An FCC map of ground conductivity in the lower 48 can be downloaded at
this link.
https://www.fcc.gov/media/radio/m3-ground-conductivity-map
On this page is a link to a 6.8 MB zip file which contains a grid of
8.5-in x 14-in pages that are sections of the full map. The contours
are fairly rough, with a factor of 2:1. This map is fairly old -- it
seems quite close to the one I worked with in Pete Johnson's consulting
office in 1961. My guess is that it does not show the poorer ground
conductivity of developed population centers described in the soil
characteristics for NEC.
N6LF shows a procedure for measuring soil characteristics on his
website, and it's been one of those things that W6GJB and have had on
our list of things to do for several years.
73, Jim K9YC
On Sun,2/7/2016 5:16 AM, jimlux wrote:
On 2/6/16 9:47 PM, Jim Brown wrote:
On Sat,2/6/2016 4:22 PM, jimlux wrote:
Using N6LF's formulas I get skin depths around 5-10 meters at 2MHz
(0.005 S/m, epsilonr = 3-10)
You might get 60 feet (20 meters) if the soil is particularly
non-conductive (rock). Dropping the conductivity to 0.5 mS/m gives
you skin depths of 22-34 m at 2MHz
Rock and sand are quite prevelant throughout the west, and pretty much
what we have in the mountains. :)
yes, but.. It's a pretty rare ham shack that sits on a granite
outcropping.
And most rock is actually ok from a conductivity standpoint.
http://emgeo.sdsu.edu/emrockprop.html
sedimentary rocks like shales have pretty high conductvity (100 mS/m).
To get down to 0.5, you're looking dolomite, limestone.
The usual granitic sort of stuff also seems to be reasonably
conductive, unless you're looking at shield, unweathered rock.
There's also a frequency dependence.. so looking up a table for low
frequencies (line frequency, etc) might not be a good baseline.
73, Jim K9YC
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