Q: What is ‘Radio Horizon’ and how is it calculated?

 With regard to VHF, UHF and Microwave Radio Frequency (RF) waves, radio horizon is a calculation of how far the RF waves can propagate given a clear path to the visual horizon. The higher the antenna, the further the radio horizon

 Although RF waves are generally thought to travel in straight lines, in fact they are subject to the refractive effects of atmospheric layers. What actually happens is that radio waves propagate a little beyond the visual horizon. For the frequencies that you will be using Wood & Douglas equipment on, it is accepted that the radio horizon is 4/3 the visual horizon, i.e. the radio waves can bend an extra third of the distance to the visual horizon, beyond it.

So, assuming no obstacles to the visual horizon and under normal weather conditions the radio horizon is calculated using the formula: -



Where d = distance in kilometres and h is the height of the antenna above ground in metres

Here are some worked examples for a shore based UHF antenna required for communication with a vessel out to sea.

1)     A shore antenna 10 metres high has a radio horizon of approximately 13km

2)     A shore antenna 50 metres high has a radio horizon of approximately 29km

3)     A shore antenna 100 metres high has a radio horizon of approximately 41km

4)     A shore antenna 250 metres high has a radio horizon of approximately 65km


In this shore to ship example we need to do the same radio horizon calculation for the vessel’s antenna and then add together the two results, shore radio horizon plus vessel radio horizon

For example, with a shore antenna 100 metres high and a vessel antenna 10 metres high, the expected range possible is 41km plus 13km, i.e.  54km

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