A dual-band frequency-tunable varactor-loaded single-layer multi-ring microstrip antenna

This paper presents a dual-band frequency-tunable single-layer multi-ring microstrip antenna (MR-MSA) fed by an L-probe with varactor diodes. In order to develop a frequency-tunable multiband planar antenna, two split ring patches with two varactor diodes mounted on each ring patch are arranged concentrically on a dielectric substrate. An L-shaped feeding probe arranged in the same layer as the ring patches is used to excite them. The antenna structure is processed with a single-layer substrate, while bias circuits to apply a DC voltage to the varactor diodes are placed on the backside of the ground plane of the MR-MSA with another thin dielectric substrate. A prototype antenna is designed and tested. The measured results reveal that two resonant frequencies of the proposed antenna can be controlled by the DC bias voltage almost independently and that fractional bandwidth of the frequency-tunable range of the test antenna is around 10% for the both modes under the condition of more than 3 dBi gain. Basic operation of the proposed antenna is demonstrated by the measurement.