[TowerTalk] vertical antenna feedpoint

[john at kk9a.com]

I have been doing the opposite for decades.  I shorten the vertical slightly
and use a hairpin coil to create a 50 ohm feedpoint.  Is one method better
than the other?

John KK9A


Jim Brown K9YC wrote:

<snip>

Once you've got a decent radial system, modeling software like NEC can 
be used to design a simple way to match it to 50 or 75 ohm coax. The 
concept is to increase the length of the antenna to move its resonance 
down in frequency far enough that the resistive component of the 
feedpoint impedance is 50 ohms, which makes it look inductive, which can 
then be tuned out with a suitable capacitor. "Suitable" is this case 
means of a type that has very low equivalent series resistance and a 
sufficiently high voltage rating for the power you want to run. I simply 
added enough length to the horizontal top of my 160M Tee to resonate it 
down in the top portion of the AM broadcast band. I think I ended up 
with capacitance in the range of 800-900 pF.

73, Jim K9YC