[BC] RCA Hardline

[Richard Fry]

IIRC, the impedance value of 51.5 ohms was not an "RCA" standard, per se, 
it was a standard coaxial impedance defined by the RMA (Radio Manufacturers 
Ass'n), which was a predecessor of the EIA (Electronics Industry Ass'n). 
Maybe it is attributed to RCA because RCA adopted the standard early on, 
and used it in the period of time when RCA was a (the?) major player in the 
US broadcast industry.

There were mechanical and electrical interface issues between the two 
standards, as a 50 ohm EIA flange has an undercut for an inner connector 
insulator at the flange joint, where the RMA flange did not.  An RMA flange 
didn't even HAVE an insulator at the flange joint.  So the RMA and EIA 
flanges couldn't just "bolt together."

The inners could be mechanically connected with an "insert bushing" 
(commonly called a whiskey cup), but that didn't transform impedances -- it 
just made electrical continuity with low DC losses.  However the SWR 
mismatch when connecting a 50 ohm system to a 51.5 ohm system that way is 
not huge -- just 1.03:1.

RF