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We'd like to install an antenna for 80m at our club/contest station. Our club, OH2K, is housed at a school, and we can't fill the roof with a radial mesh. The roof is not metallic itself, either. We already have a horizontal dipole, but would like to have something with a lower radiation angle.

We do have a 20-meter tower on the roof of the school building, but there are no guy wires attached to the top of the mast, so it's a bit hard to support a full-height 20-meter-long quarter-wave vertical. However, something in the 10-15 meter length range is easy to hang. A very short vertical would be too narrowband for contests.

We operate multi-multi, with 1 KW linear amplifiers, and stub + bandpass filters.

What practical 80m vertical antenna designs should we look at?

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  • $\begingroup$ I find a shortened helix vertical about 5m long (containing ~40m of wire wound somewhat uniformly on a fiberglass pole) sufficient to allow me to work some of the better equipped European stations from Tennessee on 3.800 $\endgroup$ – Paul Oct 25 '13 at 23:27
  • $\begingroup$ If you want a truly decent vertical without radials then you want a bottom-fed 1/2 wavelength - that is 10 meters tall, roughly. So the only problem is tower parasitics. For 80M, the best antenna for this would be a horizontal inverted V at 468/F feet length - about 66 feet per leg. You say you already have one. The radiation angle for a dipole is like a torus in free space, and a nonmetalic roof would not impede the ground wave radiation. But, of course, all straight dipoles are bidirectional. If an inverted V you can slope it to favor a certain direction. $\endgroup$ – SDsolar Jun 26 '17 at 16:11
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An angled full-wave delta loop can be hung from the tower at the height you are talking about and is very broadbanded (and can be fed from a corner so that it is vertically polarized. An advantage is that this also makes a very good receive antenna. See for example the October 1984 QST for an article on the subject.

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    $\begingroup$ I just realized that my question was not well worded (vertical polarization). We don't really care about the polarization that much. We just though that some sort of vertical antenna would be a practical choice. Delta loop could be useful too, as it'd be somewhat directional, but we'll then have to set up a couple of them. $\endgroup$ – oh7lzb Oct 23 '13 at 21:31
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    $\begingroup$ Yes, for those wondering, polarization is mostly important in VHF and line-of-sight work. It get's all twisted around in the ionosphere bounce when refraction is involved, so it's harder to predict at propagated distances. $\endgroup$ – Bill - K5WL Oct 24 '13 at 18:23
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In your situation, I would shunt feed the 20m tower on the roof and drop one 20m wire from the roof if the building is tall enough. If the building is lower than that, you can end the wire in a T-shape, and/or load it or, alternatively, complete the return path by installing two or more elevated radials. Loading will lower the bandwidth of the antenna significantly, though.

Shunt-feeding offers the advantage that you can earth your roof tower for proper lightning protection.

If you feed with coaxial cable, do not forget to also install a sheath current choke.

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  • $\begingroup$ The tower is actually some 24 meters, with the tube for 2m-70m-24cm stacked yagis on top, I'm not sure it'd resonate. There are single band yagis for 40-20-15-10-6 on it, too, and we do multi-multi. Would feeding 1kW on the tower would result in some of that power coming down the other feedlines? $\endgroup$ – oh7lzb Nov 3 '13 at 5:38
  • $\begingroup$ An antenna does not need to be resonant in order to be a good antenna. Resonance is only for feeding convenience. You most probably will need to add some series capacitance at the shunt feed to cancel out the fact that it is a bit too long. As for the other feedlines, install sheath current chokes on all of them at the base of the tower. $\endgroup$ – on4aa Nov 3 '13 at 10:13

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