[TowerTalk] Delta Loop Info and Opinions?

[Joe Giacobello]

Mike, thanks to you and Tom, I now see what's happening here.  Defining 
the arrangement as a parallel tuned circuit makes sense.

BTW, I do plan on modeling the antennas before erecting them.  I'll let 
you know how I made out.

73 and thanks for your input.

Joe

Michael Tope wrote:

>If you have a NEC program like EZNEC, Joe, you can actually
>model a detuning shunt arm and then look at the current in the
>tower and in the shunt arm. When the properly adjusted the
>current inside the shunt arm/tower loop actually peaks at the
>same time the series current below the shunt arm/tower loop
>connection goes to a minimum. It seems a little counterintitutive
>at first, but makes perfect sense after you think about how a
>parallel resonant circuit works (the section of tower and the
>parallel shunt arm form a big parallel resonant tank circuit)
>that effectively inserts a high RF impedance in series with
>the tower).
>
>73 de Mike, W4EF.............
>
>----- Original Message -----
>From: "Joe Giacobello" <k2xx at swva.net>
>To: "Tom Rauch" <w8ji at contesting.com>
>Cc: <Towertalk at contesting.com>; <antennas at mailman.qth.net>
>Sent: Thursday, September 16, 2004 9:26 AM
>Subject: Re: [TowerTalk] Delta Loop Info and Opinions?
>
>
>  
>
>>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
>>>
>>>
>>>      
>>>
>>>>     this would only work for one of the bands, and you'd
>>>>
>>>>
>>>>        
>>>>
>>>have to
>>>
>>>
>>>      
>>>
>>>>     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
>>>>
>>>>
>>>>        
>>>>
>>>trick?  Did you
>>>
>>>
>>>      
>>>
>>>>     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.
>>>
>>>
>>>
>>>      
>>>
>>>>   * Do you have any suggestions for how to accomplish
>>>>
>>>>
>>>>        
>>>>
>>>detuning for two
>>>
>>>
>>>      
>>>
>>>>     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
>>>
>>>
>>>_______________________________________________
>>>
>>>See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless
>>>      
>>>
>Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any
>questions and ask for Sherman, W2FLA.
>  
>
>>>_______________________________________________
>>>TowerTalk mailing list
>>>TowerTalk at contesting.com
>>>http://lists.contesting.com/mailman/listinfo/towertalk
>>>
>>>
>>>
>>>
>>>      
>>>
>>_______________________________________________
>>
>>See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless
>>    
>>
>Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any
>questions and ask for Sherman, W2FLA.
>  
>
>>_______________________________________________
>>TowerTalk mailing list
>>TowerTalk at contesting.com
>>http://lists.contesting.com/mailman/listinfo/towertalk
>>
>>    
>>
>
>
>
>_______________________________________________
>
>See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
>
>_______________________________________________
>TowerTalk mailing list
>TowerTalk at contesting.com
>http://lists.contesting.com/mailman/listinfo/towertalk
>
>
>  
>