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Old 07-11-2014, 11:38 AM   #1
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Will covering VHF antenna affect performance?

Does a VHF antenna radiate solely from the tip or does the shaft itself contribute? I would like to slip a light gauge metal tube over it on my launch for use as a flag staff. So the 8ft antenna would be covered 7ft but leave the top 1ft tip exposed. Any knowledgeable electricians out there know about this?
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Old 07-11-2014, 12:17 PM   #2
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I'm only planning on using it in this manner when anchored (to raise my dive flag, anchor ball, etc.). Perhaps a PVC pipe would be better and at least allow some reception?
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Old 07-11-2014, 12:22 PM   #3
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I have seen these disassembled. Definitely do not put metal around it. If you go the pvc route make sure it is not putting any stress on the antenna via a lever effect.
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Old 07-11-2014, 12:37 PM   #4
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Yes, the length of the antenna is used. Standing Wave Ratio (SWR) and other things all play into this. You can't cover it with metal and I wouldn't cover it with anything else either. Put up a second pole for the flag.

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Old 07-11-2014, 12:50 PM   #5
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Yup. . . I'll second that!! A flag flapping in the breeze can add a lot of load to a staff. Fiberglass exposed to sun and weather weakens and becomes more brittle as it ages. I've seen antennaes that have fractured due to relatively small flags and pennants attached to them.
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Old 07-11-2014, 06:15 PM   #6
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Does a VHF antenna radiate solely from the tip or does the shaft itself contribute? I would like to slip a light gauge metal tube over it on my launch for use as a flag staff. So the 8ft antenna would be covered 7ft but leave the top 1ft tip exposed. Any knowledgeable electricians out there know about this?
It does not radiate from the tip, it radiates from the entire length. A seven foot metal tube would make a pretty good electromagnetic shield and I would predict that the antenna would not transmit or receive at all.

Scrap this idea and find another way to fly your flag.
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Old 07-11-2014, 06:23 PM   #7
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I agree w/ all the above. Break the antenna, another $100+ boat buck and then you'll find another place to fly the flag.
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Old 07-11-2014, 06:30 PM   #8
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I've seen antennaes that have fractured due to relatively small flags and pennants attached to them.
But, if you must, Taylor make Charlevoix Nylon Burgee/Antenna Flag Clips that will allow you to attach small flags or burgees to an antenna mast without electronic interference. As stated by others though, best to attach them to a flag pole/staff or hoist.

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Old 07-11-2014, 06:44 PM   #9
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Don't put a metal cover over it..if it's just temporary for the circumstances or day...heck just use some cheapo wire ties. As long as you are not in a gale or running with the flag still up....you will not probably damage the antenna as large numbers of boaters and commercial firms do it all the time with no ill effects.
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Old 07-12-2014, 09:11 AM   #10
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No way should you put a 7' metal tube around your 8' VHF antenna unless you don't want the VHF radio to work. The entire length of the antenna is used, adding a metal tube around it is like putting the antenna in a Faraday cage meaning it will not work.
Either use wire ties and a small flag or find another more conventional way to mount your flag that doesn't involve your VHF radio antenna.
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Old 07-12-2014, 01:16 PM   #11
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Wow a thread where we all agree! Mark it!
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Old 07-12-2014, 01:26 PM   #12
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Wow a thread where we all agree! Mark it!

Can't allow this to stand.

Definitely stick a steel pipe on your antenna. Faraday cages are really cool and should limit damage in the event of an EMP.
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Old 07-12-2014, 02:34 PM   #13
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Wow a thread where we all agree! Mark it!
Ha!!! Spoke too soon!!

Many antennae the active part is only the last 18" or so. Noticed this when stupid operator (me) ran a skiff under a low bridge and forgot to drop the VHF ant. Busted it properly!! Got a chance to look at insides: Just coax up into bore, and last 18" or so stripped to reveal inner conductor. So you could put something metallic on it and provided it did not cover the active part up top. That might have been a POS ant, though. I'm known to go cheap!

But antenna design is part black magic.. Tried to study it but my brain does not work that way. There is matching of antenna length to wavelength, not sure what 155MHz comes out to.

PVC pipe and zip ties sounds like a good plan. I might do the same for mine. Fair weather only, of course.
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Old 07-12-2014, 03:01 PM   #14
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Greetings,
From what I understand the co-ax leading to the antenna also comes into play with the tuning circuit so if you ever install a new antenna do NOT cut the co-ax as supplied to make a "neater" job. If you HAVE to cut the co-ax there is a formula for the "best" length. I'm sure some radio hams on TF could add to my somewhat poor understanding of this black magic.
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Old 07-12-2014, 05:44 PM   #15
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Greetings,
From what I understand the co-ax leading to the antenna also comes into play with the tuning circuit so if you ever install a new antenna do NOT cut the co-ax as supplied to make a "neater" job. If you HAVE to cut the co-ax there is a formula for the "best" length. I'm sure some radio hams on TF could add to my somewhat poor understanding of this black magic.
Unfortunately, most hams do not really understand antenna principles either. :-) There are good books on it, with a whole lot of math. The good news is that for VHF on the marine bands it really doesn't make all that much difference. Oh, the antenna should be the proper length, the coax should be of good quality, no corrosion on the connectors, and doing an SWR check every now and then is a good thing to do (SWR meters are inexpensive). But line of sight communication (VHF) with 25 watts of output power and a decent antenna is pretty easy.
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Old 07-12-2014, 06:21 PM   #16
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Ha!!! Spoke too soon!!

