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)
>
--
Thank you!
David Needham, AA4VT (formerly K4AJA and K8AJA)
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