On Sat, 2 Feb 2013 22:46:41 +0000, Ian Jackson
wrote:
Some of the guys in this NG seem to be making things unnecessarily
complicated and confusing.
That would be me. Thanks.
The Slim Jim / J-Pole is essentially just a halfwave, endfed via a
quarterwave stub (in HF terms, a "Zepp" antenna).
Sorta. The original Zepp antenna was intended for use on a Zeppelin
dirigible, because the entire antenna is at DC ground and the voltage
peak is as far away from the gas bags as possible. It was in the form
of a J-Pole, with the 5/8 wave section dangling downward. I just
Googled for pictures of Zeppelins but never found one showing a
trailing Zepp antenna.
I don't have a clue how the Slim Jim design arrived, but my guess(tm)
is that it coincided when people started making J-Poles out of
twinlead, and found that the extra wire would be tolerated. Using
Google (search by date range), it seems to have arrived in about 1990.
It's a pretty good antenna.
I prefer a folded dipole and balun or a collinear dipole (as in
AMOS/Franklin) and balun antenna. I consider the complexity of
construction about the same, although I will concede that end fed
antennas are much easier to mount than center feed antennas.
For receiving, the dimensions shouldn't be that critical.
Agreed. You could make the antenna out of barbed wire and it will
work. The problem is that you won't know how good or bad the antenna
operates without measurements, simulations, or comparisons. For
comparison, I carry a simple telescoping dipole with me. When someone
claims that they're "system" isn't working well, I compare it with the
dipole. I'm often impressed with the ability of some antennas (mostly
mobile antennas) to send most of the RF towards the sky, while leaving
little towards the horizon. However, that shouldn't matter here,
where a hemispherical pattern, that covers the entire sky, is the
goal. Maybe a discone?
However, a quick Google brings up this calculator:
http://www.m0ukd.com/Calculators/Slim_Jim/index.php
Ahem. Although the author mentions using twinlead and ladder line,
there's nothing in the calculator to compensate for the velocity
factor. The design shown is apparently for one built out of rod and
tubing.
Incidentally, our local radio club (K6BJ) had an antenna construction
session during the Oct 2011 meeting. The theme was how to build an
emergency antenna. The Slim Jim variety was the most common form.
There were numerous construction articles found online and in books
and magazines. Various pieces of test equipment were available. I
didn't participate because I wasn't sure I would arrive on time, but
eventually showed up near the end. I could almost cry. Most of the
antennas were hung from the suspended ceiling, which was metal. The
most sensitive point of the Slim Jim is near the top, causing
frequency sweeps to change drastically when moved. Several people
used the same design, the same roll of ladderline, but had various
interpretations of how to cut the antenna. That had some effect on
VHF, but was fatal at UHF. I gave a fast demonstration of how
proximity to metal, and construction variables can have a big effect.
There was plenty of head scratching and head shaking, but eventually
everyone threw together something that worked. I had an entertaining
diversion diagnosing a very nicely built Slim Jim, that had conductive
shrink tube slipped over the gap. The meeting was over before I had a
chance to connect these antennas to an HT and run a live test to a
distant repeater. My guess(tm) is that they wall would have worked,
but to varying degrees.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558