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Old 16-08-2008, 07:56   #16
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Just thinking aloud for a moment. Yes, carbon tube could be worth trying although I must say I am a bit skeptical right now about the whole concept of using carbon fibre as a antenna.

I seem to recall that at various points along the antenna, there is lots of RF volts and at other points there are lots of RF amps. It is these current nodes that make me wonder about using carbon fibre as I am thinking that carbon fibre will have a far greater resistance than copper or even stainless steel. Still worth a try, doesn't have to be at HF freqs, could try it at say VHF or even UHF as the physical sizes become much easier to deal with. It is OK at say VHF, then let's try HF.

Any ideas Bill or Eric ?

Question without notice, why do we use vertical polarization for HF on boats, is there any reason other than convenience - I know I should know the answer on this one but I have burnt off that brain cell. Triatic stay antennas would have been horizontally polarized. Is there anyway of building in an horizontal radiating element into the cabin top of a cat.

BTW, I have seen copper tape glassed inside the wings and fusealage of home built composite aircraft that fitted HF for crossing the Tasman and they worked OK.
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Old 16-08-2008, 08:08   #17
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It's not so much the vertical polarization which matters on a boat, but rather the omni-directional patterns typical of vertical antennas.

All horizontal antennas exhibit some directionality, i.e., they transmit less well in some azimuthal directions. IMHO, a triatic stay antenna at HF frequencies is inferior to a well-designed and implemented antenna system based on vertical radiator(s).

Bill
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Old 16-08-2008, 08:41   #18
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Ah, now I remember, thanks Bill.
Still thinking of the possibility of a large horizontal square loop embedded into the cabin top of a cat - will probably wake up in the middle of the night knowing why it won't work .
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Old 16-08-2008, 09:06   #19
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Quote:
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I seem to recall that at various points along the antenna, there is lots of RF volts and at other points there are lots of RF amps. It is these current nodes that make me wonder about using carbon fibre as I am thinking that carbon fibre will have a far greater resistance than copper or even stainless steel. Still worth a try, doesn't have to be at HF freqs, could try it at say VHF or even UHF as the physical sizes become much easier to deal with. It is OK at say VHF, then let's try HF.
I woudn't think just the carbon tube would be wonderful being a poor conductor. It was Sandys idea to glass on a copper tape that triggered the interest. This would be the primary element and it becomes a question of whether enough signal would couple into the carbon and reradiate to maintain a good omni pattern or if there are gaping holes. I doubt there would need to be an actual connection, which would be good from a galvanic corrosion perspective.
Not sure yet but I may be using an ally sailtrack which would suffice.

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Old 16-08-2008, 15:00   #20
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Quote:
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Question without notice, why do we use vertical polarization for HF on boats
If I recall correctly the polarization of a transmitted HF signal isn't relevant since the polarization of the signal after reflecting off an atmospheric layer is indeterminate. That said, take-off angle (really the lobes of the signal in the vertical plane) is relevant and is obviously affected by the antenna pattern.

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Old 16-08-2008, 18:06   #21
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I have in the past wondered about carbon fibre used for vertical antennas at HF and haven't really found much to go on. My interest was mainly for the possible construction of a HF DSC watch-keeping antenna so for rx only. Quite a bit about use of carbon fibre as reflectors (dishes) and radiators at microwave, etc but often it is not clear what the actual structure is complemented by other conductive material (sometimes aluminium honeycomb is mentioned, for example).

But I have come across this example of an amateur carbon fibre vertical which looks as though it is possible - 2W0DAA 20 Meter band carbon pole antenna on Flickr - Photo Sharing! and the guy invites emails about it. From the photos of other antennas it appears that he has a quality approach to antenna construction. Is not altogether clear that there is no other conductor imbedded, but given the comment of "100% carbon" in one photo and the fact that it breaks down into sections it seems not. I was going to give it a go for the DSC watchkeeping antenna when and if I came across a rejected carbon fishing rod blank.

I don't think the resistivity is an issue as the cross section of a whip is large and of a mast even larger. How one makes the electrical connection for the feed point I wouldn't be sure.

I would be wary of running copper tape up the outside of a carbon fibre mast but say that without any particular experience - issues I would want satisfied beforehand would be to do with the possibility of galvanic action (but carbon is more noble than copper in seawater) and the possibility of voltage breakdown of the UV protection/epoxy layers between the copper and the carbon.

I suspect both those concerns are unlikely to matter but given the cost of carbon fibre masts I don't think I would wish to be the one experimenting with my own mast.
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Old 17-08-2008, 03:33   #22
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Have been sick as a dog for the last 10 days and haven't been near the boat. Was much better today so I went down and sat there looking at all the options. I can put a whip on the inside edge of the walkthrough transom. Directly below this is a locker for the tuner and under that is one of the tubes that provides longitudinal stiffening. This tube is 7.5 meters long and into which I could pull some folded radials. From the tuner I would end up with about 1.5 meters to connect to an external bronze disk. While it would probably be a huge pain in the arse to do, I could glue on some bronze mesh on one or both side of the stiffening tube. One side is 450mm wide and the other 900mm.

