22 deg offset dish elevation angle set up pointing.

The 22 degree offset means that the beam comes off the dish at an offset angle of 22 deg. With the feed at the lower edge, this means that the beam comes off the dish at a 22 deg angle upwards. So if the dish reflector itself is vertical, the beam elevation angle is 22 deg.

For elevation angles lower than 22 deg the dish must be tilted forwards and downwards. For very low elevation angles the lower edge of the dish and the back end of the feed support arm may meet up with the mount pole. In such cases it is necessary to mount the dish upside down with the feed at the top. This way you can go down to zero elevation angle - which is applicable if you want to use the dish for terrestrial purposes.

For elevation angles higher than 22 deg you need to tilt the top edge of the dish backwards. A backward tilt of 68 deg would mean the elevation angle was 90 deg, with the beam going straight upwards.

Offset parabolic satellite dish

If you want to make yourself an elevation angle scale you can mark the card in 5 deg increments as follows:

Angle measured with protractor	Elevation angle marked
3 8 13 18 23 28 33 38 41 43 48	25 30 35 40 45 50 55 60 63 65 70

The figure shows a "22 deg offset type" dish.

The offset dish is shown set for 63 deg beam elevation angle by tilting the dish backwards by 41 deg. This 41 deg angle may be set using an inclinometer or by using a home made inclinometer using a card as below.

A piece of rectangular cardboard has been used as an inclinometer. You make a pin hole in the top right corner of the card and then insert a string and small weight (e.g. metal nut) like a plumb bob.

Carefully draw a line parallel to the right edge and through the pin hole. Then, using a school protractor set out an angle of say:

41 deg and mark this "63 deg elevation"

If you put your eye behind the lower rim of the dish and look forwards just over the top of the feed horn you will be looking approximately directly towards the satellite. This is the line of the lower (black) ray trace in the image above. One interesting idea is to hold a thin strip of wood across the top of the feed horn and under the real bottom edge of the dish, where the feed support arm attaches. If you put an inclinometer on this strip of wood it will tell you the elevation angle of the beam. Adjust the mount till the elevation angle is as required. You may need 3 people, two to hold the wood and inclinometer and one to alter the elevation angle.

Please note that dishes come in a variety of offset angles. Some dishes have an offset angle nearer 27 or 30 deg. There is no particular number. It varies according to manufacturer. Some may choose for the beam to be partially obstructed by the top of the feed horn. This has negligible effect on gain, scattering etc but slightly improves the cross polarisation performance Short focal length (strictly short F/D ratio) offset dishes have poor polarisation isolation unless very expensive mode matched feeds are used.

Navigation link: Back to satellite dish pointing angle calculator

► Page created 21 Jan 2005, amended 1 Feb 2007 Eric Johnston