Loaded annular ring antennas for multi-frequency operation

Multi-frequency operation is obtained from the single-layer probe-fed annular ring antenna by loading it with a gap or slit. Two unloaded modes are excited around the dominant TM₁₁ mode of the ring, equally spaced on the frequency spectrum. These multiple modes are excited, if the loading is symmetric on one side in 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 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. Simulation results are confirmed by measurements.