The growing use of simultaneous transmission and reception (STAR) in full-duplex communications is literally putting the heat on some of the antennas in these systems, since signal leakage from the transmit antennas can double as interference for the receive antennas. Suppression or rejection of such transmit-antenna leakage can be challenging, leading some designers to explore the use of indented antenna arrays in full-duplex communications systems. The basic idea is to break the symmetry between the transmit and receive signal paths by indenting the antenna elements, thus creating an antenna aperture that is curved and less likely to cause self-interference.
The use of STAR signals within the same frequency band has often been the operating approach of continuous-wave (CW) radar systems and it is increasingly being used in full-duplex communications systems. The approach eliminates the need to separate the frequency spectrum into two different functional portions (while also eliminating the need for a diplexer to do the spectral separating) and clears the way for programmable use of different portions of the active frequency spectrum, such as by means of a software-defined-radio (SDR) communications systems approach.
To explore the effectiveness of indented antenna arrays in STAR full-duplex communications systems, researchers from the University of California at Los Angeles examined the design of a four-element standalone indented antenna array and compared its performance to a conventional antenna array operating in the approximate frequency range of 3.2 to 3.4 GHz. When making measurements at about 3.35 GHz, they found more than 15-dB improvement in return loss for the indented antenna array compared to the conventional antenna array, with much improved isolation between the transmit and receive functions for the antenna array operating within the same frequency band.
The researchers explored a number of different designs to improve transmit and receive isolation, including a standalone indented antenna array, a monostatic indented antenna array with distributed circulators, and a stacked bistatic indented antenna array. The radiation pattern of the standalone indented configuration was measured and confirmed as unchanged compared to a conventional antenna array, with dramatic improvement in transmit-to-receive isolation. In the monostatic indented array with distributed circulators, the circulators are placed after each antenna element to separate the transmit and receive paths, with that isolation limited by the leakage of the circulator. In the stacked bistatic indented antenna array, where transceivers use transmit and receive antennas, the isolation is limited only by mutual coupling between transmit and receive antennas.
The researchers performed experiments with a four-element quasi-Yagi antenna array operating from 3.2 to 3.4 GHz. Results include a 13-dB improvement in return loss of a single array; 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 circulators. Through the use of a standalone indented antenna array, the symmetry between transmitting and receiving paths can be disrupted by indenting the antenna elements. This achieves respectable isolation between transmission and reception without additional antenna elements or passive components, such as circulators.
See “Indented Antenna Arrays for High Isolation,” IEEE Antennas & Propagation Magazine, Vol. 60, No. 1, February 2018, p. 72.