144 is a bandpass filter with a passband from 205 mhz to 440 mhz bandpass filter 144 includes radio frequency coils l126 l128 and variable capacitors c133 c135 the coils l126 l128 have values selected to resonate at 322 5 mhz with their respective capacitors c133 c135 when the capacitors are approximately in the center of their variable range these three resonators thus comprise a filter which passes frequencies from 205 mhz to 440 mhz and rejects frequencies outside of this band using well known techniques the capacitors may be varied to adjust the impedance level presented at the output port 145 exemplary values for coils l122 l128 in nanohenries capacitors c129 c135 in picofarads and resistor r34 and r36 in ohms are given in table 2 below the bandstop filter shown in fig7 and 8 works as follows if the output impedance of bandpass filter 144 which is presented to port 2 of the circulator 126 is approximately equal to z0 over the frequency range from fp fa to fp fb and has a real part approximately equal to zero outside of this frequency range the circuit of fig7 and 8 will be a bandstop filter passing all frequencies except in the frequency range from fp fa to fp fb filter network 130 presents to the input of frequency downconverter 138 at its input port 137 an impedance which is approximately equal to a constant resistance over the.