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Re: [TowerTalk] TowerTalk Digest, Vol 158, Issue 20

To: towertalk@contesting.com
Subject: Re: [TowerTalk] TowerTalk Digest, Vol 158, Issue 20
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Tue, 9 Feb 2016 10:01:39 -0800
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On Tue,2/9/2016 9:41 AM, Donald Chester wrote:
Those maps, based on random samples, are virtually useless for predicting the 
soil conductivity at any specific site location.

How do you know that, Don? I worked in Pete Johnson's consulting office in the '60s.Pete, along with Carl Smith (of Cleveland Radio and Electronics Institute) wrote the FCC's technical rules for AM broadcasting after WWII.

I learned that those contour maps were developed by running field strength measurements of MANY radials on many broadcast stations, plotting the results on a log-log scale of field strength vs distance, and comparing the slope of those curves with theoretical curves that are part of the FCC technical regulations. These plots clearly show the soil conductivity in the region that the radial crosses. One must, of course, avoid taking measurements at points where nearby conductors (like power lines, communications towers, water tanks, etc.) are likely to skew the result.

This technique can still be used today if you have a suitable, calibrated field strength meter for the AM broadcast band. Search on Potomac Instruments.

73, Jim K9YC
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