Information on 2.3m satellite dish

[shaynes]

Hi to you all out there, I wonder if anyone can help with information?. I obtained a secondhand 2.3m diameter prime focus satellite dish - no information with it. I measured the dish depth and calculated the focus (84cm), the LNB bracket is located at 97cm. I've been using a 40mm universal LNB and got the UK SKY satellite (28.2Deg East) and others seemingly without problems even though the LNB was 8cm short of the focus. Then I read that the end of the feed horn should be at the focus!. I purchased a 23mm dia LNB with a C120 flanged feedhorn bolted on - but even this is 5.5cm short of the calculated focus at 84cm.
My question is this. Is the focal length of a satellite dish accurately given by f= DxD/16xC where "C" is the dish depth or can the feed horn be within 10 or 12 cm of the focus and still give good signals. I'd appreciate any help before I go modifing the original 3 LNB support struts in order to put the LNB bracket nearer to the calculated focus (the dish is on a 6m high tower!)
Thanks

[Chris Smith]

If you don’t have the original brackets to place it into the proper focal point, try to find some one with a Pico meter. You can move your horn by hand and find the best/perfect signal strength this way.

Formulas are fine, but absolute data is even better.

You could also try to contact the original company for the data on the focal spot.

And alternative method is to use a small one inch mirror temporarily stuck [bubble gum will do] on several places over the dish surface, one at a time.

Then standing in front of the dish at 100 yards or so use a laser to find the average best focal length or spot on a measuring stick stuck to the center acting like the feed horn base.

As you average it out from center to out side over several spots on the dish a pattern will emerge for a best spot.

Light and microwave are not exactly the same but they are very close and it might get you as close to a guess as possible.

[shaynes]

Hello Chris - unfortunately this is part of the dilemma, the 3 struts and the LNB clamp are original.It was partly because of this that I asked about the accuracy of the focus equation,it seems odd that the focus calc gives a value so far in front of the original LNB bracket - or perhaps dishes are made which are not exactly parabolic and therefore the focus calc isn't applicable. However the whole dish and backing polar angle structure is elegant and substantial,unfortunately I've no idea who made it,it's about 10/12 years old.

[Gorgon]

Hi Shaynes,
First of all, I'm no expert on this. I can recall having seen a double horn setup for both horizontal and vertical polarisation. This was a rather long contraption. It might be that your disc setup is for a general reception system.

Just an idea

TOK

<small>[ February 17, 2006, 02:28 PM: Message edited by: Gorgon ]</small>

[Chris Smith]

shaynes

The feed horns are usually worked out at the factory using the “Pico” meter method. The problem arises in different feed horn receptors or head designs. Each feed horn [and LNA] has its own chararistics and they even have diffusers, collimators, or reflectors on their face plate.

This makes placing a generic feed horn on a unknown dish even harder.

My feed horn on my 3 meter dish has a face plate reflector, and two90 degree feeds for both band frequencies, so trying to figure it out based on the math alone doesn’t work out to the last degree.

They gave me a formulae to set it up based stickily on inches away from the dish, and told me what part of the feed horn had to be at that specific distance, and if I recall it right, it was the face plate extreme that this measurement was taken from, not the LNA guts or pick up which was another six or seven inches well within the LNA assembly.

Find a satellite electronics guy [from the old days of C band] that still has a "Pico” meter and see if he can help you find the "g" spot on your dish.

If not you can buy a Pico meter for around $100 or less, and even less if its used.

[shaynes]

OK - thanks to you both for your interest and advice. I'm going to try searching by hand for the correct position using an old analogue signal strength meter that I've dug out(pico meter I think you said). It's not going to be too easy with the scaffolding that I've got but it's probably the best way. I'll let you know what I find out. Regards Stuart

[shaynes]

Hi - about my 2.3m dish depth/focus problem. My own fault, I measured the depth incorrectly, don't ask me how it happened, must be going senile. Focus is 2-3cm in front of LNB bracket, thanks everone - Stuart