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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|