Topband: Detuning a nearby Tower

[Carl Braun]

Hello Topbanders

I have a two element array 1/8wl away from my 90' Skyneedle tower with Telrex yagi atop.  I want to electrically isolate this tower from the array by dropping a shunt wire from near the top of the tower down to the panel where I have an MFJ model 931 artificial ground (input) that then connects to my earth ground.  Tuning the 931 to eliminate the RF field strength induced from the array results in an invisible tower to the array...in theory.

In the past, I have used a similar method to shunt feed the tower on 160 ala vacuum variable tuning and radials.  When I tapped the tower I found a sweet spot at the ~70' level on the skyneedle that provided a fairly good 42 ohm match and, once tuned, it worked pretty well on TX.

Here is the concern...I would prefer to use this same tap point for my detuning experiment as it seems to make sense that the same point would induce the max coupling from the array...or at least be a good starting point.  ON4UN and others have implied that, when detuning a tower, you should tap it at the TOP of the tower structure and not ¼ of the way down from the top like I's considering.

Should I just run the 3' gamma arm off of the very TOP of the tower at the 90' level then down or am I better off tapping the gamma arm at the 67' level as I did when I shunt fed the tower for TX on 160?

Comments?

Carl