A Frequency-Tunable Dual-Band Single-Layer Shorted Multi-Ring Microstrip Antenna Fed by an L-probe with Varactor Diodes

In this paper, a frequency-tunable dual-band single-layer shorted multi-ring microstrip antenna fed by an L-probe with varactor diodes is presented. In order to develop a frequency-tunable dual-band and miniaturized planar antenna, two shorted square ring microstrip antennas with vias are printed on a dielectric substrate and two varactor diodes are mounted on each patch. An L-shaped feeding probe arranged in the same layer is used to excite the shorted patches. Bias circuits to feed DC bias voltage to the varactor diodes are situated on the backside of the ground plane of the patches with another thin dielectric substrate. Two resonant frequencies of the proposed antenna can be controlled by the DC voltages applied for the varactor diodes on each patch, respectively. Thus, the proposed antenna can realize miniaturization of the patch size to a half in comparison with the conventional one and also independent control of the two resonant frequencies. A prototype antenna is fabricated and tested to show the basic operation of the proposed antenna. The measured results reveal that wide frequency-tunable ranges of the resonant frequencies with 27% and 31% fractional bandwidths for the 1st and 2nd modes are observed, respectively. Although the measured gain of the proposed antenna is reduced due to the miniaturization, the maximum gains are 2.8 dBi and 3.6 dBi for the 1st and 2nd modes, respectively. The stable radiation pattern is confirmed over the frequency-tunable range. The antenna structure can be extended to multiband design more than tri-band operation by adding more shorted ring patches.