COMPARATIVE STUDY OF HIGH PERFORMANCE GPS RECEIVING ANTENNA DESIGNS.

Juan I. Ortigosa, Neus Padros, Magdy F. Iskander University of Utah, Salt Lake City, Utah 84112

Bryce Thornberg, E-Systems, Salt Lake City, Utah 841 19

Abstract

A major factor in the performance of a Differential Global Positioning System (DGPS) ground station is the performance of the antenna. The present study analyzes and compares the performance of different GPS antenna designs. The specifications for the DGPS ground station antenna include hemispherical coverage and good multipath rejection. Good multipath rejection can be achieved by using an antenna that presents circular polarization and sharp drop-off at low elevation angles. In more detail, the specifications for the high precision GPS antenna include right hand circular polarization with a minimum of 15 dB of cross polarization rejection, hemispherical coverage presenting a variation of less than 3 dB over the main beam and a drop-off rate larger than 1 dB/deg for elevation angles from -5 to 5 deg. These specifications should be satisfied at the frequencies of 1.227 GHz (L2) and 1.575 GHz (Ll). The antenna designs examined in our study include patch antennas, helical antennas and conical spiral antennas. The analysis is based on the results of the simulations obtained using the software tool IE3D@ which is based on a full wave integral equation for solving the current distribution on 3D multilayer arbitrary structures. Results from this comparative study show that all the above stated specifications may be achieved by using a conical spiral antenna design. It was difficult to satisfy the broadband performance using patch antennas, and designs based on the use of helical antennas required the use of unacceptably large array. The numerical results from a successful conical spiral antenna design were verified experimentally.

Overview of the comparative study Based on extensive numerical simulations of the three design options of

a high performance GPS receiving antenna including patch antennas, helical antennas and conical spiral antennas, we were able to make the following observations from the comparative study:

a. Patch antennas Patch antennas present a radiation pattern with a broad lobe over one

hemisphere. This may provide the hemispherical coverage required for GPS applications. Relatively low cost and light weight are additional features of patch antennas. These features make patch antennas an attractive design possibility. Patch antennas, however, are inherently narrowband. Therefore, the analysis procedure initially considers the option of achieving the design characteristics at a single frequency. The resulting designs at L1 and L2 were then combined into

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a stacked patch antenna and the design is adjusted to obtain the dual frequency of operation. Circular polarization was obtained using a proper feed arrangement and an adequate width to length ratio [l]. Upon exciting the patch antenna, it is known that two perpendicular surface currents are excited in the rectangular patch. The ratio of their magnitude I I, I / I I, I depends on the position of the feed. In addition, the phase difference between I, and I, depends on the ratio between the side length of the patch. To achieve circular polarization, equal magnitude and 90" of phase difference in the currents I, and I, are required. This can be achieved by adjusting the feed location in the patch. The value of the dielectric constant used in our design was 2.32. Two single frequency patch antennas operating at L1 and L2 frequencies were then combined to a stacked patch antenna. After adjusting the dimensions of both patches, it was possible to obtain satisfactorily dual frequency operation. Preliminary results showed that patch antennas can satisfy some of the specifications for the high performance GPS antenna application but it is difficult to meet the required cross polarization rejection at both frequencies L1 and L2 simultaneously using this design and hence it is suggested that we consider the use of helical antennas.

b. Helical antennas Helical antennas present circular polarization, a broad frequency band and

are relatively insensitive to mutual coupling effects. Because of this features these antennas are suitable for use in GPS applications. Since the specifications include hemispherical coverage, we were interested in the axial mode of radiation of helical antennas. The center frequency of the bandwidth corresponds to an approximate tum perimeter of A [2]. The impedance matching of the helical antenna is another design consideration. The input impedance of a helical antenna should be lowered to obtain a low VSWR when connecting the antenna to a 50 Q coaxial line. This can be achieved by using a tapered feed section connected at the feeding point [3]. Several geometries with different dimensions and different number of tums have been simulated and the results showed that the single square helical antenna presents a good rejection of the cross polarized fields at both L1 and L2 frequencies. The radiation pattems cutoff near the ground, however, was not sufficient to satisfy the requirements for this GPS application. To improve this characteristic, the usage of arrays of square helical antennas is considered. Good drop off rates in the radiation pattem were obtained by analyzing planar arrays but the resulting beam was too narrow to cover all satellites. The proposed solution is a multibeam antenna based on sectoral coverage that provides sharp drop off at low elevation angles. This solution includes the study of different planar array arrangements which will compose the structure as well as an analysis of mutual coupling between helical antennas. Different configurations of planar arrays are simulated and results show that it is possible to obtain the desired drop-off rate in the radiation pattem using a 3x2 array of helical antennas and that the hemispherical coverage can be completed by using 2x2 planar arrays. The main drawback of this structure is that it requires a large number (60) of single helical antennas. This lead to consider the possibility of using conical spiral antennas for this application.

