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Topband: Fresh vs Salt Water Ground Conductivity

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Subject: Topband: Fresh vs Salt Water Ground Conductivity
From: (Bill Tippett)
Date: Tue, 16 Jul 2002 21:34:24 +0100
AG6K wrote:
>Howard -- Water has a dielectric constant of c. 79 whether it is 
salted or pure, so for RF purposes, a freshwater marsh conducts as well 
as a saltwater marsh. 

        Rich, you are mixing dielectric constant with conductivity.
Conductivity is the key parameter and there is a huge difference
between salt water and fresh water:  

Surface Type    Dielectric Constant     Conductivity (S/m)
Fresh Water             80                      0.001
Salt Water              81                      5.0

                                        73,  Bill  W4ZV


Figure R3 of 47 CFR 73.190 of the Commission's Rules contains a map of the
estimated effective ground conductivity in the United States. This data is
used to predict the propagation of AM signals across the United States. A
higher ground conductivity indicates better AM propagation characteristics.
The maps below show that the ground conductivity in the U.S. ranges between
0.5 and 30 millimhos per meter. The conductivity of seawater is 5,000
millimhos per meter, resulting in the best propagation of AM signals.
(5,000 millimhos per
meter is identical to 5.0 Siemens per W4ZV).

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