Compact Quadrature Hybrid Coupler For MRI Coil

This paper proposes the design of a compact quadrature hybrid coupler at the 300 MHz operating frequency for the Magnetic resonance imaging Birdcage coil. The desired 300 MHz operation frequency of the quadrature hybrid coupler has a wavelength of 1 meter, and then the physical size of the designed quadrature hybrid coupler will be very large. Therefore, we propose the design of a compact quadrature hybrid coupler, applying symmetrical bends in the design and reducing its physical size, maintaining the design performance in the frequency of operation, the magnitude balance below ± 0.4 dB and the phase balance below ± 0.3° by applying a specific substrate with losses. The type of the compact quadrature hybrid coupler is Branch-line hybrid and it was manufactured in micro strip board with FR4 substrate. The measured phase shift between the output ports is 90° ± 0.15° at 300 MHz. The measured magnitude balance at the output ports is ± 0.22 dB at 300 MHz. Consequently, the measured magnitudes at the output ports are -3.22 dB and -3.44 dB. The measured isolation between the input port and the isolated port is 23.2 dB at 300 MHz.