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A commonly asked question by folks new to the VHF/UHF radio community
is "How far will will my signal go?". The direct "line of sight" range between two radio stations will depend on the antenna types, the height above the ground at each end, as well as transmit power. Because of the curvature of the earth and multipath (echoes and reflections), the higher the antenna are above ground level (AGL) the better. For instance, a 32 mile point to point link over "flat" terrain, the antennas should be over 130 feet or more AGL at each end to clear the curvature of the earth. For 20 miles, at least 67 feet; for 10 miles 17 feet. This, again, is for rural and "flat ground". In urban areas, the effective range can be less by as much as one half because of "echoes" and obstructions caused by buildings and other structures. Also, there is another important factor called the .6 Fresnel Zone. Light and radio waves spread with distance. The signal will widen with distance and the spread is wider at longer wavelengths (gets smaller as the frequency increases). When a portion of the .6 Fresnel Zone is obstructed or intercepts the ground, echos (reflections) can occur reducing the effectiveness of the signal. Another "depends" is space attenuation and wavelength. Radio signals loose about 22 dB of strength at one wavelength from the antenna and 6 dB with each doubling of the distance (inverse square law). If the wavelength of the signal is increased, the per mile attenuation (loss) of the signal willl be reduced and if the wavelength is decreased the loss will increase and the signal strength will decrease following the 6 dB rule. Example: at a wavelength of two meters (150 MHz) the loss for the first mile is about -80 dB, at 70 cm (450 MHz) it is approximately -90 dB and at 33 cm (900 MHz) around -96 dB. For a given effective radiated power (EIRP) and other equal factors, a stronger signal will be received at a distance of one mile on 150 MHz than 450 MHz. See "Rfpath loss" for more details. |