We propose a robust self-interference cancellation (SIC) technique for microstrip antennas. A microstrip antenna operates as a transmitter located between two identical receiver antennas having orthogonal polarization with respect to each other. The microstrip antennas show out-of-phase electric held distribution in the H-plane. We exploit this characteristic to cancel self-interference (SI) signals through a ratrace circuit, and we achieved more than 50 dB (60 dB) SIC over 180 MHz (120 MHz) at 2.45 GHz. Furthermore, we propose a new phase-reconfigurable rat-race circuit to adjust the SI signals phase response. The proposed technique is robust and resilient to the potential phase offset of SI signals caused by fabrication errors or scatterers nearby the antennas. As a result, the 50 dB (60 dB) SIC bandwidth spreads over 280 MHz (150 MHz). This technique is a good candidate for in-band full-duplex communication systems with dimensions of 1.4λ ₀ × 0.4λ ₀ × 0.05λ ₀ and antennas edge-to-edge separation <; λ ₀ /10, and is easily applied to other frequency bands.