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Re: [TowerTalk] Fair rite materials for choke baluns

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Fair rite materials for choke baluns
From: jimlux <jimlux@earthlink.net>
Date: Tue, 5 Jul 2016 07:38:13 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On 7/5/16 7:22 AM, Jim Thomson wrote:
From: jimlux <jimlux@earthlink.net>
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Fair rite materials for choke baluns


On 7/4/16 12:26 PM, Jim Brown wrote:

An easy to build test fixture would be to use the choke as the end
insulator of an end-fed center-fed dipole, as I've shown a couple of
places, and shove high power at a high duty cycle into the dipole.

end-fed center-fed? What's that?  Do you mean coax into one side of UUT,
then of the two leads on the other side, connect one to "half
wavelength" of wire, and then what does the other side go to? The
antenna support?  Or, since you're looking at common mode, both "output"
wires of the choke go to the same place?

http://k9yc.com/VerticalDipole.pdf

##  Look at the PDF.  Think of a dipole.. but turned vertical.
With coax  going to center, and center conductor bonded to upper wire half,
BUT, the lower wire leg, is replaced by the braid of the coax itself.
A  CMC is inserted where the lower insulator would normally be placed on a wire 
dipole.
Below the  coax CMC... is just a continuation of the same coax... all the way 
back to the xcvr.


Got it..
much like a sleeve dipole, then. (or even the old 2m isopole, but with, hopefully, better decoupling at the lower hot end)

So, in any case, we're looking at stopping the unbalanced current flow, and the load impedance here is hundreds or thousands of ohms, relative to "ground". If one thinks about "where's the other conductor in the loop in which current is flowing?" that's basically capacitive coupling from the antenna assembly to earth and then back to your source, with the shield of the coax and the surroundings forming the rest of the loop.

As Jim (K9YC) had noted this is pretty easy to model if your goal is just to "reduce current" - make the impedance big, compared to the other impedances. But if you want to calculate power dissipation, that's a lot harder.




##  it’s a unique way to build a vertically  polarized dipole...using a coaxial 
CMC  as the lower
insulator. It also places the CMC at an extreme high V / high Z point...and Im 
surprised the
CMC  actually survives.

Jim   VE7RF
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