As discussed several posts above, the 41 dBu UHF signal contours, the 28 dB VHF-LO signal contours, and the 36 dBu VHF-Hi (channels 7-13) signal contours are just a general idea of what to expect, reception-wise, from a broadcast digital station as shown on the FCC signal maps; those are the minimum amounts of signal the FCC says you need to properly decode it. In actuality, as you all know, terrain and other factors contribute positively or negatively to the quality of the signal you get from a station.
When a station submits a broadcast coverage area map to the FCC for consideration, it uses a calculation known as the Longley-Rice method. This method bases the signal strength coverage of a station on the following factors:
For the broadcasting station:
1. Height of broadcast antenna
2. Power of the transmitter
3. Any "nulls" required by the transmitter to protect other stations
For the broadcast receiver:
4. Any terrain in the way between you and the transmitter
5. A "proper" receiving antenna up 30' in the air, without amplification
6. 50' of complete copper RG-6 cable from the antenna to the receiver
What is NOT considered in Longley-Rice maps:
1. Weather conditions, which can substantially extend or reduce the broadcast area at any given time
2. Interference from buildings, including multipath and signal blocking
3. Interference from electrical sources or anything else for that matter
4. Attic installations, which reduces signal strength by at least 50% in most cases
5. Antennas larger or smaller than "proper"
6. Signal amplifiers/preamplifiers placed on antenna systems
3 years ago, engineer Kyle Walker put Longley-Rice maps on his website, but didn't have the time to keep it up. I didn't notice them until just before he took them and his website down. But I now have them, and he has graciously allowed me to put them up for you to see. These are also reproducible to a very large degree using the SPLAT! software; do a Google search and you can find it.
A bit of a problem is that the maps are dated; most still read "construction permit" when in fact, all of the stations are at full power. Thankfully, with the exception of WBBM and WYIN, all of the maps are still valid, and WBBM is reasonably close.
HOW TO READ: Red is city-grade overage, easily receivable; green, yellow and orange are "Grade A" coverage; receivable with a proper outdoor antenna. Any shade of blue is marginal, meaning you may get it or you may not. And in those areas, tropo/weather/interference can very easily kill your signal. Notice how the Fox River valley really helps kill the signal along and west of it, especially in northwest Kane county.
Look for the call letters in caps. The other files are my setup, on top; and then several pix from a colleague of mine in DeKalb with his attic setup.
And if anyone has or wants to make updated versions of these, let me know and I'll gladly put them on the site. Thanks!