Site shielding: Reducing interference into satellite dish reception
The reception of satellite signals is susceptible to interference from terrestrial sources such as radio relay links and radar systems and other services sharing the same or nearby frequencies. C band satellite operation suffers interference in many places; radar, radio relay links and modern wireless systems are typical problems. What is described here is also of value in minimising interference from a satellite earth station transmit dish into terrestrial radio relay systems.
The following site shielding techniques are suggested to reduce the problem.
The idea of site shielding is to reduce the interference by physically obstructing the line of sight path from the interfering source to the dish. If the interfering source is visible from the dish as in top figure, you have an obvious problem. You need to block this red path.
The way to do this is to move the dish round the back of some building, or hill or putting one or more walls or earth banks in the way. It helps if the angle across the top of the obstruction is large, so a high wall close to the satellite dish is good. If you can put two walls so much the better. Make the heights such that from the top of either wall you can't see the dish or the interferer. The losses in this arrangement are modelled using double knife edge diffraction theory.
Consider the beam elevation angle from your dish to the satellite and make sure you don't block that. It may help to attach long string to the bottom of the dish and the other end to the top of a tall pole stood in front of the dish and some distance away so that the string is set to the beam elevation angle. This will give you some idea of how close the dish may be to the wall or obstruction.
Put your dish as low down as possible, in a dug out hole if necessary, with a high wall or earth bank all around except in the direction of the beam. The better the dish is hidden the greater the reduction in interference level. If you operate at a low elevation angle, putting the dish at the back end of a trench is a good idea with the trench in line with the beam to the satellite.
The top of the obstruction should not be made of metal but preferably some radio absorbing material (RAM). Brick or concrete at the top is better than a metal top to a wall. Rather than walls, earth banks are preferable, with the opportunity to plant dense evergreen shrubs and evergreen trees along the top.
Satellite interference filters:
If the interference is not on the same frequency as the wanted signal it may be possible to add a filter to the LNB input. Go to this website and investigate all the various microwave filter options: Interference filters for satellite Many satellite TVRO LNBs don't have good filtering on the input. This is sometimes necessary near airports, marine harbours and other radar installations. Specific filters may be used to reject interference on specific frequencies. if the interference is in-band but not co-frequency, it is possible to get narrow band filters designed to accept just your wanted carrier or transponder.
Interference source is visible from the satellite dish, interference is bad.
Single wall obstruction, small angle across the top, some reduction of unwanted signals.
Larger angle across the top, significantly reduced interference.
Double edge diffraction, interference really minimised.
Use of radio absorbing foliage improves matters even further.
Page started 10 March 2007, amended 24 Sept 2014, 19 Dec 2014 ECJ