AFrequency-Tunable Varactor-Loaded Single-Layer Shorted Ring Microstrip Antenna fed by an L-probe

This paper presents a frequency-tunable varactor-loaded single-layer shorted ring microstrip antenna fed by an L-probe. In order to miniaturize a frequency-tunable ring microstrip antenna, two varactor diodes are mounted on a shorted ring patch antenna near the shorted vias. An L-shaped feeding probe, which is arranged in the same layer as the patch, is used to excite the shorted patch. Thus, the antenna structure is composed of a single-layer dielectric substrate and the patch area of the proposed antenna is reduced to a half in comparison with the conventional one. Resonant frequencies of the proposed antenna can be tuned by DC bias voltage applied to the varactor diodes. The bias circuit is arranged on the backside of the ground plane of the shorted patch with another thin dielectric substrate. In order to demonstrate the basic operation of the proposed antenna, a prototype antenna is fabricated and measured. Fractional bandwidth of a tunable range of the resonant frequency is around 5.7% under the condition of more than 3 dBi gain. The stable radiation pattern is observed over the frequency-tunable range. The antenna structure can be extended to multiband design more than dual-band easily by adding more shorted patches.