Centre dish angle

[wium78]

Is there a formula to calculate the centre dish angle when the elevation of the satellite is known.Or to put it another way ,once the centre dish has been established, that all the dishes in the same area will have more or less the same centre dish angle.

 

[a33]

What do you mean with "centre dish", and "centre dish angle"?
I do not understand.

And are we talking about prime focus dishes, or offset dishes? (does that matter, for your question?)

greetz,
A33

 

[wium78]

Thanks Greetz.I am referring to offset dishes,however,for my question it does not matter.A formula may exist for an offset dish.I have found using dishpointer to a particular satellite and the known dish angle to that satellite in my area it does not take long to obtain a single.With the off set dish not being prime focus, by eye one can more or less ascertain a rough centre of the dish.On old dishes I have found that the elevation markings on the reverse of the dish have faded.

 

[rolfw]

The offset on every make of dish is not necessarilty the same, will depend on the manufacturer. As an installer, I can say that I very rarely, if at all, take notice of the elevation markings on the back of the dish, I align by spectrum analyser and MER readings.

 

[wium78]

Thanks Rolfw.Not being an installer I am trying to assist my fellow Africans who have decided to come to South Africa for a better life.I am helping them to point their dishes to the satellites that carry transmissions from their home countries in the rest of Africa,who have a footprint over South Africa.

 

[a33]

I am still not sure what you mean with centre of the dish.

As you are talking about the faded elevation scales: Do you wish to find the angle of the dish face against the earth surface?
Then the picture on this webpage might help: Antenna Look Angles Info
You can see the elevation angle of the satellite against the horizontal, and also the declination angle of the dish face from horizontal plane.
Declination angle of dish face = 90 - satellite elevation angle.

This, of course, is a prime focus dish in the picture. For an offset dish, the angle for the dish face must be corrected for the offset angle.
(The fact that also other angles, for a motorized setup, are shown in the picture, is not relevant for the dish angles.)

Greetz,
A33

 

[wium78]

Thanks Greetz.Centre of dish I mean placing my digital inclinomitor on the face of the dish in more or less the centre.I am trying to keep it as simple as possible being a African situation.Using dishpointer and the angle of the face of the dish it is working for dishes in the same area.Although an offset dish I was looking for a formula instead of establishing the face angle of the dish by trial and error.

 

[Lazarus]

Surely, for an offset dish, the face angle is simply

Satellite Elevation minus Offset Angle (of LNB arm)?

In the UK, many dishes have an offset of around 24 Deg so, for example, if the required Satellite Elevation (qv Dish pointer) is 28 Deg, then the face of the dish should be 28 - 24 = 4 Deg (upwards).

In any event, setting a dish face this is merely a starting point as it will inevitably need to be tweaked for optimum reception.

There is a danger in over-complicating things .....

 

[a33]

Although an offset dish I was looking for a formula instead of establishing the face angle of the dish by trial and error.

For a flatfaced offset dish, the offset angle is Arccos(width/height).
The working area of the dish must have a flat dishface, and the LNB should of course be mounted at the proper offset angle.

The calculation can also be done here (fill in width and height, hit Enter, and the offset angle is shown):

Antenna Efficiency Calculator

Satellite Offset Antenna Efficiency Calculator

www.satellite-calculations.com

Greetz,
A33