Using Prime Focus dish as Offset

nabih

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Hello everybody
Before taking a final decision to install a prime focus dish i would like to know if it would be possible to decline it and use it as an Offset
I put this question because i meet some difficulties in finding TWIN FLANGE BLACKPRO LNB with feedhorn integrated which works with Prime Focus dish.
Therefore the last alternative would be to use my prime focus dish as an Offset with Black Inverto C120
Is it possible my dear friends
Hope to hear from you very soon and best regards.
 

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No not really ...
You could in theory point the focus lower on the horizon ..so that the satellite required would form an intercept on the dish surface ...but the lnb would then no longer be at the focal point of the dish ..but would have to be placed much much lower ...
The effective gain of the dish would be considerably compromised ..and would negate the effectiveness making it worse that placing an offset feed on a PF dish at the correct focal point ...and using the dish normally ..
Don't go down this route ..its pointless ..
rgds
VS
 

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Have you tried obtaining an Inverto Blackpro from somewhere in France? I've found a firm here which is reliable. They stock a single Inverto :

_http://www.2galli.fr/boutique/fiche_produit.cfm?ref=INVSS_S__&type=65&code_lg=lg_fr&num=51

Why not try and email them and ask if they can obtain a twin for you, can they deliver to Algeria and how much. The single is only €12.80 so I guess the twin won't be that much more expensive. Having an LNB which works well on your dish is a must I would guess.
 

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There's more to an offset dish compared to a prime focus dish than the positioning of an LNB at the bottom of an off-set arm.

The f/D ratio of the dish (how shallow or deep it is relative to its diameter) is different between PF and O/S and that alone will mean that the LNB will either "look" at too much or too little of the dish depending on which pairing you have.
 

Vipersan

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Here is rough diagram ..and it is rough ..
To show that only part of the dish would be effective used in this way ..
Very inefficient ..and most likely useless ..
rgds
VS
 

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nabih

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Vipersan said:
No not really ...
You could in theory point the focus lower on the horizon ..so that the satellite required would form an intercept on the dish surface ...but the lnb would then no longer be at the focal point of the dish ..but would have to be placed much much lower ...
The effective gain of the dish would be considerably compromised ..and would negate the effectiveness making it worse that placing an offset feed of a PF dish at the correct focal point ...and using the dish normally ..
Don't go down this route ..its pointless ..
rgds
VS

Thank you so much Vipersan.
 

nabih

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sonnetpete said:
Have you tried obtaining an Inverto Blackpro from somewhere in France? I've found a firm here which is reliable. They stock a single Inverto :

_http://www.2galli.fr/boutique/fiche_produit.cfm?ref=INVSS_S__&type=65&code_lg=lg_fr&num=51

Why not try and email them and ask if they can obtain a twin for you, can they deliver to Algeria and how much. The single is only €12.80 so I guess the twin won't be that much more expensive. Having an LNB which works well on your dish is a must I would guess.

Thanks sonnetpete for the information.
 

nabih

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Vipersan said:
Here is rough diagram ..and it is rough ..
To show that only part of the dish would be effective used in this way ..
Very inefficient ..and most likely useless ..
rgds
VS
Yes i am now convinced it s no use to do it waste ot time and disapointments
Once more thank you sir for your help.
 

nabih

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PaulR said:
There's more to an offset dish compared to a prime focus dish than the positioning of an LNB at the bottom of an off-set arm.

The f/D ratio of the dish (how shallow or deep it is relative to its diameter) is different between PF and O/S and that alone will mean that the LNB will either "look" at too much or too little of the dish depending on which pairing you have.

Thanks sir.
 

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The differences between LNBs would really only come into play if you are on a marginal signal. What signals are you really going to have difficulty catching with the dish/LNB combination you already have ?
 

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Channel Hopper said:
The differences between LNBs would really only come into play if you are on a marginal signal. What signals are you really going to have difficulty catching with the dish/LNB combination you already have ?

I live in the suburbs of Algiers and my hope is to get the BBC fta channels which are very weak.
I have been advised to use a Prime Focus for this purpose and not an offset dish.
 

Channel Hopper

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So, how many channels have you already got from the satellite that are destined for this country ?

What dish do you have and are there any manufacturers gain figures ?
Did the manufacturer deliver it and offer a guarantee of no damage on delivery ?
 

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hello,
a prime focus is closer to a cassagrain . or even a gregorian.
if you have the right sub reflector and has a hole in the center of the dish it could be set up.
offests are cut different more oblong in shape.
ralph
 

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ralphmagno said:
hello,
a prime focus is closer to a cassagrain . or even a gregorian.
if you have the right sub reflector and has a hole in the center of the dish it could be set up.
offests are cut different more oblong in shape.
ralph

Not sure about any of that
 

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Iam sure that some of it is incorrect.
For example the fibo gregorian is an offset dish and nothing like a prime focus.

An offset dish is part of a hypothetical much larger prime focus dush



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ralphmagno

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here is what i was talking about.
prime focus is like a cassegrain.

the sub reflector reflects the signal back to the center of the dish gain is higher than a prime.
offset is cut different and is used in most applications.

the center of the dish of a prime is the null spot and that is what they use for most up links.
ralph
 

nabih

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Hello everybody
Very sunny and hot in Algiers today
Anybody can tell me how to transform Black Inverto Lnb for Prime Focus dish
I ve just got an aluminum feedhorn from a friend of mine as well as a 1m50 PF dish
Thanks
 

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It involves a hacksaw, and is on another thread nearby.
 

nabih

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Channel Hopper said:
It involves a hacksaw, and is on another thread nearby.
Any picture to help doing it
 
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