Calculating Focal Point ?

Breadfan

Regular Member
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#1
Hi,

I'm back in the game again with a shiny new Channel Master 90cm dish, which will hopefully go up by the weekend if the wind will drop off a little!

I'm combining the dish with an Invacom c120 / CM feedhorn and I'm not sure how to find the exact focal point so I know how far from the face to site the feedhorn. I've searched all over but no joy, the CM/Andrew website wasn't particularly helpful at all : (

Ok, I know the formula is this :-

F = (DxD) / 16d, where D is dish diamter and d is depth

Problem is, the dish isn't perfectly round. Horizontally it's about 91cm wide and 9cm deep, making 'F' about 57.5cm. Vertically it's more like 98cm and 8.5cm, making 'F' about 70.5cm !!!

That's quite a big difference and being a perfectionist I need to know where it should be !! Does anybody know which is correct, or do I use another measurement (diagonal?)

Better still, has anybody got the official focal distance recommended for this dish?

I suppose I can just experiment with signal levels when it's up and running : )

Thanks,
Breadfan
 

steffan_st

A bit enthusiastic
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#2
This is copied and pasted from a post by a guy called Benny at satdudez a while ago,maybe it can help you:

I thought I should inform about my findings and perhaps some of you like to check your own dishes chasing the last dBs.

The empiric formula for fokal distance (f) in offset antennas is f = (D x D)/(16 x d).
Where D is the width and d is the deepest measure from your edges down to dish surface. When you measure the width you should only measure the curved part. The height you dont measure.

When you have calculated f, you attach a plastic string or similar to the bottom edge of your dish. You make a mark or a knot on the string at distance f from the edge. The loose end of the string shall be attached to the upper edge of your dish. There are some guesses that can be made how long the string shall be from the knot or mark to the upper edge, but I think it is better to search the focal point with this distance flexible. You can take 1.8 x f to start with if you like to have some idea of how long string you need.

You shall not be surprised if you find that your present position gives you the strongest signal. That is what you worked for when installing. To check if you have the proper focal point higher or lower you must also change the elevation of your dish until you again get a strong signal to compare with. Elevate if you lift the focal point.
 

Channel Hopper

Suffering fools, so you don't have to.
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#3
If you are using the CM feedhorn then you do not need to calculate anything, the package does it for you.

There has been however, more than one 90cm made by Channel Master (and lately these are not even made in the same factory) so if you have a model number - usually six digits - how long and and where you have got it from, it would help.
 
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#4
Can you guys make a sketch on how to calculate the focal distance. I ve been trying to read the whole thing over and over but couldn't understand. thanks
 

Channel Hopper

Suffering fools, so you don't have to.
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#5
For prime focus dish calculation , check this link

http://www.geo-orbit.org/sizepgs/tuningp4.html#anchor879461

Offset dishes however are basically a cross secion of a prime focus parabola, and so the variety of F/d ratios means its impossible to give just one easy equition. Ask the manufacturer of your dish
 

Hareesh

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#6
Vow Its a fantastic link with huge informations to all.thnx CH for it.
 
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