I think the real relation is 50 ohm coax is just
75 ohm air line with polyethylene dielectric added.
-Chuck Harris
Rick Karlquist wrote:
Azelio Boriani wrote:
Try this for a history about the 50 OHM impedance:
http://www.rfcafe.com/references/electrical/history-of-50-ohms.htm
The reference is full of errors. The lowest loss in coax
occurs when the ratio of the diameters is 3.6 to 1, regardless
of dielectric. For air dielectric, this works out to 76.7 ohms.
For polyethylene, it works out to just over 50 ohms if you assume
a dielectric constant of 2.3. It is also worth noting that the
difference in loss between 3.6 to 1 and 2.3 to 1 is very slight
and not worth worrying about in most practical cases. What
is probably more important is that higher impedance cable
uses much less copper in the center conductor for the same loss.
The story about 2 inch and 3/4 inch pipe might be true.
The thinnest copper tubing, type M, has an ID for trade size
2 inch of 2.009 inches and and OD for trade size 3/4 inch of
0.875 inch, which is very close to 2.3 to 1. Of course there
is also type K and type L and lots of other sizes, so this
story may be a case of "data mining" (IE you can always find
some combination of pipes to support any ratio you claim).
Rick N6RK
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