مرکز منطقه ای اطلاع رساني علوم و فناوري - Ultra-short-wave propagation in the jungle

Abstract :

The predominant characteristic of propagation in the VHF frequency range is the presence of a reflected wave which tends to cancel the direct wave and results in the received field being proportional to the product of the antenna heights, and inversely proportional to the square of the distance; and the radio gain being independent of frequency. For propagation in the jungle in the meter wavelength range (VHF) between dipole antennas, the radio gain can be expressed ${P_{r} \\over P_{t}} = \\Bigg( {3h_{1}h_{2} \\over 2d^{2}} \\Bigg)^{2} \\Bigg( {R_{0} \\over R} \\Bigg)^{2} F_{s}^{2} F_{j}^{2}$ where $R_{0}$ is the radiation resistance of the dipole antenna in free space and $R$ is its value as affected by the proximity of the ground and foliage, $F_{s}$ is the shadow factor that accounts for the curvature of the earth, and $F_{j}$ is the factor that accounts for the effect of the jungle. Measurements made in Thailand indicate that above about 10 MHz, $F_{j}$ is independent of polarization. $F_{j}$ shows no systematic variation with either distance or antenna heights, the average dependence being adequately taken care of by the first factor of the equation. No measurements of ( $R_{0}/R$ ) were reported except near the ground or foliage; it can be assumed to be unity with little error in this frequency range. $F_{j}$ is a function of frequency, varying from near unity (less than $\\pm 5$ dB) at frequencies below 10 MHz to about 0.1 (-20 dB) and 0.01 (-40 dB) at 100 MHz and 400 MHz, respectively. Since $F_{j}$ is independent of distance there is no exponential attenuation of ultrashort-waves in the jungle. In fact the effect of the jungle on the direct wave must be almost identical to that on the reflected wave in order not to disturb the almost complete cancellation.