A Frequency-Tunable Varactor-Loaded Single-Layer Ring Microstrip Antenna with a Bias Circuit on the Backside of the Ground Plane

This paper presents a frequency-tunable single-layer ring microstrip antenna fed by an L-probe with varactor diodes. In order to develop a frequency-tunable ring microstrip antenna, two varactor diodes are mounted on a square split ring patch antenna. An L-shaped feeding probe arranged in the same layer as the ring patch is used to excite the ring patch. The antenna structure is processed with a single-layer dielectric substrate. Resonant frequencies of the proposed antenna can be controlled by DC bias voltage applied to the varactor diodes. The bias circuit is placed on the backside of the ground plane of the ring patch antenna with another thin dielectric substrate to reduce unwanted radiation from it. In order to demonstrate the basic operation of the proposed antenna, a prototype antenna is designed and measured. Fractional bandwidth of the frequency-tunable range of the test antenna is around 10% under the condition of more than 3 dBi gain and the radiation pattern is stable over the tunable frequency range. The antenna configuration can be extended to multiband operation more than dual-band by arranging more ring patches.