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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 |
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