A dual-band shorted annular ring patch antenna with interference rejection at the horizon is presented for GPS timing applications. It is shown that the dimensions of the annular ring can be optimized to make a null in the RHCP pattern at low elevation near the horizon for all azimuth angles. This null attenuates interfering signals originating from ground based sources. The antenna achieves circular polarization utilizing radial shunted stubs. The effect of the stubs on the resonance is analytically derived and verified through simulations. A novel feed configuration that incorporates a coplanar waveguide transition improves the impedance match for both L1 and L2 GPS frequency bands compared to previous designs that present compromises between the feed impedance of the two bands. Additionally, since the shunted stubs reduce the number of required electronic components compared to other antennas with similar horizon nulling capability, the cost is reduced. A prototype antenna operating at GPS L1 and L2 bands has been fabricated and validated through measurements.
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