Indented Antenna Arrays for High Isolation: The Growing Interest in Simultaneous-Transmit-and-Receive-Based Full-Duplex Communication Systems

In full-duplex communication systems, reflections of transmitted waves as a result of the antenna mismatch may leak into the receiver and create strong on-site interferences. Such reflections cannot be suppressed by conventional frontend isolation techniques even with ideal circulators. Indented antenna arrays are thus proposed to destructively combine the reflection from each antenna element and prevent them from negatively impacting the receiver sensitivity. The essential idea is to break the symmetry between the transmitting and receiving paths by using an antenna aperture that is curved by indenting the antenna elements. Because of this asymmetry, all of the elements contribute to the wave transmission and reception in phase, whereas the reflections from those elements are out of phase and cancel each other out. With indented antenna configurations, broadband isolation between the transmitting and receiving paths can be achieved for both monostatic and bistatic antenna settings. The improvement is obtained without sacrificing antenna radiation patterns or polarizations. Measured results with a four-element indented quasi-Yagi antenna array operating from 3.2 to 3.4 GHz are presented. The results have demonstrated a 13-dB improvement on the antenna return loss of a single array, a 15-dB transmit/receive isolation improvement in a monostatic array, including circulators, and a 20-dB transmit/receive isolation improvement for a bistatic array without using circulators.