Contents I. Introduction
Multiband and wideband planar antennas are studied intensively for various wireless applications such as mobile communication, broadcasting, and sensing. A multi-ring microstrip antenna (MR-MSA) fed by an L-probe has been proposed by the authors as a multiband planar antenna [1], [2]. The MR-MSA consists of multiple ring patches arranged concentrically on a dielectric substrate and a feeding L-probe placed under the patches [3], [4]. The MR-MSA exhibits excellent multiband performance as well as design flexibility because operating frequencies and their polarization (linear/circular) can be assigned easily. Furthermore, unidirectional radiation patterns can be obtained at all resonant frequencies. Only disadvantage of the MR-MSA is that bandwidths of the operating frequencies are narrow. In order to solve the narrow band problem, a frequency-tunable microstrip antennas with varactor diodes [5], [6] or with variable reactance circuits [7], [8] for single-band operation have been presented. Furthermore, frequency-tunable varactor-loaded multiband MR-MSAs with a double-layer substrate [9], [10] and with a single-layer substrate [11]–[13] have been proposed. In order to miniaturize the frequency-tunable MR-MSAs, varactor-loaded single-layer shorted MSAs fed by an L-probe have been reported [14]–[17]. In these antennas, the ring patches can be miniaturized by a quarter with shorted via holes connecting between the patches and the ground plane. However, cross polarization is increased because the ring patch is replaced with an L-shaped geometry. In order to reduce the cross polarization due to the L-shaped shorted patch, a varactor-loaded dual-band MSA fed by an L-probe with straight shorted elements is presented [18].