Many antennae the active part is only the last 18" or so. Noticed this when stupid operator (me) ran a skiff under a low bridge and forgot to drop the VHF ant. Busted it properly!! Got a chance to look at insides: Just coax up into bore, and last 18" or so stripped to reveal inner conductor. So you could put something metallic on it and provided it did not cover the active part up top. That might have been a POS ant, though. I'm known to go cheap!

But antenna design is part black magic.. Tried to study it but my brain does not work that way. There is matching of antenna length to wavelength, not sure what 155MHz comes out to.

PVC pipe and zip ties sounds like a good plan. I might do the same for mine. Fair weather only, of course.
Likely a crappy antenna, but what size and model was it? I have twice attended antenna dissection exercises and the really low end ones can be as you describe. Gee, nobody on TF would have one of those would they..... uh, whoops, maybe it would be OK!
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Old 07-12-2014, 06:24 PM   #17
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Ha!!! Spoke too soon!!

Many antennae the active part is only the last 18" or so. Noticed this when stupid operator (me) ran a skiff under a low bridge and forgot to drop the VHF ant. Busted it properly!! Got a chance to look at insides: Just coax up into bore, and last 18" or so stripped to reveal inner conductor. So you could put something metallic on it and provided it did not cover the active part up top. That might have been a POS ant, though. I'm known to go cheap!

But antenna design is part black magic.. Tried to study it but my brain does not work that way. There is matching of antenna length to wavelength, not sure what 155MHz comes out to.

PVC pipe and zip ties sounds like a good plan. I might do the same for mine. Fair weather only, of course.
That's about a 1/4 wavelength at VHF marine frequencies. You have a no gain antenna. Also be careful the coaxial shield you see may need an additional what I call fold over of the shield for the ground plane for impedance matching. Most marine antennas that have gain are collinear design antennas they are typically 5/8ths wavelength and require chokes for impedance matching. If you cut one of these apart you would see gold plated rods and what look like round collars.
It's still not a good idea to install a 7' piece of metal even assuming your antenna is only a 1/4 wavelength. You would still be covering 4-6 inches of the radiating element with metal, metal around an antenna can lower it's resonant frequency (it acts like an RF capacitor) and will raise the VSWR to the point your VHF radio may fold back it's output power to protect it's transmitter from the terrible return loss (high VSWR).
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Old 07-12-2014, 06:55 PM   #18
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I wouldn't want to jeopardize my $100+ antenna with flying a flag...especially one as important as a Dive Flag. It's easy to get a simple flagstaff to connect to a rail for proper flag display. I have a spare SS antenna rail mount that would work well for that.

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Old 07-12-2014, 07:31 PM   #19
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Unfortunately, most hams do not really understand antenna principles either.
And I include myself in that category, although I have an Extra Class ham license (AF4WM), have been a ham for some 47 years, and have a few QRP awards stuck away in a desk drawer somewhere.

But I was thinking about the OP's question some, mostly because it is a rainy Saturday afternoon and nothing better to do. An RF signal is a sine wave, and when it leaves the transmitter, hustles through the coax, and through the wiring in the antenna, then in a perfect situation it will exit the antenna as an acceptable part of a sine wave. That is, not cut off too soon or too late. Channel 16 of the marine band is 156.8 Megahertz. At that frequency, one wave-length (how long the sine wave is before it starts to repeat) is about 75.3 inches, or 6.275 feet.

VHF Marine antennas typically use 1/4 wavelength, 1/2 wavelength, and 5/8th wavelength ratios. A 1/4 wave antenna then would be an antenna with an electrical length one-fourth of the actual signal's wavelength, or about 18.8 inches. Lots of sailboats (mine included, back when I had a sailboat) just stick a 19-inch whip at the top of the mast and that works just fine.

Trawler people typically use 8-foot antennas, which one might think would be a full wave-length, but actually are not necessarily so. Usually the coax just continues on up into the fiberglass, and the actual antenna is -- surprise! -- only 19 inches.

All of which is by way of thinking that I am not absolutely certain that putting a piece of PVC pipe over the antenna, with some of the antenna sticking out above, would not work perfectly well. Not saying it would, mind you, and if others more knowledgeable about the subject want to correct me I shall not take it amiss.
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Old 07-12-2014, 07:32 PM   #20
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I have a spare SS antenna rail mount that would work well for that.




If you ever feel compelled to part with that spare keep your old pal Craig in mind. We can hammer out the details over some ice cold IPA's.
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