In a microwave device, for example a microwave local oscillator with a harmonic mixer for feedback control, problems of coupling the mixer circuit to the R.F. cavity (10) are overcome by providing a d.c. blocking capacitor of the mixer on a planar surface of a support (20) extending across an aperture (14) in one wall of the cavity. In preferred embodiments the capacitor is a planar device formed by a plurality of interdigitated fingers (26, 28). These fingers (26, 28) are oriented to couple magnetically with the R.F. energy at the aperture in the cavity (10). The support conveniently comprises a printed circuit board having a ground plane (22) on the same surface as the capacitor, which is located in a small opening (24) in the ground plane. A second ground plane (38) is provided on the opposite side of the printed circuit board, overlying the opening. The two ground planes are interconnected by plate-through-holes (40).