Physical modeling of em wave propagation over the earth

To obtain experimental data for correlation with the theoretical results obtained from mathematical ground wave propagation models, it is often expedient to conduct experiments on laboratory models using microwaves. Besides the obvious convenience and reduced cost, practical difficulties encountered in full-scale experiments are avoided (e.g., weather and other uncontrolled parameters such as path nonhomogeneities, earth curvature, buildings, right-of-ways, etc). There is no need to use exact similitude scaling; the media and geometries are generally chosen to test the limits of validity of the theory. The designs of several such models in the frequency range of 4-5 GHz are discussed, and typical experimental results are compared with the corresponding theory. Specific examples are propagation in the earth-ionosphere waveguide and propagation over planar and curved surfaces (including stratified media, for example, layered earth and sea ice), mixed paths (including media having abrupt discontinuities such as land-sea boundaries and gradual nonuniformities such as sloping beaches), uniaxially anisotropic media, and media having an index of refraction near unity (e.g., heavy vegetation). When using such models, instrumentation systems and field probing techniques are key factors in their success. These are also discussed.