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Author Topic: 2Mtr Antenna from tubing (I'm a newbie, be gentle)  (Read 1098 times)
GM6WCF
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Posts: 28




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« on: March 29, 2012, 04:54:45 AM »

I have the material to build a 2Mtr J pole or Slim Jim antenna from copper or aluminium tubing. All the designs I've seen so far require the use of elbows at the 'corners'. Is there any reason why I shouldn't cut slits across the tubing at places where bends are required and then flatten the tube in those areas? The 'strips' so formed could then be bent around a former of the correct dimension. Is the change in shape from tube to strip likely to effect propogation?
Dave B (Scotland)
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AB4TI
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« Reply #1 on: March 29, 2012, 06:09:58 AM »

I think as long as the measurements are the same the method of construction doesn't matter. Just go for it and post your results. Experimentation is what it's all about!
Good luck, & 73 de Dan AB4TI
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WB6BYU
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« Reply #2 on: March 29, 2012, 09:15:44 AM »

The elbows are faster because all the cutting can be done with a tubing cutter.

But I'm sure a thrifty Scot would find a less expensive way to do it.


It won't affect anything significantly.  You can make them out of wire, angle stock,
tubing, or solid rod just as easily, or anything else you happen to have handy.

In my experience the ideal method is to make the matching section somewhat longer
than required, and provide some sort of sliding short circuit across the bottom to allow
you to adjust the length.  That's because the stub length depends on the characteristic
impedance, which depends on the diameters and spacing of the conductors.  Being able
to adjust both the length of the stub and the tap point makes life much easier when
you are building a new design (rather than copying an existing one.)
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KE4DRN
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Posts: 3734




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« Reply #3 on: March 29, 2012, 06:51:08 PM »

hi Dave,

you can put some fine sand or table salt inside the tubing
and seal the ends with tape, this keeps the tubing from
collapsing when you bend it with larger diameter pipe as the form.
Then remove the sand or salt and your good to go.

Not to talk you out of the Jpole, I'm a big Moxon fan,

http://www.moxonantennaproject.com/wb5cxc/wb5cxc.htm

http://www.moxonantennaproject.com/

73 james
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GM6WCF
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« Reply #4 on: March 30, 2012, 04:39:40 AM »

Thanks for all the interest and replies. I'm going with the Slim Jim because my initial interest is in receiving to check out activity in the area after a long absence - then portability to higher terrain for 'exploring'.
Thanks for the tips on bending the tube. Such will be of use if I build a copper loop.
Re matching, I've seen some interesting ideas including sliding feed points as well as variable capacitors, made from strip material, across the gap for fine tuning. These ideas are in my mind for possible incorporation.
I've read up some on the Moxon and from what I have read it does look like a candidate for a later project, probably when I have constructed a mast.
I have a question concerning a shielded loop I'm making and a diagram in the ARRL Antenna Book. If anyone cares to follow the thread it's under ARRL Antennas - Circuit Question.
Thanks again
Dave B
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WB6BYU
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« Reply #5 on: March 30, 2012, 06:39:58 AM »

The standard J-pole and the "Slim-JIM" are equivalent, since the two radiator wires of the latter are
effectively in parallel.  I'd go with the standard version because it requires fewer bends and less
material.  Performance should be the same in either case - subject to the usual issues with common
mode current that are endemic to all J-pole type antennas.
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