Compact dual-frequency circular microstrip antenna with an offset circular slot

This paper presents a new, compact dual-frequency design of single-feed circular microstrip antennas with an offset circular slot placed close to the patch boundary. It is found that, when the radius of the offset slot is about 0.38-0.62 times the radius of the circular patch, dual-frequency operation can be achieved using a single probe feed. The two operating frequencies have orthogonal polarization planes, and have a low frequency ratio of about 1.15-1.18. The two frequencies can also be much lower than the fundamental resonant frequency of a corresponding simple circular patch antenna without the offset slot. This behavior makes the proposed antenna have a reduced antenna size as compared to a simple circular patch antenna operated at either one of the two frequencies. Details of the antenna design and experimental results are presented. (C) 1999 John Wiley & Sons, Inc.