Abstract: A waveguide seal (16) which provides reliable mechanical contact and which reduces the electrical breakdown in the area of a waveguide joint (10). The seal (16) includes a dovetailed groove (40) into which is inserted a helical coil (30). During assembly, flanges (18) of the interconnected waveguides (12, 14) are then compressed against the helical coils (30) on each side of the seal (16). The helical coil (30) is then partially compressed and provides reliable mechanical connection between the waveguides (12, 14) and the interposed seal (16). Further, a gap (58) in predefined areas between the seal (16) and the waveguide flanges (18) provides for significantly reduced electrical breakdown in the joint area, thereby providing a better electrical connection.