making up a short BNC 50ohm cable

[AndrewT]

I was looking for 50ohm cable to fit screw on BNC plugs.
The bore of the plug is ~4.8mm, 5mm is too big.
The bore is "threaded" and will probably be a good tight fit for a 4.95mm or 5mm coax with the outer insulation removed.

Looking up Wiki, I have found two coax with this diameter

RG58u
RG142u

RG8x is probably too big @ 6.1mm

Which do you recommend?
and why?

 

[AndrewT]

another question:
I have a few pre-made BNC cables.
They have 50ohm plugs, but use 75ohm RG59 cable.

In what situations will these not work?
And where will the impedance not affect the operation and connection?

 

[cliffforrest]

RG58u -but only because I have 1/3 of a 100m spool left, and it has never let me down. After 50 years it is getting a bit stiff, but then so am I!

"short lengths" and you will not see any mismatch or reflection problems below 50MHz or so. Even then very marginal.

For audio it is absolutely not an issue.

 

[AndrewT]

How good is the screen coverage?
How much copper is in the screen?
single layer braided or double layer braided?

wiki lists it as 0.81mm solid core (I would prefer solid core to get good contact inside the BNC)
CPC sell it as 7off 0.32 for the core
which is your's?

 

[Einric]

I am fond of RG400.
It is RG58 sized 50Ω cable with stranded silver plated copper center conductor, dual silver plated copper braided 95% shields with Teflon dielectric.
Solder the pin on if possible.

Also look in to CNT-195 cable.

BNC's are available in 50Ω and 75Ω.

 

[cliffforrest]

My filed drawing attached. I've no idea how "universal" those details are.

I have been happy with solid inner because I have only used soldered contacts.

About 40% screen cover, but if that is a worry, look up "Faraday cage"

 

[AndrewT]

...........
Solder the pin on if possible...............

I don't think the pin is removable.
It is a blank drilled hole, only accessible by poking the core down through the back end of the plug. A bit less than 1mm diameter.

Thanks for the other cable designations.
RG400 is only 2.95mm diam. too small.
CNT195 is only 2.79mm diam. also too small.

 

[Einric]

How long of a BNC cable do you need?
Do you want BNC Male on both ends?
I can make one for you with RG-400 if you'd like.
I am an RF guy by trade and I'd build one for you.

 

[AndrewT]

Thanks, but you're too far away.

I'm thinking very short 150mm to 1000mm long and maybe some long test cables.

 

[counter culture]

And where will the impedance not affect the operation and connection?

Any time the cable is electrically short.

 

[AndrewT]

Any time the cable is electrically short.

using a 100MHz oscilloscope, where the wavelength of that frequency is 3m, then what is "short"?
Length = wavelength, or wavelength/10, or ?

Or should the cable, with incorrect characteristic impedance, be related to twice that frequency?

 

[DF96]

'Short' depends on context, but can be wavelength/10 or maybe wavelength/40. It is definitely much much less than wavelength/4, as a quarter-wave cable acts as an impedance transformer. If you want to look at high frequency signals using a scope then you don't want an impedance mismatch.

 

[AndrewT]

So for a 100MHz scope, 150mm is short, 300mm might be short.
Everything longer will benefit from impedance matching, i.e. 50ohms for cable, 50ohms for connector and if the end is open a 50r load.

My generator only goes to 3MHz and the sqw shows it's limitations in slowish rise/fall times.
So longer cable can be used, say <=5m and still be termed "short".