Hi Mikie and all,
Never mind the radio theory, you can study that after I have given you a SIMPLE answer. St. Lou like any urban area most likely uses repeaters and it just could be you're listening to the input frequency rather than the output. Just as likely is the possibility the repeaters employ "satellite" receivers and a "voter" to feed the repeater with the best signal heard on one out of all the remote receivers. A receiver in the immediate neighborhood will pick up the mobile loud and clear while you being across town miles away won't hear it at all.
Unless it's some oddball semi-duplex system, by listening on the output you hear everything they hear. My local FD uses one of those, not all of the mobiles go through the repeater, some transmit on it's output frequency. If this weren't such a small town I wouldn't hear those particular units but thankfully they're never more than a mile and a half away and I have an excellent antenna system.
Hopefully you have or can get the callsigns of the systems in question and run the licenses through the FCC database at
http://wireless2.fcc.gov/UlsApp/UlsSearch/searchAdvanced.jsp which will tell you both the input and output frequencies and which is which. That is the complete skinny, however the RR database may be simpler to use since in most cases only lists the outputs, the frequencies you want to listen to plus the squelch codes.
Now what was I saying about a simple answer? (;->) Well, just listen to the output frequencies, there's your simple answer.
