Question on prime focus dish efficiency

bodge

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#1
I bought this prime focus dish second hand earlier this year and am in the process of converting it for a ku band feed. I've now measured this dish several times and have come up with figures that seem "impossible". Its 219.2 cm across but only 24.25 cm in depth. THis gives an f/d ratio of 0.56 which as you are going to tell me is consistent with an off focus dish. When I bought it it was described as a test model. I can assure you that the measurements are correct; does the fact that this dish has very little depth mean that it will be less efficient than " normal" prime focus dishes ?
 

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Likvid

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#2
Depends, a long focal distance is good when transmitting like in VSAT situations due to better XPD performance.

Problem is getting a feed for that odd f/d
 

Likvid

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#3
bodge said:
I bought this prime focus dish second hand earlier this year and am in the process of converting it for a ku band feed. I've now measured this dish several times and have come up with figures that seem "impossible". Its 219.2 cm across but only 24.25 cm in depth. THis gives an f/d ratio of 0.56 which as you are going to tell me is consistent with an off focus dish. When I bought it it was described as a test model. I can assure you that the measurements are correct; does the fact that this dish has very little depth mean that it will be less efficient than " normal" prime focus dishes ?
Can you post some pictures from the back as well? easier for me to tell who made the dish, it says Luxor in front but Luxor never made any 2.2M dishes if it wasn't some prototype then.....
 

bodge

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#4
I'm planning to use the feedhorn conversion kit at the bottom of this page

http://www.satellitesuperstore.com/lnbs.htm

Just out of interest where exactly would the focal point be for this type of feedhorn - exactly at its entrance or slightly inside ?


Likvid said:
Depends, a long focal distance is good when transmitting like in VSAT situations due to better XPD performance.

Problem is getting a feed for that odd f/d
 

bodge

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#5
THis is the only picture I have available right now. I can take more tomorrow if needed

Likvid said:
Can you post some pictures from the back as well? easier for me to tell who made the dish, it says Luxor in front but Luxor never made any 2.2M dishes if it wasn't some prototype then.....
 

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Likvid

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#6
I don't recognize the dish.......
Seems solid built with a good mount.
Try out the SMW feed i talked to earlier, it doesn't cost too much i beleive.

That prime focus feed from the Superstore won't help you much as it is designed for dishes with f/d of around 0.40
 

bodge

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#7
I understand what you are saying but this dish is a prime focus dish despite the crazy measurements. I have in fact managed to get the superstore feed to work but need to extend the arms to reach the exact focal point. I can't see how I could attach the SMW feeds to this dish either.

Likvid said:
I don't recognize the dish.......
Seems solid built with a good mount.
Try out the SMW feed i talked to earlier, it doesn't cost too much i beleive.

That prime focus feed from the Superstore won't help you much as it is designed for dishes with f/d of around 0.40
 

Likvid

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#8
bodge said:
I understand what you are saying but this dish is a prime focus dish despite the crazy measurements. I have in fact managed to get the superstore feed to work but need to extend the arms to reach the exact focal point. I can't see how I could attach the SMW feeds to this dish either.
Yes i know it is a prime focus dish but you won't get any good results with the feed you are going to order, it is designed for dishes with F/D or 0.40 or close to that.

You need to find a feed for 0.55 or close to that.
 

bodge

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#9
Ok but how can I attach the feed ? Could the feed boss and plate supplied by the superstore be used with the SMW feedhorns ?

Likvid said:
Yes i know it is a prime focus dish but you won't get any good results with the feed you are going to order, it is designed for dishes with F/D or 0.40 or close to that.

You need to find a feed for 0.55 or close to that.
 

Likvid

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#10
bodge said:
Ok but how can I attach the feed ? Could the feed boss and plate supplied by the superstore be used with the SMW feedhorns ?
After checking the SMW feeds i don't beleive you could use that either as the flanges are constructed for a Offset dish.

Sorry, i can't help you in this matter anymore as you have a very strange F/D for a prime focus dish.

I don't beleive you will find a suitable feed to your dish and using that feed you was thinking of ordering from the Superstore isn't good for that dish as it will perform like a 1.5M dish or something close to that.

I would buy another dish if i were you.
 

bodge

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#11
I'm sure you are right. Does anyone else have any suggestions ??

Likvid said:
After checking the SMW feeds i don't beleive you could use that either as the flanges are constructed for a Offset dish.

Sorry, i can't help you in this matter anymore as you have a very strange F/D for a prime focus dish.

I don't beleive you will find a suitable feed to your dish and using that feed you was thinking of ordering from the Superstore isn't good for that dish as it will perform like a 1.5M dish or something close to that.

I would buy another dish if i were you.
 

Likvid

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#12
How much do you want to spend on a new dish? can you take a 3 meter dish if possible and how much are you willing to pay?
 

bodge

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#13
My limit is about 2.4m . As little as possible as far as cost is concerned

Likvid said:
How much do you want to spend on a new dish? can you take a 3 meter dish if possible and how much are you willing to pay?
 

dave branson

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#14
If its previously been used for C band reception,the original feed would have been for C band.maybe the f/d was different.just a thought.
 

ouagadougou

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#15
bodge said:
I'm sure you are right. Does anyone else have any suggestions ??
I would try building my own Circular or Conical Feed although you will have to do a lot of reading and some maths to get it right !! if you are up for that then try this

http://www.qsl.net/n1bwt/contents.htm
 
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