|[AMRadio] Inverted V antenna|
k4kyv at charter.net
Wed Jan 27 15:07:39 EST 2016
From: Jim Isbell, W5JAI [mailto:jim.isbell at gmail.com]
> My proposal is shown it the two attachments. The JPG shows the actual location and the PDF shows the
> dimensions. The orientation is NW to SE ie the house faces SW.
Looking at the diagram, I see a couple of problems.
Firstly, assuming you are making this a centre-fed dipole, the feed point appears to almost lie right on the metal roof. This would make it an extremely ineffective radiator. It needs to be elevated as high as possible; simply pull enough tension on the wire to get rid of the sag, to raise the feed point as high as it will go. It doesn’t matter that the dipole would be tilted somewhat, in fact some people do that intentionally to make their antenna a “sloper”, which supposedly has some gain towards the low end, IIRC.
Secondly, the only band that a tuner could take care of and still get usable RF to the antenna, would be 15m and perhaps 17m. A coax-fed dipole works well on odd harmonics, but becomes essentially a dummy load on even. Even with a tuner, it would still be a dummy load on 20m and 10m. This is the same problem as I posted earlier about the guy who wrote to QST (Jan 2016, p. 66) wondering why his SWR was sky-high on 20m after he had extended the lengths of the legs of his 20m dipole to make it a half-wave on 40m, but it worked OK on 40.
Since you have a tuner and the two support structures that would appear to take as much pull as you could possibly exert (a telephone pole and a lighthouse), I would suggest using balanced open wire feedline and make it a half-wave dipole, and pull the antenna as tight as possible. The reduced weight of the OWL would make it all the more easy to pull out most of the sag, and with the OWL and tuner, you could use the antenna on 40m through 10m, and you might even get it to work pretty well on 75-80m.
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