[Elecraft] 50 and 75 ohm BNC Connectors

[Jim Brown]

On Mon, 06 Aug 2007 13:46:21 -0400, Jack Smith wrote:

>I would be amazed if a single 50 ohm barrel would "stop" a digital 
video 
>signal.

This IS way off topic, but the right answer lies in studying reflections 
and losses. Any real world video cable will have significant loss in the 
GHz range, so reflections are unlikely to travel very far from one 
termination to another and still be strong at the other end. 

In other words, lets say you had two video monitors on a line, one at 
the end, terminated, and the other bridging somewhere along the line. 
The bridging one would see reflections from a mis-match at the end 
monitor, attenuated by the loss in the line between the two monitors at 
the frequency at which the mismatch was significant (>500 MHz), and the 
monitor at the end of the line would NOT see its own mismatch. And I'm 
talking small mismatches here -- a 50 ohm connector instead of a 75 ohm 
one, not a MISSING 75 ohm term, or a double one. And a double term would 
be far less problematic than a missing one. 

In his EMC lectures, Henry Ott correctly observes that a mismatch 
produces a reflection that is observable only at a distance from the 
mismatch, not at the point of a mismatch. So if I were building a 
precision video or data plant that might need to work well into the 
hundreds of MHz range, I would be pretty careful about connector 
impedance at passive patch points, but not at terminations. And the 
input of an ACTIVE router is a termination!  

73,

Jim Brown K9YC