posted 03-09-2008 08:57 PM ET (US)     

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[Provided a link to a webpage containing a calculator for estimating the radio range between two stations given the height of their antennas above ground]

http://www.qsl.net/kd4sai/distance.html

posted 03-10-2008 08:41 AM ET (US)            

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From my article on the topic of the range of VHF Marine Band radio communication:

d = 1.42 X h^0.5 where d is in miles and h is in feet

This gives the exact answer. The calculator mentioned above seems to round down to the nearest mile.

posted 03-10-2008 04:23 PM ET (US)            

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Jim - the distance considering two antennas would be the sum of the distances for each antenna as calculated with the formula you give.

But - rekindling an earlier calculation I made when this subject came up a long time ago - from just trigonometry and using a mean earth diameter of 7926.584 miles (12756.6 km) gives a constant of 1.501 instead of the 1.42 - so there must be some refraction - or something else as I don't know squat about radio transmissions. -- Jerry/Idaho

posted 03-10-2008 07:23 PM ET (US)            

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There is a good explanation of the visual horizon versus the radio horizon at

http://en.wikipedia.org/wiki/Radio_horizon

Radio waves will travel farther than light waves due to some refraction. This makes the radio horizon longer than the optical horizon.

posted 03-12-2008 09:10 PM ET (US)            

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Hey everyone - I slipped a cog when I gave the 1.501 mentioned above - it should have been the square root of the 1.501 or 1.225 - so if anyone was trying to duplicate the number - I made a mistake. Sorry if I misled anyone.

Regardless - in all probability, operators of two boats communicating will be within their optical line of sight. -------- Jerry/Idaho

posted 03-12-2008 10:58 PM ET (US)            

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The radio range calculations are based on a perfect globe and no terrain. The presence of any terrain in the path will affect the range, generally tending to reduce it although in some unusual circumstances intervening terrain can enhance radio range due to scattering of the radio waves.

Also, the calculated path lengths are based on good minimum signal levels, but it is often seen in marine installations with outboard motors that there can be a significant radio noise floor created by the motor. Signals which arrive below the radio noise floor will be difficult to detect and use for voice communications, so this will also have an effect on range.

I agree with Jerry's summary: anytime two vessels are in sight of each other they should be able to easily communicate via their VHF Marine Band radio sets, providing each vessel has a good installation with properly working transmitter, receiver, and antenna. However, in my experience with other recreational vessels it is quite common for there to be significantly degraded elements in the all-too-common installation, producing a situation in which a vessel has very poor radio range and can barely communicate a mile away.