Propagation Over "Line-of-Sight" Radio PathsAuthor: R.J.Edwards G4FGQ © 5th September 2001
This program covers just the essentials. Refraction in the Earth's atmosphere, ground reflections and antenna radiating efficiency are not taken into account.
Antenna gains are relative to a hypothetical isotrope. Relative gain in the preferred direction of a 1/2-wave dipole is 2.1 dBi. A short dipole 1.8 dBi. Gain of an array of N dipoles = 10*Log(N)+2.1 dBi. Gain of a circular parabolic dish = 20*Log(N*Pi) dBi, N = diameter in wavelengths. A rectangular aperture of N*M wavelengths has gain = 10*Log(4*N*M) dBi. The gain of a short vertical above a perfect ground along the horizontal is 4.771 dBi. Note: Horizontal radiation of a ground-mounted vertical is groundwave. Over sea water and good ground the extra loss at MF is quite small and the program can be used to estimate ground path loss at distances up to the radio horizon of 50/Cuberoot(MHz) miles. As a crude approximation, estimate signal strengths via ionosphere reflections by adding 6dB to the line-of-sight data for each up-down hop.
Equivalent antenna apertures and beam-widths are included in the results. They allow better understanding of the performance of narrow-beam antennas at UHF. But beam-widths are meaningful only at antenna gains more than about 10 dBi.
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