The invention proposes a transition between a microstrip technology circuit and a waveguide, the waveguide being furnished with a probe linked electrically to the microstrip circuit. The transition comprises at least one first resonant cavity coupled by a first hole placed level with the said plane. The transition furnished with the cavity behaves as a bandstop filter. The transition is placed in an outside unit of a transmission system comprising a transmit circuit embodied in microstrip technology and an antenna of waveguide type. A transmit circuit comprises at least one local oscillator and the resonant cavity is tuned to the frequency of the local oscillator.