A Compact, High Power Capable, and Tunable High Directivity Microstrip Coupler

A coupler is an indispensable component to sample the forward and reflected power for the real-time radio frequency (RF) power monitoring system. The directivity of a coupler is a critical factor to achieve accurate RF power measurements. This paper proposes a microstrip coupler with a tunable high directivity circuit to accurately measure the reflected RF power. The directivity tuner composed of passive components adjusts the phase and amplitude of the coupled RF signal, and cancels out the leakage signal from the RF input port at the coupled reflection port. The experimental results, which agree with the simulation results, show that the microstrip coupler with the directivity tuner circuit has a compact size ( ∼ 0.07λ_g × 0.05λ_g), high power capability (up to 1 kW), and high directivities (more than 40 dB) at the operating frequency bands ( f = 297.3, 400, and 447 MHz, respectively) for magnetic resonance imaging applications.