In message , Michiel Kappeyne
writes

Hi all,

New to this forum, I am so impressed with the level of expertise I find
here.

Thanks for welcoming me.

I want to build a Slim Jim antenna for receiving airband (118-136MHz)
out of 3/8" or 1/2" soft copper tubing. This antenna won't see any
ouside use--it will hang away from any metal against my window facing
the Hudson River towards Newark Airport, 15 miles away. Amzingly, my
little 8" helical rubber duck works as well as a ladder cable-based
125MHz Slim Jim I bought, so I want to step it up...!

There are many, many stories floating around the Internet about
homebrew
Slim Jim projects, but what strikes me is that everybody seems to
arrive
at different lengths for a 3/4 wave in the 144MHz band (where I find
most Slim Jims), even allowing for differing propagation speeds in the
various materials these builders use. What am I missing here? I thought
the calculation was as straightforward as the wave length in open air
adjusted by the velocity factor, about 0.96 for copper tubing. My
antenna should center around 125MHz, as the most interesting
frequencies, Tower, Ground, tend to group at the lower end of the
airband spectrum.

I understand that the bigger the diameter of the tubing is, the wider
the antenna's bandwidth. What would be a good trade-off diameter? Will
the resulting bandwidth be good enough?

I am leaning towards 3/8" as happy medium, unless one of you tells me
I'm crazy.

Anyway, I won't solder any elbows at the two ends; I will loop this
tubing using a bending spring. Should I compensate for these round ends
in my length calculations, or won't that matter? How critical is the
distance between the two parallel sides? I can't find any information
on
that either. Can I leave the air gap bare, or should I treat the tube
endings to some kind of solid, perhaps adjustable, plug?

I'll devise a slide for the coax connection so I can adjust it for best
matching.

I am a complete novice when it comes to building antennas, and for now
can only contribute my thanks for your advice.

Some of the guys in this NG seem to be making things unnecessarily
complicated and confusing.

The Slim Jim / J-Pole is essentially just a halfwave, endfed via a
quarterwave stub (in HF terms, a "Zepp" antenna). It's a pretty good
antenna. For receiving, the dimensions shouldn't be that critical.
However, a quick Google brings up this calculator:
http://www.m0ukd.com/Calculators/Slim_Jim/index.php
--
Ian

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

On Saturday, February 2, 2013 6:10:44 PM UTC-6, Jeff Liebermann wrote:
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?

Dunno.. I've done a lot of air band listening, and myself, I
think it's best to concentrate of the tower, and letting the
airborne chips fall as they may. Unless one is real close to
the airport, the tower, ATIS, and other ground signals will
usually be the toughest to hear.
The idea of using a *short* yagi actually works quite well in
most cases. You can hear the ground stations, and still most
of the others in the air also. Most of the airborne signals
will be fairly stout, and a rubber ducky would pick them up.
The more gain the yagi has, and the tighter the pattern,
the better the tower, but the worse the airborne.
This is why I would stick with short 2-3 el yagi's if going
that route. You don't want too much gain.

I've got a simple 3 el yagi cut for 2m, and I'd often use
it for air band. I could tweak the direction for best tower
signals, and with the antenna out of tune, the pattern is
not sharp, and would pick up most everything well enough.

Using that, I could receive ACARS from cruising jets up to
about 300 miles. From Houston, to about the OK border or so..
I could follow Southwest jets from KHOU to KDAL, and I could
hear them going into Dallas until they descended to about 9000
feet or so. And that's with the 2m yagi in no particular direction,
or aimed to favor Hobby.. With an out of tune yagi, sometimes
the max signal does not jive with the normal direction..

Anyway, I've found that it's best to favor the ground signals,
and not worry much about airborne. You will hear those with
little trouble. And this is the main reason I take the
decoupling into consideration if using a simple vertical.
You will want good decoupling to do well on the low angle
ground signals. A sky warmer will not be your friend in this
case.