Coaxial Waveguide Adapter, 3.5 mm (f), 18 to 26.5 GHz
K281C - Keysight Technologies
The Keysight K281C adapter transforms waveguide transmission line into 50-ohm coaxial line. Power can be transmitted in either direction, and each adapter covers the full frequency range of its waveguide band with SWR less than 1.07.