Problem with offset angle

[begarkitta]

Hi,

Most Ku Band dishes have an offset angle ranging between 20-24 degrees -- which means that even when the dish is vertical with reference to ground (90 degrees to ground), the LNB would be receiving at aroung 20-24 degrees. What do we do if we want to point this setup towards a satellite at 15 degrees? Theoretically, then, the dish would have to be tilted towards ground, right?

Can somebody direct me to a site or source where I can get some information on the the geometry of the Ku band dish. Are there Ku band dish with 0-10 degree offset?

 

[_stu_]

Theoretically, then, the dish would have to be tilted towards ground, right?

Exactly right!

If you don't want your dish pointing down, then perhaps a prime focus would be in order?

 

[rolfw]

As far as I know, the reason for the offset, is to prevent the LNB shading the dish. so anything much less than 20 degrees would not be efficient.

Any particular reason why you don't want the dish pointing downwards?

 

[begarkitta]

Thanks for the reply Stu.
Rolf,
I dont care where and how my dish is facing if only I can receive signals from Telstar-18. From where I live, the Elevation firgure for this satellite is 21 degrees. Now, assuming that the offset angle of my dish is 24 degrees, my dish would have to be looking "down" by 3 degrees. Is this alright? Would not obstruction in the line-of-sight (such as trees) come in the way? I am slightly confused; I though you would avoid obstruction by "looking up", right? I did some calculations using java-based programmes and the results show that I can receive signals from the above Sat. But then these programmes do not consider the offset angle parametre!

 

[Park Royal]

This diagram might help you understand the mechanics of an offset dish.
PR

 

[johnsattuk]

Thanks for the reply Stu.
Rolf,
I dont care where and how my dish is facing if only I can receive signals from Telstar-18. From where I live, the Elevation firgure for this satellite is 21 degrees. Now, assuming that the offset angle of my dish is 24 degrees, my dish would have to be looking "down" by 3 degrees. Is this alright? Would not obstruction in the line-of-sight (such as trees) come in the way? I am slightly confused; I though you would avoid obstruction by "looking up", right? I did some calculations using java-based programmes and the results show that I can receive signals from the above Sat. But then these programmes do not consider the offset angle parametre!

You have answered your own question really

You must have line of sight to the sat. If the sat. is behind a tree or other obstruction, that is where it is. You cannot move the dish up and down to see it, as it is effectively a point source, if you move the dish you are just looking at clear sky.

The type of dish used , and the apparent angle of the face of the dish don't really matter, it is the look angle of the dish which decides how it appears to you. ie: with 15 deg sat. and prime focus dish (0 deg offset) you would see the dish at 15 deg. up, with a 25 deg. offset dish you would see the dish at 10 deg down, but they would both be looking at the sat.

An offset dish is just a cut down prime focus dish, I think they were mainly developed because they are much easier to mount and fit on to domestic properties physically and cosmetically. The 20 - 30 deg offset puts them close to vertical in this part of the world, for the sats which most non sat-nutters want to recieve. ;)

If you have not got line of sight, you will have to chop down the obstrutions or change the location of the dish.

 

[begarkitta]

johnsattuk,

Thanks. I think I am beginning to understand: the point is not to get confused between the "apparent" angle (the angle the dish may appear to be looking at) and the "actual" angle.

By the way, do we have any method whereby we can measure the offset angle of a dish -- the off-the-shelf dishes that we get here do not carry any technical specification. Thanks again.

 

[johnsattuk]

johnsattuk,

Thanks. I think I am beginning to understand: the point is not to get confused between the "apparent" angle (the angle the dish may appear to be looking at) and the "actual" angle.

By the way, do we have any method whereby we can measure the offset angle of a dish -- the off-the-shelf dishes that we get here do not carry any technical specification. Thanks again.

I think it easy to try and get too technical with these angles, charts with declination angles to 3 decimal places, motor angles to a few seconds of arc, dish ofset angles to 2 decimal places, not many of us I suspect have any way of measuring these with any degree of acuracy.

Whilst declination and motor angles are the actual angles required, the dish offset is a design figure which will be subject to some tolerances in manufacturing the dish and the precise positioning of the LNB. It obviously helps to have some idea where the dish should be looking, but at the end of the day I think most of us will use the sat signal itself for final alignment since this is what it's all about. The same applys to skew angle, however carefully you set it, sig. strength on a particullarly weak sig. can often benefit from a careful tweak.

Whats your location

 

[begarkitta]

johnsatuk,

Thanks for the reply. I entirely agree with you. I am from south India, 74.51E, 12.45N.

There is a part of my question which I might have put wrongly/misleadingly. What I mean is: suppose I give you a 90cm offset dish. Is there anyway you can get to know the offset angle?
Thanks again.

 

[johnsattuk]

What I mean is: suppose I give you a 90cm offset dish. Is there anyway you can get to know the offset angle?
Thanks again.

I suspect that you mean, is there a relatively simple way, since with the right measuring equipment and formulae, all would be revealed. but a bit beyond me nowadays

If the dish has the LNB mounted, it would be fairly easy to measure that angle.

On further thought, if you measure the shape of the dish in the vertical plane, (since it is symetrical only the vertical plane is of interest), and draw it out accurately (CAD perhaps), you should be able to see where the focus point would be, and therefor measure its angle.

For practical purposes a string stretched over the rim in the vertical plane, and measurements to the dish face at 5cm intervals would probably give a result.

Again if your maths is up to it the above measurments could no doubt be resolved