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 ~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 × 100 × 6.4 mm
³
and is fabricated on a dielectric substrate (ε
_r
= 2.2). The feed point and matching circuit for an impedance match to 50 Ω 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.