[Elecraft] 50 and 75 ohm BNC Connectors

[Jack Smith]

Bob:

No... at least according to Amphenol, who makes the connectors, the 
mating end of the center pins are identical diameter in both the 50 and 
75 ohm BNC versions and are mechanically interchangeable.

Not so with Type N connectors, where the pins are different diameters 
and can cause the problems you mention.

Jack K8ZOA



Bob Nielsen wrote:
> 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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