c. Conical spiral antennas The conical spiral antenna is another design option for GPS applications,

since its features include circular polarization, a broad main lobe, broad frequency band and good front-to-back radiation ratio for small cone angles. Different geometries for the conical spiral antenna were simulated. Initially, the dimensions were obtained using the criteria given by Dyson [4][5]. The obtained results showed a polarization ratio of -20 dB over the main beam, and a front-to-back ratio of -17 dB for the frequency of 1.227 GHz. For the frequency of 1.575 GHz the front-to-back ratio was -12 dB and the polarization ratio over the main beam is -13 dB. To improve the performance of the antenna at the higher frequency, a more restrictive criterion is used to determine the diameter d of the truncated section at the vertex. The value of d is restricted not to be greater than A/14, where A is the wavelength at the frequency of 1.575 GHz. The obtained results showed that the polarization ratio is improved by 3 dB but the front-to-back ratio remained unchanged. Better results were obtained by using a lossy material at the base of the cone. The thickness of the material used in the simulation is 5 cm and an absorbing material of loss tangent 1 was assumed. The results showed that the polarization ratio is less than -20 dB at both the L, and the L, frequencies. The -6 dB beamwidth is 130" at the frequency of 1.227 GHz and 145" at the frequency of 1.575 GHz. The drop off rate is 1 dB/deg and 2 dB/deg at the frequencies of 1.227 GHz and 1.575 GHz, respectively.

As a conclusion, it can be mentioned that some of the specifications are satisfied by using a stacked patch antenna, but it is particularly difficult to obtain good cross polarization rejection at the L l and L2 frequencies simultaneously. Good performance is obtained by using a 3D array of helical antennas based on the sectoral coverage principle. The main drawback of this design is the large number of helical antennas required for the total structure. The conical spiral antenna design provides the most promising results from the performance- complexity tradeoff point of view. Prototypes of the single helical antenna and the conical spiral antenna have been constructed and used to experimentally validate the results of the simulations. Figure 1 shows a photograph of the constructed conical spiral antenna placed on a rectangular section of lossy material. Figures 2 and 3 show the simulation and experimental results, respectively. From figures 2 and 3 it may be seen that measurements showed good agreement with the numerical results.

REFERENCES AND BIBLIOGRAPHY [ 11 Desphand, M. "Rectangular microstrip antenna for circular polarization", IEEE Trans. on Antennas and propagation, Vol. AP-34, No.5, May 1986. [2] Kraus, J.D. "Antennas", McGraw-Hill 1988 [3] Kraus, J.D. ' I A 50-Ohm Input Impedance for Helical Beam Antennas", IEEE Trans. on Antennas and propagation, Vol. AP-25, No.6, Nov.1977 p.913. [4] Dyson J.D. "The Unidirectional Equiangular Spiral Antenna", IRE Trans. on Antennas and Propagation, Vol. AP 7, Oct 1959, pp.329-334. [5] Dyson J.D. "The Characteristics and Desig n of the Conical Log-Spiral Antenna", IRE Trans. on Antennas and prop., Vol. AP-13, Jul 1965, pp. 488-499.

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Figure 1 Constructed conical spiral antenna

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- _ meas RHCP meas LHCP slmul RHCP simul LHCP

Figure 2 Simulation and exprriniental results for the conical spiral antenna at the L1 frsquency.

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_ _ meas RHCP . simul RHCP

meas LFC; simul Lsc? - _ - -

Figure 3 Simulation and experimental results for the conical spiral antenna at the L1 frequency.

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