The reason for 50 ohms is that it is pretty close to 51.5 ohms. The
reason for 51.5 ohms is that it is possible to make rigid coax with
standard sizes of copper tubing at that impedance. (And, as a result,
51.5 ohm 1 5/8 line used to be considerably cheaper than 50 ohm.) The
77 ohm lowest loss value is more or less correct. I don't know why 70
ohms. The use of 65 ohms was not uncommon in the ' 40's (I've seen
quite a bit of Western Electric provided 65 ohm line used in AM
antenna feed and sample systems over the last 40 years). The Russians
(and also the Chinese and some former "east block" countries) often
use 60 and 120 ohm open wire quasi-coax. 120 ohm open wire quasi-coax
is also a quasi-standard in some European-built high power MF systems
as it is a nice voltage/current compromise at high power.
ben dawson
Benjamin F. Dawson III, P.E.
Hatfield & Dawson Consulting Engineers
9500 Greenwood Ave. N.
Seattle, WA 98103 USA
email: ben-dawson_at_hatdaw.com
Phone (206) 783-9151
FAX (206) 789-9834
Received on Thu Mar 15 2001 - 05:13:12 EST
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