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Author Topic: Dipole question  (Read 463 times)
N9DO
Member

Posts: 55




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« on: March 23, 2004, 04:39:54 PM »

This may seem a like a trivial question, but I've always believed in following the instructions to the letter and then deviate from them if necessary if it doesn't work.
 
I'm putting together a unique dipole antenna from drawings I found in a magazine.
The transformer line from the center insulator to the coax feedline is 75 ohm with a length of 9' 8 3/16". The drawings give this length from the tip of the pl-259 to the bottom of the center (dogbone) insulator, yet showing the wires reaching out toward each leg of the dipole.
 
My question is: there will be approximately 3" of the transformer line that will go thru the eye on each side of the center insulator to each dipole leg. Should this length be considered as part of the total length, (9' 8 3/16"), or is the piece of transformer (75 ohm coax) to reach each leg of the dipole in addition to the given length.
 
Or does it matter at all. I've built many dipoles in the past and have never considered the 3" or so necessary to reach the dipole legs. Maybe that's why
they (in some cases) haven't worked too well.
 
Thank you for your help.
 
73,
de N9DO
George H
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KC8AXJ
Member

Posts: 303




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« Reply #1 on: March 23, 2004, 07:26:39 PM »

I beleive the design factor of the coax length (being it's not the common 50 oHm) is a impedence matching trick.

As far as the dipole fed end, yes when you "split" that coax open it becomes a dipole (or a part of your dipoles' length electricly) of sorts. It's length though (3 inches) if you are in HF, as you have found in the past, really doesn't have much of a factor,resonance wise. Now in VHF/UHF, 3 inches is HUGH.

Good Luck !
Steve  
       


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WB2WIK
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Posts: 20666




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« Reply #2 on: March 24, 2004, 12:58:58 PM »

The answer depends on the operating frequency, and what percentage of a wavelength is represented by that six inches (3" per side).

On 80 meters, a 6" change in dipole length is insignificant, as it will change resonance by only about 15 kHz; however on 10 meters, a 6" change in dipole length is quite a bit, close to 1 MHz, which can dramatically change how the antenna tunes.

WB2WIK/6
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