Chicago O'Hare International Airport
From The RadioReference Wiki
Owners of these fine radios have probably discovered by now that they require some "extra" programming information when setting up systems. In the past, in addition to the system frequencies and talk groups you needed a "base frequency", an "offset" and a "spacing".
On the new radios this has evolved into;
Lower Base Frequency Upper Base Frequency Offset Spacing Polarity
Uniden came out with a neat little spreadsheet which calculates the Upper Base Frequency for us if we know the lower, offset and spacing. These are generally listed in the profiles and on radio reference. For Polarity, all known systems use Positive "+". I'll upload a copy of the Spreadsheet to our files folder here. It works correctly, though I can't get their published formula for calculating upper base frequency to work, and I do hate to have rely on spreadsheet stuff when I'm out in the field. I'll work on that, too.
But I encountered another problem which I wanted to bring to your attention. It had me rather befuddled for months trying to program in a couple of local UHF Motorola TRS's...
You have to use the correct mode (duh). The default settings on these radios when configuring a new UHF Motorola TRS is to use the "Auto" modulation setting. Beware! This will configure your entries to use narrow FM on the actual system frequencies. Unless the system has been narrowbanded your new system will not track talk group activity on P25 digital channels.
If you are using narrowband on a standard bandwidth P25 Motorola UHF TRS you will hear the control channel - but rather weakly. The radio will (sometimes) even produce the system ID. If you sit on the control channel you may even see (very rarely) talk group activity flashing on screen. And this is even on full quieting strongly received systems. But so to ID scan or search and you'll hear nothing.
If this is what's been happening to you go back and reprogram the system(s) as regular FM mode. Voila. Utterly reliable tracking and P25 demodulation on even very weak systems. Problem solved.