A study of microstrip antennas for multiple band operation

In this investigation two approaches have been studied, both theoretically and experimentally. The first investigation is made on annular microstrip antennas. As seen from the cavity model theory an antenna of this type, if excited for (1,1), (1,2) and (11,3) modes, will have predominantly broadside radiation of the same polarization. However, unlike disc or rectangular patches, the resonant frequencies for these modes are not fixed ratios, being dependent on the dimensions of the inner and outer radii. Sets of curves and tables which show this dependence are presented. In addition, the feeding methods and techniques for suppressing undesirable modes are discussed. For most microstrip antennas, such as disc and rectangle, the computer resonant frequency from the cavity model theory is always higher than the measured, due to the fringing field effect. However, for annulus, this may not be true since it has two unconnected boundaries. This is also investigated in some detail. In the second approach, a rectangular patch, excited particularly in the (0,1) and (0,3) modes, is investigated since they both produce broadside radiations of the same polarization. However, the operating frequencies for these modes are at a fixed ratio, approximately 3:1. To remove this restriction, electrically shorting pins are inserted along the modal lines in the (0,3) modal over a wide range while that of the (0,3) mode is kept unchanged.