Cavell Mertz FM Coverage Maps

[WOHO]

Using the Cavell-Mertz FCC Info links to get to your local FM station or translator and there's a link to give you a nice 2D coverage map of your FM station signal. How accurate are these maps? They seem to be extremely overly-generous in coverage, and in my experience you would be lucky to get the station at the location in blue in mono, yet alone FM RDS or HD?
How are these coverage maps generated?
I didn't see one like it for the AM's, just the FM and translators, and LPFM.
Likewise, on the LPFM plots, they must be assuming that they have a fully efficient 100 watts output, and
that their antenna is actually 95 feet in the air and not 20 foot off the ground on a pipe?

Anyone that can shed some light on how much to deduct to get true FM coverage?
(or another site that is more accurate for real-life station coverage areas?)

[cckadlec]

I can't speak too much on the engineering side of things and how they make all this, but I know a lot of work goes into it when such a firm does it for the FCC when requesting upgrades, changes, new stations, etc. and naturally it does not take tropo into account, and everyone knows that. But usually the coverage maps are far less than the actual coverage. I can't remember seeing many, if any, where coverage areas extend BEYOND what is normally heard aside from if there is an interfering station somewhere that takes the coverage area down in areas. If you take a look at the submitted coverage maps done by engineers to apply to the FCC for licensing changes, they're pretty damn detailed. But those are independently-produced and they won't match to what you get on a site like FCC Data or Radio-Locator, where I'm really not sure how they gather that data to display the maps. Little mystery to me too! I feel there is some programming in there, like it connects to software that generates maps based on the power output for each set of compass/degree values, which you can see in engineering details for most stations. On FCC Data, you need to wait a little while before the full map is displayed (with anything more than just city grade coverage shown), which is why I assume it's fetching data to make the map as opposed to it just finding a map on a server, which would be almost instantly-downloaded otherwise.

AM coverage maps are made by other sources and the signals naturally behave differently than AMs and have night, critical hours, day, etc. coverage areas that become more complex. I would advise using this site for decent coverage maps for AM.

I'll leave the "how do they do that?" sorta question for those with more experience in these things as I deal with propagation, not engineering-related stuff.

[MWmetalhead]

The Cavell Mertz maps at fccinfo.com simply use the FCC projected contour plots. These plots completely fail to take into consideration terrain conditions. In other words, they do not use Longley-Rice modeling. So, in flat land regions where broadcasters are using the facilities for which they are actually licensed, they are fairly accurate. In mountainous and very hilly areas, they are pretty much useless.

I want to say the CRTC once offered free plots of all North American FM stations using Longley-Rice, but I believe that database no longer exists.

Rabbitears.info offers Longley-Rice plots of all TV stations in the United States (and perhaps other regions, too?) but does not offer such maps for FM, as far as I can tell.

[Ben Zonia]

It would be more accurate to say that the F(50,50) prediction is more like a truncated version of more sophisticated models, in the absence of readily available and powerful computers and using calculations for the whole exact signal path at specific point to point distances. The FCC tried over the years to add more factors and terms, like the Terrain Roughness Factor, to predict signals, but pressure from NAB type organizations caused the FCC restrict their use. Concern about "move ins" and "squeeze ins" and the overzealous non engineer attorneys using taboos and rules like First Local Service and the Rural Radio Initiative that limit some logical moves.
"
Hopefully the Cavell Mertz and FCC sites will stay up this weekend, as the REC fccdata.org site will be down. The Cavell Mertz site has more historical engineering data in the lost data era between 1980 and 2005, when office moves and overzealous cleaning out of files occurred. Some of the FCC engineers did manage to dumpster dive some, but not all, of the FCC historical data.

[MWmetalhead]

Have any Buzzboard lurkers tried this?

https://www.qsl.net/kd2bd/splat.html

[Kennelly Heaviside]

And the Earth isn't spherical either. At first approximation it is an ellipsoid, but a highly irregular one.

Will any of these plot out F(50,50) and F(50,10) contours for reference and comparison? I guess Nautel also has a site that is something like this.

[ftballfan]

Have any Buzzboard lurkers tried this?

https://www.qsl.net/kd2bd/splat.html

There aren't any instructions on how to use it!