Thank you very much for all your responses! The spectrum of thought, and wide array of opinions, are fascinating and educational.
There are clearly several roads that lead to Rome.
Distilling the narrative I extract the following:
- Antenna height dominates. It is hence a good thing I live in a Manhattan apartment building at 250ft elevation, with a large window facing West and an unobstructed view of New Jersey. (Of course I can forget about listening to JFK or LGA.)
- With neither antenna-building expertise nor dedicated rf measuring equipment, it is better to stick to a straightforward design such as a dipole or Yagi with two or three elements, or even a piece of coax, where dimensions either require less precision or can be realized more easily. I'll definitely continue the experimentation. And oh, toss my copper tubing lunacy...!
- Even so, I should not expect jaw-dropping improvement over my airband rubber duck or even the $30 ladder line Slim Jim I bought. After moving that around a bit it's working quite well now and pulls in airplanes taxiing around Newark Airport, 11 miles out, depending on atmospheric conditions. Newark Tower, which transmits with more power at higher elevation, comes in intelligibly most of the time. Both the Slim Jim, its horizontal gain notwithstanding, and the duck capture nearby airplanes up to 8,000 ft or so. Most air-to-ground communications are at lower altitudes anyway.
Many thanks again for all your comments! I'll post updates to this thread in case anybody is interested in my novice attempts.
Cheers,
Michiel
P.S. On a different note, I'll be putting a perfectly restored Sony Pro80 in as-new operational and cosmetic condition on eBay in the next couple of weeks.
Quote:
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
|