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).
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