Figure 1: Rotary joint with a coaxial section
Rotary joints are used to transmit high-frequency energy from a fixed to a rotating part of the system (e.g., to the antenna). Within the joint, one uses either round waveguides or a short coaxial section in the waveguide system which is located in the center of the rotary joint.
A probe is used to couple the RF energy from the waveguide into or out of the coaxial section, as shown in Figure 1. However, such a connection is quite narrowband due to resonance from the probe.
A circular waveguide section is suitable for a rotary joint also. The round waveguide section is dimensioned to transmit the electromagnetic waves as an E0,1 wave. The connections are designed to prevent the circular waveguide from operating in the wrong mode. The length of the resonant pieces is 3/4 λ so that a high impedance for undesired modes is achieved.
|The rotating section of the joint uses a choke joint to complete the electrical connection with the stationary section.
Figure 2: Rotary joint with a circular waveguide is the most common type
The rotatable waveguide sections are called either I-type, L-type, or U-type, depending on which direction the face of the flanges.