Abstract: A transmission line filter having two conductors with insulation therebetween. A third conductor of a length related to the wavelength of the signal being filtered is spaced from one of the conductors but electrically coupled thereto. Terminal connectors are located at each end of the filter to interconnect the filter into the regular transmission line of the system in which the filter is being used. The filter can be formed of coaxial cable and can be a single frequency filter a multiple frequency filter of a bandpass filter arrangement. In a filter trap arrangement, one end of the third conductor is conductively connected to the other of the main conductors.