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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