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Old 01-03-2009, 05:48   #1
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VHF antenna optimum height

I've been looking over old threads concerning vhf antenna heights and the consensus, from apparently knowledgeable persons seems to be that the higher the better. Question--does this assume that the tallest sailboat mast is less than the optimum elevation of the antenna? In other words assuming you have 25 watts of power there must be some point as you increase elevation of the antenna where you loose signal. What is that height?
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Old 01-03-2009, 05:51   #2
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Much higher than your mast, that's for sure!

VHF is line of sight plus a little bit. The higher the better. Aircraft have much greater range than land-based stations. Satellites have much greater range than aircraft.

25 watts is plenty to operate from planes and satellites. A 1-watt handheld on a mountain can communicate over very great distances.

On a boat, get the main VHF antenna as high as you can.

Bill
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Old 01-03-2009, 06:03   #3
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What Bill said .

The only drawback in going higher (assuming the transmitter stays at sea level) is the loss in the coax for a very long run; but this won't be a problem on any sail boat mast using good quality coax (RG 214 etc).

5 watts being transmitted at the top of the mast will give a much greater range than 25 watts at deck level.
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Old 01-03-2009, 06:29   #4
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Heres a question for the radio geeks.

With a masthead antenna, is it worth the extra db gain going with a six foot fiberglass antenna on the mast, over the traditional short 3db stainless whip antenna?

I sometimes see sailboats with the big glass whip antennas aloft
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Old 01-03-2009, 06:43   #5
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I don't think so, no real gain in range for extra weight and windage aloft (exactly where you don't want it).
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Old 01-03-2009, 07:23   #6
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Quote:
Originally Posted by westsail42 View Post
... With a masthead antenna, is it worth the extra db gain going with a six foot fiberglass antenna on the mast, over the traditional short 3db stainless whip antenna? ...
Not on a sailboat.

The greater the gain of the antenna the more directional it becomes. Everything will be fine so long as a 6 or 9 db antenna is vertical and stable. However, as the vessel rolls and pitches the antenna will no longer be vertical. The signal radiated from the more directional 6 or 9 db antenna may then be directed upward into the sky or downward into the surrounding sea, rather than in the direction of the horizon.

Hence, the general rule for antenna selection is to use a 3 db antenna (3' long) for sailboats and either a 3 db or 6 db antenna (8' long) for power boats. In general, 9 db antennas (19' + long) should be reserved for use on land. The directional characteristics of the antenna operate in the receive mode as well as when the antenna is used for reception of incoming signals.
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Old 03-03-2009, 15:31   #7
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I appreciate your input everyone.
Let me tell why I'm pondering this question----A few years ago I had an expensive experience with lightning and (maybe I'm, a little paranoid) but IMHO the 3' antenna (being mush like a lightning rod) on top of the mast head increases the likelihood of a lightning strike by, my guess 10%. My previous 3' whip was vaporized and I replaced it with a 8', 6 DB mounted at the deck level. My normal boating area is the Chesapeake Bay and this arrangement works OK but this time next year I plan to be in the Bahamas and will need the additional sensitivity mostly for cruiser /weather nets(?). I'm thinking about hoisting a temporary antenna up the backstays. What do you think?
Bob
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Old 03-03-2009, 17:03   #8
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Bob,

I''m a native Chesapeake Bay sailor, too. With long experience in the Caribbean, the Bahamas, South Florida, etc. Where lightning strikes are more frequent than here.

Put a 3' VHF antenna atop your mast. You can't estimate the "additional attraction" to lightning it might cause. That's just nonsense. I've seen catastrophic lightning strikes on a small power cruiser sandwiched between two large sailboats, destroying the little cruiser and leaving the sailboats intact!! Lightning will do what it's gonna do, and there's very damned little you can do about it.

You're powerless. In the current parlance, 'get used to it' :-)

When in the presence of lightning storms, just disconnect your VHF antenna. And your SSB antenna if you have one. And any other's which might be susceptible. That's about all you can do, IMHO.

Bill

BTW, there's no way an 8db antenna low down will match the range of a 3db antenna atop the mast.

B.
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Old 03-03-2009, 17:10   #9
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I go along with Gord with one exception: for a 3dB vertical one is not limited to the not so efficient bottom fed 3' stainless whip... but it is cheap. I installed this one: Shakespeare Marine Antennas Specifications: Galaxy Little Giant™ 5400-XT VHF Marine Band

and the performance difference is stunning, which is logical considering we now have a center-fed vertical dipole. It is 4' high.

To Bob: Put it up the mast like you had before and know that there are much less lightning storms in the Bahama's!!

ciao!
Nick.
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Old 03-03-2009, 17:42   #10
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Bob,

I don't think the antenna will make that much difference in regards to lightning strikes. If a storm is approaching, disconnect the antenna from the radio. As for the gain, you will most likely be saving the loss in the coax and about one or two dB gain in signal strength. You probably loose one or two dB in the coax now.

As for omni directional antennas somehow becoming directional just because they have more gain, don't believe it. An omni directional antenna has a radiation pattern like a donut. The more gain, the flatter the donut. The higher the antenna, the greater the line of sight distance. An extra 3dB gain is like doubling the transmit power on both ends. As for not being vertical when healed over, this would reduce your signal on both receive and transmit due to cross polarization of the antenna. With more gain, this effect would be less noticable.

As long as you can maintain a good mechanical mounting to the mast and the antenna itself is strong enough to handle being slapped around with the pitching and rolling and whatever wind is available, I would say go for it. You can never have too much gain on an omni directional vertical. It is the mechanical problems that I would look at closely.

Hal
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Old 03-03-2009, 18:07   #11
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Hal, you write that with more gain, the donut gets flatter. That is the same as what really matters: the vertical opening angle gets smaller. Now, when you heel, your signal can go over the target (windward side of you) or hit the water before reaching it (leeward side of you). You want a decent vertical opening angle on the antenna and that is where "they" decided 3dB is optimal.

cheers,
Nick.
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Old 03-03-2009, 20:48   #12
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Hey! Is this another reason to get a catamaran. You can put on a higher gain antenna because you don't have the concerns about healing?

Bob, my boat is at Long Cove Marina right now. Sailed right past Ken Narrows/island...
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Old 03-03-2009, 23:48   #13
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Hey! Is this another reason to get a catamaran. You can put on a higher gain antenna because you don't have the concerns about healing?
The movements of the masthead on a cat are so violent that an 8' fibreglass antenna will quickly break into pieces ;-)

(seeking cover)
ciao!
Nick.
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Old 04-03-2009, 10:38   #14
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Nick,

Your arguments are valid. My point was that an omni directional antenna is always an omni directional antenna. There is give and take in every installation or type of antenna. Everyone makes their own choice. Your antenna sounds like a good one. I might have two. One on the mast and a spare on the deck in case the one on the mast vaporizes
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