A new mixer/modulator for millimetre-wave applications is presented. The circuit is implemented with a combination of finlines, coplanar lines and microstrip lines. By applying the described mixer/modulator to millimetre-wave systems a twofold use of active elements such as the Gunn element and Schottky diodes is possible. The size of the whole planar structure is about 4 × 4 cm and there is no need for a variable short.