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Motorised sat dishes: DiSEqC or 36V? If your zeal to find everything you can in the skies is growing, it may be time to motorise a dish to track the heavens You can cater for several different satellites with an LNB switching system but if you want access to everything within the grasp of your location and dish size, then moving one dish and LNB to the satellite required has to be a better bet. How does a motorised dish work?
01 The principles of a motorised dish
Most DiSEqC 1.2 and USALS mounts are used to motorise a fixed dish (left) by inserting the motor unit between the wall mount and the dish bracket (right)
are pretty simple. Just one dish, with an LNB at the primary focus on a special mount is moved under control of the receiver to point at the satellite carrying the required transponder or channel. The problem is that the satellites are located in an arc across the sky, so to go from one to another the dish moves not in a straight line but following this curve. Although there have been some products that can aim a dish anywhere in the sky (and these are used in tracking systems for caravans, boats, and so on), the normal system has a special mount with has just one motorised axis set parallel to the Earth’s own North Pole-to-South Pole axis. This means that simply rotating the dish about this axis moves its aim along the satellite arc from satellite to satellite. How far it is rotated determines which satellite it points to. Should you go motorised?
02 To access more than one satellite position you need either multiple dishes/ LNBs and switches (see ABCs in issues 274
and 275) or a motorised antenna. LNB switches have the advantage that changes between satellites are nearinstantaneous so you can channel-hop through all your channels regardless of their source. A motorised dish takes time to travel between satellites, especially if they are far apart in the sky, but if you’re just going from Astra 19.2°E to Hot Bird 13°E, it only takes a few seconds. A switched system allows you to gradually build up the complexity of the system, while with a motorised system you need to get everything right from the start. But a motorised dish will also pick up any satellite along the arc (if the dish is big enough) whether it’s there now or has yet to be launched. A motorised dish is more expensive to buy and to have installed, but it will work out cheaper than several fixed dishes. There comes a point when switched dishes cannot compete with a motorised one in terms of both the number of satellites and the overall cost.
DiSEqC motorised mounts
Traditional motorised mounts 03 The original motorised dishes used an open framework ‘polar mount’ to provide the rotational axis and installation adjustments and a ‘jack’ or actuator to provide the motive force. This system is still widely used with larger dishes (over 1m). The jack is connected to the equipment indoors with four or five
Motorised axis Altazimuth mount
DiSEqC 1.2 motor
2 What Satellite & Digital TV
connections – two for the motor and two or three for the pulse generator providing feedback to the indoor electronics on where the dish is pointed. These are usually included in a special multi-core cable. The jack’s motor uses 36V DC and such a motorised mount is usually referred to as a 36V or a DC mount. The dish can be powered to precisely the right spot for each satellite by counting pulses from a zero point (one end or the middle of the arc). A ‘positioner’ – built into the receiver or separate – keeps the pulse count and powers the motor the right distance and direction to find the required satellite. Some polar mounts enclose all the mechanics in one unit with a worm-screw drive, as a ‘horizon-to-horizon’ mount. It’s much neater but the multi-core cable and the positioner remain. Polar mounts are available to accommodate almost any size of dish but receivers that can drive them are now few and far between.
04 In 1997 motorised antennas were revolutionised by the invention of the DiSEqC 1.2 motorised mount, and ease of installation and operation mean that for domestic systems DiSEqC now reigns. The USALS system makes installation easier but is mechanically identical. A DiSEqC motor obtains both its power and its commands from the receiver using the LNB cable (which is looped through the motor to the LNB). No multi-core cable is needed. The motor incorporates a positioner – the positions for the satellites to be received are stored in the mount and the receiver selects the one required with a simple number code. Powered by the LNB supply, the DiSEqC motor is not very powerful, so it’s highly geared and therefore very slow to move between satellites. DiSEqC motors are also not very strong and can only safely accommodate dishes of up to about 1.0m diameter (some manage 1.2m if in a very sheltered position). A typical DiSEqC motor draws about 500mA, so the receivers must be capable of supplying
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DiSEqC 1.2 receiver
DiSEqC 1.2 mount
DiSEqC 1.2 receiver
DiSEqC jack on polar mount
DiSEqC 1.2 receiver
that. Whereas most modern receivers are DiSEqC-compatible, some PC-based systems can’t supply that current. How to motorise a fixed dish
05 Both DC and DiSEqC motorised mounts are available to motorise an existing fixed dish. Because a DiSEqC mount needs no extra cable it is simple to fit a motor to an existing dish in situ. The motor is mounted to the wall or patio mounting pole and the dish’s altazimuth mount clamps to a short rotating pole on the motor pointed up or down (and suitably angled to compensate for the dish’s elevation offset). Such motorisation kits can be tricky to set up completely accurately and are rather ungainly (and more vulnerable to wind) but are practically the only means to provide a DiSEqC motorised mount for a small dish (and the only option for an unusual antenna such as a flat plate). There is no real advantage to installing such a system from scratch over converting an existing installation but the wall mount must provide a pole that is as close to true vertical as possible. How to convert DC to DiSEqC
06 If you have an old DC motorised dish or need a large dish beyond the capabilities of a DiSEqC mount but you use a DiSEqC receiver, there are two ways to ‘convert’ DiSEqC commands to operate the mount. A DiSEqC jack can be fitted to the polar mount. These are mechanical fits for a DC
A DiSEqC 1.2 receiver can be used to control a DiSEqC mount, a polar mount with DiSEqC jack, or a polar mount and DC jack with a DiSEqC positioner to convert the signals
DC jack on polar mount
DiSEqC 1.2 positioner
jack but appear to the receiver as a DiSEqC mount. The DiSEqC jack is as strong as a DC jack but still as slow as a DiSEqC mount (or even slower). A DiSEqC positioner (often called a V-box) will convert DiSEqC 1.2 commands from a DiSEqC receiver to DC signals for a polar mount. The positioner will usually supply the high currents required by a DC jack and so, although the system requires the multi-core cable to the antenna, it combines the strength, large dish capacity and high speed of a traditional mount with the simplicity of a DiSEqC receiver n Geoff Bains
Glossary DiSEqC
Digital Satellite Equipment Control. Digital codes modulated onto the LNB power supply from a receiver to control antenna equipment such as switches and motors.
Positioner
Equipment that stores the required positions for the dish and directly controls the motorised mount’s motor using electrical feedback to tell it where the dish is pointed.
Motorised axis
‘Jack’ actuator
The traditional polar mount and jack is now used in the UK only for larger dishes What Satellite & Digital TV 3