General > General Technical Chat
How to tell the difference visually on BNC connectors
G7PSK:
Is there any way to tell the difference between 50 & 70 ohm BNC connectors where there is no markings as to the impedance on them. Without test gear that is.
vlf3:
Not easily unfortunately... the design can vary internally, centre core pin or shoulder screen within the
insulated spacer; it's a very small diameter milling that set's the through path impedance, but visually
can be very hard to tell.
If your going to use an unknown ! 75 Ohm BNC for Tx on a short feed-line, it's miss-match should not be too great
a VSWR. However, for much longer feed-lines and if placed halfway in the run, it would show-up with a high VSWR...
at the end of the day, fit them and check the reflection on short Tx bursts.
Used on Rx, will not be a problem.
xygor:
In all the ones I have seen, In the 50-ohm, the dielectric extends to flush with the end of the pin. In the 75-ohm, the pin protrudes from the dielectric. The is true for both the plug and the jack.
edpalmer42:
--- Quote from: G7PSK on March 25, 2013, 12:48:28 pm ---Is there any way to tell the difference between 50 & 70 ohm BNC connectors where there is no markings as to the impedance on them. Without test gear that is.
--- End quote ---
Sure. Go to Wikipedia and look at the pictures of 50 & 75 ohm connectors.
http://en.wikipedia.org/wiki/BNC_connector
They can typically be connected to each other without damage but, of course there could be reflections depending on the frequency of the signals. There is a legend that sometime in the dim, dark past there was one brand of BNC connector that would cause damage if it was interconnected to a connector of the other impedance. I don't know the details.
FYI, the situation with N connectors is completely different. If you try to connect 50 & 75 ohm N connectors you can damage the center pin or have a really poor connection because the diameter of the pins is different.
Ed
vlf3:
The Wiki image would depend on the manufacturer... my experience with British BNC's 75-50 Ohm some time ago now, their was no visual dramatic design change, as shown; well you live and learn. ???
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