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I'm a newly-licenced ham and currently starting out with HF. My home doesn't allow for a permanent setup. I can therefore only operate portable from parks and such, with limited TX power (my rig goes up to 20W), so an efficient antenna seems important to ensure the best rate of success.

Of all portable antenna configurations I've tried, I've had the most success with a monoband 20m vertical consisting of a 5-meter fiberglass fishing rod with a wire spirally wound(*) around it top to bottom and 4 radials laying on the ground.

I would now like to try working the bands that lie lower down (for now 30, maybe 40 meters), but for that I'd have to get myself a longer mast, and all the 7+ meter fishing rods that I could find have at least some carbon fiber content.

CF is conductive, so logically it must interfere with the antenna's operation, possibly throwing off the tuning and/or causing resistive losses in the carbon. I'm also considering hoisting a speaker wire dipole on such a mast. In that case, again, it seems that a conductive mast must interfere with the twinlead section of the feedline.

How significant is this effect in practice, and how exactly does a poorly conductive mast affect antenna performance for the configurations mentioned?


clarifications: (*) the spiral is a very spread-out/slow one, pitched just enough so that the wire would sit tightly against the mast without flapping around in the wind. It is nowhere near tight enough to count as a loading coil, so the antenna should be considered equivalent to one where the wire runs up in a straight line.

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    $\begingroup$ I think it's pretty difficult to say without testing the precise antenna you have in mind. $\endgroup$ Aug 26 at 12:55
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You should definitely wrap a wire, or perhaps three wires, on the outside of the carbon mast. These could be wound in a slow spiral to stop them flapping. The efficiency will not be compromised much by having the carbon fibre in parallel with the copper wire. Make sure the feedpoint is not "shorted out" by the carbon - use an insulating bush or pure-fibreglass section there of course.
The wire spiral would also be useful to prevent vibration induced by vortex shedding.

For a two-wire transmission line running down the mast - it's not ideal to have the two wires touching the carbon fibre. The field is concentrated mostly between the wires and about 1 x the spacing of them. If you can space the wires away from the carbon by about 1.5 x the spacing of the wires, then the losses will be small. See some pictures of the fields here.

If you need any loading coil - carbon fibre would cause significant losses if used as the core of a long spiral. It's important to keep conductive (and magnetic) materials out of the core of the inductor, and at least one diameter away from the ends too. So don't wind it on the carbon part of the mast. It is slightly more efficient to spread the coil over bottom half of an antenna than to concentrate it at the bottom, but this could be made up by using a larger diameter, more efficient coil - say 10 cm diameter instead of the 2 cm of the pole. An inductor would on a ferrite rod or core could also provide a high Q solution at these freqencies. If you have a full 7 m monopole length, the antenna will not demand too much from the coil, and will have good efficiency and bandwidth.

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  • $\begingroup$ It's not going to be that good a conductor that it will make a good radiating element. Carbon fiber is "quite conductive" compared to an insulator, but still has between 100x and 1000x as much resistivity as copper. I do see the idea of "helping it out" with some parallel wire but I don't think that will work as well as you want it to; most of the current will then be in the wires, and the rod will, again, just contribute loss. $\endgroup$ Aug 26 at 14:25
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    $\begingroup$ To be clear, I wasn't suggesting winding the wire as an inductor/loading coil on the mast; it's a full-length vertical. Rather, it's wound "just tightly enough" so that the wire would sit tight against the rod and not sag/flap around in the wind without using too many zipties. Maybe I should include this clarification in the question as well. $\endgroup$
    – Ivan R2AZR
    Aug 26 at 14:53
  • $\begingroup$ @IvanR2AZR OK, that's better then. I've flipped the answer around. $\endgroup$
    – tomnexus
    Aug 26 at 15:16

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