A tunable second-resonance cross-slot antenna

Second-resonance slot antennas, due to their ability to transmit and receive on both sides of a microstrip ground plane, are often employed in planar active antenna arrays. For example, in an amplifier array, the input and output of an amplifier circuit are connected to orthogonally polarized antennas. The purpose of using orthogonal polarizations is to ensure amplifier stability. We present a 7 GHz cross-slot antenna designed to be used in a mixer/phase detector active antenna. By incorporating a varactor diode into the microstrip feed line, the cross-slot resonance can be electronically tuned over a 10% bandwidth. By mechanically varying the feed line (tuning stub) length, a 45% 2:1 VSWR bandwidth is possible.