A frequency-tunable varactor-loaded single-layer ring microstrip antennas fed by an L-probe with a reduced bias circuit

This paper presents design of a varactor-loaded frequency-tunable ring microstrip antenna with reduced bias circuits. Two varactor diodes are mounted on square ring patch element fed by an L-probe to realize a frequency-tunable microstrip antenna. An L-shaped feeding probe is used to realize impedance matching of the ring microstrip antenna. Resonant frequency of the proposed antenna can be tuned electronically by a DC bias voltage applied to the varactor diodes. In order to develop a frequency-tunable multiband microstrip antenna with multiple ring patch elements, reduction of the bias circuits is an important issue. In this paper, one of the bias circuits for negative bias port of the frequency-tunable ring microstrip antenna is removed by using a via hole connecting the ring patch and the ground plane at the zero-voltage plane. Fractional bandwidth of the frequency-tunable range of the proposed antenna with the reduced bias circuit is measured to be around 10% under the condition of over 3 dBi gain and the reflection characteristics less than -10 dB.