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I have this BNC jack to jack adapter. I want to measure its impedance but I am reading around 1mΩ. The datasheet says that it is a 50Ω adapter.

Why am I measuring such a low resistance?

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4 Answers 4

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Why am I measuring such a low resistance?

It's the resistance between centre pin and shield that is important to measure then:

  • If you connected an infinite number of these together end-to-end and measured the input impedance (centre pin to shield) it would measure 50 Ω for the rest of time.
  • If you connected enough to make a length of around 200,000 km, it would read 50 Ω for about 2 seconds (corrected)
  • This is because the velocity that electricity propagates in a 50 Ω environment is roughly 200,000,000 metres per second
  • The 50 Ω reading will become disrupted by the reflection coming back from the open end when it gets back to the start
  • The forward travel time is 1 second and the return reflection time is 1 second

In your actual scenario it would read 50 Ω for a few pico-seconds. I've assumed ideal lossless connectors for this thought experiment (added words).

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    \$\begingroup\$ I like the picture your answer conjures up, but I think the 200k km stack will read 50 ohm for two seconds, not one. It might be safer to specify ideal superconducting adapters, as if you plug real ones into the Telegrapher's equation, you'll get a different answer. \$\endgroup\$
    – Neil_UK
    Apr 3 at 8:36
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    \$\begingroup\$ Yes, it's two seconds. Thanks @Neil_UK \$\endgroup\$
    – Andy aka
    Apr 3 at 9:05
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mΩ are a good reading for the resistance between the opposite pins! You are measuring a low resistance because the inner pin is a solid piece of metal.

You can't measure the 50 Ω with a DMM, it depends on the geometry of the inner and outer conductors, and the dielectric between them. Rest assured, those dimensions look pretty much 50 Ω to me.

A hint, if the PTFE sleeve had been cut back somewhat at the inner pin, you would be looking at a 75 Ω connector - often used in video applications. It's easy to get them confused, and usually not too much of a problem except at very high frequencies or for precision work.

You can measure the impedance with RF measurement gear, though how is a different question.

The conductor goemetry and dielectric between them controls the way electric and magnetic fields behave inside the coax. The 50 or 75 Ω figure is the ratio of the electric to magnetic field inside the coax, which happens to have the dimension of Ohms. Plastic has a dielectric constant roughly twice that of air, so plastic inside the coax reduces the electric field, leaving the magnetic field unaltered, reducing the ratio of electric to magnetic fields. A plastic insulated coax therefore has a lower impedance than an air-spaced one.

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  • \$\begingroup\$ Thank you very much for the tip! I actually have a bunch of these adapters on the lab and I just identified two of them with les PTFE (I guess you mean and the white plastic that surrounds the conductor) which according to what you are saying they should be the 75ohms right? \$\endgroup\$ Apr 3 at 9:14
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    \$\begingroup\$ @SantiOspina Yes, much less white plastic between the conductors, in both the plug and the jack, means higher impedance, usually 75 ohms. It almost doesn't matter, as BNC is not used for precision measurements. But it does make a small difference, and it doesn't hurt to have it correct, especially if you have multiple connectors. \$\endgroup\$
    – Neil_UK
    Apr 3 at 11:15
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Multimeters are for measuring DC characteristics. RF cables and connectors are not intended for use with DC signals, but are intended for use with AC signals.

If you'd like to measure the properties of this connector, you'll instead need a different instrument known as a vector network analyzer, or VNA. Unfortunately this procedure is a bit more complicated & expensive than just using a multimeter.

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The pin on a 75 ohm BNC plug has a diameter of 1.4mm, whereas on the 50 ohm plug it's 1.1mm (result of a Google search).

Not easy to measure on the socket!

Using a 75 ohm plug in a 50 ohm socket can damage the socket, causing a loose connection with a 50 ohm plug (I remember this from the days when we had BNC video cables and thinwire Ethernet cables knocking around in the office).

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  • \$\begingroup\$ Thats something very interesting to consider next time I use these adapters, thanks! \$\endgroup\$ Apr 4 at 19:21

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