In one embodiment, a modular feed assembly for an antenna has (i) a hub adapter for mounting the feed assembly onto the antenna hub and (ii) a distinct waveguide transition configured to be selectively mated to the hub adapter. By providing a modular design, the hub adapter can be selectively used with different waveguide transitions having different frequency characteristics to form feed assemblies for different antennas having different operating frequency ranges. The hub adapter and each waveguide transition have timing features that limit the rotation orientation between the two components to, for example, horizontal and vertical polarizations that are 90 degrees apart. The hub adapter has a resilient compression element that forms an annular seal between the hub adapter and a mated waveguide transition to inhibit RF leakage and keep the two components in place. The hub adapter has openings that allow the compression element to be formed in place.