Originally Posted by Michiel Kappeyne

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, 11 miles away. Amazingly, 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