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Old 29-11-2007, 18:48   #1
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Need some technical advice....antennas.

On our trawler we have two of the very tall two piece VHF antennas. They came with the boat.... They are a bit on the old side and original fiberglass coating has given up the ghost. The raw fiberglass is showing and some of it is coming loose. The antennas work fine.

My question is this: Can these antennas be painted? If so, what kind of paint would work without impairing the efficacy of the antennas?

The antennas cost over $500.00 each and have to be truck freighted....and yes, I'm trying to save a few dollars at the moment.

Thanks!!
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Old 29-11-2007, 21:31   #2
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Hey Just,
Yes, they can be painted as long as the paint has nothing which would partially attenuate them. VHF RF waves go through plastic paint like it's going through plastic! Paint containing lead is long gone so that is not a factor. The lower part of your antenna is probably just a mount which raises the antenna that gets it higher off the waterplane...effectively increasing its range. Are they VHF antennas? The lower the frequency the better RF penetrates. There is good reason not to paint the face of radar arrays relating to this.

Given people are probably just going to notice the lower half and chances are the mounts have nothing to do with RF transmission, then why not just paint the lower halves and leave the upper parts unpainted?

You can get spray on marine grade enamel white paint for a few bucks a can. I would take that route and save yourself the cost of a professional spray job with some sort of expensive LPU. Sand off the loose coating and other loose crud while wearing a respirator and you should be fine.
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Old 29-11-2007, 21:50   #3
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Yes, you can paint them. Epoxy might be best, but a regular enamel should do fine to keep the fiberglass splinters under control for a few years. A couple of coats won't have any significant detuning effect on the antenna.

My ham friends used to joke that black paint increases the gain by 2dB. Not true, of course, but it *looks* like it ought to!
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Old 29-11-2007, 21:54   #4
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Actually, black absorbs very high RF...like light. That should be self-evident. How do these myths get started?
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Old 30-11-2007, 06:21   #5
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I used to wax my fibreglass MOB pole, which kept it from slivering/splintering.
A light sanding, followed by waxing might temporarily restore the antenna surface.
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Old 30-11-2007, 07:34   #6
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If your down to actually seeing (and feeling) the glass fibre threads then 3 coats of epoxy like International’s Interprotect should seal up everything nicely and look like new (Do the whole lot!)
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Old 30-11-2007, 19:33   #7
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Everybody.... Thank you for your replies!! Just saved me a good solid "boat unit"!!
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Old 30-11-2007, 19:42   #8
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black absorption

Black paint not containing any conductive materials will not affect RF until the RF frequency approaches that of infra-red, a LOOOONG way from any UHF, radar, or GPS frequencies.
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Old 30-11-2007, 20:41   #9
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How about using some gelcoat (polyester enamal) on it?
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Old 30-11-2007, 23:14   #10
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Quote:
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Black paint not containing any conductive materials will not affect RF until the RF frequency approaches that of infra-red, a LOOOONG way from any UHF, radar, or GPS frequencies.
I hope everyone realizes we were kidding about the black paint. Regular paint? No effect to speak of.

As for polyester gelcoat, that will work fine. If it was my antenna, I'd just go to the hardware store and pick up a can of spray enamel, probably white (for aesthetics -- any color will work, though).
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Old 01-12-2007, 07:36   #11
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Some years ago I had the same problem and went to the hardware store and bought a length of shelf paper (that plastic lining stuff for cabinets) and unrolled it, removed the backing and rolled the antenna in it. It produces a nice smooth job and the fiberglass shards are encapsulated.
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Old 01-12-2007, 12:17   #12
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black paint

Actually black paint and pigment has the greatest UV resistance because it reflects UV light whereas white absorbs UV. This is why black tie wraps last longer than white ones above deck.

Fiberglass antenna degeneration is due to the UV degradation of the materials at the surface. Long has it been recognized that the best thing to do is paint a new antenna to make it last longer without those nasty fibers showing up.
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Old 01-12-2007, 12:28   #13
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The characteristic black colour of outdoor Cable Ties, comes from carbon black, the UV Stabilizer in W.P. Nylon ties.
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Old 01-12-2007, 12:34   #14
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Quote:
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If your down to actually seeing (and feeling) the glass fibre threads then 3 coats of epoxy like Internationalís Interprotect should seal up everything nicely and look like new (Do the whole lot!)
Epoxy is not a good idea for external use unless it is covered with paint. It will not hold up against the UV's.


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Perfection Undercoat followed by Perfection Finish can be applied to gelcoat or to a properly prepared epoxy substrate (if the surface is in very poor condition then an epoxy primer such as Internationalís Interprotect will be required first). As a rule, two-pack paints cannot be applied over single-pack paints. Such paints may not provide a stable substrate so they should be removed.
yachtpaint.com - the website of International and Interlux paints
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Old 01-12-2007, 12:46   #15
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I suspect that the "very long two-piece VHF antennas" you mentioned are actually composed of:

1. the vhf antenna itself (top portion); and
2. a 6 or 8' long fiberglass extension...not part of the antenna itself.

This is/was a very common thing found on motor yachts.

The extension pieces can be found at West Marine or Boat U.S. I've found in the past that the easiest thing to do is to replace them, not worry about painting them.

If my suspicion is correct (which you should be able to tell by inspection), then it should be fairly easy to replace the extension pieces with whatever length you want. If the coax is easily accessible, just cut the end off at the radio, pull it back out, replace the lower section, feed the coax back to the radio, and solder on a new PL-259 connector. If the coax is not easily accessible, you can put a "splice" in the coax by using two PL-259 connectors and a "barrel connector"; often, this splice will fit inside the fiberglass extension tube.

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