What is the waveguide rotary joint and how does it maintain low VSWR during continuous rotation?

The waveguide rotary joint maintains low VSWR during continuous rotation by using a circular waveguide section operating in a rotationally symmetric mode (TM01) at the rotating interface, so the electromagnetic fields are unchanged regardless of the angular position of the rotating section. The rotary joint connects a stationary waveguide (from the transmitter/receiver) to a rotating waveguide (connected to the antenna on a rotating pedestal). The design: the input rectangular waveguide is transitioned to a circular waveguide using a rectangular-to-circular transition. The circular waveguide operates in the TM01 mode (which has azimuthally symmetric fields: E_r and E_z components only, no E_phi component). Because the TM01 mode's field pattern is rotationally symmetric: the coupling between the stationary and rotating sections is independent of the rotation angle (zero VSWR variation with rotation). The rotating interface consists of: a precision choke joint (a quarter-wave choke groove in the flange that creates an electrical short circuit at the waveguide wall, ensuring low-loss, low-reflection contact even with a small air gap between the rotating surfaces; the choke eliminates the need for sliding metal-to-metal contact, which would cause wear and VSWR degradation), and a bearing system (precision ball bearings that maintain the alignment and concentricity of the rotating section with tight tolerances). Performance: VSWR variation with rotation: less than 0.01 (essentially constant). Insertion loss: 0.1-0.3 dB. Power handling: 1-1000+ kW. Frequency range: single-band or dual-band (using nested rotary joints).