27MHz Marine Aerial mounting
Submitted: Tuesday, Dec 13, 2005 at 23:04
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impeter
Hey.
I've got a bit of a dilemma here. I want to install a 27MHz Marine radio into my boat but I don't want the clutter of an antenna poking out on the surface. What I was thinking was to mount the antenna below the front hull. This leads to two questions that I need answered:
1. Is there much of a difference between mounting horizontally or vertically? Or is the gain zero if the antenna is horizontal and non-zero when it's vertical?
2. Does fibreglass isolate the signal? Will the fibreglass greatly drop the signal strength? I know that a metal cage would kill the signal but i'm not sure about how fibreglass will affect it.
Thanks in advance for your help
Reply By: Mad Dog (Australia) - Tuesday, Dec 13, 2005 at 23:25
Tuesday, Dec 13, 2005 at 23:25
If the antenna of the station you wish to communciate with is vertical there will be a massive signal strength penality, typical cross polarization losses are around 20db
Fibreglass wont hurt
AnswerID:
143764
Reply By: impeter - Tuesday, Dec 13, 2005 at 23:58
Tuesday, Dec 13, 2005 at 23:58
Wow. That's A huge loss!!! :oS And I guess most people have their aerials poking up. I can't afford to lose that much gain especially not if i'm far from anyone and my boat runs out of fuel. :oS (It's happened before - Stupid fuel gauges.)
But i guess it's good that fibreglass won't do anything bad.
Thanks heaps for the superfast reply!!
:o)
AnswerID:
143767
Reply By: signman - Wednesday, Dec 14, 2005 at 09:26
Wednesday, Dec 14, 2005 at 09:26
Definately vertical- both for gain and for signal range.
If you intend to mount it on/thru/under the deck you will also have problems with no ground plane.
I would consider the mounting and location of your 'emergency' radio antenna be of top priority- regardless of how it looks!!!
Option 2. Why not got to marine VHF?? The antenna need not be as long.
AnswerID:
143789
Follow Up By: impeter - Wednesday, Dec 14, 2005 at 22:46
Wednesday, Dec 14, 2005 at 22:46
With the no ground plane, couldn't that problem be solved by purchasing a ground plane independant aerial? Does it really make a big difference even though the boat is all fibreglass? I thought that the ground plane was only helped if the surface that the aerial was mounted on was metal.
I agree with you that emergency equipment is the most important. With the VHF marine radios, you can't communicate with the 27MHz Radios can you. We have a few friends that are running 27MHz, so it would be good to be able to communicate with them. My handheld UHF CB's have quite a limited range. :oS
I guess I could try making the aerial ready to pull out from a secret location if really needed. But then again it's good to be able to have the radio on the whole time just in case someone else needs help.
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Follow Up By: Member - Jerry C (WA) - Thursday, Dec 15, 2005 at 00:13
Thursday, Dec 15, 2005 at 00:13
impeter,
The surface of the water will be your ground plane, even with a metal deck the surface of the water is so large that it would predominate .
Cheers,
Jerry
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Follow Up By: impeter - Thursday, Dec 15, 2005 at 11:07
Thursday, Dec 15, 2005 at 11:07
But if I mount the aerial under the deck (not above the deck) so that it is hidden away, would this affect the signal in terms of the ground plane?
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Follow Up By: Member - Jerry C (WA) - Sunday, Dec 18, 2005 at 23:50
Sunday, Dec 18, 2005 at 23:50
Impeter,
There are probably a few million aerials covered by fibreglass radomes around the world and they give satisfactory performance so give it a go.
Cheers,
Jerry
FollowupID:
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Reply By: Footloose - Wednesday, Dec 14, 2005 at 13:15
Wednesday, Dec 14, 2005 at 13:15
Yep, mount it vertical. Nothing worse than your main motor gone bang in a 6ft swell at night with the lightning striking all around you, foamy bits on those waves, and thinking gee I wish I could yell help :))))
AnswerID:
143822
Reply By: Member - Camper (SA) - Wednesday, Dec 21, 2005 at 18:35
Wednesday, Dec 21, 2005 at 18:35
As I understand it your signal radiates out in a plane at right angles to the antenna and from near its tip. If you have the antenna horizontal your signal will radiate vertically into the sky not to the station you want to reach. Unless it is an airplane directly above or a station directly to port or starboard of the antenna (or a submarine below you?).
So vertical is the way to go and as much height as you can get. Height means the signal travels further.
I used to have one of those bases with a hinge so that it could reside horiz when in the shed and on the road and vertical after launch. Can you leave it horiz till you need it?
Run out of fuel at sea? Thats not a popular mistake to make!!
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