RE: NEC-LIST:earth conductivity measurements at VLF

Grant:

Here are several comments on the measurement of ground constants at VLF.

1. For all but the most highly conducting media, the skin depth at VLF is so
great that small probes at the earth's surface give misleading data. They give
you a number all right, but it is only applicable to the volume being sensed.
And that volume is concentrated VERY near the surface in terms of the skin
depth. The volume of soil beneath the antenna that counts is the area on the
surface, which depends on the size of the groundplane, multiplied by the skin
depth at the frequency of interest.

2. There are some meters, based on coplanar loops, that work at 10 kHz. These
meters measure the mutual coupling between the loops. This method is described
in the recently developed IEEE Std 356, a standard effort led by David Thiel
of Australia. I am a coauthor of the standard. The draft of this standard can
be purchased from IEEE (See the IEEE website, http://www.ieee.org, and click
on Standards). A Canadian company used to sell a meter based on this principal
and I used one at Ace Lake, near Fairbanks, on a job for the University of
Alaska Geophysical Institute who was under contract to the FCC (Dr. John Wang)
studying long range propagation in the AM broadcast band. That meter was very
good for delimiting the permafrost at the UA/GI Ace Lake site. The surface
soil was melted since it was summer, so you couldnt tell the permafrost
location without either digging or using a remote sensing device.

I hope this helps.

Sincerely,

George Hagn

P.S. If you have any data on the soil moisture profile, I have a program that
I developed to use the volumetric soil moisture to soil conductivity in S/m
and relative permittivity (the real part of the complex conductivity is what
the texts call the dielectric constant, or the relative dielectric constant).
I would be glad to run my model for you if you give me some soil moisture
values to use as input data. I am readying this model for market from Hagn
Associates Ltd., a small company I started last spring to sell various niche
market items (such as ham antennas for transmit and receive that can be
situated in an attic, to mitigate complaints from condo associations,
neighborhood associations and city and county zoning authorities).

P.P.S. I also developed a ground constants measurement kit during the war in
Vietnam, which I called the open-wire line (OWL) kit. That kit is useful for
measuring the surface soil values (down to a skin depth) of conductivity and
relative permittivity (AKA relative dielectric constant) at HF and VHF. Those
were the frequencies of interest when we were modeling jungle propagation and
the effects of jungle on antennas back in the 1960s. I now sell the OWL Kit.
If you have any interest in HF or VHF let me know.

Original Message:
-----------------
From: Grant Bingeman DrBingo_at_compuserve.com
Date: Tue, 4 Sep 2001 12:47:30 -0400
To: nec-list_at_ee.ubc.ca
Subject: NEC-LIST:earth conductivity measurements at VLF

The FCC has a set of curves covering 530 to 1700 kHz that simplify the
determination of effective ground conductivity near a transmitting site.
This assumes you have an operating antenna, RF source, field intensity
meter and good maps and/or GPS receiver.

This indirect method does not work so well at VLF frequencies such as 30
kHz, since the losses in the antenna cloud the losses in the earth, even
with a very large antenna and ground system.

Is there a reliable method for determning VLF ground conductivity? The
devices made for 60 Hz AC power systems don't work above about 200 Hz. I
have investigated a number of these "megger" type boxes, and can't get a
custom version made for 20 or 30 kHz.

Grant Bingeman

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