[TowerTalk] Force 12 C31XR Modeling

[David Gilbert]

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 at 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)
>>
>