At the satbroadcasts site there was recently published an article entitled, Minimum carrier to noise ratio values (CNR, C/N) for DVB-S2 system. This handy article and chart explains why, on any given dish, you might have no problems receiving certain signals without problems, but much more difficulty receiving others. It also explains why some people have successfully used a relatively small dish (4 to 6 feet in diameter) to receive certain C-band signals, but for other signals you might be out of luck if you can’t put up a full 10 foot (or even larger) dish.
Basically, there are two things that can make a difference with DVB-S2 signals – the modulation (QPSK, 8PSK, 16APSK or 32APSK) and the FEC (Forward Error Correction) code rate. A QPSK signal will be easier to receive than an 8PSK signal, which will in turn be easier to receive than a 16APSK signal, and so on, assuming the FECs are equal (by the way, PSK stands for Phase-Shift Keying). And as you can see from the chart at that site, a signal using a forward error correction code rate of 1/4 will be much easier to receive than a signal with a FEC of 9/10.
So, while you might get away with a 4 foot dish with a C-band LNB and a conical scaler ring when trying to receive a QPSK signal with a FEC code rate of 1/4 or 1/3, there’s virtually no chance that setup will work to pick up an 8PSK signal with 9/10 FEC. And if you want to receive a 32APSK signal with 9/10 FEC (if such a signal actually exists), you’d better have a really big dish, a super high end LNB, and the ability to aim it all precisely. I may exaggerate just a little here, but receiving any 16APSK or 32APSK signal with anything smaller than a 10 or 12 foot dish may not be possible, depending in part on the FEC used. In my experience, getting reliable reception on an 8PSK 9/10 FEC signal with anything smaller than a 10 foot dish is not possible, but maybe if you have a super LNB and are very patient in getting everything positioned and aimed just right, you might make it work (note I say might, and I certainly don’t recommend trying it unless you absolutely can’t install a larger dish for some reason).
If you’re new to the hobby of receiving free-to-air signals, you might not have been aware that all signals are not created equal, and may have been confused by the fact that your DVB-S2 receiver or tuner reports a high quality reading on some signals, but a low or non-existent quality reading on other signals from the same satellite. Now you know a couple of possible reasons why.