Most often, NMD connectors are 3.5mm, 2.9mm, 2.4mm, and 1.85mm connectors. They include a large threaded body that is specially designed to stabilize the test port cable when attached to the front of the analyzer or test set using MegaPhase test port cables.
The NMD design allows the use of standard female cables and adapters. Since the test port connectors are male connectors, they have standard inside threads that can accomodate the female connectors. It is best to use the recommended cables with NMD connectors. NMD stands for Network Measurements Division, an outdated term for the original HP division that produced the Agilent 8510A and 8720A analyzers.