Corotor performance question

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N0QBH

Thread Starter
SatelliteGuys Pro
Jul 13, 2006
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Central MN
Anyone know how much of a difference there is between a C band only feed horn and one with C/Ku ?
I'm sure there must be some degradation with that Ku "antenna" in the middle of the C band feed horn throat.
Since I have both a BUD and a Ku dish now, would it be worthwhile to switch back to a C band only feedhorn on the BUD?
 

Anole

SatelliteGuys Master
Sep 22, 2005
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L.A., Calif.
The only member with instruments and posting results is Pendragon.
He's measured lower C-band performance on orthomode feeds which include Ku capability.
It would follow that corotors would, too.

The thing about corotors AND ortho feeds, is that you can fit them with better LNBs.
Perhaps you should try a spare LNB, and see if performance changes?
Tell us what you are using.

Don't overlook the free improvement of a full realignment of the scalar on the dish. :)

Which signals are giving you troubles?

As a last resort, an inexpensive C-band only ortho feed and two modest LNBs would surely help. ;)


edit: oh, a 7 foot dish
you might well be suffering from adjacent bird interference
i have no idea how to rate that particular dish, but even if it were the best 7' ever, it's a little small for satellites parked 2 degrees apart
 
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pendragon

SatelliteGuys Pro
Oct 13, 2008
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With stock parts, the best you can do for dual C/Ku band reception off a prime focus dish is to use a dual orthomode feed. My measurements across a wide variety of dishes have come up with a consistent loss of 0.5 dB. That's not peanuts when you have a smaller BUD and DVB-S2 increasing in use. Still I ran this combination for many years off a 1.8m with excellent results, although I had to get the alignment nailed to grab high FEC S2. It's much easier to have a little margin.

Corotors generally cause higher losses than dual orthos on C-band. I haven't made as many measurements for corotors, but it's more like 1 dB. Others have confirmed this in private correspondence. Ku losses are invariably high with either design, on the order of 3-7 dB. This usually negates the larger aperture size unless one can minimize the Ku feed losses and under-illumination. A C-band only prime focus, ideally with a single ortho feed, and an accurate Ku-only offset is a hard combination to beat.
 

nhulst

SatelliteGuys Guru
Pub Member / Supporter
Dec 20, 2007
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Ku losses are invariably high with either design, on the order of 3-7 dB. This usually negates the larger aperture size unless one can minimize the Ku feed losses and under-illumination.
Are there techniques that can realistically be used to minimize the Ku feed losses you spoke of? The discussion I recall has always pointed to the fact that you can't have a proper Ku scalar without obstructing the feedhorn for C-band reception. This would imply that we have to use any possible tips or tricks to gain back a little bit of performance elsewhere, if that's possible.
 

pendragon

SatelliteGuys Pro
Oct 13, 2008
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Are there techniques that can realistically be used to minimize the Ku feed losses you spoke of? The discussion I recall has always pointed to the fact that you can't have a proper Ku scalar without obstructing the feedhorn for C-band reception. This would imply that we have to use any possible tips or tricks to gain back a little bit of performance elsewhere, if that's possible.
This is partly a case of "You can't fool Mother Nature", but there are a few avenues to suggest:

1. Minimize the distance from the feed mouth to the Ku LNBs. Unless you make your own feed, the only options are to select commercially available feeds that do this. Dual ortho feeds tend to be better than corotors in this regard.

2. Custom mount a dual polarization Ku LNB to the back of a dual ortho. This saves a little distance and allows the use of C120 flanged LNBs, some of which are much better performers than the WR flanged single polarization LNBs. See my ancient thread on modifying my dual ortho into a Frankenstein feed as an example.

3. Get a single ortho C-band feed and cut an offset hole in the C-band scalar to fit a proper prime focus Ku feed/scalar, for example an Invacom ADF-120. You want something with an adjustable f/D to match the Ku illumination to your dish, rather than the fixed f/D Ku prime focus LNBFs that are relatively inexpensive. There will be some loss on the Ku side because of it being offset from the prime focus main axis, but this should only be around 0.5 dB if one butts the Ku scalar against the C-band feed mouth. That's far less loss than a dual ortho or corotor exhibits.

4. Modify or build your own dual ortho to better match the Ku side to your dish. I'm not going into any particulars because if you don't have the mathematical tools this could be like shooting a fly in the dark with a pistol.
 

N0QBH

Thread Starter
SatelliteGuys Pro
Jul 13, 2006
204
53
Central MN
Took advantage of the (probably) last warm weather on Wednesday and swapped my corotor C/Ku feed horn with the original C band only.
Noticed the signal levels jumped about 5% on the Micro HD meter:) Who knew going backwards would bring me forward?
I'd have to say from my own experience, if you own a mesh BUD and want Ku, buy a 90cm solid Ku only dish. The cost and hassle are about a wash.
The performance and flexibility, however, are heads and shoulders above.
 

Anole

SatelliteGuys Master
Sep 22, 2005
11,819
9
L.A., Calif.
one last gasp

Linuxman once talked about old patent-infringing corotors.
I *think* they were Chaparral brand, and hotter on *Ku* than the popular/commercial ones they wound up shipping for decades.
I didn't pay a lot of attention, and put corotors behind me as I once did tubes, many decades ago! :)
Search the forum if you still hold out an urge for relics of the past. ;)
 

pendragon

SatelliteGuys Pro
Oct 13, 2008
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63
FWIW, the measurements I provided were based on the older style Chap corotors, one of which I still own. I also used to have a newer style Chap corotor; it certainly was no better and for the most part lower in performance. I happily gave it away.
 
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