Method and apparatus for reducing radio frequency interference (RFI) using
a dual-element patch antenna [10]. The antenna possesses two antenna elements
[13, 14] having distinct radiation patterns. Either element may be independently
selected using a DC bias voltage. Diodes [20] connected to the elements
serve to disable one element when the other is selected. In one selected mode,
a nominal radiation pattern provides a broad, hemispherical shaped sensitivity
that is designed for acquiring and tracking all navigation satellites above the
horizon. This nominal radiation pattern, however, is susceptible to interference
that is present near or below the horizon. The second selectable radiation pattern
of the dual-element antenna has comparatively higher gain toward zenith, and lower
gain at and below the horizon to mitigate interference. This combination of features
is packaged in a single antenna unit that can be a direct replacement for existing
antennas. The dual-element antenna unit has a low vertical profile and is suitable
for mounting on high-speed moving vehicles.