Spinning Into Control with Coaxial Rotary Joints
Coaxial rotary joints are necessary inline components when a static RF system must be connected to a rotating RF line. These rotary joints are used in military, aerospace, and commercial applications, most often with rotary antennas and radar. Specifically, coaxial rotary joints are used in Air Traffic Control, image transmissions, medical/industrial, telecommunications control, and ground-, ship-, or air-based radar. As all of these applications require high performance interconnect, reliability, and long-life operation, the same standards are applied to coaxial rotary joints.
Like all coaxial systems, the coaxial connectors used at the ports of the rotary joints limit the maximum frequency of operation. Hence, rotary joints to 18 GHz may use 2.92mm female coaxial connectors, where coaxial rotary joints to 6 GHz are likely to employ standard SMA connectors. Some specialized rotary joints may also come with PCB pin connectors, which could be used as end-launch microstrip, or stripline, waveguide to coaxial rotary converters. Other types of rotary joints include 90 degree coaxial rotary joints. Some waveguide rotary joints also incorporate a coaxial intermediate stage within the device, though the end-port may be waveguide or coaxial.
Unlike other coaxial converters or inline interconnect, coaxial rotary joints have additional mechanical considerations. These include an average rotational speed limit, in revolutions per minute (RPM). These limits are typically dictated by the bearings and mechanical assembly of the rotary system. Exceeding the average maximum RPM limit for any significant amount of time would likely lead to early failure of the rotary capability, and likely reduce RF performance as well.
Because of the additional rotary system to the inline coaxial interconnect, there may be very critical input power and peak power specifications for a coaxial rotary joint. If the power limit is succeeded, or if the temperature of operation is exceeded, then the rotary performance of the joint may be diminished, and the joint may be more easily damaged. Typically a coaxial rotary joint will have slightly lower VSWR performance than typical inline interconnect.
Lastly, the rotary nature of a coaxial rotary joint will also introduce RF performance variation based on the rotation parameters. Typically, there is a max VSWR, insertion loss, and phase variance provided with the datasheet, which is specified at the recommended RPM. Deviated from the recommended RPM could influence this variance, as would any excessive vibration or mechanical force being applied to the joint. Moreover, the variance to VSWR, insertion loss, and phase are also likely a function of frequency, and may need to be taken into consideration when making a selection.