As satellite communications (Satcom) moves ever upwards in terms of frequency, so systems tend to move down the scale when it comes to weight and size. Whilst Naval platforms, such as ships, retain their overall size, reducing the above the water mass leads to increased stability. Whilst airborne platforms have always sought the lightest components, in-vehicle and man-pack communications systems are the latest users of lightweight components. Howard Venning, managing director of Aspen Electronics, explores.
When reviewing Satcom system design, a key component in the antenna feed assembly is the rotary joint. Waveguide, as opposed to a coaxial feed system, is used to handle the microwave frequency power levels generated by the transmitter, whilst coax is typically used for the receive path. The rotary joint provides the angular flexibility to allow the antenna to be pointed to any position in the sky, no matter what the position of the transceiver system.
Being a component in the antenna feed assembly the waveguide path of the rotary joint needs to demonstrate excellent performance, meeting both electrical and mechanical specifications. Electrical specifications are primarily focused on the operating frequency and RF power levels.
Taking a Ka band Satcom system as an example, operating frequencies would typically be 29-31GHz, transmit and 19-21GHz receive, with the transmit feeder system being realised in WR-28 waveguide, and the receive side using low loss co-ax. The key specification for both channels is insertion loss as additional loss in the transmit path has to be compensated for by higher power transmitters, whilst the receive path will suffer from reduced signal to noise performance. An additional insertion loss parameter only found in rotating joints is “insertion loss wow” this being the variation in insertion loss as the joint rotates. Naturally, the VSWR performance needs to be considered, including “VSWR wow”, plus “phase wow” i.e. the phase variation with rotation.
Turning to the mechanical specifications, the latest platforms, especially “Satcom on the move” (SOTM) applications need small mass, that is light weight and as small a size as possible. For certain applications this translates into a compact design where the distance between waveguide input and output is kept to a minimum, allowing antenna radome heights to be reduced. However, in reducing the size and weight we should not forget that bearing and seal designs must ensure smooth operation with the minimum of torque, which should be maintained during the lifetime of the rotating joint.
Available from Aspen Electronics is the Diamond 20-22124-0. This particular rotary joint was developed for airborne Satcom applications requiring a low profile antenna system to minimise drag on the exterior of the aircraft. Because of its size it can be used in any application where light weight, compact, ruggedised and high performance would be advantageous. Material and finish selections, design mechanics and the use of precision preloaded ball bearings allow this model to be used in almost any ground, sea or airborne applications.
The Diamond 20-22124-0 is therefore ideal for new applications, however, where this design is being considered to replace an existing design using an older, larger waveguide rotating joint, additional waveguide feeders can be attached and matched to maintain overall RF performance, and interface with the existing waveguide runs.