important to note that the increase indirectivity for the RSW patches, relative to the conventional de-sign, is a consequence of the larger patch size required for re-duced-surface-wave excitation. (While the larger structures re-sult in a slightly lower efficiency than for the conventional patch,current research efforts involving RSW patch miniaturizationare an attempt, in part, to improve this situation.)

IV. A DUAL-BAND ANTENNA DESIGN FOR GPS APPLICATIONSUsing the linearly-polarized SAR and ISAR reduced-surface

wave designs presented in Section III, a RHCP dual-band an-tenna designed for the L1 (1.575 GHz) and L2 (1.227 GHz)GPS frequencies is considered. The dual-band RSW antenna is aplanar design consisting of an SAR-RSW element designed for

Fig. 3. A dual-band reduced-surface-wave patch antenna (top view).

the higher-frequency band of L1 lying within the interior of anISAR-RSW element designed for the L2 band. The dual-bandRSW design is shown in Fig. 3.

As mentioned previously, since (1)(3) are derived assuminga perfect cavity resonator [3], a slight shift between the de-sign and measured resonant frequencies is observed when theRSW patch dimensions are obtained directly from these ex-pressions. Thus, for the sake of completeness and to also im-prove the design for the inner and outer radii that would re-sult in a resonant SAR-RSW and ISAR-RSW antenna at theL1 and L2 frequencies, non-ideal cavity effects are taken intoaccount. (Although (1)(3) are convenient expressions, for anarrow-band application such as GPS, a full cavity-mode anal-ysis is more appropriate.) These non-idealities include fringingat the radiating edges of the patch, higher-order mode excita-tion, and losses associated with the dielectric substrate, patchconductor, and radiation. As with the previous designs, the RSWantennas were fabricated on a Rogers RT/Duroid substrate witha dielectric constant of 2.94, thickness of 0.1524 cm, and a losstangent of 0.0012. Using these substrate parameters and a fullcavity-mode analysis, the corresponding patch dimensions for aRSW antenna designed for 1.575 GHz were found to be 3.03cm and 5.58 cm for the inner and outer radius, respectively.Using a similar analysis [9], [11], the inner and outer radiusof an ISAR antenna resonant at 1.227 MHz were calculated as7.16 cm and 11.15 cm, respectively. The antennas were eachprobe-fed at their approximate 50 match points occurring at

cm and cm. For this initial dual-banddesign, right-hand circular polarization was realized by using adual-orthogonal probe feed arrangement on each of the antennas(Fig. 4). In the pattern results that follow, the probes were fedthrough two microstrip quadrature hybrids designed for the L1and L2 frequencies. (Similar performance was realized using asingle broadband microstrip hybrid [11].) The quadrature hy-brids were used to divide the input power fed into each antennainto two equi-amplitude components with a phase difference of90 between them.

The measured RHCP radiation (co-polarization) for the dual-band GPS-RSW design at the L1 frequency, on a one-meterdiameter circular aluminum ground plane, is shown in Fig. 5.(Throughout this study, the feed circuits (the quadrature hybridsin this case) are shielded by placing a Styrofoam layer approx-imately 2.5 cm thick over the circuit and using aluminum foilto wrap the entire package.) This pattern was obtained by mea-suring the receive pattern of the antenna when illuminated by


Fig. 4. The feed configuration used to realize a right-hand circularly polarizeddual-band RSW antenna.

two orthogonal linear polarizations from a transmitting horn an-tenna, and then forming a linear combination with a 90 phaseshift from the two resulting data sets. (In this manner, the polar-ization purity of the dual-band RSW antenna is examined inde-pendently from the characteristics of the transmitting antenna.)As demonstrated with the linearly-polarized RSW designs pre-sented in Section III, the RHCP pattern for the dual-band RSWdesign is relatively smooth in the forward region and is ex-tremely small on the backside. Thus, in accordance with the de-sign radii at the L1 and L2 frequencies given by (1), the patternshows very little evidence of lateral space-wave diffraction. Aspreviously discussed, it is the reduction in the lateral radiationfor both the SAR and ISAR designs that is primarily respon-sible for the characteristic patterns of each (while a reductionin surface waves also occurs for the SAR design, in the case ofthe ISAR design in Fig. 1, surface waves are not excited andthe characteristic pattern can only be attributed to a reductionin lateral radiation). Consequently, it is worth pointing out thatthe dual-band RSW design, a composite of an L1 SAR and L2ISAR, is intrinsically immune to surface waves.

