- Working out the range

While VHF and UHF communications are normally limited to visual line-of-sight distance between two transceivers with antennas mounted as high as possible to maximise range, the actual process of radiation is much more involved.

The direct wave can be bent and scattered so that stations not actually in line-of-sight contact can communicate successfully.

Signal radiated at low angles to ground is reflected towards space (Space Wave) and can combine with the direct ray to affect signal strength. The phase of the reflected wave is reversed on contact with the ground.

Where the distance in wavelengths travelled by the direct and space waves is the same or virtually the same, for example at low frequencies where wavelengths are long (below 20MHz), the signals cancel each other out to greater or lesser degrees depending on the degree of phase difference. The closer the lengths of the two paths (in wavelengths) the greater the degree of decrease in signal strength.

However, at higher frequencies above 50 MHz, wavelengths are shorter. This means that the difference (in wavelengths) between the paths travelled by the direct and the space wave is much greater, with the result that signal strength is increased.

In practice, as the phase shift on reflection is never exactly 180 degrees, the two waves will never cancel out completely, and some energy will always be lost as the wave is reflected from the ground. Ground reflection losses can be almost eliminated at UHF frequencies by using highly directive antennas so that almost all the signal is radiated in the extremely concentrated direct wave.

For simple VHF/UHF antennas, as used in a system of base to mobile (or portable) stations, it is possible to calculate the approximate range taking this into account. The formula is based on direct line-of-sight distance with the modification of increasing the Earth's radius by one third:

(Note: if there are any obstructions in the signal path these will need to be taken into account)

This gives the distance of the transmitting antenna to the horizon. Calculating similarly for the receiving antenna and adding the results will give the maximum possible range between two antennas.

In achieving maximuim possible range it is important to site the antennas above any obstructions (eg buildings, trees, high ground, etc.) and in hilly terrain to take into account the required communications paths. Hill tops give good allround visibility but if the communcations paths are restricted to one direction, below the crest is better.

For more precise range predictions, station gain and path loss need to be taken into account. Station gain assesses the overall system performance of the transmitter, receiver and antenna system and comprises the gain and height of both the transmitting and receiving antenns, transmitted power, receiver sensitivity and the required signal-to-noise ratio.

Nomograms are available for finding estimations of path loss, receiver sensitivity and determining antenna-height gain.