In article om,
wrote:

I read in W1FB's Antenna Notebook that you can take a 1 wavelength
piece of wire, feed it with 75 Ohm coax 1/4 wavelength in from one
end, with the hot side going to the 3/4 wavelength piece and the
shield going to the 1/4 wavelength piece. It looks like you don't
need any type of tuner.

Has anybody tried that? I'm looking for a simple antenna for PSK-31
QRP that the homeowners association won't even see, and this looks
like the deal, but I can't find any corroboration for the concept.

That sounds like a form of off-center-fed resonant doublet. Since the
radiator is 1 wavelength long, it will have current maxima 1/4 away
from each end (which is where you're feeding it).

I'd guess that it will provide a reasonable (although imperfect) match
to a 75-ohm coax... you might see an SWR of 2:1 or so on the coax.
The impedance seen by the rig will depend on the length of the
feedline, and a rig designed to drive a 50-ohm load might see a fairly
low SWR, or perhaps one as high as 3:1 or 4:1 (at a guess). If your
rig has a built-in ATU, it's probably adequate to flatten this sort of
SWR down to the point where the rig's finals are happy with it.

There's likely to be some amount of RF current present on the outside
of the coaxial feedline, due both to conduction at the feedpoint and
to induction from the wire (since the feedline is not located
symmetrically in the center of the radiating element). If this causes
sufficient "RF in the shack" to be a problem, you might want to add an
isolating choke where the feedline enters the building... and if there
isn't enough RF in the shack to cause problems, then don't worry about
it. In either case, it's probably going to be less RF-in-the-shack
than you'd get with an end-fed longwire, fed against the station
ground or a counterpoise.

Seems like a reasonable candidate for a "stealth" antenna, if you can
conceal the coax running up to the feedpoint, and use a thin-gauge
wire as a radiator (e.g. thin-gauge enamel-insulated magnet wire).

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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