In terms of C-band losses, I've measured a single orthomode feed (C-band only) against a dual orthomode feed (C/Ku) a few times in reasonably well-controlled tests, including one time where I swapped the C-band LNBs and another where I moved one LNB back and forth. The results indicated about a 0.5-1.0 dB loss in CNR for C-band on the dual ortho vs. a single ortho.
I have not tested combo feeds and corotors. But in public and private correspondence with a few people who have the equipment and know-how, they all concluded a corotor has at least a further 1 dB loss on C-band over a dual ortho. Unfortunately the performance of low-cost combo feeds has been all over the map to the extent it makes any generalizations on performance meaningless.
We now come to Ku-band losses of dual orthos, combo feeds and corotors. I wish I had a bag of quantitative information, but it's very hard to come by. I've tested my dual ortho with its stock LNB mounts on all my BUDs. I was able to get good reception but never amazing on Ku. My 1.2m offset beat the BUDs with the dual ortho in nearly every shoot-out. I have recently been half-heartedly trying a Ku prime-focus feed a bit off-axis on one of my BUDs, but I've found no reason to doubt that the Ku BUD losses are partly because of an imperfect parabaloid.
Last year I spent some time adapting a dual ortho feed to an Invacom C-120-flanged LNB, with the intent of improving the performance of my 1.8m solid petal (prime-focus) dish over a 1.2m offset using an integral Invacom LNBF. In a perfect world the 1.8m should produce 3.5 dB better CNRs than the 1.2m. After tweaking the 1.8m to the point of exhaustion, I was only able at best to see a 1 dB improvement. Was the missing 2.5 dB caused by surface deviations of the reflector or losses in the feed? I have no way of knowing because I ran out of time and was unable to compare a prime-focus feed on the flanged Invacom.
I have certain suspicions. Any type of C/Ku combo feed must by reality lack an effective scalar at Ku frequencies. If you've ever compared reception on C-band with and without a scalar, and/or Ku-band with feeds with effective scalars vs. something with little or none (like a 121 Superdish feed), you'll already suspect a good reason. The C-band scalar on a combo feed coupled with the outside C-band waveguide is probably better than nothing, but not by a lot. There is also the problem of getting the Ku energy from the front of the feed to the back where the Ku LNB(s) are located.
Having recently put up a 1.8m Prodelin offset for Ku-only reception, I've been running simple experiments on Ku feeds that may provide some other clues. This reflector came with a massive Tx/Rx box sitting at the end of a conical scalar that's a bit more than 5" in diameter at its mouth. I machined an adaptor from this feed to one of my C120-flanged Invacoms and this 1.8m combination measured about 3.8-4.0 dB better than the 1.2m/Invacom system. While on the surface this is better than theory, there is a possible explanation. The f/D of the 1.2m reflector is 0.5, which is somewhat less than the 0.6 of the feed design employed. This would cause the 1.2m to be underscanned, leading to some loss of gain.
What is intriguing me is the Prodelin has a f/D of 0.6, but the feed it came with seems to be outperforming some other examples I've substituted. I'm perfectly willing to believe it is (1) a better feed, or more likely (2) is better matched to the reflector. I don't know where this will go, but I'm curious about trying some drastically different feeds on my prime-focus dishes to see if I can improve the match at Ku. But this is only of academic interest to me because the 1.8m Prodelin is so outstanding on Ku that it makes we wonder what benefit I could possibly gain by further horsing around.
with an 8 - 10 foot bud.
