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post #1 of 13 Old 06-19-2012 Thread Starter
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radio question

I just bought a second boat . Oh yeah , I'm a fancy two boat having man .
( just had to brag about that ) Anyway , this boat has a 25 watt VHF radio in it . My other boat has a 5 watt radio . Also on this boat with the 25 watt radio the antenna is at the top of the mast instead of on the back of the boom gallows . I can way better hear & understand people on the 25 radio . My question is what should I expect for improvement in regards to range on the 25 watt radio ???
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post #2 of 13 Old 06-19-2012
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Re: radio question

VHF is radio line of sight, so most of the receiving range difference is the height of the antenna. Receiver sensativity will also affect receiving range.
25 watts of transmit power will travel farther than 5 watts of power, plus the range difference of the higher antenna. To calculate the ranges you need to plot the antenna heights on a 4/3 arc to see if your signal clears the horizon. If you transmit over land, included the tree height at the horizon as they will attenuate the signal.
I have forgotten the formula for transmit signal attenuation at sea level, but it is pretty great so that it is hard to get beyond the horizon. Atmospherics can produce some large deviations though, but dependable communications will be limited to radio line of sight.
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post #3 of 13 Old 06-19-2012
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Re: radio question

Ball park we get on average 10nm+ with 25 w on a short antenna 25' above the water.
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post #4 of 13 Old 06-19-2012
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Re: radio question

Ok,
Go here: http://search.yahoo.com/r/_ylt=A0oGd.../distance.html
and plug in your numbers.

Last edited by ccriders; 06-19-2012 at 12:03 PM.
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post #5 of 13 Old 06-19-2012 Thread Starter
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Re: radio question

thanks guys
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post #6 of 13 Old 06-20-2012
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Re: radio question

Quote:
Originally Posted by kjango View Post
I just bought a second boat . Oh yeah , I'm a fancy two boat having man .
( just had to brag about that ) Anyway , this boat has a 25 watt VHF radio in it . My other boat has a 5 watt radio . Also on this boat with the 25 watt radio the antenna is at the top of the mast instead of on the back of the boom gallows . I can way better hear & understand people on the 25 radio . My question is what should I expect for improvement in regards to range on the 25 watt radio ???
D (Distance to the Horizon in miles) = 1.42 x (H--Antenna Height)^.5. EG, for a 50' antenna, the theoretical line of sight range would be 1.42 x 7.07 = 10.04 miles. One must, however, add the height of the receiving antenna. If the receiving antenna is also at 50', the range is increased to 2 x 10.04 = 20.08 miles. There is also some "bending" in the atmosphere that increases the range slightly (some say a coefficient of 1.47 is somewhat more accurate taking into account the "bending" although whether that is correct or not is unknown).

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post #7 of 13 Old 06-20-2012
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Re: radio question

You should be able to make most calls on 5 watts. As stated, range is usually limited by the curvature of the earth, not the power of the radio. In emergencies, go ahead and blast away on 25 watts.
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post #8 of 13 Old 06-20-2012
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Re: radio question

Since your old antenna is already somewhat elevated, i would suggest that you are getting more clarity with the new system mostly because it is better quality or better connections, at least for calls within 5 miles or less. More height gives you more distance but not more clarity in shorter spans. My guess is that a 5 watt hard wired unit was not a top quality product originally (at least in my experience) and hardware quality makes a huge difference, as does teh quality of teh various connections.

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post #9 of 13 Old 06-20-2012
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Re: radio question

Since your old antenna is already somewhat elevated, i would suggest that you are getting more clarity with the new system mostly because it is better quality or better connections, at least for calls within 5 miles or less. More height gives you more distance but not more clarity in shorter spans. My guess is that a 5 watt hard wired unit was not a top quality product originally (at least in my experience) and hardware quality makes a huge difference, as does the quality of the various connections.

Mike sullivan
1985 Marshall 22 catboat
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post #10 of 13 Old 06-20-2012
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Re: radio question

Fixed radios really aren't expensive, for example the SH 1150 Eclipse has DSC+, 25W, and gets good reviews. I've seen it for sale for $120 on the interweb. Good time to replace the 5w one?

Bristol 31.1, San Francisco Bay
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