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