High Performance GPS Pinwheel Antenna

There is a need for improved antenna performance in view of a recent decision by the DOD to disable SA at L1 GPS channel. For single frequency receivers SA had been the main cause of the largest errors of code and carrier phase measurements. With SA turned off, the multipath, ionosphere and antenna phase center variations are now the biggest contributors to code and carrier phase measurements. Ideally the antenna must be smaller and lighter than a typical “choke ring” antenna for surveying and man pack applications. In this paper, a new high performance GPS antenna is proposed using “Pinwheel” (patent pending) technology [16], characterized by a stable phase center and very good immunity to multipathgenerated replicas of GPS signal. The antenna is a fixedbeam twelve-element phased array antenna comprised of aperture-coupled spiral slots optimized to receive L1 GPS right-hand polarized signal. In addition there is an array of eleven concentric slot rings located outside the perimeter of spiral slots in order to suppress the diffraction and reflection from the edges of the antenna, and to achieve much improved right-hand circular polarization at very low elevation angles. This allows further reduction of the predominantly left-hand polarized multipath signals generated at low elevation angles. The antenna has a very simple mechanism of establishing a proper phase gradient for all spiral slots in order to achieve right-hand polarization for the whole upper hemisphere. A spiral slot antenna element has a wide-band performance, therefore preventing unwanted phase (group delay) and amplitude modulation of the spread spectrum GPS signal typically found in narrow band antennas. In order to limit the antenna to out-of-band interference, the GPS signal is band limited with RF filters in the LNA section of the antenna. 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 much sharper than the conventional GPS patch antenna. The sharp pattern roll-off allows further to reduce the antenna’s susceptibility to multipath-generated replicas of the GPS signal. The antenna does not require any alignment with respect to a given direction (such as North) due to its natural symmetry.