In order to evaluate the quality of the circular-polarizationfor the RSW antenna at 1.575 GHz, the left-handed circular po-larization (LHCP/cross-polarization) was measured in a similarfashion, and the result is also included in Fig. 5. The left-handedpattern is extremely low relative to the right-handed pattern (ap-proximately 25 dB down at broadside), suggesting that the GPSantenna design can provide a high-level of discrimination be-tween direct and multipath signals on the basis of polarization.This feature complements the ability of the GPS-RSW antennato reject ground-bounce multipath by reason of the low-levelhorizon and backside radiation.

The RCHP and LHCP radiation for the dual-band RSWantenna was also measured at the resonant frequency of 1.231GHz (again, a one meter ground plane was used). It is im-portant to note here that the slight shift in the ISAR resonantfrequency could be a result of a small level of mutual couplingbetween elements, which was not taken into account in thecavity-mode analysis used in this study. With the pattern resultsalso provided in Fig. 5, the performance of the antenna at thisfrequency was found to be very similar to the performance at1.575 GHz. At this frequency, the pattern is again characterizedby a smooth front-side and low radiation levels along the

Fig. 5. The measured RHCP and LHCP radiation patterns for a dual-bandGPS-RSW antenna on a 1 m diameter ground plane. The patterns have beennormalized to the maximum RHCP value at each frequency.

horizon and towards the backside of the antenna (of both theright- and left-handed type). The similar low cross-polarizationlevels at L1 and 1.231 GHz indicate that the dual-band RSWantenna is equally capable of rejecting multipath signals basedon polarization at both frequencies. It is shown in [11] that,due to the similarity in the E- and H-plane patterns character-izing the RSW elements, these types of antennas exhibit muchbetter cross-polarization characteristics than conventional patchdesigns. Comparing to the theoretical reduced-surface-waveCP patterns which are presented in [11], it is found that aGPS-RSW antenna mounted on one meter ground plane pro-vides nearly the optimum cross-polarization performance thatcan be expected from the RSW design [11].

In comparing the radiation patterns of the dual-band RSWantenna to a single-frequency SAR-RSW and ISAR-RSW el-ement designed for L1 and L2, respectively, there is little ap-parent difference. Hence, the radiation pattern associated withthe mode of the ISAR patch at 1.227 GHz is unaffectedby the presence of the SAR-RSW element in the dual-banddesign. From the input impedance response shown in Fig. 6,it is evident that while a relatively good match exists for theISAR antenna at 1.227 GHz, the reflection coefficient associ-ated with the SAR-RSW element at this frequency is approx-imately 0 dB (corresponding to an input impedance of

). At the higher frequency of L1, there is a slightoverlap between the mode of the SAR-RSW patch andthe mode of the ISAR-RSW patch (the input impedanceof the ISAR-RSW antenna at this frequency is equal to

and hence, the reflection coefficient is notequal to 0 dB). The identification of the modes is determinedfrom the cavity-mode analysis presented in [9]. However, sincethese measurements were obtained by manually switching thefeed ports for each frequency, at the L1 frequency the feed portsare on the SAR antenna and the mode of the ISAR is not


Fig. 6. Measured (a) input resistance and (b) input reactance as a function offrequency for the dual-band RSW patch antenna with a substrate relative per-mittivity of " = 2:94.

directly excited. Rather, it is excited only through the mutualcoupling between the elements, which is very small. It is im-portant to note that, although the lack of a significant frequencyseparation between the ISAR mode and the SARmode does not affect the radiation patterns of the antenna, thisparticular modal response becomes important when designing asingle-input feed network capable of simultaneously feeding theSAR-RSW and ISAR-RSW elements in the dual-band design.A dual-band design including a diplexer feed circuit (comprisedof a 3-dB three-branch broadband hybrid and stub-line combi-nations for tuning and detuning the SAR and ISAR elements) ispresented in [11].

