[TowerTalk] Fair rite materials for choke baluns

[jimlux]

On 7/5/16 7:22 AM, Jim Thomson wrote:
> From: jimlux <jimlux at earthlink.net>
> To: towertalk at 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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