A frequency tunable ring microstrip antenna fed by an L-probe with varactor diodes

This paper presents a frequency tunable ring microstrip antenna fed by an L-probe with varactor diodes. In order to control a resonant frequency of the ring microstrip antenna electronically, two varactor diodes are mounted on the rectangular ring microstrip patch antenna. The resonant frequency of the proposed antenna is controlled by a DC bias voltage applied to the varactor diodes. The fractional bandwidth of the tunable frequency range of the proposed antenna is around 7.3% under the condition of 1 dB gain reduction from the maximum and the reflection less than -10 dB. The antenna structure can be easily extended to multiband operation with concentrically arranged multiple ring patch configuration.