From: WarmSpgs@aol.com
Message-ID: <82.7633112.27cbf6b4@aol.com>
Date: Mon, 26 Feb 2001 13:13:08 EST
Subject: Re: LF: Antenna measurements/losses/insulators
To: rsgb_lf_group@blacksheep.org
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII; format=flowed
Content-Transfer-Encoding: 8bit
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
Sender: <majordom@post.thorcom.com>

In a message dated 2/26/01 10:28:03 AM Eastern Standard Time, 
j.r.moritz@herts.ac.uk writes:

<< ...my favorite theory for 
 the major cause of loss resistance at the moment is that it is 
 caused mainly by dielectric losses in the ground, where the electric 
 field of the antenna penetrates to some depth at LF. This is 
 contrary to the conventional view that the major losses are due to 
 the resistance of the ground system. I don't think there is really a 
 contradiction, just that amateur antennas have relatively high 
 dielectric losses because they are smaller than conventional LF 
 antennas. A bit of thought shows that a predominance of dielectric 
 loss would explain lower loss resistance at higher frequency, and 
 G3AQC's "footprint" effects, among other things. >>

For what it's worth, Jim's theory is entirely consistent with (pardon the 
expression) commercial practice and theory.  I agree that there is no real 
contradiction.  What we as amateurs lump together under the term "ground 
loss" are actually a number of factors, and at LF the dielectric properties 
of the ground certainly are a major component; perhaps _the_ major component 
at 136kHz.

The soil conductivity charts we use to predict groundwave coverage at MF over 
North America differ greatly from the ones used at LF.  Skin depth is a major 
reason usually cited for this discrepancy, along with different assumptions 
about dielectric constant.  The interaction between the ground system and 
losses in the earth extend to a greater depth, where subsoil and rock 
properties may diverge widely from those near the surface.  Thus, not only is 
the apparent conductivity different for MF and LF at any given location, but 
the shapes of the conductivity contours don't bear much resemblance to each 
other, either.

All this appears consistent with what amateurs are finding with practical 
antenna systems at LF.

73,
John