[TowerTalk] multiband vertical dipole with a tuner

[Jim Lux]

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