I made quite a few vertical 1/2 wavelength dipoles from 30 MHz up but
the same principles should work at lower bands albeit much larger!
What I did was to run the coax down inside the lower 1/4 wavelength
tube. To make this work, three things had to be done. The lower tube
had to be at least 3-5 times the diameter of the coax or at least
1.5" inside diameter if using RG8 coax. Next, the coax had to be
centrally supported within the lower tube and thirdly, I placed
several ferrite beads on the coax at both the feed end (in the center
of the antenna) and where the coax exited the lower tube. The later
beads can be external to the tube. The VSWR was about 1.5:1 but that
is not a problem.
At 12:38 PM 12/29/2009, David Gould wrote:
>This may be slightly off-topic but hopefully not too much!!
>I would like to build a vertical dipole for 20m with the bottom about
>1m off the ground, to compliment a beam, to give me a quick
>What are the potential problems, or issues to be aware of if I were
>to take the coax feedline down the centre of the tubing that makes
>the lower section of the dipole. I have seen several alternatives
>where the coax is taken away from the centre of the dipole parallel
>to the ground, but I would prefer not to do this.
>Any thoughts gratefully received. I guess a good feedline current
>choke might be a good idea at the base.
>Happy New Year to everyone and thanks for the various help over the years.
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