Micro BNC 50 Ohm 

Micro BNC 50 ohm connectors offer excellent performance with a package density far greater than can be achieved with standard or mini BNCs.  Micro BNCs can be mounted on a 8mm pitch, offering a packing density more than 4 times greater than standard BNCs.

These 50 ohm versions of the popular Micro BNC, also sometimes know as HD BNC, are ideally suited to applications in test equipment and other industries where the simple quarter turn BNC bayonet coupling and high performance are required.  Note : 50 and 75 Ohm versions of Micro BNC are not compatible and permanent damage to the connector may occur if mated.

For details; http://www.coax-connectors.com/products/connectors/bnc-micro-50-ohm.aspx