Dave,
One of my past projects was a no-radial multi-band vertical with a feed
similar to what you propose. I found two alternatives for isolating the
feedline. The first was a 1.77:1 transmission line transformer that improved
the match and also served as a "current balun" or line isolator. The second
method was simply buried coax with the shield grounded immediately below the
antenna.
I should note that the bottom of the tubing was not the high voltage
point -- this was a "bent" vertical dipole with a portion of the bottom
element extending horizontally at about 4 ft. height. The coax exited the
tubing roughly 2/3 the distance from feedpoint to tip. With the latter of
the two isolating methods noted above, the distance from ground to feedpoint
was very close to 1/4-wavelength on 20M, providing some "natural"
decoupling. (The design of the antenna was such that feedline isolation was
of greatest importance on 20M.)
Hope this helps!
73 & HNY,
Gary, K9AY
> 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
> omni-directional alternative.
>
> 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.
>
> 73,
> Dave, G3UEG
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