Tunable and high directivity coupler for MRI applications

This paper proposes a tunable and high directivity coupler that is used a RF power monitor in ultra-high field magnetic resonance imaging (MRI) systems. High directivity of a coupler is a critical factor to assure the accurate output in the reflected power measurement. To obtain a tunable and high directivity coupler, a novel directivity tuner includes tunable capacitors and resistors in a passive circuit. A well-tuned capacitance and resistance compensates phase difference and improves directivity in MRI operating frequencies. Therefore, this method significantly reduces the uncertainty of power measurement outputs and can accurately determine reflected power with load variation. The measured results of the proposed coupler agree with the simulation data and achieve excellent isolations (more than 65 dB) and directivities (more than 40 dB) in a compact size of 40 mm × 25 mm.