[TowerTalk] Feedline (choke) question

[Jim Brown]

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