pass band as shown in fig3 c by capacitive coupling at open ends of the input line 18 and the output line 19 and the mutual inductance m this trap serves to suppress an image disturbance which would otherwise occur in a second mixer the band pass filter of this type is disclosed in the u s patent application ser no 186738 entitled 34 band pass filter 34 filed on sept 12 1980 and assigned to the present assignee an equivalent circuit of one of the resonators of the band pass filter is shown in fig4 numeral 21 denotes a terminal which is connected to the input terminal 16 or output terminal 17 of fig3 or the input line 18 or output line 19 numeral 22 denotes a coupling capacitor c 3 between the one half wavelength microstrip resonator 20a and the input line 18 or between the resonator 20b and the output line 19 and numeral 23 denotes a one half wavelength microstrip resonator having a charactersitic impedance z 2 and a length l 2 which is open ended a resonance frequency f o of the equivalent circuit is approximated by equ3 where c o is a light velocity u03b5 is an effective dielectric constant and l 2 is the length of the one half wavelength microstrip 20a or 20b from the formula 7 a variation of a center frequency f if1 of a pass band of the narrow band band pass filter by a.