Reception problems in Cyprus.

D

divibi

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Zorba said:
I think its the other way round.
Click to expand...

Thank you, Zorba. I am not sure, again, as I read somewhere that big PF dishes see Astra 2A, 2B and 2D separately- and you have to motorise the dish for that!!! I am really not sure. We need Channel Hopper, Ralph, Robbo, Analoguesat and others to visit this thread here.
 
Analoguesat

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Did someone call my name ?? :D


I cant help much with big dishes - the biggest one Ive ever had to play with is just 1m. SWMBO wouldnt mind me getting a big dish but I think the neighbours would complain. They all think Im nuts anyway.

I do know that as the dish diameter increases the boresight (acceptance angle) decreases, but after that it gets a bit hazy :D
 
PaulR

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It's too late in the evening but I'll try to pull it all together.

From another post here

If a prime-focus dish and off-set dish have the same f/D ratio then, by the laws of physics, they will have the same gain. There will be a slight advantage to the off-set as the LNB and arm won't be shading the dish but otherwise they will be the same.

The thing is that they generally aren't the same. The deeper a dish is, the lower is its gain. Why would anyone not want a dish to have as much gain as possible? Well, the problem is that a shallow (high gain) dish has poorer rejection of unwanted signals from the side compared to a deep dish and sometimes it's more important to reject the interference.
Click to expand...
The smaller the f/D ratio, the deeper the dish. O/S dishes have an f/D of about 0.6 - 0.65 while PFs tend to be about the 0.5 mark I think.

Putting it another way, O/S dishes are generally flatter than PFs so that means that O/S dishes are optimised for gain rather than signal rejection.

Hope I've got it the right way round.
 
Robbo

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Pretty sure you got it the right way round Paul.

The Focal length of a PF dish can be calculated by the equation F= (D^2)/(16xDepth), where D= diameter.

Therefore if the dish is deeper the focal length will be shorter.


The biggest dish Ive messed with is a 1.8, rather miniscule compare to a 3.7m, LOL.
 
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