A field assessment of HF/VHF wire antenna impedance changes in rain forests

This work addresses the effects of vegetation coupling to HF/VHF wire antennas used in dense woods. A preliminary theoretical evaluation was carried out for wire dipoles, in which the electrical parameters of a thick forest were considered in the antenna resistance and reactance equations, instead of the vacuum properties. Then, impedance measurements of a few wire antennas for tactical radio links were performed in a vegetated site of Manaus, in the Brazilian Amazon. Comparing the open area to the forest results, moderate frequency shifts to the left were observed in the latter, as expected from the predictions.