[TowerTalk] Delta Loop Info and Opinions?

[Joe Giacobello]

Tom, thanks for your reply.  In all honesty when I read the article, it 
made sense to me because if one were trying to minimize the interaction 
between the delta and the tower, minimizing the RF current flow on the 
tower seemed logical.  I went back and rechecked the article to make 
sure I hadn't gotten it backwards, and I hadn't.  I am going to have to 
reconsider the whole issue in light of your explanation.

BTW, I did mention that the deltas were for 30 and 40M and I had planned 
on vertically polarizing them and feeding them with balanced line.  The 
reasons for using the loops are that I am looking for some gain and 
directionality on 30 and 40M with a low takeoff angle, simplicity and I 
have a lot of experience with quad loops.

I appreciate your input.

73, Joe

Tom Rauch wrote:

>>    * ON4UN in his book recommends detuning the tower by
>>    
>>
>running a wire
>  
>
>>      down the side in series with a variable capacitor
>>    
>>
>and tuning for
>  
>
>>      minimum current with an RF current probe.  However,
>>    
>>
>it seems that
>  
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>>      this would only work for one of the bands, and you'd
>>    
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>have to
>  
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>>      accept some interaction with the tower on at least
>>    
>>
>one of the bands.
>
>I always wonder where some of this stuff comes from.
>
>That's generally the opposite of how you should tune if you
>are measuring current. You would generally tune for MAXIMUM
>current in the sectionalizing drop wire, NOT minimum. That
>insures the sectionalizing area is resonant. Since it is a
>"loop", current flowing up equals current flowing down, and
>that canels radiation. The end-to-end impedance is highest
>at resonance, so the section you tune acts like a
>high-impedance parallel tuned network isolating the tower
>above and below that area.
>
>The exception to tuning this way is if the area below or
>above the tuned area is self-resonant. It also is not the
>preferred way to detune a structure, but the preferred way
>requires a FS meter.
>
>  
>
>>    * Does anyone have any experience with this detuning
>>    
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>trick?  Did you
>  
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>>      find that it was really necessary?
>>    
>>
>
>Well, I've done hundereds of them commercially. I'm not sure
>how necessary it is in a Ham installation, especially since
>you never said if the antenna was vertically polarized or
>what band it was on.
>
>  
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>>    * Do you have any suggestions for how to accomplish
>>    
>>
>detuning for two
>  
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>>      bands?  Two wires and two capacitors?
>>    
>>
>
>That works, but it interacts if the wires overlap the same
>area of the structure.
>
>  
>
>>    * Does a crankup tower alter this detuning requirement
>>    
>>
>in any way?
>
>Only if the connections are intermittent.
>
>I have a question, why are you using a delta loop or a loop?
>There are good mechanical reasons for using a loop, but
>generally electrical reasons are not good. Will this be
>vertically polarized?
>
>73 Tom
>
>
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>
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