MICROSTRIP RING ANTENNAS OPERATING AT HIGHER-ORDER MODES FOR MOBILE COMMUNICATIONS

An experimental study of microstrip ring antennas suitable for use in mobile communication systems is presented. Conical radiated beams are generated by the TM(21) and TM(41) modes. Dual mode concentric rings have been designed which switch the beams in elevation as the vehicle traverses up and down hills or changes latitude within a continent, while maintaining omnidirectional azimuth coverage to optimise signal reception. The authors have successfully designed and tested a stacked array which gives beam coverage in elevation from 20 degrees to 60 degrees with a gain >6 dBi. Mode switched antennas are a cheap and viable alternative to an expensive phased array antenna with continuous beam steering. Various feeding techniques have been investigated for exciting these modes. Bandwidths of 5% or more were attained for each mode. Coaxial probes gave good coupling efficiency and, with the addition of impedance compensating cylinders, bandwidths of 11% and 5% for the TM(21) and TM(41) modes, respectively, were obtained. Electromagnetic coupling provided a less mechanically complex feed system and control over the input impedance via the length of the feed overlap under the patch. Bandwidths of 7% were obtained for both modes and the resonant frequency was tunable by about 6% by varying the feed overlap length.