Reducing the Effects of the Superstrate on the Microstrip Omnidirectional Antenna With an Annular Ring

In this communication, a new method of embedding an annular ring into the superstrate is proposed and studied to reduce the negative effect of the superstrate on the microstrip omnidirectional antenna. The working mechanism of the annular ring is analyzed based on the E-field distributions and an equivalent model that can approximately determine the dimensions of the annular ring. The effect of the parameters of the superstrate and the annular ring on the antenna performance is analyzed to verify the equivalent model. With the help of the annular ring, the negative effect of deep ripples in the radiation pattern, the strong radiation at the lower hemisphere, and the sensitiveness of the radiation pattern to the ground size from the superstrate are mitigated. A fabricated prototype shows that the measured -10 dB impedance bandwidth is 0.7 GHz (12.9-13.6 GHz). Within the frequency range of 13-13.8 GHz, the measured radiation patterns have no deep ripples and the measured radiation at the lower hemisphere is weak. The proposed method of embedding an annular ring to the superstrate, which does not involve the optimization of superstrate's parameters, can serve as a promising candidate in practical applications.