On 9/26/2025 3:21 PM, Jack Brindle via TowerTalk wrote:
Jim’s method of installing verticals is to use a single piece of coax, remove the jacket and
shield for 1/4-wave to form the top portion of a dipole, then add a choke 1/4-wave down from the
“feed” point. No connectors needed until it gets down to the shack entry. Large
toroids are very useful here. Having said that, I am pretty sure Jim placed connectors at the lower
end of the dipole.
That's only one method I've shown for building a vertical, and that
choke is placed 1/4 wave (common mode) down the feedline from where the
shield is removed. The result is a center-fed vertical dipole,
eliminating the concern of the feedline distortion the pattern.
The antenna doesn't provide a predictable feedpoint Z without the choke,
which is really the bottom end insulator. The required choking impedance
depends entirely on the transmit power -- because it's at a very high
voltage point, dissipation can get high enough for destructive failure
choking Z is not high enough.
Here's my applications note on that design.
ttp://k9yc.com/VerticalDipole.pdf
73, Jim K9YC
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