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29-11-2007, 17:48
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#1
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Registered User
Join Date: Nov 2007
Location: SW FL
Boat: Gulfstar 44 Motor Cruiser
Posts: 93
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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!!
__________________
Forrest Gump was a true philosopher...
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29-11-2007, 20:31
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#2
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Moderator Emeritus

Join Date: Sep 2007
Location: Eastern Tennessee
Boat: Research vessel for a university, retired now.
Posts: 10,406
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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.
__________________
David
Life begins where land ends.
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29-11-2007, 20:50
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#3
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Moderator Emeritus

Join Date: Sep 2006
Posts: 4,666
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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!
__________________
Paul Elliott, S/V VALIS - Pacific Seacraft 44 #16 - Friday Harbor, WA
www.sailvalis.com
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29-11-2007, 20:54
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#4
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Moderator Emeritus

Join Date: Sep 2007
Location: Eastern Tennessee
Boat: Research vessel for a university, retired now.
Posts: 10,406
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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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David
Life begins where land ends.
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30-11-2007, 05:21
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#5
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Senior Cruiser

Join Date: Mar 2003
Location: Thunder Bay, Ontario - 48-29N x 89-20W
Boat: (Cruiser Living On Dirt)
Posts: 47,904
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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.
__________________
Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"
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30-11-2007, 06:34
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#6
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CF Adviser
Join Date: Oct 2007
Boat: Van Helleman Schooner 65ft StarGazer
Posts: 10,282
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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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30-11-2007, 18:33
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#7
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Registered User
Join Date: Nov 2007
Location: SW FL
Boat: Gulfstar 44 Motor Cruiser
Posts: 93
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Everybody.... Thank you for your replies!! Just saved me a good solid "boat unit"!!
__________________
Forrest Gump was a true philosopher...
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30-11-2007, 18:42
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#8
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Registered User
Join Date: Jan 2005
Location: Seattle area (Bremerton)
Boat: C&C Landfall 39 center cockpit "Anahita"
Posts: 1,077
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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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"I don't think there'll be a return journey Mr. Frodo". Samwise Gamgee
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30-11-2007, 19:41
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#9
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Registered User
Join Date: Nov 2006
Location: Back in Northern California working on the Ranch
Boat: Pearson 365 Sloop and 14ft.Whitehall pulling skiff.
Posts: 10,455
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How about using some gelcoat (polyester enamal) on it?
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"Life is not a journey to the grave with the intention of arriving safely in a pretty and well-preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming: Wow - what a ride!"
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30-11-2007, 22:14
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#10
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Moderator Emeritus

Join Date: Sep 2006
Posts: 4,666
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Quote:
Originally Posted by Rick
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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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).
__________________
Paul Elliott, S/V VALIS - Pacific Seacraft 44 #16 - Friday Harbor, WA
www.sailvalis.com
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01-12-2007, 06:36
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#11
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Registered User
Join Date: May 2004
Location: Cruising on the hook
Boat: 34’ Marine Trader
Posts: 752
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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.
__________________
Jim
We are what we repeatedly do.
Excellence, therefore, is not an act but a habit."
--Aristotle
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01-12-2007, 11:17
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#12
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Registered User
Join Date: Jan 2005
Location: Seattle area (Bremerton)
Boat: C&C Landfall 39 center cockpit "Anahita"
Posts: 1,077
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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.
__________________
"I don't think there'll be a return journey Mr. Frodo". Samwise Gamgee
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01-12-2007, 11:28
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#13
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Senior Cruiser

Join Date: Mar 2003
Location: Thunder Bay, Ontario - 48-29N x 89-20W
Boat: (Cruiser Living On Dirt)
Posts: 47,904
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The characteristic black colour of outdoor Cable Ties, comes from carbon black, the UV Stabilizer in W.P. Nylon ties.
__________________
Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"
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01-12-2007, 11:34
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#14
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Registered User
Join Date: Mar 2003
Location: Philippines in the winters
Boat: It’s in French Polynesia now
Posts: 11,369
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Quote:
Originally Posted by Pelagic
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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Epoxy is not a good idea for external use unless it is covered with paint. It will not hold up against the UV's.
Quote:
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 International’s 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.
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yachtpaint.com - the website of International and Interlux paints
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Faithful are the Wounds of a Friend, but the Kisses of the Enemy are Deceitful! ........
The measure of a man is how he navigates to a proper shore in the midst of a storm!
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01-12-2007, 11:46
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#15
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Eternal Member
Join Date: Jul 2006
Posts: 4,046
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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.
Bill
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