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From what I understand, a BNC connector and it's cable has an inner conductor to route the signal and an outer conductor(cylindrical) to provide a shield to inner conductor and to provide a return path to signal. Triaxial BNC connectors have two shields, cylindrical connectors for better protection. But there's one conductor on these connectors, as shown in pictures below, whose purpose I don't understand.

Coaxial Connector

Triaxial Connector

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  • \$\begingroup\$ Have you done a continuity great between the two outer ones instead of just assuming? \$\endgroup\$ – PlasmaHH Sep 13 '18 at 6:37
  • \$\begingroup\$ That's what I thought, but I'm looking to buy some cables, and don't have any with me at the moment, so can't do test \$\endgroup\$ – Salman Sep 13 '18 at 6:39
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You're off by one.

Normal coaxial BNC connectors have two conductors: shield (often ground) and signal (or force). The points you have labelled as 1 and 2 are both part of the shield contact -- they both touch opposite sides of the shield on the female BNC connector -- and the point labelled 3 is the signal contact.

Triaxial BNC connectors have three conductors: shield, guard, and signal. As before, the points you have labelled as 1 and 2 are both part of the shield; 3 is the guard, and 4 is the signal. The added guard conductor in a triaxial BNC cable is used to help minimize the effect of leakage currents which would otherwise pass through the dielectric material, typically by placing the guard at a similar potential to the signal. Here's a video from Keysight which explains this in more detail.

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