Frequency Control & Microwave
Ka-band SOTM systems drive rotary-joint miniaturisation
Size reduction is becoming one of the key issues in microwave rotary joint technology as designers of Ka- band satellite-on-the-move systems push for ever more compact antennas. Steve Cranstone, managing director of Link Microtek, looks at the latest developments
W
hile large satellite-on-the-move (SOTM) communication systems operating at S, C and X-band
frequencies have been widely deployed in naval applications, the latest SOTM developments are focusing on much smaller configurations for tanks, armoured personnel carriers and airborne platforms such as unmanned aerial vehicles (UAVs). As well as a reduction in physical size, there is also a requirement for higher capacities and data rates, particularly for real-time high-definition video feeds, so Ka-band frequencies (26.5 to 40GHz) are becoming the preferred choice. Each moving vehicle is equipped with a satcom antenna system mounted on a three-axis stabilised pedestal. As the main axis of rotation has to have unrestricted 360-degree movement for satellite tracking, cables cannot be used for feeding signals to and from the antenna reflector, so the only option is a microwave rotary joint.
Antenna topology
A key design goal for SOTM antenna manufacturers is to reduce the footprint and profile of antenna systems for the emerging Ka-band applications. This is
being achieved by using small reflector antennas or flat panels and by changing the traditional antenna topology, moving the block upconverter (BUC) – which converts data at L-band frequencies into a Ka-band signal for transmission to the satellite – from the rotating side to the fixed side below the antenna pedestal (inside the tank, say). Such an approach means that the microwave rotary joint now has to handle the transmit frequency at full power with, of course, minimal loss. 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.
Dual-channel configuration To address these Ka-band SOTM requirements, Link Microtek has developed a new microwave rotary joint, the AM28RJD (Figure 1). This miniature device features a dual-channel configuration, with Ka-band frequencies handled by a high- power transmit channel, implemented in right-angle WR28 waveguide, while the L- band receive channel uses two female SMA coaxial connectors.
Designed to be located on the main axis of rotation of the antenna, the rotary joint measures just 36.0mm (D) x 90.3mm (H), excluding the 50mm-diameter UBR320 standard bulkhead flange, and normally has to fit within a confined space at the centre of the bore of a slipring assembly that powers the antenna’s DC motors and other parts. The central transmit channel of the rotary joint covers Ka-band frequencies from 29 to 31GHz and delivers excellent microwave performance, with an average power rating in excess of 50W, a typical insertion loss of only 0.5dB and a
maximum VSWR of 1.7:1. Meanwhile, the L-band receive channel, which normally operates over the frequency range 950 to 2150MHz, offers a microwave power rating of 1W CW, a maximum DC current rating of 2A (for powering the LNB), an insertion loss of 0.25dB and a typical VSWR of 1.5:1.
Insertion loss
Ka-band SOTM systems utilise expensive solid-state power amplifiers to produce the output that is necessary to cope with adverse weather conditions or when the satellite is low down near the horizon. It is vital to avoid any significant loss of power in the path between the amplifier and the antenna, so insertion loss is a critical parameter for the rotary joint. The use of electromagnetic simulation
(Figure 2) as part of the rotary-joint design process allows optimisation of the insertion loss, power-handling capability and other specifications. The simulation shows the internal waveguide-to-coax transition – a key section of the rotary joint – with the various colours representing field strengths ranging from high (blue) to very low (red). Akin to a mechanical watch, the internal construction of the rotary joint consists of over 40 very small individual parts, which are crafted to high precision and tight tolerances before being assembled and tuned by hand.
Robust construction Figure 2: Electromagnetic simulation of internal waveguide-to-coax transition 46 June 2016 Components in Electronics
Despite its intricate design, the AM28RJD is robustly constructed to withstand the particular mechanical stresses associated with SOTM systems – namely, occasional
Figure 1: The AM28RJD microwave rotary joint addresses Ka-band SOTM requirements
rapid movement when locking on to the satellite or if the vehicle turns a corner, combined with continual dither as the stabilised platform adjusts to maintain best lock on the satellite. Fabricated from aluminium to minimise weight, the rotary joint has an Iridite finish and is designed to meet the typical military environmental conditions of MIL-STD-810G. Other configurations of the Ka-band dual-channel rotary joint can also be supplied, customised to suit specific antenna requirements. For example, some applications may call for the high-power transmit channel to be implemented in a right-angle WR28 waveguide on the fixed (input) side but with a female coaxial connector on the rotating (output) side. Link Microtek has many years’ experience in the design and production of microwave rotary joints, from simple single- channel waveguide and coaxial devices to large complex multi-channel assemblies. The Ka-band SOTM terminal market is still in the early stages of prototype system build, test and qualification, but it will continue to develop over the coming years and Link Microtek is well placed to participate with its rotary-joint offering.
www.linkmicrotek.com www.cieonline.co.uk
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