Impact of neutralization on isolation in co-planar and back-to-back antennas

The concept of neutralization or RF cancellation is a promising technique for isolation improvement between two closely spaced antennas. In this contribution, the neutralization technique is studied for increasing the isolation between two closely spaced dual-polarized patch antennas and between dual-polarized back-to-back patch antennas at 2.6GHz. The back-to-back antennas are proposed for in-band full-duplex relaying, where the antennas have to be isolated such that self-interference from its own transmission is cancelled with additional analog and digital cancellation. Off-the-shelf components are used and achievable isolation levels and realized isolation bandwidth are measured. Measurements show that isolation levels of 75 dB across a 5MHz band can be achieved between patch antennas on opposite sides of a back-to-back antenna using off-the-shelf power dividers/combiners and phase shifters connected between the feeds of the two antennas. When power is sampled directly from the second port of the dual-polarized antenna instead of using separate couplers, the realized isolation bandwidth is much smaller. Results show that neutralization can be used to improve antenna isolation in back-to-back antennas for enabling in-band full-duplex operation but the operational bandwidth is still limited.