prime focus VS offset dish

[rodscha]

what is the comparable efficiency of prime focus vs offset for fringe reception of 7w,26 east in South France?

And for 28East,weakest channels?
Would it providegreater stability against weather /seasonal variation?

And,why are there so few offset dishes in France/ spain for example?

 

[rodscha]

sorry I meant,so few PRIME FOCUS dishes around France/Spain/UK!!!

what is the comparable efficiency of prime focus vs offset for fringe reception of 7w,26 east in South France?

And for 28East,weakest channels?
Would it provide greater stability against weather /seasonal variation?

And,why are there so few offset dishes in France/ spain for example?

 

[ozumo]

A prime focus dish is more likely to fill with snow. Small offset dishes generally result in a more compact and neat installation which is more acceptable to those who must be obeyed. I wouldn't worry about comparing efficiency, dish size is king, offset or prime focus.

 

[Channel Hopper]

sorry I meant,so few PRIME FOCUS dishes around France/Spain/UK!!!

Perhaps you are not looking hard enough

I've installed perhaps 20 or so, from Cherbourg to Abbeville and beyond.

 

[rodscha]

Hello CH
and what are the reasons for these Normands and Picards to have opted for Prime Focuses rather than offsets?
Here in Languedoc ,not seen one ,although snow is much less frequent than in North France!

 

[Channel Hopper]

Aesthetics, plus most clients wanted something around 1.5m in diameter.

 

[rodscha]

and here it would mean less sensitivity to high winds,wouldnt it?
what would be equivalent to 80 cms(offset) in Prime Focus?
are the same lnbs,including monoblocs,usable?

 

[ozumo]

80cm, if you can find one. No you can't use the same LNBs. Well you can but the performance will be poor. You'd have to cut the feed horn off and attach a feed horn appropriate to the dish.

If you think more dish elevation = less wind resistance then mount an offset dish upside down.

 

[a33]

are the same lnbs,including monoblocs,usable?

Usable, yes.
But giving a much worse performance, as the typical LNBf illumination angle doesn't match the normal opening angles of prime focus dishes. (Edit: So you use only part of the PrimeFocus reflector surface for your signal, using an LNBf (including monoblocks) that is meant for offset dishes.)

So not advisable.

greetz,
A33

 

[sonnetpete]

Here in Languedoc ,not seen one ,although snow is much less frequent than in North France!

Ahem....We haven't had snow for the last three winters..and I've only ever seen one Prime focus dish within my area of Manche...though as CH says, perhaps I'm not looking hard enough...

 

[Channel Hopper]

Ahem....We haven't had snow for the last three winters..and I've only ever seen one Prime focus dish within my area of Manche...though as CH says, perhaps I'm not looking hard enough...

Prime focus dishes have rarely stood out as something that should not be on the wall, or roof of a building unless they really are of a size that is visibly out of place.
The public view of a 'dish' has of course changed to allow offset shapes as a viable alternative , the manufacturers responding with 'pressings' out of a design, as opposed to multilayer mesh/ fibreglass /curing to keep costs down.

 

[Channel Hopper]

If you think more dish elevation = less wind resistance then mount an offset dish upside down.

Unless you are in locations where there is a good chance of prolonged snowfall.

There are further options with heaters and the likes, (or just go online when one can't be bothered to clear off the snow)

 

[rodscha]

Thank you CH,sonnepete,ozumo,a33 for all your answers!
so,it seems the only advantage of prime focus dishes is,better aesthetics.
And I'd need specialty LNBs on top of that...
Maybe when dishes began being accessible to a larger public,the 1st ones were Prime focus?
It seems that C-band users use more often Prime focus dishes,maybe just older dishes,because C-band is an older technology?

 

[a33]

so,it seems the only advantage of prime focus dishes is,better aesthetics.

And cross-polarization of linear signals is a bit better than with offset dishes, I have read. (Don't know about circular signals.)
For small PF dishes, the LNB is blocking the center of the dish, so that is not advantageous when the dish surface is already small.
Finding (or checking) the focal point (for placing the LNB ) however is much much easier with a PF dish.

And I'd need specialty LNBs on top of that...

Well, I guess it started the other way round: The normal LNBs (for PF dishes) have a separate feedhorn, adjustable to the f/D of the dish.
The 'special' LNB is for offset dishes, where they put a fixed feedhorn to it (called LNBf; f added for feedhorn, though most people simply continue to call them just LNB ). This feedhorn would be for the 'normal' f/D ratio for offset dishes: about 0.60 to 0.65.
However as most people receive Ku-band with offset dishes, those 'special' LNBs are the ones that are in the majority nowadays...

Greetz,
A33