[R-390] R-390A RF DECK COAX CABLES IMPEDANCE
Jacques Fortin
jacques.f at videotron.ca
Mon Oct 24 19:58:26 EDT 2016
Hi Charles,
Thanks for your answers.
But I whish to explain why I wanted to know....
"All of the coax runs in the 390A are too short to support wave propagation
modes at the frequencies they are handling, so they are really operating as
simple unbalanced shielded cables, not as transmission lines. "
Agreed, as far as only a pure sine wave signal is traveling thru.
I wanted to use a replacement coax that is the closest to the original one,
especially regarding the capacitance.
" Note that the source and load impedances of the connected circuits are not
matched to the coax's characteristic impedance."
OK, so mismatched both sides, then my concern about the capacitance again.
So if the original cables have a determined specific impedance, a
replacement one having the same impedance should have a quite close value of
capacitance per length, for the same external diameter and dielectric used.
>From measurements now: the R-390A original coax cables (at least from my
samples) have a 11 mils solid center conductor and a 57 mils external
dimension of the center conductor insulator.
This insulator being made of Teflon with a relative permittivity of 2, this
computes to 70 ohms of impedance...
I just wanted to know if someone already seen a specific impedance value
specified for those cables.
I was not even hoping for a RG number.
For the shielding, I guess that any good teflon coax with a 95% minimum
shield coverage will be OK.
73, Jacques, VE2JFE
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