1 is fixed by the inductance of the posterior antenna loop that inductive value being a function of the geometry of the coil further the value c j is determined by the manufacture of the diodes 88 and fixed within a range for a given diode type although the inductance l 1 is tunable it cannot compensate for incorrect values of c 1 or c j as is apparent from equation 7 therefore as a practical matter it is necessary to calculate the proper values of the network 92 components prior to construction of the coil 10 they cannot be 34 tuned 34 once in place for this reason it is unlikely that the correct values would be arrived at accidently or by trial and error the received nmr signal is transmitted to the mri apparatus by cable 76 which is connected to the pickup loop 70 a parallel capacitance 94 connected across cable 76 through a series capacitance 98 between one wire of the cable and the parallel capacitance 94 provides impedance matching of the pickup loop 70 to the 50 u03c9 cable a second pair of back to back diodes 96 shunt the capacitance 94 as well as short the pick up loop 70 capacitor 98 also prevents dc bias which may be present on cable 76 from inadvertently switching diodes 96 the diodes 96 serve to detune the pickup coil so as to reduce direct pickup of the rf excitation signal by the pickup coil despite decoupling of the anterior and posterior coils 12 and 14 the above description has been that of a preferred embodiment of the present invention it will occur to those who practice the art that many modifications may be made without departing from the spirit and scope of the invention for example.