Multifrequency operation of microstrip-fed slot-ring antennas on thin low-dielectric permittivity substrates

Operating frequencies and radiation patterns of microstrip-fed ring slot antennas are investigated. The microstrip feed is shown to strongly affect the operating frequencies of the slot antenna. As the length of the microstrip stub is increased to exceed half the guided wavelength of the first operating frequency, then multiple operating frequencies with broadside radiation patterns become available. By extending the single microstrip feed to couple to two slot locations, the antenna´s operating frequency ratios and polarizations can be tuned. A number of multifrequency microstrip-fed slot ring antennas are demonstrated. Numerical analysis based on the moment method is compared with the experimental results. A dual-frequency antenna array is also reported.