Focal Point

wolsty

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Kernow
#1
Is there any easy way to calculate the focal point of an offset dish? I'm using an old Sky Analogue 90cm dish and suspect that the LNB is not in its optimum position. Are all makes and dimensions of antennae built with the same focal length?

wolsty
 

Channel Hopper

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#2
Answers are no and no

Offst dishes are in effect a section of a prime focus parabola, and as such the length of feed rod is approximately the focal distance of a much larger prime focus antenna.

Testing by adjustment is long winded as adjusting the distance of the LNB from the antenna can alterthe focuspoint of the dish, and the maximum gain achieved in testing could be affected by non adjustment of the elevation of the antenna to the satellite.

Also the offset dishes are rarely the size quoted, some manufacturers use the horizontal dimension as the advertised size, and many installations fall foul of planning permission limits because of this.

What make is it, I may be able to pull the critical dimensions you need from the suppliers if I have some further details
 

wolsty

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Kernow
#3
Thanks, CH. Pretty much what I expected. As for make, I haven't a clue. I 'rescued' the dish from my local recycling site, because it is bigger than the one I began with, and have just been experimenting to see if I can improve reception. In fairness to the dish, it's not bad - I can pull in 1 deg W, some Sirius channels and 5 deg W quite well. Both Astra sites and Hotbird are as strong as anyone could reasonably wish. Don't go to any trouble, I just wanted to be sure that I couldn't squeeze more out of it.

wolsty
 

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#4
Great find - especially if it was totally free

If you can describe colour whether its solid or mesh, type of back plate, if it has a plastic surround or so

Maybe a digital photo

I can then give you some answers
 

wolsty

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Kernow
#5
Thanks, CH. When the wind and rain go away, I'll jump up on the roof and have a look.

wolsty
 

wolsty

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Kernow
#6
Well, we've just got a window in the gales and rain of the last few days, so I popped onto the roof to have a look at the dish. Sorry, I don't possess a digital camera; I'll have to do my best with words.

It's oval, mesh, grey painted, with a plastic trim around the circumference. The dish is fixed to the mounting with four round-headed bolts, slotted for a screwdriver. The mount itself has angle markings for declination adjustment. The LNB arm is tubular and is fixed into the mount behind the dish with a lug and single 10mm bolt. The LNB bracket is black plastic and is bolted onto the arm.

I managed to scrounge (FOC) another dish, slightly bigger in the horizontal axis and in much better condition. Unfortunately, someone had removed the LNB arm and the bracket looks as if it's been used on a horizontal pole. Would it be possible to trabsfer the arm and mounting from the old dish to the new one?

wolsty
 

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#7
The slot screws suggest its an older style Concentric unit.

If the alloy edge of the antenna simply bends through 90 degrees backwards to reinforce the surface then its unlikely it will perform well

If youhave the one where the dish is shorter than it is wide with the aluminium being brought forward first then bent back, then these are excellent (especially for motor upgrades)

The feed support should fit either without trouble
 

wolsty

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Kernow
#8
It looks very much like the Concentric type you describe. And there's a lot of rust on it - although that's normal in this part of the country, where it rains a lot and there's always plenty of salt spray to corrode anything metallic.

I've just had a look at the second dish. It looks almost new - no rust or corrosion and the anodised parts are still bright. The edges are, however, like the Concentric type you've just described and I wonder if it's worth going to the trouble of trying to make one good dish out of spare parts from two, especially if the model doesn't perform well.

I think I might just revert to the dish which came with the motor - it's slightly smaller but is a rigid, solid metal type. It's much heavier, but the analogue performance wasn't at all bad.

When I can afford it (ie when I can persuade my wife of the benefits of an upgrade) I'll go for a 1.2 metre Channel Master.

Thanks for the advice, though.

wolsty
 

wolsty

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My Location
Kernow
#9
Spent a day on the roof playing with the dishes and opted to stay with the rusty old 90 cm Sky model. Reception of everything within the dish size limits is now very good, although 1 deg W is still weaker than I would like.

Since I'm in the far west (almost on the 5 deg W line of longitude), I used 5 deg W as my due South reference, rather than 1 deg W and the tracking has improved considerably.

However, the position of our house, on the east side of a steep hill with trees on top, means that anything west of about 10 deg W is below the horizon. Goodbye Hispasat, as my wife won't agree to a house move.

:(

wolsty
 

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#10
I have a catalogue of excellent telescopic posts, Some extend to 35m with attached guy lines. You could always suggest to the other half its for the washing to improve drying times
 

w hole

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#11
Hey Woltsy

If you've got trees on top of your house as per. your posting, why not put the dish up in one of the branches?

Regards

W.H.
 

rolfw

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Berkshire
#12
LOL

"However, the position of our house, on the east side of a steep hill with trees on top"

Yes it could read like that, must look pretty :)

Rolf
 

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#13
I once put a dish up in a tree, fine for three months but these things have a tendencey to move and then it goes off line.

That and the leaves of course

Clark Masts was the company, not cheap but a possible solution.

Whats wrong with moving anyway ?
 

wolsty

"Satellite Expert"
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Kernow
#14
Trees on top of the hill, silly! And as for 35 metre telescopic masts, I daren't even ask.

Wolsty
 
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