[TowerTalk] Zo of romex

[Lux, Jim]

On 2/22/23 2:06 PM, Leeson wrote:
> On 2/22/23 11:46 AM, Jim Lux wrote:
>
> > I'd try an epsilon of 2.  Most plastics are around 3 (depending on
> > water content), but there's some air space too.
> >
> > Twisted pair with PVC or PTFE insulation has a Z0 of about 100-120
> > ohms -
> > You could measure it if you were ambitious.  It's a short piece, isn't
> > it? Way less than a wavelength? it might not make much difference.
>
> If you're really curious about the Zo of a transmission line, you can 
> measure it with almost any antenna impedance instrument: At one end, 
> put a resistive load you expect will be a mismatch, say ZL = 50 ohms. 
> Connect the other end to the SWR instrument, and measure the highest 
> impedance you see over a range of frequencies high enough to include 
> at least one quarter wavelength. There will be a peak impedance, which 
> is Zo^2/ZL. The rest is up to your calculator.
>
> Of course there will be potential inaccuracies from frequency 
> variation of Zo, and from the parasitic reactance of, for example, any 
> lead length at load or instrument. But you'll be plenty close enough. 
> If the impedance at 1/4 wave is too high for your instrument, try a 
> higher ZL.
>
> "Experiment trumps theory"
>
> Dave, W6NL/HC8L
> _


I'm waiting for the discussion on preferred ways to install a connector 
on NM-B (Romex), whether placing a plastic owl near it changes the Z.
My NanoVNA has SMA, that's going to be a bit challenging, but I think I 
can make it work.  (yeah, or use a SMA/BNC adapter to a BNC-binding post 
adapter - but then we'll have all those arguments about whether the 
adapters are adding loss or parasitics, etc).