On Thu, 20 Feb 2014 20:38:22 -0800, "Sal" salmonella@food
poisoning.org wrote:


"Jeff Liebermann" wrote in message
.. .

I'm not sure of the frequency of your J-pole. The inductance of about
6 cm of #12AWG solid wire is about 0.05 uH.
http://www.consultrsr.com/resources/eis/induct5.htm
To resonate at 146Mhz, that would be about 24 pF. 50 Mhz would be
about 200 pf.

That's enlightening.

One must suffer before enlightenment.

I need to admit that no such science was applied. I just tried small caps
from my stash to see what would happen. When something good happened, I
remembered it for next time.

Well, I was a bit more scientific when I tried it about 10 years ago
in an effort to improve the "American Legion" J-pole. This is from my
memory, which is notoriously unreliable.

I had the help of a return loss bridge, sweep generator, and
oscilloscope.
https://www.google.com/search?q=return+loss+bridge&tbm=isch
I could tweak the antenna, and see the results to the VSWR
immediately. I was finding that the J-pole has a rather narrow
operating frequency range and I wanted to know if something could be
done about it. Mounting the connector on the base resulted in the
narrowest usable bandwidth (at VSWR = 2:1) but also the lowest VSWR.
Using the more conventional feed points furthur up from the base, the
minimum VSWR was horrid, but the bandwidth was wider. Inserting a
series trimmer or wrapping the insulated wire around the 1/4 wave rod
reduced the minimum VSWR, but narrowed the bandwidth slightly.
Replacing the 1/4" dia aluminum rods, with 3/4" copper water pipe
increased the usable bandwidth somewhat, but made the insulated wire
wrap capacitor very small and critical. This may sound very
scientific, but in reality, it was all done in about an hour, with no
photos or recorded data. I had to return the equipment that
afternoon.

About a year ago, the local radio club had a beginners session on
building "emergency antennas", which means a VHF Slim Jim and J-pole
type antennas made from 300 ohm twinlead. The only test equipment
available was an HT and VSWR bridge. I predicted problems and was
invited not to show up. I did anyway, near the end. The problem was
that the antennas were VERY sensitive to their surroundings. It was
very difficult to cut one to length for minimum VSWR, while people and
metal objects moved around the room.

The j-pole that seems to benefit most is the 2m version, which I've built
the most of. Perhaps we're not dealing with just the inductance of that 6
cm of wire. There could be some residual inductance in the rest of the
antenna which is being brought to resonance. Maybe when my element lengths
are not optimal, some inductance would be found there. Not sure why the
residual reactance would always be inductive. The first time I add a cap
and it gets worse, then I'll know it isn't so.

Dunno. I can throw together a 4NEC2 model of a j-pole and see what
might be the feed point impedance for various styles and locations. No
time for that for a few days.

Maybe I should experiment with varying the spacing across the stub and see
what that does, with and without a cap.

It's more fun to fire up the 4NEC2 optimizer, and see what it comes up
with for the best possible antenna given a collection of restraints.
Yet another project...


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558