Polarization Characteristics of Multiband Loaded Microstrip Annular Ring Antennas

Multifrequency operation can be obtained from the single-layer probe-fed annular ring antenna by loading it with a gap or slit. Two types of loading are studied: vertical and horizontal gap. For each case, two unloaded modes are excited around the dominant TM₁₁ mode of the ring, equally spaced on the frequency spectrum. These multiple modes are excited, when the loading is symmetric on one side of the ring along one axis, and the excitation is off-centered on another side, or vice-versa. By choosing the location of the vertical or horizontal gaps along the y- or x-axis respectively, the polarization of each mode can be controlled. One of the loaded mode frequencies is much lower than the dominant TM₁₁ mode frequency of the annular ring, indicating a miniaturization of antenna to a great extent. Effects of substrate parameters are investigated on the loaded and unloaded mode frequencies, and a design guideline is presented. Simulation results are confirmed by measurements.