V. COMPARISON TO FREQUENTLY-USED GPS ANTENNAS

A. Choke-Ring Antenna ComparisonAlthough the commonly-used configuration of a microstrip

patch antenna mounted on a choke-ring ground plane is fairlysuccessful in reducing the edge diffraction effects from the patchsurface-wave and lateral radiation, the ground plane itself is rel-atively bulky and can add significant cost and weight to thesystem. Thus, while keeping in mind the simplicity and light-weight nature of an RSW antenna, the first step in evaluating thedual-band RSW design as a viable alternative to the dual-bandchoke-ring design is to directly compare the RHCP and LHCPradiation performances of the two structures. (In [4] a choke-

Fig. 7. The measured RHCP and LHCP radiation patterns at 1.575 GHz and1.227 GHz for a a 35.6 cm diameter commercial L1/L2 choke-ring antenna.

ring antenna designed specifically for L1 operation was shownto be characterized by a broad, smooth RHCP pattern with lowbackside levels and also appreciably low levels of LHCP radia-tion.)

The L1 patterns for a commercial dual-band (L1/L2) choke-ring antenna are shown in Fig. 7. (Here and for the remainder ofthe radiation results that follow, the patterns have been normal-ized to the maximum RHCP value at each frequency.) For thischoke-ring design, dual-band operation is realized by compro-mising on the design specifications (specifically on the depthsof the choke-ring corrugations) so as to realize a moderatelysoft electromagnetic surface at both the L1 and L2 frequen-cies ( and are approximately zero at both frequencies).However, as seen in Fig. 7, a consequence of this is that an ex-tremely large cross-polarization pattern results (LHCP). Thus,unlike the single-band choke ring antenna shown in [4], thedual-band choke-ring antenna is fairly susceptible to receivinghigh-angle, odd-bounce multipath signals, which will signifi-cantly limit its performance. The right- and left-handed circularpolarization patterns at 1.227 GHz for the dual-band choke ringantenna are also provided in Fig. 7. Comparing the patterns tothe corresponding patterns at 1.575 GHz, it is evident that thedual-band choke-ring design has a very similar performance atboth frequencies. In contrast to the dual-band RSW designs (ona 1 m ground plane (Fig. 5) and a 25.4 cm ground plane tobe presented shortly), the RHCP for the dual-band choke ringis characterized by higher levels of radiation along the horizonand towards the backside of the antenna at both frequencies. Inaddition, as a result of the design compromises implementedto realize reasonable performance at both the L1 and L2 fre-quencies, the choke-ring design becomes severely de-polarized.While the LHCP levels for the dual-band RSW antennas are ap-proximately 25 dB down from the RHCP level at broadside atboth frequencies, there is only about a dB difference in thedual-band choke-ring patterns.


Fig. 8. The measured RHCP and LHCP radiation patterns at 1.575 GHz and1.233 GHz for a dual-band GPS-RSW antenna on a 25.4 cm square groundplane.

