Theory and experiment of electromagnetically excited microstrip antennas for circular polarization operation

Feed structures using electromagnetic coupling for microstrip antennas have been proposed and investigated because of their ease of fabrication and integration with active devices. Aperture-coupled, slot-fed, coplanar waveguide-fed, and slot line-fed microstrip antennas are categorized as electromagnetically excited microstrip antennas. Since the slot is common to these electromagnetically excited microstrip antennas, its applications to circular polarization (CP) operation are studied. Using simple design formulas for an arbitrarily oriented slot-fed microstrip antenna, the CP performances for the first three types of electromagnetically excited microstrip antennas are investigated and tested experimentally. The measured CP radiation patterns and the feed structures are shown for the slot-fed, coplanar waveguide-fed, and aperture-coupled cases at about 2.7 GHz. As can be seen from the CP radiation pattern plots, the best CP is obtained for the slot-fed case because the theory was developed for this geometry. However, for the coplanar waveguide-fed case and the aperture-coupled case, the axial ratios are still below 3 dB for some range of observation angle.<>