Proper modeling using an FEM electromagnetic simulator leads to the design of a low-cost, lightweight GPS patch antenna capable of excellent multipathrejection.
Global Positioning System (GPS) receivers (Rxs) are becoming ubiquitous, as part of the electronics packages of new automobiles, in cellular telephones, and in compact electronic devices. Unfortunately, multipath errors continue to plaque the performance of discrete and embedded GPS Rxs, even with advances in signal processing. However, an innovative low-profile, lightweight antenna may offer a possible solution for reducing the multipath error in GPS systems. As will be shown, it may be possible to achieve high-performance GPS requirements with the aid of a shorted annular patch antenna.
In recent years, the number of GPS applications requiring an augmented accuracy is considerably increased spanning from geodetic surveying to aircraft landing control and satellite attitude determination. The major limitation affecting the precision of the system is the multipath error. Multipath interference is generated by the reflections and diffractions of the GPS transmitted signal from surfaces around the antenna. Since multipath effects are dependent upon the surrounding environment, they are difficult to quantify, and available signal-processing techniques do not help in solving the problem completely under all conditions. A more effective way to limit the deleterious effects of spurious reflections is by means of an antenna with superior multipath rejection capability.
At the radiator level, multipath can be essentially controlled in two ways. Since GPS signals are right-hand circularly polarized (RHCP), odd reflections are left-hand circularly polarized (LHCP). Hence, the use of antennas with a good rejection of LHCP signals can potentially eliminate multipath effects arising from direct reflections. Effects due to double reflections will remain, but these are normally much weaker than the direct reflections. Additionally, considering that reflections often impinge on the antenna at low elevations, multipath rejection performance can be improved by shaping the antenna gain pattern to reject low-elevation signals while ensuring adequate hemispherical coverage.
Several low multipath GPS antennas have been proposed in the past. Unfortunately, most of the available solutions, including arrays1 or choke rings,2 are impractical in aerospace applications due to the operational requirements in terms of size and weight. A more effective design has been proposed in ref. 3 where a novel compact radiator, namely the shorted-annular-patch (SAP) antenna, has been introduced as a possible solution for low-multipath GPS applications. In what follows, the main characteristics of SAP antennas will be discussed and a detailed review of a SAP design procedure will be presented.
The SAP antenna geometry is presented in Figure 1. At variance of a conventional disk the inner boundary of this patch is shorted to the ground plane. The presence of the conductor in the central zone of the antenna makes this geometry much more flexible with respect to other microstrip geometries allowing for a larger bandwidth and easier matching.4
The essential feature of the antenna is that the low-multipath radiation pattern requirements can be fulfilled using a single radiator, as the pattern of the shorted annular patch can be easily controlled varying the antenna geometry without degrading the radiation characteristics. In fact, it is easy to show that,5 when working on the TM11 mode, the shorted ring has the same magnetic current distribution of a conventional disk and therefore a similar radiation pattern. As a consequence, with a proper choice of the external and internal radii, narrower radiation patterns that maintain the radiation characteristics of a circular disk can be obtained.
To design an SAP antenna, the first step is the selection of the patch outer radius. As it will be shown, this parameter essentially controls the antenna amplitude pattern toward the horizon and, in case of high-precision GPS applications, its choice must be the optimal compromise between the specific coverage requirements and the low multipath constrains. Once the external boundary of the shorted ring has been fixed, the inner radius has to be adjusted to make the patch resonating at the desired frequency.
As a proof of the SAP peculiarity, three shorted annular patch antennas resonating at the nominal GPS L1 frequency, 1.57542 GHz, with an external radius of 35, 45, and 55.7 mm, have been designed considering a substrate with dielectric constant, εR, of 2.55 and thickness of 3.2 mm. Adequate circular polarization purity is attained by feeding the antenna by means of two 50-Ω coaxial probes located 90 deg. apart and having 90 deg. of phase difference.
To simplify the design process, a simple analytical model4 was used as a starting point to roughly estimate the antenna resonant frequency and feed location. The design was then optimized through extensive finite-element-model (FEM) based simulations using commercial High-Frequency Structure Simulator (HFSS) software from Ansoft Corp. (Pittsburgh, PA).6 Accurate simulations were obtained by manually refining the mesh for each geometrical element of the antenna. The inner radius and the feed location for each of the three patches are shown in the table.
The effect of a larger external radius is shown in Fig. 2 where the copolar radiation patterns of the three SAP antennas have been compared with the one of a conventional circular patch resonating at the same frequency and designed using the same substrate. As expected, a larger outer radius of the antenna results in a narrower beam.
It should be noticed that the shorted annular patch antenna having external radius of 55.7 mm was designed by considering the surface's wave reduction principle described in ref. 5. According to this criterion, the outer radius of the ring must be selected in such a way that the TM0 surface-wave emission is inhibited avoiding the radiation pattern deterioration due to the diffraction of the waves on the dielectric truncation. However, the choice of the external radius is also influenced by other requirements such as the extension of the radiation pattern coverage and the acceptable level of crosspolar interference.
The characteristics of the antenna and the reliability of the simulations were experimentally verified fabricating a prototype of the SAP antenna having an external radius of 35 mm.
The simulated and measured input impedances of the antenna were first compared. A comparison between numerical and experimental data often serves as a source of errors, however, due to false assumptions or poor estimations. In this case, in particular, it should be noticed that the measured input impedance is generally taken at a reference plane arbitrarily set during the calibration while the computer-generated data are calculated at the microwave port defined within the simulation environment (Fig. 3).
To avoid this phase uncertainty, the two results presented here have been compared choosing the antenna ground plane as the common reference. The calibration reference plane was set by measuring and analyzing in the time domain the reflections arising from a short-ended SMA connector of the same type of the one used to feed the antenna. The calculated input impedance was coherently evaluated at the antenna ground plane by employing the de-embedding procedure included within the simulation tool environment.
In Fig. 4, the measured antenna input impedance is compared with the simulated data before and after the deembedding procedure was applied. As it can be seen, the predicted result is in excellent agreement with the experimental values but this outcome can be appreciated only if a coherent de-embedding method is used.
Due to the precision of the simulator and the accuracy of the fabrication process, it was possible to achieve a fairly precise design that provided predictably high performance. In fact, the antenna resonates at the nominal GPS L1 frequency and is very well matched. The multipath rejection performances of the SAP prototype have been evaluated considering both the sharpness of the antenna pattern toward the horizon and the circular polarization purity over the whole radiation hemisphere.
The measured and simulated radiation patterns (Fig. 5) show that the proposed SAP antenna has, as expected, an amplitude roll-off from boresight to horizon of about 15 dB. It is important to note that this result, which provides a wide hemispherical coverage while sufficiently rejecting grazing signals, has been obtained using a 14-cm-square ground plane so without increasing the overall dimension of the antenna. In addition to the sharpness of the radiation pattern, the SAP prototype proposed in this paper fully satisfies the polarization purity constrains required for high-precision GPS applications. In fact, the axial ratio stays below 2 dB within the entire coverage hemisphere.
The final antenna design, based on an SAP geometry, is inexpensive and light in weight, but offers an extended radiation pattern flexibility which can be used to optimize the multipath rejection performances in consideration of the specific application constrains. The characteristics of the antenna have been verified performing both numerical and experimental tests. Where properly considered, the simulated results are in excellent agreement with the experiments.
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