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 Post subject: 50 Ohm BNC vs 75 Ohm BNC
PostPosted: Tue Feb 20, 2018 7:19 am 
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According to Wikipedia, 75 Ohm BNC can be recognized from 50 Ohm BNC due to reduced or even absent dielectric material a8also according to some other websites I found).
https://en.wikipedia.org/wiki/BNC_connector
https://upload.wikimedia.org/wikipedia/ ... 75_Ohm.jpg

But Wikipedia also mentions that this is by no means reliable.
Is there another reliable way to make 100% sure if it is 75 Ohm or 50 Ohm?

I suppose I could try and measure it somehow.

I have access to Oscilloscope and colour bar generator.
Any thoughts on how best to connect a test-setup to determine 50 Ohm or 75 Ohm?

Or another way of reliably recognizing one or the other.


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 Post subject: Re: 50 Ohm BNC vs 75 Ohm BNC
PostPosted: Wed Feb 28, 2018 1:24 pm 
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most 50 ohm is RG-58. Most 75 ohm is RG-59 or RG-6. physical specs can be googled. RG-58 & RG-6 should have one center conductor, RG-58 has many strands

The physical specs are considered to be more of a guideline than a contract in many asian countries. ebay BNC crimp on connectors don't crimp right on CCTV siamesed cable, which is apparently RG-58 1/2

you could make a TDR with a pulse generator and a scope. If you have to ask, don't bother, it would take way too long to explain

If you have a big spool of cable, connect the color bar generator to the monitor and the spool, with a T on the connection and a 75 ohm load on the far end of the cable. It it's 75 ohm cable, no reflection. It it's 50 ohm, you get a ghost image down and right of the real image

Image

50 feet of cable is enough to see this effect


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 Post subject: Re: 50 Ohm BNC vs 75 Ohm BNC
PostPosted: Thu Mar 08, 2018 7:27 am 
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Cool. I'll try the color bar generator, I got one of those.

I'll look more into Time-Domain Reflectometry, and when I learn more about it, I'll give that a go as well.
You pointed me in the right direction for that one.
Thank you for that.


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