In general these problems are very complex, requiring the kind of
solutions below, on a very individual basis. Modeling everything,
including tower, guys, etc in a vicinity, will allow you do an
effective job of breaking things up. Trying to accomplish this with
trial and error would be a real stroke of luck.
The more cluttered the landscape, the less chance of effectively
accomplishing decoupling. Phillystrand wherever possible definitely
helps.
Special cases of decoupling have been designed into plans. One of
these very few is having the tower in the center of a 4 square.
Detuning the tower can be accomplished by various means, including
inducing an off-phase current into the tower until zero current is
measured. Actually depending on the tower, the f/b of the array can be
improved over a standard 4-square out in the open, just by varying the
amplitude and phase of the current fed to the tower in the center.
In the case of a phased array (fixed?) with a near conductor, theory
suggests that active detuning for a single fixed array is possible.
But if you do something like switch the direction of the array, the
current fed to the offending conductor is now all wrong for the other
direction. It gets complicated very fast.
On 7 Dec 1999 05:06:33 -0000, you wrote:
>
To: <towertalk@contesting.com>
>> Date: Mon, 06 Dec 1999 10:37:48 -0600
>> From: "Kris Mraz, N5KM" <mraz@aud.alcatel.com>
>> Subject: [TowerTalk] Influence of tower in the near field
>>
>> Can the influence of a tower in the near field of an 80m array really
>> be removed by detuning the tower?
>>
>> I know that there are methods of detuning a tower to make it
>> invisible but I'd like to hear how affective it really is. Can those
>> of you who have done this provide any anecdotal evidence one way or
>> the other?
>
>Any floating conductor much over a quarter wavelength will have substantial
>coupling. If the conductor is grounded, then it only has to be somewhat
>more than 1/8 WL. Simply "detuning" a long tower away from resonance doesn't
>keep current from flowing; it merely changes the phase, as in tuning
>a Yagi element to be a reflector or director. To keep my 40 meter
>phased array from being affected by nearby 80 and 160 verticals, I have
>had to break those verticals up into 30 foot sections with insulators
>which are bridged with relays that close when on 80 and 160. I modeled
>this with EZNEC. The array is under constuction so no physical measurements
>yet. When on 80, the 40 meter verticals float from ground.
>
>Rick Karlquist N6RK
>rick@area.com
--. .-..
73, Guy
Guy Olinger, K2AV
k2av@contesting.com
Apex, NC, USA
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