On Jun 7, 2:11*pm, Gaius wrote:
In article fda40c34-830f-438d-830e-0c1c23d2bc44
@c10g2000yqi.googlegroups.com, says...

BNC's are still 50 or 75 Ohm. Old versions had different diameter
centre pins in order to acheive the correct characteristic impedance.

Can you (or anyone !) provide some evidence of this, in terms of actual
measurements or manufacturers' specifications showing a difference in
the "Old" versions ? You are also confusing the effects of conductor
diameter versus mating surface diameter. 75R and 50R STILL have
different inner conductor diameters to achieve correct Zo; it's the
MATING SURFACES which are the same.

If you plugged a 50 Ohm plug into a 75 Ohm socket the larger centre
pin would splay out the female centre connector, so when you plugged a
75 Ohm back in again it wouldn't make contact. It used to be fairly
common practice to paint the body of the connector purple or green in
order to quickly identify the type and so prevent damage.

I (professionally) used many thousands of BNCs during the timescale you
mention. While it IS true that some 75R sockets could be damaged, in
EVERY case it was due to the inner conductor of the plug being off-
centre. 75R sockets were/are inherently rather fragile.
The problem was caused by a mechanical design weakness in some brands of
plugs (I don't remember which manufacturers).
AFAIR, the BBC used 50R plugs/sockets in some 75R video equipment,
simply because the 50R socket was more robust, by design.

For the past 20 or so years most manufacturers have used the same
sized centre pin, but the 50 Ohm version has a PTFE skirt in order to
provide the correct charateristic impedance. So they are now
mechanically interchangeable.

I have lots of old stock and recovered BNCs dating back at least 20
years. All the 50R and 75R components mate without problem.

Well I've got some old 75 Ohm female to female adaptors in front of
me, marked RS part number 455-933 (but the type associated with the
part number has changed many times over the years). I also have one
marked G37534 which looks fairly similar. Most have been opened up
because they have had a 50 Ohm shoved into them.

However on the one pristine 75 Ohm adaptor I could find, the outer
part of the 75 Ohm centre receptacle is so narrow it would easily fit
the inside of a 50 Ohm centre receptacle.

Unfortunately I can’t find any old Greenpar (or similar) mechanical
drawings on the web only the newer types.

UKM