"Richard Harrison" wrote in message
...
Walter Maxwell, W2DU wrote:
"Do you think any soil characteristics could be determined by such
data?"

Kraus has an interesting figure, No. 11-20 on page 305 of his 1950
edition of "Antennas". It is the feedpoint resistance versus height in
wavelengths over perfect ground (a copper sheet?) of a resonant 1/2-wave
dipole.

It varies from zero ohms at zero height to a maximum of about 100 ohms
at 0.35 wavelength above ground. The resistance settles down to just
above 70 ohms at infinite height (free space radiation resistance
value).

Clean dry sand may be a very good insulator. If it were deep enough, a
dipole lain on it might have a feedpoint of about 70 ohme. Better soil
conductivity might shift the drivepoint of the dipole to a lower
resistance as the Kraus figure indicates for perfect ground next to the
dipole.

Several measurements at slightly different locations and times may need
to be made and averaged for reliable results. The curve in Fig. 11-20
oscillates around the free space radiation resistance so that at some
altitudes feedpoint resistance goes down as altitude increases. One
would need to know which part of the curve the measured resistance fell
upon.

Walt may be on to something with his method for determining earth
constants. There are so many broadcast stations in the USA that soil
conductivity has been already determined in nearly all areas. For
unknown areas, one could lay out radial paths from existing stations and
measure feild strengths along the radial at several places and see how
much more attenuation there is versus the "unattenuated" values expected
and determine average ground conductivity by the loss added by the
ground.

Best regards, Richard Harrison, KB5WZI

Richard, your indication that the dipole input resistance of 100 ohms at 0.35
wavelength above ground is interesting, in that if you look at Kraus' graph of
mutual impedance of parallel side-by-side radiators in his Fig 10-12, Page 266,
you can see the reason for this.

At this height above ground the the dipole is spaced 0.7 wavelengths from its
image in the perfect ground plane. The mutual resistance at this spacing
is -24.8 ohms, as shown in Fig 10-12 and in Table 10-1 on Page 267. Table 10-1
also shows the self resistance minus the mutual resistance at this spacing to be
97.9 ohms. There is the approximate 100 ohms shown in the graph of Fig 11-20.
Note that 97.9 - 24.8 = 73.1 ohms, the nominal resistance of a thin half-wave
dipole in space.

Of course this data relates only to the condition of perfect ground, with total
reflection and no attenuation. I would hope that the delta R and delta X values
vs height taken from my measured data might shed some light on the ground
conductivity and permittivity under my antenna. My soil is very sandy. Knowing
that, the delta values might show some tendency to verify that condition.

Walt, W2DU