To ensure as fair a comparison as possible between the dual-band RSW and the choke-ring designs, the RHCP and LHCPpatterns for the dual-band RSW antenna were measured on a25.4 cm square grounded substrate with no additional groundplane. It is significant to note that, as a consequence of the11.15 cm short-circuit ring radius of the IRSW antenna in thedual-band design (corresponding to L2), a 25.4 cm ground planeis nearly as small a ground plane as can be used for this design.The patterns at L1 and L2 are shown in Fig. 8. The dual-bandchoke-ring ground plane is approximately 30.5 cm in diameter.Thus, for approximately the same size structure, the dual-bandRSW antenna demonstrates much less backscattered radiationand maintains a relatively small axial ratio for all angles abovethe horizon (the LHCP radiation levels are much smaller thanthe RHCP levels). In comparison to the measurements madeon the one-meter ground plane (Fig. 5), the RHCP radiationremains relatively unchanged with only slightly higher levelsof radiation occurring along the backside of the antenna. TheLHCP component of the overall radiation has increased; how-ever, the axial ratio is still greater on the 25.4 cm ground planethan that demonstrated by the larger commercial L1/L2 choke-ring design. As demonstrated in [11] for a single-band RSW de-sign, further reductions in the cross-polarization levels for thedual-band RSW antenna can be realized with improvements inthe feed design. Since the increase in LHCP levels for the RSWantenna on the smaller-sized ground plane is occurring mainlyat low-elevation angles and along the backside, a more sophis-ticated feed design for this smaller patch may indeed be neededto improve the multipath-rejection performance of the antenna(LHCP multipath signals are more likely to come from these an-gles).

B. Pinwheel Antenna Comparison

In [12] a GPS antenna based on an array of coupled spiralslots in a pinwheel type configuration has been introduced.

Fig. 9. The measured RHCP and LHCP radiation patterns at L1 and L2 for thepinwheel antenna.

While a short description of this antenna design can be found in[4], the key features to note here are that, like the RSW design,the pinwheel design can be used as a standalone element (notrequiring the use of a choke-ring ground plane) and thereby alsoeliminates the weight and bulk associated with a choke-ringantenna. However, it is also important to recognize that, relativeto the GPS-RSW structure, the pinwheel design is a much morecomplicated antenna.

The radiation patterns measured for the pinwheel design atL1 and L2 are shown in Fig. 9. Here it is apparent that, with theexception of slightly higher backside levels and RHCP levelsalong the horizon, the pinwheel design provides similar RHCPradiation characteristics as the choke-ring design at both fre-quencies. (Relative to L1, slightly higher levels of backscat-tered and horizon-level radiation are observed for this antennaat the L2 frequency.) Both antenna designs exhibit significantlyless RHCP radiation below the horizon compared to a conven-tional patch design and thus, both are capable of reducing the re-ceiver susceptibility to multipath events. As with the dual-bandRSW design, the cross-polarization levels of radiation measuredfor the pinwheel antenna are significantly lower at both fre-quencies than those of the dual-band choke ring (particularlyalong the front-side of the antenna), but higher than those forthe single-band choke ring antenna [4]. The radiation patternsfor the standalone pinwheel antenna (the pinwheel is printedon a square board of approximately 16.5 cm) can be directlycompared to those of the standalone RSW element presentedin Fig. 8. (Since, as previously discussed, the RSW antenna isconstrained to a minimum size ground plane of 25.4 cm and thepinwheel antenna is encased in a radome packaging, this is asfair a size comparison as can be made in this study.) As withthe choke-ring design, the most striking difference between thepinwheel design and the dual-band RSW antenna are the de-creased RHCP radiation levels along the horizon associated withthe RSW design. The two antennas have comparable LHCP pat-terns at both frequencies. Although the pinwheel antenna has a


size advantage over the dual-band RSW structure, the RSW de-sign, as previously noted, is a much simpler and easier-to-man-ufacture design.

VI. SUMMARY

Based on a reduced-surface-wave design concept, a newdual-band L1/L2 GPS microstrip patch antenna has been fabri-cated and tested. The dual-band RSW design is comprised oftwo concentric elements known as the shorted-annular-ringreduced-surface wave antenna (designed for L1) and the in-verted shorted-annular-ring reduced-surface wave antenna(designed for L2). The microstrip antennas are printed on thesame substrate and hence, form a planar patch design. The keyto the dual-band design is that each of these antenna elements,when printed on an electrically thin substrate, produce only asmall amount of lateral radiation. As a result, the dual-bandRSW antenna, a lightweight, low-profile design, is character-ized by broad, smooth front-side radiation pattern with verylow backside levels.

