Towards a Practical Single Element Null Steering Antenna

The GNSS signal properties which make it successful also make it vulnerable. Being an open signal transmitted from distant satellites allows it to be easily adopted and used worldwide. However, it also means that it is easy to interfere with or spoof. In the literature, antenna arrays or numerous receiver-based techniques have been proposed to detect and mitigate the interference. But, these approaches usually have complicated architectures and require significant additions or changes to the existing hardware. This makes these solutions difficult to adopt for safety of life applications. In previous work, a single element, dual feed antenna was designed to be capable of sensing the general direction of signal source. Its capabilities can then be used for spoof detection as well as interference mitigation. For instance, in aviation, the terrestrial interference sources usually originates from below the aircraft (i.e. the lower hemisphere). The single element dual feed antenna can distinguish signals from low and high elevations. Furthermore, it can then create a null toward the source direction to mitigate it. The antenna consists of a dual-feed patch, 90 degree hybrid coupler, variable phase shifter, power combiner, RF switches and microcontroller. Both right-hand and left-hand circular polarization signals (RHCP and LHCP, respectively) can be received by the dual feed patch. The body of aircraft extends the ground plane of antenna and alters the polarization of low elevation signals adjusting the RHCP and LHCP components. Hence, the body aids in discriminating high and low elevation sources. Null-steering is accomplished by combining the phase-shifted RHCP signal with the LHCP signal. The technology is adaptable to existing installations, replacing the antenna itself without changing the receiver or requiring additional antennas and their accompanying holes and cabling. Several iterations of components were constructed, tested and programmed to develop a miniaturized printed circuit board (PCB) version of the antenna with desired functionality. This paper describes the PCB design and implementation in detail. Several important modifications are made to solve potential issues on PCB and to improve performance. It also provides results of tests conducted to validate the miniaturized version of antenna. Results indicate that the antenna can clearly distinguish signals from different elevation and azimuth satellites.