On Fri, 1 Feb 2013 05:52:10 +0000, Michiel Kappeyne
wrote:

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

Oh-oh. We're in trouble now.

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...!

Altitude will get you better results than gain in this case. The idea
is to get over the ground clutter so that you can hear aircraft and
service vehicles on the ground. 15 miles is a bit of a stretch but
possible. The trick is to create an antenna that will hear well
overhead but also has most of the gain in the direction of the
airport. Presumably, your station and the airport are not moving.

What I suggest is an aircraft band yagi antenna pointed at the
airport. That should take care of the ground stations. However, that
does nothing for hearing airplanes flying overhead. For that, I
suggest a simple coaxial antenna, as Jim Lux has suggested. A length
of coax with 1/4 wave of braid peeled back over the coax will sorta
work. If you want something better, a simple 1/4 wave ground plane
antenna or a horizontal 1/2 wave dipole. Don't bother with a J-Pole,
Slim Jim, or anything with gain. The problem is that such antennas
"borrow" gain from the upwards direction, and use it to improve gain
at the horizon. That's NOT what you want for the "overhead" antenna.
Obviously, it is necessary to combine the two antennas. A Wilkinson
combiner will have about 1dB loss on receive, which can be easily
tolerated.
https://www.google.com/search?q=wilkinson+combiner&tbm=isch
One potential problem is that if both antennas receive the same signal
180 degrees otto phase, there will be cancellation. To reduce (but
not eliminate) this problem, try to position the overhead antenna
where the path to the airport is blocked by some obstruction (i.e.
chimney).

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?

The diameter of the elements has some effect on the cut length. So
does spacing between elements, coax impedance, where they're measuring
from, and how the end of the elements are cut. For example, a square
cut tubing will have a different effective length than one with the
end crimped and rounded, or with a hemispherical shaped end cap. The
idea behind the non-square tubing end is to increase the usable
bandwidth of the antenna.

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.

Calculations are nice, but when it comes to actually sweeping an
antenna for where the minimum VSWR is located, I often get surprises.
If you had a sweep generator, direction coupler, detector, and scope,
you could see the VSWR sweep change as the antenna is waved around.
The calculations are generally under near ideal conditions. Real
installation tend to be not so ideal.

My
antenna should center around 125MHz, as the most interesting
frequencies, Tower, Ground, tend to group at the lower end of the
airband spectrum.

Yep. However, I think you'll find that with a receive only antenna,
you can have quite a bit of VSWR and it will still work just fine.
Therefore, the antenna cuts and trims are far less critical for a
receive antenna than one that's also used for transmit. You might be
trying to over-optimize the design without any obvious benefit.

I understand that the bigger the diameter of the tubing is, the wider
the antenna's bandwidth.

True.

What would be a good trade-off diameter? Will
the resulting bandwidth be good enough?

Well, the -3dB bandwidth of the aircraft band is about:
20/150 = 0.15 = 15%
The -3db bandwidth of my 2m J-Pole is about:
http://802.11junk.com/jeffl/crud/144-Jpole-50-VSWR.jpg
17/146 = 0.08 = 12%
I could recalculate the antenna for 125Mhz, but I'm lazy. The percent
bandwidth at 125 would be the same as at 146Mhz. The smaller
bandwidth for the J-Pole would seem to be a problem, but isn't because
receiver can tolerate a higher VSWR without (much) loss in gain.

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

You're crazy. How will you know that your antenna is actually
optimized? Without test equipment, you don't have much of a chance.
Better to build something really simple, where the tolerances are not
as important or critical.

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?

The only way I know how to make sure your estimates are correct is to
either construct an NEC2 model, or to just build it and measure the
antenna gain and VSWR. Since a computer model tends to be less
expensive than copper, methinks that would be best.

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

Cut-n-Try? Well, good luck.

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

Well, you're off to a bad start. Got any RF test equipment?

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