Analysis of Broadband Slot Cut Semi-Circular Microstrip Antennas

Broadband microstrip antenna is more commonly realized by cutting slot inside the patch. While designing slot cut antennas at a given frequency, slot length is taken to be either quarter wave or half wave in length. However this simpler approximation does not give closer result. In this paper, an analysis to study the effect of slot like, half U-slot and rectangular slot on the broadband response of Semi-circular microstrip antenna is presented. It was observed that the slot does not introduce any additional mode but reduces the resonance frequency of second order TM21 mode of the patch and along with fundamental TM11 mode yields broadband response. The slot also modifies the surface current directions at TM21 mode and thereby yields broadside radiation pattern over the complete bandwidth. Further by studying the surface current distributions at modified TM21 mode, the formulation of resonant length is proposed. The frequencies calculated using the proposed equations agrees closely with the simulated result with % error less than 5% over the complete slot length range.