Contents I. Introduction
Multiband or wideband antennas are intensively studied for various wireless communications and radars such as mobile phone systems and vehicle electronics. In order to satisfy these demands, the authors have been proposed a multi-ring microstrip antenna (MR-MSA) for multiband operation [1], [2]. The MR-MSA is composed of multiple ring patches printed on the upper dielectric substrate concentrically and an L-shaped feeding probe (L-probe) in the lower substrate [3], [4]. The MR-MSA brings excellent performance as well as flexibility in multiband design because operating frequencies and its polarization (LP or CP) can be easily designed. A unique disadvantage of the MR-MSA is narrow bandwidth of each operating frequency. In order to overcome this problem, frequency-tunable varactor-loaded MSAs [5], [6] and with variable reactance circuits [7], [8] have been developed. These frequency-tunable MASs are mainly for single frequency operation. A frequency-tunable multiband MR-MSA with variable reactance circuits has been proposed by the authors [9]. The variable reactance circuit is arranged on the backside of the MR-MSA and the T -shaped probes are placed under each ring patch elements to couple to the ring patches. The problem of the frequency-tunable MR-MSA with variable reactance circuits is that the variable range of each resonant frequency becomes narrow. In order to improve the frequency tunability of the MR-MSA, a frequency-tunable varactor-loaded ring MSA with a double-layer dielectric substrate has been proposed [10], [11]. On the other hands, a frequency-tunable varactor-loaded ring MSA composed of a single-layer dielectric substrate has been also proposed [12]. In this antenna configuration, the L-shaped probe is arranged in the same layer as the ring patch. The fractional bandwidth of the both frequency-tunable ring MSAs with double- and single-layer substrates are around 10% under the condition of over 3 dBi gain and of the reflection less than −10 dB, respectively.
