On 7/25/2024 5:01 PM, Brian Beezley wrote:
That's a good use of the FCC map, Jim. I had completely written it off
for ham use once I discovered that it applies only to the AM broadcast
band. Values will be quite different at HF. But ranking 1 MHz ground
conductivity by location should yield the same HF ranking.
I couldn't open the dropbox file -- it wanted me to sign in. But I do
have some background on this -- one of my summer jobs while in EE was
working for Pete Johnson in his consulting office using that map to plot
contours of patterns we'd computed for directional arrays he designed.
Those plots of curves of field strength vs distance on a log-log scale
are called a "nomograph," and my freshman EE curriculum included a
course in creating and using them.
This was in 1961-2. I strongly suspect that the map is at least a decade
older than that. And I do think I know how the data were obtained -- by
running radials (that is, measuring field strength along a radial line
extending from an AM broadcast transmitter at a large number of points
and matching graphs of attenuation vs. distance for measured data to
corresponding curves that are part of FCC AM Technical Regs.
I mention Pete Johnson, because he and Carl Smith wrote those technical
regs after WWII. Carl was better known as the proprietor of an
electronics and radio technical school in Cleveland.
And yes, the data are quite coarse -- but I was able to say that
conductivity on low HF is poor in that area, because everything around
it is low. :)
73, Jim K9YC
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|