If you're referring to the notations next to station listings in a TVFool report, the overwhelming majority of co- and adjacent-channel warnings can be safely ignored. The reports list them even if there's no chance that the weaker station of each pair is receivable -- i.e., below a noise margin (NM) of maybe -10
dB or so.
Looking first at co-channel interference, let's say you have a local channel with a 50+
dB NM, and another station a hundred miles away broadcasting on the same channel, but with a projected NM of -20
db. The 70+
dB difference means the local station's signal is stronger than the more distant station by at least seven orders of magnitude --
10 million times. That's an extreme example, perhaps, but a lot of these station pairs have signal-strength differentials in the five- and six-orders range.
Adjacent-channel assignments used to be a big deal in analog broadcasting, particularly three decades ago when UHF tuners were a lot less precise than they are now. In addition, digital signals are less prone to cause interference to broadcasts on adjacent channels because of their differing nature. (Calling Piggie: Got any screenshots comparing analog and digital signal analyzer traces? They'd be educational about now!) Where interference might be a possibility, the
FCC typically orders one of the stations (usually the low-power outlet) to install an additional filter at the transmitter to ensure it doesn't happen.
FCC regulations also sharply reduce the incidence of adjacent-channel interference by specifying how much distance is allowable between two such transmitters. The regs are involved, but to oversimplify, the two stations' antennas may be located at the same place, or within maybe 10-12 miles of each other. If that's not possible, they must be separated by much greater distances -- in most cases, 68+ miles (for UHF) and well over 100 miles (for VHF).
What are Co-channel and Ajacent channel warnings?
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Linear Mode
