[Elecraft] 50 and 75 ohm BNC Connectors

[Bob Nielsen]

The reason it *might* stop the signal is that a 75 ohm center pin  
would not necessarily make contact with the 50 ohm center contact of  
the barrel.  The diameter is quite a bit smaller in order to achieve  
75 ohm impedance.  Likewise, a 50 ohm center pin could damage a 75  
ohm receptacle.

Bob, N7XY

On Aug 6, 2007, at 10:46 AM, Jack Smith wrote:

> Richard:
>
> I would be amazed if a single 50 ohm barrel would "stop" a digital  
> video signal.
> A high bit rate digital waveform, such as digital video, is subject  
> to destructive interference due to reflected signals--if the bit  
> rate is sufficiently high and the rise/fall time sufficiently  
> short, and if the noise margin of the signal is sufficiently low  
> for reflected signals to cause bit errors. I really doubt that a  
> single 50 ohm connector, however, would cause a major problem.  
> There's a much larger problem with high frequency attenuation in  
> sending data over coaxial cable, which is normally fixed via  
> equalizers and regenerators in the telecom world. That's one reason  
> fiber has proven to be desirable even for relatively low bit rates.
>
> There's a lot of "ifs" in that statement, but still if I were  
> building a video plant with 75 ohm coaxial cable and 75 ohm  
> equipment, I would certainly use 75 ohm BNC connectors to reduce  
> reflections and because they fit 75 ohm cable mechanically.
>
> When distributing a 10 MHz reference sine wave over 50 feet,  
> however, the impedance bumps from mixing 50 and 75 ohm impedances  
> do not prove to be a significant issue. In fact, I could probably  
> use 50 feet of zip cord connected via alligator clips to distribute  
> timing signals if I had to. (My measurements on zip cord show it to  
> have an impedance around 100-125 ohms but to be on the lossy side.)
>
> Jack K8ZOA
>
>
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