Investigation of Wave Propagation in a Dielectric Rod Array: Toward the Understanding of HF/VHF Propagation in a Forest

Radiowave propagation inside a forested environment in the HF/VHF frequency ranges has long been of interest for communication and sensing. We report on the measurement of an array of water-filled straws as a simplified, scaled model of a forest. A periodic array is first constructed in the laboratory based on an approximate 1:50 scaling with respect to a typical forest. Transmission loss inside this structure versus frequency and distance is measured. The data are then processed to extract the dominant propagation mechanisms in the structure. The results are compared to the full-wave numerical simulation for an array of infinite rods. To provide physical interpretations for the wave propagation inside the array, an effective medium theory is also applied to calculate the effective permittivity and permeability of the structure. It is found that: (i) Passband/stopband behaviors exist in the measurement and approximately agree with the full-wave simulation for infinite rods. (ii) The water straw array can be described as a -negative (ENG) medium within a certain frequency band in the long wavelength limit. (iii) The extracted propagation constants from measurement, simulation and effective medium theory show good agreement. The positions and heights of the water rods are next perturbed to study the effect of randomization. Finally, the predictions from the dielectric rod array are correlated with the measurement data collected in a real forest in Bastrop, Texas.