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 TM21 and TM41 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 to 60 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 TM21 and TM21 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. Introduction: Many new satellite based services for road vehicles are planned to come into service in the late 19%. For example, cars and trucks will have access to satellite telecommunications and global positioning systems. Compact antennas, mounted inconspicuously on the vehicle, will be required to receive these services together with roadside traffic information data. Not only are these services likely to be operated at different frequencies but the radiation pattern requirements from the antennas will also be different. For example, telecommunications may be provided via a geostationary satellite system requiring antenna beams pointing at elevation angles between 20 and 60 at European latitudes, i.e. 30-70 from zenith. Alternatively, continuously orbiting satellites, as proposed in the Motorola Iridium system, may provide a similar service network. One difficulty is that the antenna beam is required to track the satellite while the vehicle turns or travels up and down hills. Antenna gains of 3 dB or more are specified for the telecommunications systems addressed here, depending on the application and in all cases the antennas are to be circularly polarised. Three solutions to the antenna problem can be envisaged. The first is to use an omnidirectional antenna, but this can be discounted at present due to low gain. Secondly, the satellite may be tracked using a phased array; however, this is an expensive option and as such can be ruled out for standard consumer terminals. The third solution, which is reported here, is to generate a conical beam, which can be switched in elevation to optimise the gain. This beam configuration gives omnidirectional coverage in azimuth allowing a vehicle to turn without problems and