Interference on satellite TV

"I have been noticing 'jitter' on some adverts and programmes where there is a white background and little else on it."

Before buying a TV set, it's important to see it working with a Sky Digibox. There are many TVs that are not fully compatible. (No idea why because, in theory, the Sky box Scart outputs give standard levels of video etc.) But the fact is that some TV sets don't like the Digibox signals.

Try turning the contrast down in the Digibox "picture" menu. Try a fully-screened Scart lead and, if that doesn't help, try a different Digibox and see if the problem remains. If it does then your TV set is probably at fault. Cutting the wires to the video and audio return pins at the TV end of the Scart lead might help.

There are many sources of interference and they all have different effects on your satellite TV reception. The effects can even vary with distance. It can affect one channel multiplex or several.

The interference may be local. It might even be coming from your TV or Screen itself! Make sure that ALL connecting leads are fully screened. Try moving the equipment as far away from the TV screen as possible. It can be caused by a DECT portable phone, a mobile phone, a "Snooper" type radar detector in a nearby car, high power lighting or faulty fluorescent lighting, "energy saver" bulb, dimmer switch, a microwave oven, switching transients from any sort of switch and arcing from, for example, a boiler thermostat. In an apartment block, it's not unusual for the electricians to run poor quality coaxial cable alongside mains power cables in the conduit. This is a recipe for disaster as, each time someone switches an appliance on or off, the resultant voltage spike is induced into the coaxial cable where it travels down to your Digibox, often causing a momentary "freeze", popping noise or a complete "lock-up". It has to be said that digital satellite receivers are less susceptible than terrestrial receivers but it can still happen.

The interference may be coming from outside your house and the source may even be several kilometres away. This means that it is often possible to block or reduce the interference. Street lights can go faulty and cause problems (usually at night). If this is a possibility, take a walk with a portable AM radio tuned off-channel so you can hear a background hiss. If you live near an electric railway, look for interference when a train passes. If you live in the country, interference from electric fences is common. You'll see momentary flashes every second or two on an analogue TV. (Don't throw that old portable TV away - it might come in useful!)

One source of interference, which is becoming increasingly common, is the transmissions from mobile phone repeater masts.

Here's one man's description of a problem which might be caused in this way:

"First of all some of the channels started freezing. It only happened every now and then, usually nights and weekends. It was always the same channels - mainly the channels beginning with UK (UKFood, UKStyle etc.) movies 1, 2, 3 & 8 and certain others. I rang Sky who sent an "engineer" out. Typically the system was working fine when he came out. He replaced the digibox and went and a couple of hours later the channels froze again."

The signals fed from the LNB, down the cable to the digibox, lie between 950-2150 MHz. Cellphones can use 1800 MHz and their signals could get into the system or the coaxial cable.

Of course you should first eliminate all possibility that the signal is simply being blocked by trees or similar! Even a few leaves can have a very bad effect on the weak satellite signals.

A cable with better screening might help (double screened). If that fails to help then you should test the system, if possible, by placing a dish temporarily at ground level and connecting to that. If reception is trouble-free, try raising the dish higher. If the problems return, that's a good sign that there is a beam of horizontal radiation.

The trick is to determine where it's coming from and position the dish so there is an obstacle in the way. The most convenient obstacle is your house. Consequently, if you can position your dish so the house blocks the interfering signal without blocking the satellite signal, you should get a big improvement.

This sketch shows one example. The dish would normally be mounted out of sight at the rear. To avoid the interference, it is mounted on a pole at the front of the house, looking up the roof. The satellite signal can still reach the dish but the roof partially blocks the signal from the 'phone mast. A further improvement could be made by lining that section of the roof (inside the loft) with metal or metal foil. In addition, the cable should also be on the side of the house away from the source of interference. I know of one instance where the cable had to be fed through earthed steel conduit to eliminate interference.

It may, instead, be possible to mount the dish on the left wall or near the ground, as in the picture, where it would be shielded from the 'phone mast by the house or by a garden wall or rising ground. You could even recess it in a hole in the ground. The only criteria are that it must be able to see the satellite but not see the 'phone mast.

It is important to understand that the interference may be getting into the cable itself, even if it is good quality "double-screened" satellite grade. For this reason, the cable run should be kept as short and as low as possible. It should itself be shielded by a wall or even by threading it through steel conduit. It is also important to realise that the 'phone signals may be bouncing back from other buildings; so what you think is a shielded position may not be! Unfortunately, success often comes only after a lot of trial and error. There's no point in asking "should I put my dish here?" Nobody knows.

Another remedy is to mount the dish upside down so the LNB arm is at the top. This has the effect of tilting the dish so it is at an angle and is less susceptible to collecting the horizontal interfering signal.

Alternatively, install a prime-focus dish. This, also, will be at a much steeper angle.

There are many things to try but you will obviously want to achieve the simplest solution that achieves the desired result. Carry out tests as suggested and you should be able to make at least some improvement.

Do you live near an airport?

Sometimes the flight path is changed so that a plane will obscure your signal for a couple of seconds every minute or two. Take a look outside!

Airport radar sweeps a beam around every few seconds. If you get this type of interference, the only answer is to mount your dish in a position where it can not see the airport radar signal or reflections of it. Trial and error will be necessary and you are more likely to be successful if the dish is at ground level behind a wall or solid structure.