A comparison between the dual-band GPS-RSW radiationpatterns and those for the choke ring and pinwheel antennasreveals similar or better radiation performance while being atleast an order of magnitude smaller in terms of volume andweight than the choke-ring designs and easier to manufacturethan the pinwheel design. Although reduced backside radia-tion is common to all three designs, the dual-band RSW an-tenna demonstrates superior low-angle and ground-bounce mul-tipath rejection compared to the choke-ring and pinwheel de-signs. While, admittedly, a more directive radiation pattern forthe receiving antenna does limit the ability of the receiver to de-tect direct signals from low-angle satellites, it is also importantto recognize the higher gain corresponds to an increased signalintegrity from the overhead satellites. Furthermore, with the on-going efforts to increase the number of satellite constellations inorbit (i.e., Glonass), the required field of view for the receivingantenna may decrease, with signals from satellites closer to thehorizon becoming less important.

REFERENCES[1] E. D. Kaplan, Understanding GPS Principles and Applications. New

York: Artech, 1996.[2] L. I. Basilio, J. T. Williams, and D. R. Jackson, Effective point of

radiation considerations for a microstrip patch antenna, IEEE Trans.Antennas Propag., submitted for publication.

[3] D. R. Jackson, J. T. Williams, A. K. Bhattacharyya, R. L. Smith, S. J.Buchheit, and S. A. Long, Microstrip patch designs that do not ex-cite surface waves, IEEE Trans. Antennas Propag., vol. 41, no. 8, pp.10261037, Aug. 1993.

[4] L. I. Basilio, J. T. Williams, D. R. Jackson, and M. A. Khayat, A com-parative study for a new GPS reduced-surface-wave antenna, IEEEAntennas Wireless Propag. Lett., vol. 4, 2005.

[5] L. Boccia, G. Amendola, G. Di Massa, and L. Giulicchi, Shorted an-nular patch antennas for multipath rejection in GPS-based attitude de-termination systems, Microw. Opt. Technol. Lett., vol. 28, no. 1, Jan.2001.

[6] L. Boccia, G. Amendola, and G. Di Massa, A shorted elliptical patchantenna for GPS applications, IEEE Antennas Wireless Propag. Lett.,vol. 2, 2003.

[7] L. Boccia, G. Amendola, and G. Di Massa, A dual frequency mi-crostrip patch antenna for high precision GPS applications, IEEE An-tennas Wireless Propag. Lett., vol. 3, 2004.

[8] J. M. Tranquilla, J. P. Carr, and H. M. Al-Rizzo, Analysis of a chokering groundplane for multipath control in global positioning system(GPS) applications, IEEE Trans. Antennas Propag., vol. 42, no. 7, pp.905911, Jul. 1994.

[9] L. I. Basilio, J. T. Williams, and D. R. Jackson, Characteristics of aninverted-shorted-annular-ring reduced-surface-wave antenna, IEEETrans. Antennas Propag., accepted for publication.

[10] C. A. Balanis, Antenna Theory: Analysis and Design, 2nd ed. NewYork: Wiley, 1997.

[11] L. I. Basilio, New GPS antennas designed for reduced multipath sus-ceptibility, Ph.D. dissertation, Univ. Houston, Houston, TX, 2003.

[12] W. Kunysz, High performance GPS pinwheel antenna, NovatelInc., [Online]. Available: http://www.novatel.com/Documents/Pa-pers/gps_pinwheel_ant.pdf [Online]. Available

Lorena I. Basilio (S96M03) was born inPasadena, TX, in 1970. She received the M.S. andPh.D. degrees in electrical engineering from theUniversity of Houston, Houston, TX, in 1998 and2003, respectively.

