"Michiel Kappeyne" wrote in
message ...
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.

Cheers,
Michael

In simple terms the velocity factor of the antenna is determined by the
insulating material around the antenna.
There is another factor called the diameter to wavelength. That is how big
around the antenna is. That number usually runs from .94 to .98. The
larger the diameter the antenna is in relationship to the wavelength will
shorten the antenna also. When you go from a wire such as twin lead to
copper tubing for the slim jim there is a big ratio change. The graph here
may help you see that effect.
http://www.radio-electronics.com/inf...on-formula.php

So not only does the diameter effect the bandwidth, it also effects the
length.
For receiving I doubt that you will notice much differance in the bandwidth
as far as the signal strength going from a piece of number 14 wire to 3/8
inch tubing.