A joining device to enable the fastening of a rigid flat radome onto a circular-opening reflector comprises an upper brace applied to one outward-turned side of the radome and covering the peripheral segment of the reflector and radome, a lower brace placed against the face of the reflector opposite the radome, and a rigid spacer comprising and joined with a pad disposed between the reflector and the radome, at least one cylinder leaving a through-hole for a means of fastening the radome onto the reflector, and at least one peg mating with the radome. The upper brace covers the peg and comprises an orifice for putting the cylinder through. In an antenna comprising a circular-opening reflector, a rigid flat radome comprising a plurality of holes on its periphery, and such a joining device, only some of the holes of the radome are used to allow a means of fastening through.