Jim,
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
dipole.
John - WA1JG
On 11/15/2013 1:13 PM, Jim Lux wrote:
So..
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)
radiator.
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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