A Shallow Varactor-Tuned Cavity-Backed Slot Antenna With a 1.9:1 Tuning Range

A shallow (0.025 wavelengths) microstrip-fed cavity-backed-slot (CBS) antenna has been demonstrated that tunes from 1 to 1.9 GHz with better than -20 dB reflection coefficient using a single varactor diode (0.45-2.5 pF). This is possible because the slot and the cavity combine to form a single resonance, and therefore, do not need to be tuned independently. The cavity is 72 × 72 × 3.18 mm
³
, and is dielectrically loaded (¿
_r
= 2.1). The impedance match over the tuning range has been achieved by searching for combinations of feed and varactor locations for which the impedance is matched to 50 ¿ by a single series inductor, and the input impedance is nearly identical whether the antenna is in free-space or conformally mounted on a 1.2 × 1.2 m
²
ground plane. The cross-polarization is better than -25 dB at 1.0, 1.5, and 1.9 GHz. To the authors' knowledge, this is the first demonstration of a varactor-tuned CBS antenna.