Coax connectors and accurate / repeatable measurements

Much is written about the virtues of some types of coax connectors over others.

Whilst much of the discussion focusses on mythically based characteristic impedance and loss of the connectors or adapters, little focus is given to the reliability / repeatability of connections.

UHF series

The UHF series is often maligned for its departure from uniform 50Ω characteristic impedance, but for most non-measurement purposes at HF and even low VHF, the Insertion VSWR is quite small in the context of practical antenna systems, as is the Transmission Loss and Insertion Loss. What is a real concern with these connectors is that the outer conductor connections depends critically on the tightness of the coupling ring, and that they are subject to incorrect or false tightening, and loosening with vibration or movement of the cables.


UHF series connectors can be used effectively if the plug is wiggled to locate the two V points in the bottom of the notches on the socket whilst the coupling ring is tightened. Note that it is easy to get a false tightening where the sharp V point engages a rough spot on the slope of the notch instead of at maximum depth. It helps to tighten the collar with a tool, the one pictured above is a pair of multi-grips with soft jaws so that sharp burrs are not raised on the collar.

Unfortunately, though a properly tightened UHF connector might be ok for a practical transmitter on 146MHz, it is not suitable for high accuracy measurements at that frequency and instruments like antenna analysers that use the connector are compromised.


BNC, even if clean and new, are just not reliable as anyone who has used them with a VNA will attest. Convenient, but not measurement grade.


SMA connectors are very popular, but suffer the problem that proper connection depends critically on the torque of the coupling nut

SMA Essentials

The upper and lower tools are torque wrenches for SMA, the others are 5.5mm and 8mm wrenches for SMA nuts and bullets.

N type

N type connectors use a tapered sleeve to make the outer conductor connection. They works reliably so long as they are clean, and the connector is properly tightened.