LNB focusing

[crabber]

I have resurrected an old motorised dish today which had been in a shed for years. It was abandoned because the arm that holds the LNB had disintegrated because of rust.
I needed a dish to test a cheapish LG TV which has one satellite tuner and can drive a motor with USALS. I am getting away from the point but I have seen both Sony and Philips TV which cost far more yet cannot drive a dish. Is this common with TV's now?
So I made a new arm with a pivot underneath the dish so I could adjust the LNB height/angle in relation to the dish. I have it working but probably only by good luck, and obviously I could buy a new dish but I am keen to understand the principles involved.
I would have thought that the LNB should be directed at the centre of the dish, however mine appears to give the best signal when pointing below the centre. Also the clamp has been left on the pole so the rotation is still the same but the elevation marks are completely rusted away so have to be set by guess work.
If the lnb is "pointing" to the wrong spot on the dish could this error be made good but also having the dish elevation incorrect and vice versa ?
The dish is around 80 cm perforated although I have no idea of the make and cant see any labels on it.
If anyone has made it to the end of this post many thanks for your time and I hope you can throw some light on the issue for me.

[cosworth4x4]

offset or prime focus .
Im guessing offset due to one arm ?

[crabber]

Yes only offset, I have two others and tried to compare the arms but both the length (which I guessed) and angles are different yet there all appear to be very similar dishes.

[ian]

http://www.john-legon.co.uk/offsetdish.htm

At first glance it looks complex but it is really pretty simple maths, so don't be put off. I used similar formulas years ago to calculate the optimum position of my feedhorn.

Ian.

[cosworth4x4]

http://www.john-legon.co.uk/offsetdish.htm

At first glance it looks complex but it is really pretty simple maths, so don't be put off. I used similar formulas years ago to calculate the optimum position of my feedhorn.

Ian.

I was going to sent the same

[crabber]

Thanks, it is too dark and cold to measure now so I guessed the height to 800 width 700 and depth 45 got out the calculator and used Focal length = (width ^ 3) / (16 * depth * height) as the article suggests and it comes to 595mm. I did some more searching (finding dish offset angle) and found an online tool that works it out for you, I was amazed to see that this gave the same result, so I am dead chuffed. I will measure it tomorrow and see how far out my guess work is, so thanks once again.

[ManikM]

glad you brought this up - in the summer i bought a new Laminas 1.2m dish, its been ok, but i thought i would have seen signal levels much higher than my old 1m Gibertini.

i have peaked alignment at 1w, but i do wonder if i would benefit playing around with LNB position in the holder \ possibly getting a c120 feed horn along with LNB - just in the case the LNB isnt see all of the dish face...opinions....?