Abstract: In a filter network a common coupling element provides input of radio frequency energy to both an odd quarter wavelength resonant cavity and a variable reactance isolating the coupling element from ground. The resonant cavity presents an open circuit to a selected pass frequency and the variable reactance shunts energy to ground at a selected reject frequency. In a duplexer for radio transmitter and receiver, a filter network attaches to the transmission line between transmitter and antenna junction; the cavity is tuned to the transmitter carrier; the variable reactance is tuned to the receiver frequency. A similar filter network attaches to the transmission line between receiver and the same antenna junction; the resonant cavity is tuned to receiver frequency and the variable reactance is tuned to the transmitter carrier. The networks are spaced one quarter wavelength from the antenna junction.