A non-magnetic low-loss wideband quadrature hybrid for front-end applications in magnetic resonance systems

The quadrature hybrid described below was developed for circular polarizing antennas in magnetic resonance systems. It is based on a 180°/3dB wideband hybrid cascaded with a 90° two-phase network. Due to this combination a quadrature hybrid (90°/3dB) with very little phase and amplitude imbalance (<±1° and <±2.5%, respectively) over a bandwidth of more than two octaves is achieved. Except one semi rigid cable balun, all transformers and inductors are realized on a double layer low-loss PTFE-board in symmetric strip line technique. The hybrid is thus easy to fabricate with high reproducible data. Since ferrite material is strictly avoided 15 kW peak power in close proximity to the strong main field magnet can be handled. Insertion losses in the receiving paths are about 0.15 dB to maintain the original signal to noise ratio of the antenna.