Frequency control of multi-band ring microstrip antennas with variable reactance circuits

Multiband or wideband antennas are desired for various wireless applications such as mobile communication systems and car electronics. In accordance with these trends, multiband planar antennas with multi-ring microstrip antennas (MR-MSAs) have been developed [1], [2]. The MR-MSAs consist of multiple ring patches arranged concentrically on a double-layered dielectric substrate and an L-probe in the middle layer for feeding. The MR-MSAs exhibit excellent multiband performances and design flexibility because operating frequencies as well as its polarization (LP or CP) are specified very freely. One of disadvantages of the MR-MSA is narrow band width of each operating frequency. On the other hand, frequency-tuneable MSAs with varactor diodes [3], [4] or with variable reactance circuits [5], [6] are reported.