PURPOSE: To obtain about 40dB attenuation at 800MHz and to obtain sufficiently practical trap effect reducing loss at 900MHz±25MHz less than 30dB, by connecting a 1/4 wavelength leading and opening line and capacitive reactance to a transmission line.

CONSTITUTION: The 1/4 wavelength leading end opening line 2 of 800MHz is connected to a transmission line 1 of 50Ω. A capacity reactance 3 is connected to the same coupling point P. Since the impedance of the line 2 approaches zero at 800MHz, the line 2 is acted as a trap circuit. However, the line 2 is acted as an inductive reactance at the wavelength of frequency higher and close than/ to 800MHz. Therefore, selection of the value of capacitive reactance 3 makes it possible to cancel the inductive reactance and make the reactance approach specific impedance. When the impedance of a transmission line 1 approaches the specific impedance, the loss of insertion can be regarded as approximately zero.