Three Techniques for Measurement of Ground Constants in the Presence of Vegetation

Foliage degrades the usefulness of conventional wa"e tilt and propagation loss techniques for estimating in situ the RF ground constants in forested regions, therefore, an investigation was made of three other techniques that should provide more accurate input data for modeling, mathematically, radi'.o antenna performance in forests. Two of these techniques are RF methods providing complementary information: the open-wire transmission-line probe method gives independent measurements of the near-surface conductivity and the dielectric constant; the dipolc feed-resistance (measured an a function of height) method allows a coarse estimate of the effective conductivity to the skin depth. The latter technique, however, requires theoretical refinement before it can be applied singly. At present there is no mathematical basis for evaluating numerically the dipolo .feed-resistance over a stratified earth. The third technique, the dc geophysical resistivity method, yields a measure of the earth conductivity stratification beneath the vegetation. The dc resistivity method is supplementary to the RF "nethods, but its usefulness may fco limited to estimation of stratification since the accuracy with which conductivity can be extrapolated from dc to RF is not known. Available literature indicates that the conductivity will be highly dependent on physical-chemical properties (especially moisture content) of the soil and near-surface rocks. This special technical report describes the three techniques, giving the theory, instrumentation, and methods of interpretation for each. Results of the tests conducted during this investigation are analyzed briefly.