A Differential Dual-Polarized Cavity-Backed Microstrip Patch Antenna With Independent Frequency Tuning

A dual-polarized cavity-backed microstrip patch antenna has been demonstrated with independent tuning of both polarizations from 0.6 GHz to similar to 1.0 GHz using varactor diodes (1.2-5.4 pF). The 10-dB impedance bandwidth varies from 0.9% to 1.8% over that range. Differential coaxial feeds and the placement of the tuning varactors in the appropriate symmetry planes are employed to achieve a differential-mode port-to-port isolation of > 28 dB over the tuning range. Accordingly, the cross-polarization level is < -20 dB at broadside. The antenna is 100 x 100 x 6.4 mm(3) and is fabricated on a dielectric substrate (epsilon(r) = 2.2). The feed point and matching circuit for an impedance match to 50 Omega are determined using the transmission line model. This model underestimates the sensitivity of the resonance frequency to the loading capacitance, but leads to a good initial design. To the authors' knowledge, this is the first dual-polarized antenna with independent tuning over a 1.7:1 bandwidth and single-sided radiation.