>From: km1h@juno.com (km1h @ juno.com)
To: <topband@contesting.com>
>Date: Thu, 26 Mar 1998 10:04:55 EST
>
>>You could install a low dipole, and measure the impedance of the
>>dipole as yopu raise and lower it and calculate conductivity that
>>way. That's what I did when I measured my elevated radial system. My
>>actual soil conductivity was 4 mS/m, while the ARRL Handbook method
>>indicated 0.5 mS/m. That's because the ground surface was mixed with
>>rock and dry, while five feet down and lower there was wet clay and
>>rock.
>>
>>
>>73, Tom W8JI
>>w8ji.tom@MCIONE.com
>
>Tom that agrees right on the nose with the ARRL Antenna Manual ( 17th
>Edition here) ground conductivity chart. Most of GA is rated at 4 mS/m
>on the chart wheras my QTH is listed as 1 mS/m.
>
>Would you agree that chart is at least a good starting place to begin
>ground system planning?
>
>It is interesting to note how good the soils in AZ, NM and TX actually
>are portrayed in the Manual. And all those guys are complaining!
>
>73 Carl KM1H
>
Yeah, I get a chuckle when I hear the locals grousing about how
poor the dry desert ground is for RF. I'm quite happy with this
earth compared to my 3 milliSiemens northern Maryland boyhood
home for RF ground. Even the crummy 8 mS stuff up around Phoenix
is better than that.
But bear in mind that those charts are _very_ low resolution
(spatial resolution) averages. The local conductivity variations
can be and frequently are quite large and can occur on a very
small scale relative to those charts. For example, here in the
Tucson area the chart shows us at about 15 mS. And on the
relatively flat regions between mountains where we are on
alluvial soil, this is pretty much correct as an _average_. But
there are significant small (spatial) scale variations even in
the flat regions and in the mountainous areas over hard rock, the
RF ground can be _very_ poor. When it is very good, they put a
copper or silver mine there and try to restore it to poor.
No matter where you are located, it is a good idea to do a
measurement of your site ground constants. Particularly if you
plan on modeling how antennas will work there. Or if you want to
know if it is _really_ necessary to put in that expensive,
difficult, inconvenient ground screen.
73, Eric N7CL
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