Aerospace & Defence
A microwave technology revolution
The latest generation of satellite-on-the-move systems are squeezing complex hardware into an ever-more restricted space. Here, Steve Cranstone, managing director of Link Microtek, looks at the impact on a key component of such systems - the microwave rotary joint
T
he last few years have seen a significant upsurge in demand for microwave rotary joints from the
defence and aerospace sector, driven largely by the development of satellite-on- the-move (SOTM) technology. SOTM communication systems are deployed on mobile ground vehicles or naval vessels or in aircraft or unmanned aerial vehicles to provide broadband network access for high data rate applications such as real time high definition video or transmission of telemetry data. In order to track the satellite, each moving vehicle or vessel is equipped with a satcom antenna system mounted on a three-axis stabilised pedestal. Since it is essential for the main azimuthal axis to have unlimited 360 degree movement, it is not possible to use cables to feed signals to and from the antenna reflector - the only option is a microwave rotary joint. While there are many SOTM systems designed for use at Ku-band frequencies (12 to 18GHz), the increasing demand for satcom bandwidth has shifted the emphasis to the Ka band (26.5 to 40GHz), and a variety of different rotary joints are now available for these higher frequency applications as well.
Low profile One of the key requirements of most SOTM systems is minimising the height of the antenna radome. For this reason, the block upconverter (BUC) - which converts data at L-band frequencies into, for example, a Ka-band signal for transmission to the satellite - is usually located below the antenna pedestal, which means that the Ka-band signal has to pass through the rotary joint. The reverse process, downconverting the incoming signal from Ka-band to L-band, is performed by a low noise block converter (LNB), but since this is quite a small device, it can usually be accommodated above the pedestal without any difficulty. In other systems where space is not a constraint, it is quite possible to use separate rotary joints for the transmit and receive channels. However, in SOTM applications there is always a strong emphasis on size reduction, so a typical SOTM rotary joint would be a dual channel
20 July/August 2015
device capable of handling Ka-band frequencies on the transmit channel and L- band frequencies on the receive channel. While the receive channel would always use coaxial connectors, the higher frequency transmit channel could be implemented with either coax or waveguide, depending on the performance specifications that needed to be achieved. Whatever the configuration, excellent isolation between the channels is always necessary and values typically exceed 50dB. Where the requirement for a low profile antenna is the overriding consideration, the rotary joint can be designed with right angled connectors or waveguide flanges to reduce the vertical space needed for cable bends or waveguide runs. Minimising the overall weight of the system can also be crucial, especially for airborne systems, so aluminium tends to be the material of choice for a rotary joint.
Slip rings
In every SOTM antenna system, there are a number of elements above the pedestal that require electrical power to operate. In addition to the LNB, there will be motors that continuously adjust the position of the antenna reflector to keep it locked onto the satellite regardless of the motion of the host vehicle. There will also be encoders and a GPS.
Although the most common method for powering these various elements is to integrate a slip ring with the rotary joint, in some small systems DC power can be fed via the rotary joint itself, using the central conductor of the coaxial receive channel. This approach opens up the possibility of eliminating the slip ring altogether, thereby further reducing the overall size, cost and complexity of the system. One such rotary joint is the AM28RJD from Link Microtek, which offers a high current rating of 4A at 24V DC on the coaxial receive channel.
Insertion loss
Insertion loss is a critical parameter for any microwave rotary joint. The latest compact Ka-band SOTM systems, in particular, utilise expensive solid state power amplifiers to produce the output that is necessary to cope with adverse weather
Components in Electronics
conditions or when the satellite is low down near the horizon. As it is vital to avoid any significant loss of power in the path between the amplifier and the antenna, rotary joints are typically designed to achieve an insertion loss of just 0.5dB. For optimum microwave performance, a waveguide transmit channel will provide the lowest insertion loss and can handle higher power than a coaxial alternative. However, rotary joints have to be protected from excessive mechanical loading, so if a waveguide transmit channel has been specified, it is usually necessary to connect it via a section of flexible waveguide, rather than rigid, in order to alleviate any stresses that might arise. However, flexible waveguide inherently has a higher insertion loss than rigid waveguide - as in many aspects of engineering design, there are inevitably some trade-offs that have to be made.
Long service life The deceptively simple external appearance of a rotary joint belies the complexity of its internal design, which consists of over 40 separate precision engineered parts, including connectors, pins, cages, spring mounts and bearings - all of which need to be assembled and tuned by hand. Despite its intricate design, it is essential for the rotary joint to achieve a high level of reliability - it has to cope with prolonged operation, sometimes in tough environmental conditions, without wear or degradation that might cause a significant increase in insertion loss.
Future trends
As the demand for high data rate satcom services continues to grow, system frequencies will likewise continue to rise. Ka-band satellite services are now coming on-stream, and in the longer term the next phase of development will be at Q band, which covers frequencies from 33 to 50GHz. For the rotary joint manufacturer, these higher frequencies will inevitably mean designing and producing even smaller components. While this will certainly be possible, the major challenge will be to reduce the amount of skilled manual crafting and tuning required, so that the unit price can be lowered to meet market expectations.
www.linkmicrotek.com www.cieonline.co.uk
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