If you use a razor blade to make a shallow slit in the Romex directly
over the ground wire, you can just pull the ground wire out without
significantly affecting the insulation. If you do that on a long enough
section, you could just measure the transmission line impedance.
73,
Dave AB7E
On 2/22/2023 10:32 AM, David Needham wrote:
Anyone's guess on the transmission line impedance as described? I've found
calculators for 2 conductors in free space, but I'm scratching my head on
what the Romex covering will do for the dielectric constant. So far my 120
ohms seems like a decent guess, but I wouldn't mind having something better
than my guess.
Dave AA4VT
On Wed, Feb 22, 2023 at 12:29 PM David Needham <aa4vt.dave@gmail.com> wrote:
Thanks Jim and others for your replies and ideas.
Agreeing with Jim, the transmission line is made from 14/2 with ground
Romex. The ground wire is pulled out leaving the dead space. I measured a
random piece I have here and it is about 5.5 mm between the centers of each
of the conductors, with about 3 mm of dead space between them when the
insulation is included. It is pulled relatively tight when installed with
the ring terminals, so I'd assume the spacing is somewhat maintained.
I'll be sure he uses a good CM choke setup per Jim's advice as well.
On 2/21/2023 9:47 AM, David Needham wrote:
I came
up with that the impedance of the transmission line must be about 120
ohms
to get the approximate curves with the stock dimensions.
A pair of #12 THHN taped together is in the range of 105 ohms (two
colors to keep track of polarity). Twisting the pair would likely raise
Zo slightly. I see zero reason for a third conductor. With a line that
short, I would not expect Zo to be critical.
We've often used a C3SS when operating portable for CQP and 7QP county
activations, and found that it is very picky about chokes. It does like
the right number of turns of #12 THHN around a 2.4-in o. d. #31
Fair-Rite toroid.
73, Jim K9YC
--
Thank you!
David Needham, AA4VT (formerly K4AJA and K8AJA)
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