A Novel GPS Survey Antenna

A novel GPS antenna for surveying applications is proposed. It is a fixed beam phased array of aperturecoupled slots optimized to receive a right hand polarized signal. The proposed antenna is made out of a single PCB board. Another PCB board is placed underneath the antenna to act as a reflector in order to reinforce the antenna directivity and reduce the back-lobe radiation. The radiation pattern roll-off of this antenna is sharper than the conventional GPS patch antennas mounted in the “choke ring” configuration. The sharp pattern roll-off allows reducing the antenna’s susceptibility to multipath generated replicas of the GPS signal. The antenna is much smaller and lighter than a corresponding typical “choke ring” antenna. There is no phase center offset between the L1 and L2 GPS frequencies and the antenna does not require any alignment with respect to a given direction (such as North) due to its natural symmetry. Due to its planar structure it can be easily buried in the vehicle or aircraft skin. INTRODUCTION GPS antenna requirements differ in various applications. For precise surveying applications, ideally the antenna should receive only signals above the horizon and reject all signals below the horizon plane of the antenna, have a known and stable phase center that is co-located with the geometrical center of the antenna, and have perfect circular polarization characteristics to maximize the reception of the incoming right hand polarized (RHP) signal. A typical measure of merit of antenna polarization characteristics is Axial Ratio (AR). For dual frequency operation (L1 and L2), the antenna should also have a common phase center at both frequencies and ideally the same radiation pattern and axial ratio characteristics. There is, of course, no antenna that could meet all these requirements. The closest antenna that meets most of these requirements, to some degree, is a patch antenna mounted on a choke ring ground plane. Such an antenna is, however, large, bulky, heavy and relatively expensive, prohibiting its use in various applications. The new antenna presented in this paper is light and small and does not require a choke ring ground plane to achieve performance similar or better to a patch antenna in the choke ring ground plane configuration. CHOKE RING GROUND PLANE ANTENNA In order to see the benefit of the new antenna, we need to understand what type of performance is achieved with a patch antenna mounted in the choke ring ground plane configuration. We will use the following figures of merit: Axial Ratio, antenna amplitude directivity pattern roll-off (elevation plane), antenna amplitude directivity pattern variation in azimuth plane, phase center location and phase center variation. Choke ring ground planes are a circular shaped ground plane with quarter wavelength slots that are shorted at the bottom and open at the top. This translates to a very high impedance ground plane that does not support image currents generated within the ground plane that normally would interfere with the currents generated within the patch antenna itself. This feature translates to very low side-lobes underneath the antenna horizon and very smooth amplitude and phase patterns generated by the antenna. In addition, very good Axial Ratio values (less than 3 dB) above 10 deg. elevation angle are achieved. The large size of the ground plane, on the other hand, translates to sharper amplitude roll-off (from zenith to the horizon) and increased main beam directivity of the antenna. Typical roll-off is in order of 10 to 12 dBi from zenith (90° elevation angle) to horizon (0° elevation angle) as compared to 3-5 dB roll-off of the patch antenna itself. Typical increase in the main beam directivity is in order of 1-2 dBi as compared to a stand-alone patch antenna without the choke ring ground plane. This performance enhancement is paid for with the size and weight of the additional ground plane. Typical diameter size of the choke ring is in the order of 14” to 16” and a weight of 10-20 lbs. In dual frequency operation, the phase center offset is inherited from the patch antenna itself. Since patch antennas use a stack configuration in order to resonate at two frequencies, they have a natural vertical offset between L1 and L2 phase centers that cannot be compensated for with a choke ring ground plane. Good surveying antennas currently available on the market have a typical L1-L2 offset in the order of 5-20mm. For highend applications such as tectonic movement monitoring this offset must be corrected for, especially for very long GPS baseline measurements. Microstrip patch antennas with stable phase centers must be fed in at least two points, preferable in four points (for all four edges of rectangular/square patch). Since the antenna must be circularly polarized, a 90° phase gradient must be established between the feeding points. The feed network to establish a 90° phase gradient for a two point feed system is relatively simple (90° hybrid will do), however as the number of feeds is increased the feed network becomes more complex and lossy.