I read up TVFool's own explanations, but can someone explain:
1. how TVFool derives the NM figures, so that might further explain what exactly NM means...
2. how TVFool derives the Pwr figures, what the relationship is to ERP, and why it is expressed as negative values (higher power is a smaller negative value)...
TVFool uses the Longley-Rice Irregular Terrain Model (ITM) developed by the U.S. Govt,
which is widely embedded into a variety of free and commercial VHF/UHF Propagation
Prediction Programs (PPP).
ITM uses physical calculations for line-of-sight, diffraction, refraction and troposcatter
path loss, based on empirically derived statistical models, which in turn are derived
from a very large database of many decades of actual on-air signal level measurements.
On-line TVFool uses hi-rez SRTM (Shuttle Radar Topography Mission) terrain database
to calculate very detailed signal level estimates along the path to your location.
The torrent downloadable signal level maps are medium resolution, whereas a very
low resolution is used for TVFool's on-line signal level maps. To reduce calculations,
signal level maps use fairly large (e.g. 1km x 1 km or larger) "calculation blocks".
Transmit ERP (kW) is the amount of Effective Radiated Power towards the azimuth
with the HIGHEST transmit antenna gain. When available, TVFool uses the reported
antenna pattern to calculate the ERP towards YOUR location....otherwise it assumes
an omni-directional pattern.
The Receive Pwr(dBm) level is a statistically derived estimate of the signal level delivered
by a 0 dB gain antenna....hence it is a large negative number, since signal power levels
are on the order of femtowatts (dBf) rather than milliwatts (dBm):
dBm = 10*log(power/1mW) so 0.001 mW = -30 dBm.
ATSC requires more than about 15 dB SNR and Thermal Noise level is about -106 dBm.
Hence the PERFECT, THEORETICAL sensitivity is about -91 dBm. The actual sensitivity
is found by SUBTRACTING the Antenna Gain (Ga) and ADDING the overall System
Noise Factor (NFs). Thus, for a 10 dB Gain antenna and 3 dB NFs with Preamp,
actual sensitivity is -98 dBm. [Compare to -83 dBm CECB spec without Preamp.]
NM (Noise Margin) after ADDING Antenna Gain and SUBTRACTING System NF,
is an estimate of the amount of EXTRA signal level over and above the sensitivity level.
For more reliable reception, NM should be in the range of 10+ dB.
[I also suspect that TVFool did not include Land Use Clutter Loss per FCC OET-72.]
Reception of signals with negative NM are possible if your location is "better than most"
and/or extraordinary conditions are in effect, such as tropo-ducts over and near water.
However, bear in mind that ITM is a STATISTICAL model guesstimating the signal
level that will provide "acceptable" reception 90% of the time for 50% of user
locations (in "similar" propagation conditions...I did say it was a statistical model).
Andy has additional explanations in the TVFool thread on AVS Forum.