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techjunkie02

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I have a 6 foot prime focus dish that is currently only set up with a C-Band LNBF.

I am thinking about changing it to a dedicated Ku dish, and have these questions;

    Would it perform as well, or better or worse than a 4 foot offset dish, on Ku only?

    Which LNB and Feed or LNBF would you say would give the best performance?

and, any other tips or comments you wish to give [smile]


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satlink

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Reply with quote  #2 
techjunkie02

I'am using a 7.5 paraclipse Hydro prime focus Ku only 
works great. matched up with a Invacom snh-031 universal
8 deg w/to 85 w
GeeZer

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I have a 6 foot prime focus with a Chaparral C/Ku feed horn.  I don't use the C much, but the Ku does very well.  Has an old Cal Amp LNB and I have absolutely no idea where it came from.  Do I think it out performs a 4 foot dish of any type...yes.
techjunkie02

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Quote:
Originally Posted by satlink
techjunkie02

I'am using a 7.5 paraclipse Hydro prime focus Ku only 
works great. matched up with a Invacom snh-031 universal
8 deg w/to 85 w


What kind of mount/support/scaler do you use to attach the snh-031 to the dish?

my dish has 3 support arms going to the scaler ring

Thanks,

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satlink

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Used pvc split different sizes until arrived at collar diameter needed.
used a single nut scaler ring.
pendragon

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Reply with quote  #6 
If you want to maximize the performance of a prime-focus dish for Ku-band, you MUST use a LNBF or feed designed for prime-focus dishes. If you use something from an offset dish, it will only 'see' a smaller portion of the prime-focus dish and you will be throwing away size advantage. There are Ku LNBFs specifically designed for prime-focus dishes. I have not tried any of these, but they should work. Instead I have used Invacom's ADF-120 feed on an appropriate LNB. This in my experience gets everything one can from a prime-focus dish.

To put this in stark terms, if your 6 foot prime-focus dish has a f/D = 0.35 (fairly typical) and you install an offset LNBF (almost always f/D = 0.6), it can only use the center 3.5 feet of the 6 foot dish, wasting the outer 2.5 feet where most of the area is. Thus you will be only using 1/3 the area of the whole dish, and this corresponds to a loss of nearly 5 dB compared to using the right feed/LNBF.
techjunkie02

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Quote:
Originally Posted by pendragon
If you want to maximize the performance of a prime-focus dish for Ku-band, you MUST use a LNBF or feed designed for prime-focus dishes. If you use something from an offset dish, it will only 'see' a smaller portion of the prime-focus dish and you will be throwing away size advantage. There are Ku LNBFs specifically designed for prime-focus dishes. I have not tried any of these, but they should work. Instead I have used Invacom's ADF-120 feed on an appropriate LNB. This in my experience gets everything one can from a prime-focus dish.

To put this in stark terms, if your 6 foot prime-focus dish has a f/D = 0.35 (fairly typical) and you install an offset LNBF (almost always f/D = 0.6), it can only use the center 3.5 feet of the 6 foot dish, wasting the outer 2.5 feet where most of the area is. Thus you will be only using 1/3 the area of the whole dish, and this corresponds to a loss of nearly 5 dB compared to using the right feed/LNBF.


Question; If I use the Scalar and the feed tube from an old LNBF (c-band), wouldn't it work if I keep it at it's proper focal length, and just adjust the position of the Ku-lnbf inside of the feed tube (removing the C-band guts and just using the tube as holder/feed for the Ku-lnbf)??  ... and thanks again for your help.

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pendragon

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Quote:
Originally Posted by techjunkie02


Question; If I use the Scalar and the feed tube from an old LNBF (c-band), wouldn't it work if I keep it at it's proper focal length, and just adjust the position of the Ku-lnbf inside of the feed tube (removing the C-band guts and just using the tube as holder/feed for the Ku-lnbf)??  ... and thanks again for your help.


You'll get signal, but nothing any different than mounting the Ku offset LNBF in the open.  Ku-band frequencies are 3X higher than C-band, and a scalar properly designed for Ku-band will be 1/3 the diameter and the rings will be at 1/3 the spacing. This makes the C-band scalar nothing more than a bigger signal blocker than a Ku-band scalar. Not just useless, but more than useless. Furthermore, if you take the cover off of your Ku LNBF, you'll find it has its own scalar in the shape of a reasonably deep cone. This is what establishes the 'look angle' for the LNB, and this is tuned to the f/D of offset dishes, not prime-focus. That is what causes the huge loss on a prime-focus dish.

If you're looking to mount a true prime-focus LNBF or feed on a three-legged prime-focus support, there is a device sometimes referred to as a 'K2C3' adapter that will do this for you. It's not hard to fashion one yourself, but they're only about $10 on the Internet.
techjunkie02

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Reply with quote  #9 
Quote:
Originally Posted by pendragon


You'll get signal, but nothing any different than mounting the Ku offset LNBF in the open.  Ku-band frequencies are 3X higher than C-band, and a scalar properly designed for Ku-band will be 1/3 the diameter and the rings will be at 1/3 the spacing. This makes the C-band scalar nothing more than a bigger signal blocker than a Ku-band scalar. Not just useless, but more than useless. Furthermore, if you take the cover off of your Ku LNBF, you'll find it has its own scalar in the shape of a reasonably deep cone. This is what establishes the 'look angle' for the LNB, and this is tuned to the f/D of offset dishes, not prime-focus. That is what causes the huge loss on a prime-focus dish.

If you're looking to mount a true prime-focus LNBF or feed on a three-legged prime-focus support, there is a device sometimes referred to as a 'K2C3' adapter that will do this for you. It's not hard to fashion one yourself, but they're only about $10 on the Internet.


Yes, Rick sells that K2C3 adapter. I'll look for a prime focus Ku LNBF. Thanks again!


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Captain_Kurtz

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Reply with quote  #10 
Quote:
I have a 6 foot prime focus dish that is currently only set up with a C-Band LNBF.

Would it perform as well, or better or worse than a 4 foot offset dish, on Ku only?


A lot depends upon the surface accuracy of the 6' dish vs. the 4' dish.
If it's a 6' fiberglass or hydro-formed metal dish (that isn't warped), then yes it should be appreciably better than the 4' dish.
However, if it's a 6' mesh dish or a Fortec Star stamped-metal-panel type dish, that are made to be just good enough for C-band, then I would bet no, it won't be any better - probably even worse than the 4' dish on Ku band.
(I am assuming the 4' dish doesn't have any problems, and that it is designed for Ku band.)
YMMV.
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