Abstract: A diplexer for electromagnetic signals of higher and lower frequency is formed of a common waveguide channel for both signals, the common channel branching into a through waveguide channel and a side waveguide channel. The through channel includes a filter having a pass band for propagation of the lower frequency signal and inhibiting propagation of the higher frequency signal. The side channel is formed as a waveguide below cut-off frequency with respect to the lower frequency signal for inhibiting propagation of the lower frequency signal while permitting propagation of the higher frequency signal. A coupling aperture formed as a slot resonant at the higher frequency is located in a waveguide wall at an integral number of quarter guide wavelengths in front of the filter for coupling the higher frequency signal between the common and the side channels.