Experimental characterization of the forest as a medium for HF radio propagation

The forest is often modeled as an anisotropic homogeneous medium filling a slab above the lossy dielectric ground. As with a true dielectric material, a section of forest may be characterized by a complex permittivity or equivalently by a real relative permittivity and conductivity. Measurement of these properties in the forest has been problematic, with no existing technique able to appropriately sample a large enough section of forest. Methods such as extrapolation via a dielectric mixing formula [1], transmission-line-based sampling [2], and inference from an analytic model [3] have been tried. We have devised and performed a novel technique to directly sample a fairly large section of the forest and retain phase information, which allows us to extract a medium permittivity. Because of the primarily vertical orientation of tree trunks, the forest is highly anisotropic at HF and VHF frequencies, and our method is able to extract the forestpsilas permittivity for both horizontal and vertical polarization. Results of the measurements are surprising but may be understood by considering the forest as a composite medium comprised primarily of vertically-oriented scatterers.