Re: NEC-LIST: Unusual modeling problem

Alan Nott hit upon an important point in his post:
>
> Soil parameters vary considerably with frequency as well as moisture
> content (recent rain etc), and the 'accuracy' of the conductivity
> figures represents a bit of wishful thinking!!! {snip} If the
> ground has been significantly disturbed, as I suspect is so in your
> case, soil compaction will also change the parameters.

Dielectric constant is a terrible term to use as it is rarely ever
constant with real materials. The complex permittivity is generally a
significant function of frequency (and with plastics, temperature
too).

With materials like moist soil and biological samples, the
conductivity at low frequencies is dominated by ionic conduction. As
the frequency is increased, the inertia of the ions prevents them from
moving quickly enough to provide high conductivity. A crude "rule of
thumb" is that conductivity drops off as the reciprocal of frequency
in this region. Usually by a few hundred kiloHertz to a few
MegaHertz, ionic conductivty ceases to be a significant factor.

For soils, possible references are the tables in:

    A. R. von Hippel, "Dielectric Materials and
    Applications," MIT Press, Cambridge, MA, 1954.

Data for several soil types are presented as a function of frequency
(into the microwave region) and as a function of moisture content.

    73, Barry L. Ornitz WA4VZQ ornitz_at_tricon.net
Received on Thu May 25 2000 - 11:59:32 EDT