In article , wrote:

What kind of metal was behind the window frame?

The house was old enuf that I'm sure it was made of wood, and
since it was a PICTURE window, there were no sash weights, either.

So the question remains: are J-poles sensitive to nearby materials?

That depends on the materials. Conductive, or lossy-dielectric
materials seem to have a significant effect especially when located
near the high-impedance points.

When I built a ladder-line J-pole and hung it next to my house's
outside wall, it de-tuned quite badly. Its performance stank so badly
that I could barely reach a nearby repeater.

When I hung it by an inside wall, it was not de-tuned enough to affect
the radio's performance appreciably (I didn't have an MFJ meter at the
time and so don't know the actual degree of de-tuning). I was able to
hit the repeater quite well even though the antenna was further inside
the house.

The first (outside) wall was stucco... with embedded chicken wire.
The second (inner) wall was drywall and lumber.

I suspect that exterior brick, or stone, or wet wood, would fall
somewhere in between the two in terms of de-tuning potential.

My guess is that the classic J-pole may be somewhat more vulnerable to
being de-tuned by nearby materials than a center-fed half-wave
radiator. The quarter-wave matching section is often fairly critical
of adjustment - I've read reports that even small changes in the
configuration of the upper potion of the matching section (e.g.
material near the open end of the stub, or changes in the spacing) can
cause some fairly large changes in the J-pole's feedpoint impedance.
Standard J-pole antennas seem to work most stably when built of rigid
materials and mounted "in the clear".

I haven't read any reports of actual experiments which attempted to
quantify the tuning stability of the classic J-pole, compared with
variants like the Arrow (a.k.a. Cebik's nontypical design), the
open-sleeve, and the sperrtopf coaxial design, or compared with a
center-fed dipole or other varieties of halfwave radiator. It'd be an
interesting study to try to put together.

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