This paper proposes a phase offset canceler for an array antenna, which comprises multiple low-cost commercial of the shelf (COTS) software defined radio (SDR) devices. The main challenge is the phase synchronization of SDR devices because each SDR devices has its own phase offset when it produces IQ stream. We devise a simple yet effective method to cancel the phase offset of individual SDR device using a double-pole, double-throw (DPDT) switch which is implemented between antenna and SDR device. The DPDT switch is controlled according to the frame timing of each IQ stream to produce two set of measurements, from which we can compute angle of arrival(AoA) with a simple signal processing. We refer this mechanism as 'Monopulse Switching'. We evaluated the working principle experimentally to reveal Monopulse switching successfully cancel phase offset to detect AoA without any initial calibration and achieves 2.8 degree measurement accuracy.