Abstract: A microwave bandstop filter comprises a waveguide segment of cross-section that presents longitudinal variation of sinusoidal type modulated by an amplitude function that is continuous, the period of said longitudinal variation of sinusoidal type being the Bragg period for the fundamental guided mode at a center frequency of the band to be stopped. A filter assembly comprises a microwave lowpass filter presenting a cutoff frequency and at least one interfering passband at frequencies higher than said cutoff frequency, and at least one bandstop filter as defined above, connected to the output of said lowpass filter, in which the amplitude and the period of said longitudinal variation, and also the length over which it extends are such that they stop said interfering passband of said lowpass filter. An output multiplexer for a multichannel microwave transmitter includes such a filter assembly.