[TowerTalk] ground plane problem help needed

[Jim Pruitt]

About 4 years ago I picked up 2 bottom sections of what I think were 11 
meter ground planes.  The section looks like the bottom section for a 
HyGain 18AVQ vertical or similar. The only difference is the mounting 
flange has a plate that holds what looks like about ½" diameter ground 
radials.  The issue I am having is that I used that mounting section to 
make a 20 meter vertical.  Easy enough, right? WRONG.  I used mating 
tubes to make the ¼ wave 20 meter vertical but it resonated at...ready 
for this.... 39mhz!  Then I grabbed the second one and used it.  Now it 
resonates at 13.5mhz but shortening the vertical does not bring the 
frequency up.  I am wondering if perhaps someone (CBer maybe) ran way 
too much power through them and broke down the dialectic of the plastic 
(or whatever that is) insulator that isolates the vertical from the 
mounting bracket and ground.  That is the only explanation I can come up 
with as to why it is so far off.

Does anyone have any suggestions as to how to fix the problem?

Thank you.

Jim Pruitt
WA7DUY