Tower "De-tuning"

[Frank Donovan]

Hello Lex!

I detune my towers so that they do not interfere with nearby HF
vertical arrays.  I'm not sure that this will be helpful in your
application of trying to avoid receiver overload from a nearby high power
AM broadcast station.

The approach I would take in your case is to electrically isolate (ie
insulate) the driven element of your antennas from the supporting boom or
mast.  This should be easy to accomplish by replacing the existing 
mounting mechanism with a piece of high pressure phenolic laminated sheet,
drilled for u-bolts to mount to the element to the plate, then the plate
to the boom.  

In the industrial plastics world this material is known as "Grade LE
Phenolic Laminated Sheet" and any industrial plastics company should have
it.  The material is extremely strong and completely impervious to weather
(I've had many pieces in use for over 25 years with no significant
deterioration!) Standard thicknesses include .250  .312  .375  .437  .500"
If u can't find a supplier, I purchase mine from Read Plastics in 
Rockville MD, (301)881-7900.  

In addition to isolating the driven element from other metallic objects,
I'd also take steps to assure that power from the broadcast station is not
induced onto the shield of your feedlines, then induced into your driven
element and receiver!  Such steps might include feedline chokes (ferrite
beads or coiled coax) at each end of the feedline, as well as 1/4 wave
intervals (at the BC station freq) in between.  You might also consider
burying your feedlines, or at least running them on the ground.

To get back to your question regarding how I detune my towers, I do it by
using computer models of my towers and HF vertical arrays.  In the model, 
I attach a horizontal wire to the tower at a convenient attachment point
(I usually attach it at 75-85% of tower height).  I then very the length
of the wire until the pattern of the HF vertical is unaffected by the
nearby tower(s).  After determining the required wire length with a
computer model, I then implement the results of the modelling on the real
tower, and I've always had excellent results!!

Good luck!

73!
Frank
W3LPL
donovanf@sgate.com