During the course of her studies, she worked withDr. Jeff Williams and Dr. David Jackson primarilyin the area of microstrip antenna design and anal-ysis, with a concentration on reduced-surface waveantennas during her Ph.D. work. In September 2003she joined the Electromagnetics and Plasma Physics

Department at Sandia National Laboratories in Albuquerque, NM. Her currentresearch interests are in the area of microstrip patch antennas for GPS applica-tions, photonic bandgap antennas, and plasmon structures with enhanced opticaltransmission.

Dr. Basilio is a member of the International Union of Radio Science (URSI)Commission B.

Richard L. Chen , deceased, (S00M04) received the B.S. degree in physicsand the M.S.E.E. degree from Southeast University, Nanjing, China, in 1992and 1995, respectively, and the Ph.D. degree in electrical engineering from theUniversity of Houston, Houston, TX, in 2003.

After graduation, he joined the Department of Electrical and Computer Engi-neering, University of Houston, as a Postdoctoral Research Associate and thenas a Research Assistant Professor. His research interests included microstrip an-tennas and arrays, nano-scale frequency selective surfaces and signal integrity.

Dr. Chen was a member of the IEEE Antennas and Propagation and the IEEEMicrowave Theory and Techniques Societies. He was a coauthor of a paper thatreceived a best presentation award for a presentation given at the ION GPS/GNSS 2003 conference in Portland, OR.

Jeffery T. Williams (S85M87SM97) was bornin Kula, Maui, HI, on July 24, 1959. He receivedthe B.S., M.S., and Ph.D. degrees in electricalengineering from the University of Arizona, Tucson,in 1981, 1984, and 1987, respectively.

He joined the Department of Electrical andComputer Engineering at the University of Houston,Houston, TX, in 1987, where he is now an AssociateProfessor. Prior to that, he was a Schlumberger-DollResearch Fellow at the University of Arizona. Hespent four summers (19831986) at the Schlum-

berger-Doll Research Center in Ridgefield, CT as a research scientist. During1981 to 1982, he worked as a Design Engineer at Zonge Engineering and Re-search Organization in Tucson, AZ, and as a summer engineer at the LawrenceLivermore National Laboratory in Livermore, CA. His research interestsinclude the design and analysis of high frequency antennas and circuits, highfrequency measurements, the application of high temperature superconductorsand leaky-wave propagation.

Dr. Williams is a member of the International Scientific Radio Union (URSI)Commission B. He is a former Associate Editor for the IEEE TRANSACTIONSON ANTENNAS AND PROPAGATION and Radio Science


David R. Jackson (S83M84SM95F99) wasborn in St. Louis, MO, on March 28, 1957. He re-ceived the B.S.E.E. and M.S.E.E. degrees from theUniversity of Missouri, Columbia, in 1979 and 1981,respectively, and the Ph.D. degree in electrical engi-neering from the University of California, Los An-geles, in 1985.

From 1985 to 1991, he was an Assistant Professor,from 1991 to 1998, he was an Associate Professor,and since 1998, he has been a Professor in the De-partment of Electrical and Computer Engineering at

the University of Houston, Houston, TX. His current research interests includemicrostrip antennas and circuits, leaky-wave antennas, leakage and radiation

effects in microwave integrated circuits, periodic structures, EMC, and bioelec-tromagnetics.

Dr. Jackson is presently serving as the Chapter Activities Coordinator for theAP-S Society of the IEEE, and as the Vice Chair for URSI, U.S. CommissionB. He is currently an Associate Editor for the International Journal of RF andMicrowave Computer-Aided Engineering and a member of the Editorial Boardof the IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES. He is apast Distinguished Lecturer for the IEEE AP-S Society, and also a past AssociateEditor for the IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION and thejournal Radio Science. He has also served as a past member of AdCom for theAP-S Society.