It all actually works out quite neatly. So I am thinking this may be both the easiest and have the best chance of performing well. Thus radials plus the bronze disk under water. I will do a test to see how difficult it is to glue up some mesh and if it works use a piece about 450mm by 6 meters. For the connection I will solder on a copper pad that can be exposed later then through bolted to inside the tuner locker.

For the radials I'm thinking of two for each band at 1/3 and 2/3 the frequency range rather than just one at mid freq. Just need to remember the way to work out the wave length

What does everyone think

Mike
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Old 17-08-2008, 03:47   #23
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Freq (in Hz) x wavelength (in metres) = 300,000,000 (i.e speed of light in metres/ second)
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Old 17-08-2008, 04:30   #24
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Have been sick as a dog for the last 10 days and haven't been near the boat. Was much better today so I went down and sat there looking at all the options. I can put a whip on the inside edge of the walkthrough transom. .......
For the radials I'm thinking of two for each band at 1/3 and 2/3 the frequency range rather than just one at mid freq. Just need to remember the way to work out the wave length

What does everyone think

Mike
Considering you could be using freqs from say 4 MHz to say 16+ Mhz, I wouldn't fuss to much on the length on the radials. I would rather be thinking the more the merrier whatever size they are and let the tuner take care of the matching ('cause that's job).

Have you any idea of what sort of whip / tuner you plan to use (or HF for that matter).

FWIW, these local tuners and whip are reportedly very good. I have only seen a couple and the ones I saw looked good (and worked very well). Almost literally bullet proof. http://www.qmac.com/mobile_antennas.html
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Old 17-08-2008, 05:31   #25
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Considering you could be using freqs from say 4 MHz to say 16+ Mhz, I wouldn't fuss to much on the length on the radials. I would rather be thinking the more the merrier whatever size they are and let the tuner take care of the matching ('cause that's job).

Have you any idea of what sort of whip / tuner you plan to use (or HF for that matter).

FWIW, these local tuners and whip are reportedly very good. I have only seen a couple and the ones I saw looked good (and worked very well). Almost literally bullet proof. QMAC - Mobile antennas for HF and VHF transceivers.
300,000,000 that was the bit I couldn't remember, thanks.

How many do you mean by the more the merrier. 10, 15, 20, 25, 40? 14 would give a radial for each Mhz step 28 1 for each half meg step. They have to be cut to some length so why not to something specific.

No real idea of SSB, tuner or whip other than probably an Icom 802 with an sgc tuner which seems to be the popular choices. I would opt for the sgc as it will work with any make. Elecraft make some nice gear but it would probably need to be a marine set.
I tried an online test for a ham licence a couple of months ago and passed easily, although I had to make a few guesses. Been a long time since my radio theory days I was quite surprised. I think a ham license would be good when cruising.

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Old 27-08-2008, 22:48   #26
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Love the discussions on this topic and wish to throw my two cents in. My rather simple, yet effective solution on my ketch, was to feed my main mast at the top of the compression post and attach the ground foil to the base of the post to form a very effective dipole. I have made contacts from the west coast of Canada throughout N. America and south pacific ocean. There was a little trial and error determining the distance between the feed and ground and so far I've settled on about 8.5 feet.

All this to say that sometimes the simple solution is to experiment with what you have in the interests of simplicity.

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Old 28-08-2008, 08:43   #27
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I just checked a carbon fiber mast in our rigging shop and the continuity between any metal fasteners that are screwed into the mast is zero ohms (dead short). Even from the very top to the bottom using one of the windex light wires as a return. So the conductivity seems very good. There is no conductivity on the surface as there are layers of epoxy and paint. Getting to the conductive parts is as easy as using one of the existing fasteners or drilling and tapping for a screw at a convenient point for a dedicated connection. Some of these mast have the so called "porcupine" static discharge device for lightning protection and they are just screwed to the mast and use the mast as the conductor. I have not seen what would happen if one of these masts was actually struck. I have seen a 23 foot SSB antenna get struck. These are fiberglass with three embedded wires running the full length. It was split open like a banana peel.

Eric
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Old 28-08-2008, 09:59   #28
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Whimsical;

Did you look into the screw-type antennas? I am just reasearching this stuff, not a ham, but they look interesting.

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Old 29-08-2008, 02:30   #29
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Don't know what you mean by a screw type antenna.
I haven't researched any as yet, I am only trying to decide on what I will do for the ground side. So many strong differing opinions to connect to water or avoid it. I will be using a whip of some sort. Not the time to experiment with the mast.
I have decided to pull some radials into the stiffening tubes under my bridgedeck and to put some mesh on the bridgedeck as well. I will also put in some copper foil down to where I would put a bronze plate through the hull. This way if the radials/mesh are no good I can add a water connection quite easily. Now just need to find out how much mesh to use. I am thinking of about 3 square meters.

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Old 29-08-2008, 03:14   #30
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Getting to the conductive parts is as easy as using one of the existing fasteners or drilling and tapping for a screw at a convenient point for a dedicated connection.
Thanks for that, it was the bit that I have been wondering about.

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I have not seen what would happen if one of these masts was actually struck.
"Carbonised", perhaps?
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