How to point satellite dish.
Focal length of parabolic dish
How to make inclinometer
Sub reflector adjustment
Use rear side struts
VSAT antenna anti-icing
Dish interference: Site shielding
Correcting VSAT dish distortion
Antenna panel alignment

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.   See here for example C band interference filters for satellite use.   Many satellite TVRO LNBs don't have good filtering on the input.  This is sometimes necessary near airports, marine harbours and other radar installations.

 

Back to general VSAT information            C band spectrum interference

Terrestrial interference into satellite earth station
Interference source is visible from the satellite dish
Using a single obstruction to block terrestrial interference
Single wall obstruction, small angle across the top
Additional height of the obstruction increases the dioffraction angle and reduces the interference
Larger angle across the top, reduced interference
Dual ridge obstruction reduced interference much more
Double edge diffraction, interference minimised
Site shielding using evergreen shrubs and trees
Use of radio absorbing foliage improve matters further

Page started 10 March 2007 ECJ