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PostPosted: Jan 3rd, '18, 11:52 
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The latest thing to break on my boat was the deck-mounted VHF whip aerial. The thread in the mounting bracket sheared off. So, I'll need a replacement.

Given that VHF is almost line-of-sight, the most efficient place to mount it is on top of the mast. Also, my mast is likely to stay up most of the time, but the antenna will need to be one more thing to take off when the mast comes down. I would probably need a mast mounting bracket to put it on.

If it goes on the mast, what's the shortest length? Would a coiled aerial be OK, or should I get a straight one?

Finally, I'm becoming aware of how fast electrical stuff deteriorates when left in the sun, and the top of the mast is probably the most exposed place to put anything on the boat. Any tips?

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PostPosted: Jan 3rd, '18, 14:05 
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A little bit of antenna theory first.
Vertical antennae generally radiate in all (horizontal) directions equally unless they are built to be directional. But different antennae claim different gains, that is to say some more sophisticated antennae radiate more horizontally than others do.
How?
By radiating less "not horizontally", by concentrating the signal into a flatish disk with less signal going above or below horizontal. This works really well when you're antenna is bolted to a house or a car. And not so well when it is bolted to a yacht mast which is leaning over 30 degrees!
So high gain antennae aren't really a bonus on a boat; you really want a basic low gain or unity (eg: x1) gain antenna mounted as high as you can practically get it.

The antenna you have linked to is described as:
This super light-weight black VHF marine antenna is specifically built for sailboat racers. It won't add much weight, while delivering the performance you need. Its 15" center-loaded design delivers Unity Gain, and it comes with a black L-bracelet for mounting atop your mast. Its connector is gold-plated too.

So; it's light, it's already got a mounting bracket, it's relatively short and it is unity gain. It is indeed designed for a yacht mast; it's just the thing you need. Gold plated connectors should be less prone to corrosion too.

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PostPosted: Jan 3rd, '18, 14:07 
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Additional tip; don't let the weight of the antenna cable hang on the connector; the cable needs to be supported by something else near the top of the mast.

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PostPosted: Jan 3rd, '18, 14:13 
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You will need a ground plane and some antennas will have this addressed with the base, a coiled antenna, within reason, will work as well as a straight as the length is electrical as opposed to lineal.
The length can also vary depending upon what type of RF length, 1/4, 1/2, 5/8 or full wave.
Another type of antenna has a coiled base, looks like a thick tube with a short whip, they work but not as efficient as the above.
All plastics will eventually suffer the rigours of UV but the better antennas (GME etc) are pretty good.

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PostPosted: Jan 4th, '18, 07:21 
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I did the mast top arial thing but found I couldn't communicate with other boats if close by when cruising in company. In future I'll save the mast top arial for solo expeditioning.


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PostPosted: Jan 4th, '18, 08:04 
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It's all over my head. :wink:

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PostPosted: Jan 4th, '18, 08:09 
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Three issues with mast top trailer sailer antennas:

- they are more likely to get damaged in the mast up mast down we all must do, the antenna of course, but more likely the cables/plugs required on deck
- they 'may' as per mob's comment above have some issues with nearby stations
- will likely have a poorer signal if you don't use a lower loss cable such as LMR400 or other coaxial cable (think 12mm thick, hard to install) NOT the 4mm thick cable you probably have in your present set up (RG58 or similar). The losses in a few extra plugs/sockets and the 10m run of thin/high signal loss coax to the mast may well present a lower signal to the radio than a handheld at deck level (with antenna mounted ON the radio)

However if you use quality coax, connections and a mast top mount you will increase the horizon you antenna 'sees'. This is of course not as relevant if you are using a channel that is linked to a hill top repeater site.

PaulS

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PostPosted: Jan 4th, '18, 20:58 
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And a pro for a mast top mounted VHF antenna:

They make an excellent deterrent for birds to land up there, perhaps wrecking your wind vane in the process and smudging the deck below.
AZ100


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