Abstract: A birdcage knee coil operated in quadrature in an NMR imaging system includes a pair of like butterfly coils each formed from a strip transmission line formed on a common dielectric substrate. The pair inputs the quadrature signals created by the birdcage coil to a combiner output circuit. Each butterfly coil of the pair comprises a pair of wings of like area connected by a common transmission line so that currents induced by a magnetic field on the ground conductors of one wing oppose and cancel the currents induced by that field on the ground conductors of the other wing. The transmission line forming each butterfly coil has opposing ground conductors of the same transverse width with a center conductor equally spaced between the ground conductors and a transverse width of about one tenth that of the ground conductors. The pair of coils are arranged symmetrically relative to the birdcage coil to provide an electrically balanced system.

Abstract: A first circular ring of a given inductance is coupled to second and third circular rings by a plurality of equally spaced parallel rods to form a birdcage coil. The second and third rings have a combined inductance the same as that of the first ring and are located adjacent to each other at an end of the rods opposite the first ring. A tune-detune circuit is located at the end of and couples each of the rods to the second and third rings. This circuit includes parallel capacitances whose combined value is the same as the capacitance coupling the rods to the first ring to selectively tune the coil to a given radio-frequency. PIN diodes selectively rf couple the parallel capacitances respectively to the second and third rings. Direct current diode bias voltages are applied to the second and third rings which act as busses for the bias voltages. The second and third rings are split in half and capacitively coupled to permit applying relatively higher (DC) bias voltages to the diodes.

Abstract: A solid-state switch for coupling an antenna to either a transmitter or a receiver. The switch includes a pair of oppositely poled diodes and a low voltage power supply connected between the diodes for providing a forward biasing voltage to the diodes. Reverse biasing voltage to stop RF current flow through the diodes is provided by the RF signal itself by means of a voltage doubler. The same voltage doubler and a power detector reverse bias the receiver diode during overload conditions.