Topband: Tower detuning dilemma

[David Raymond]

I have an issue involving tower detuning for the wisdom of the group.

I have a 190' Rohn 25 tower which supports a 160m four square and, nested inside of that, an 80m four square.  The elements are full length, conventional ground mounted wire verticals supported by catenary ropes.  The arrays are sitting on top of a fairly substantial copper radial field.  I initially had the 160m array in K8UR configuration (bent dipole or lazy Vee).  While the array performed OK I was disappointed with the f/b ratio (about 12 db f/b measured, confirmed by EZNEC).  I subsequently reconfigured the array to ground mounted verticals and saw a substantial improvement in f/b (also confirmed by EZNEC).  I was seeing about 16 db or 17 db f/b.

EZNEC also showed that by eliminating the bottom 100' of tower (detuning) I could improve the f/b ay about another 5 db.  I mounted a side arm at the 100' level on the tower and ran a #18 Copperweld wire spaced about 12' from the tower down to a control box at the four foot level.  I resonated the wire with about 86 pF of capacitance.  My f/b improved by the predicted 5 db showing about 22 db f/b.

All was well for several years.  Sometime in the past 12 months I noticed about a 14 db drop in the f/b for the array.  I did a number of things including inspecting each element, the coax feeding the elements, measured power dumped to the dummy load, etc.  I finally replaced the Comtek controller with a spare.  In the process of installing the spare controller I left the ground return wire from the detuning network loose.  I noticed I had my original 17 db f/b ratio back.  When I connected the ground wire of the tuning network I lost about 10 db of f/b.  As soon as I remove the ground side of the detuning network, I pick up the lost 10 db of f/b.  The situation is the same with both the original and spare controllers.  

I looked further, suspecting maybe something like an open or partially open shield on a coax connector or perhaps a problem with the ground radial system.  Everything seems to be normal.  Further inspection shows that the connectors are all tight and coaxes are normal.  Just to make sure I unscrewed all coax connectors and reinstalled them.  Connections between the elements and radial system are all solid.  

I have good continuity up through the detuning wire and down through the tower (maybe an ohm or two).  Anyone have an idea what might be going on?  

73. . Dave
W0FLS