Band stop filter and circuit arrangement for common antenna

摘要

In a band stop filter having a plurality of series resonating circuits, and coaxial cables connected between the series resonating circuits, the length of each of the coaxial cables is made either shorter or longer than quarter wavelength at the center frequency. With this arrangement, asymmetry is introduced in the insertion loss vs frequency characteristic curve so that the curve is sharper than the symmetrical curve in a given range, resulting in a reduction of insertion loss at a transmission band which resides either above or below the center frequency. Two band stop filters may be combined to constitute an antenna coupler for connecting a transmitter and a receiver to a common antenna. Each of the series resonating circuits used in the band stop filter may be constructed of a quarter wavelength coaxial resonator, which functions as a parallel resonating circuit, and a loop-like coaxial cable, which functions as a coupling capacitor.