FOR COMMUNICATIONS and sensing applications, engineers are increasingly trying to determine radio-wave propagation inside a forested environment in the high-frequency/very-high-frequency (HF/VHF) range. If scaled model measurements are made in the lab, designers can collect large quantities of data under well-controlled conditions. At the University of Texas, for example, Yang Li and Hao Ling measured an array of water-filled straws as a simplified, scaled model of a forest.
First, the researchers constructed a periodic array based on an approximate 1:50 scaling with respect to a typical forest. Each element of the array was a plastic straw sealed at the bottom and filled to the top with water. Next, they measured transmission loss inside this structure versus frequency and distance. The data was processed to extract the dominant propagation mechanisms in the structure. The engineers then compared the results of the full-wave numerical simulation for an array of infinite rods.
To provide physical interpretations for wave propagation inside the array, an effective-medium theory was applied to calculate the structure's permittivity and permeability. The passband/ stopband behaviors roughly agreed with the full-wave simulation for infinite rods. The positions and heights of the water rods were then perturbed to study randomization effects. Finally, the predictions from the dielectric-rod array were correlated with data from a real forest in Bastrop, TX. See "Investigation of Wave Propagation in a Dielectric Rod Array: Toward the Understanding of HF/VHF Propagation in a Forest," IEEE Transactions On Antennas And Propagation, Dec. 2010, p. 4025.