![](https://www.eevblog.com/forum/rf-microwave/looking-for-some-waveguide-expertise-here/?action=dlattach;attach=314886;image)
That's a double ridge waveguide. The ridge increases the bandwidth that the waveguide can transmit without multiple modes (ie, without dispersion). Same goes with the ridges inside the horn flare - these extend the bandwidth of the horn. They work just like a
Vivaldi antenna. You don't often see double ridge waveguide in radar, so this could be X-band satcom equipment, or EW or radar warning receiver antennas. Very cool find!
Edit: septum polarizers are not normally flared, they usually have a 'stair-step' appearance. They're used to create circular polarization. You'll often find them inside satellite TV LNBs.