Topband: Topband ground characteristics

[Jim Brown]

On 10/3/2012 9:08 AM, VE7MID wrote:
> I have not been able to locate these FCC curves (and it appears that they
> may have been withdrawn).

They have been part of the technical section of the FCC Rules that cover 
AM broadcasting. There is a family of curves for every 40 kHz or so of 
the AM broadcast band, each curve plotting the drop-off in field 
strength vs distance for the range of ground conductivities on the FCC 
ground conductivity maps.

Last I looked, the Rules require that the map values be used, except 
that an applicant may, at his option, make a prescribed number of 
measurements along one or more radials from an existing station to 
obtain more detailed data, and use that data in place of the FCC soil 
data.  The guy I was working for (Pete Johnson, who along with Carl 
Smith wrote the FCC technical Rules for AM broadcast after WWII), had a 
station on 680 kHz in Charleston, WV, and was trying to bump up his 
daytime power.  The limitation was that he could not cause interference 
to second adjacent WLW's contours (about 200 miles away).  He ran radial 
measurements over that distance to support his application, showing that 
WLW's field strength fell off more than predicted by the FCC ground 
conductivity data.

73, Jim K9YC