Topband: How To "De-Tune" A Nearby Tower ?

[W8AV@aol.com]

aa4nu@ix.netcom.com writes:

<< Anyone on here accomplished this "de-tuning" using
 a similar process ? Suggestions ?
 
 Is this the best method to use to try and "remove electrically" 
 the other vertical structures so that the new antenna idea can 
 be tested without undue influence from the other towers ?
  >>

Hi Billy,

    I have a base insulated 150 tower that is used as my 160 antenna right 
smack dab in the middle of the 80 meter array.  As a matter of fact the big 
tower supports the 80 meter array.  I used a coil between the tower and 
ground to detune it on 80 meters so that my triangle array would generate the 
proper pattern.  One thing that I had to do, though, was to put a parallel 
resonant trap tuned to 1830 or so in front of the detuning coil so that I did 
not short the tower out on 160.  I then used a b'cast field intensity meter  
to detune the big tower.  Excite the desired antenna element with a low level 
signal, say 10 watts or so.  Set the field meter up so that it points at the 
tower to be detuned and at a right angle to the  excited element and tap down 
the coil until the signal gets to a minimum.  The culprit is then detuned.  
You can do this with grounded towers too but you need to drop one to three 
skirt wires off the tower, bond them to the tower at the top and insulate 
them down the tower.  Then make a ring buss at the bottom and attach your 
detuning coil between the ring buss and ground.  Use the same procedure to 
detune the tower as mentioned above.  I needed about 8 uH of coil to detune 
my 160 antenna on 80.  I arrived at this value by modeling the tower to be 
detuned with EZNEC.

73.......de Goose W8AV



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