Transmitter Profile Detail data seem Strange when applied to a location

[Wroxter]

Hi,

Has anyone else noticed the profile data being strange at report's site.

For example note the very high Effective ERP for a station 77.5 miles away from my location in Ottawa.

Transmitter Profile Detail
WNPI-DT (Digital)
Channel: 23 (18.1)
Network: PBS
Maximum ERP: 40.000 kW
Coordinates: 44.491453 -74.857138

NOTE: The vertical dimensions in this profile have been exaggerated in order to make the terrain features more visible. This is merely an approximation based on the information available from the FCC database.
WNPI-DT
Effective ERP: 40.000 kW (Adjusted according to your location)
Distance: 77.5 miles Azimuth: 138 degrees Compass: 151 degrees


Another example (which tweaked my look at the above example) was comparing a friend's report of a closer (Canadian) station 15 miles farther away or more than double the distance from one of the station's I pick up. ERP data was different for the two locations and at the transmitter but there was only 5% difference between the two reports. There is not much elevation difference between the two sites which are both LOS.

W

[mtownsend]

I guess I'm not clear on what the question is asking.

Many TV stations use directional antennas. Even though some people might refer to a transmitter as a "1000 kW" source, it does not mean that it emits 1000 kW of power in all directions. The "1000 kW" is usually in reference to the max emitted power from the antenna in its strongest direction.

The "effective" power being broadcast in your direction depends on your location relative to the azimuthal (compass direction) and vertical location within the antenna's radiation pattern. Most ERP variations are dominated by the azimuthal change in antenna pattern because there is negligible change in the vertical direction when you are miles away from the transmitter (a few 10s of meters change in elevation makes almost no difference when your distance to the transmitter is 10,000 meters or more, which ends up being much less than 1 degree of angular change).

The azimuthal changes in antenna pattern can be far more noticeable. If the broadcaster is using an omni-directional antenna setup, the ERP will be nearly constant everywhere you go. However, if the broadcaster uses a highly directional antenna, the ERP in "off axis" directions can be significantly less than their peak ERP.

The ERP alone does not take into account terrain blockage. The propagation modeling takes the ERP as a starting point, and then factors in terrain effects (blockage, diffraction over ridge tops, Earth curvature, etc.). This is how the tools figure out the available signal strength at your home.

[Wroxter]

I guess the question wasn't clear as I did not understand what "Effective ERP .. (Adjusted according to your location)" meant,

Thanks for your explanation, it makes things clearer and explains why a very closestation can have lower ERP than one many times the distance even if they have the same maximum ERP.