AbstractA microwave directional coupler is reported whose coupling can be varied with externally applied magnetic field without any mechanically moving parts. Variable coupling is achieved by using evaporated n‐type InSb films. A simple, single‐hole coupler is discussed first with regard to its performance, e.g., coupling, variation in coupling, VSWR and insertion loss in mm‐wave region. These quantities are related to the properties of the thin film, in particular, to magnetoresistance of InSb. Second, an improved variable directional coupler using InSb‐Sn‐Ag films, and which has an insertion loss and VSWR less magnetic‐field‐dependent, is introduced and discussed. In the improved coupler, the coupling varies from 19.3 dB to 42.4 dB at 33.8 GHz as the magnetic field is varied from 0 to 10 kGs. The insertion loss and VSWR are 1.18 dB and 1.27, and the directivity is in excess of 19.8 dB. Over the frequency range of 33.8 ± 1.0 GHz, the 2‐hole coupler shows a directivity higher than 14.0 dB, insertion loss lower than 1.23 dB, and VSWR less than 1.33.