[TowerTalk] multiband vertical dipole with a tuner
jh.graves at verizon.net
Fri Nov 15 14:03:49 EST 2013
I will admit to some prejudice in this discussion but you have peaked my
interest. I do use a 10-40M 1/2 wave vertical. Why a vertical dipole
instead of a half wave end fed vertical? The patterns should be similar
and the feeding in unquestionably simpler. Also you have an easier time
feeding multiple bands, and because it is end fed, the matching is
simpler as well. I will admit that an 80 to 160 Meter vertical becomes
"slightly ungainly" but again, it no harder to handle than the vertical
John - WA1JG
On 11/15/2013 1:13 PM, Jim Lux wrote:
> I'm a big believer in simple antennas, and dealing with matching
> separately from the mechanical designs. Jim's vertical dipole,
> basically a wire with coax and a choke is sort of the epitome of
> "simple design".
> The "outside" of the coax is the second half of the dipole, and this
> has been seen forever in various and sundry "sleeve dipoles" of one
> sort or another.
> One can go fancier with "hats" at the top and bottom to make something
> that presents a better match with a short (compared to wavelength)
> So here's the question. Say you have some sort of tuner at the
> feedpoint, so feedline losses are minimized. How do you go about
> implementing the "1/4 wave of wire + 1/4 wave of coax" in that
> context, particularly when you are operating well away from resonance?
> If you just hang the tuner with the choke, then you pick up the loss
> in the 1/4 wave of coax. Maybe that's a lot, maybe not (I'd have to
> go model it). Maybe using some sort of air dielectric for low loss
> might be better.
> What about the bottom half of the antenna being open wire line? I'm
> thinking that this is less "sleeve-like" so it won't work.
> One advantage of a physically short radiator is that the pattern
> doesn't change as you go up the bands. A 40m dipole has a pretty odd
> pattern on bands above 20, because the current distribution is strange.
> Something that is, say, 25 ft () long is "short" on 40, but not
> hideously short. You've got a tuner so the narrow bandwidth isn't a
> problem. ( a lambda/8 dipole has a radiation resistance of 3 ohms.. 25
> ft is about 8 meters, so you're at lambda/5, so I'd expect Rrad to be
> around 8 ohms
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