Disturbing the thing being measured – coax line

An issue that often arises in online discussions inability to reconcile the VSWR indicated by a transceiver (or possibly an inline VSWR meter) and an antenna analyser.

Is this Segal's law at play?

There are several common contributors including:

  • faulty, dirty, or not properly mated connectors and cables;
  • VSWR meters that are not accurate at low power levels; and
  • influence of the common mode current path on VSWR.

The first and obvious question is, are all the cables and connectors sound and properly tightened? Some types of connector (eg UHF series, SMA) depend on tightness of the screw ring / nut for proper electrical contact of the outer conductor. An aerosol can of Isopropyl Alcohol (IPA) and some cleaning swabs / brushes are very handy to ensure connectors are clean.

Should you trust your VSWR meter – detector linearity discusses the second issue.

To the third issue mentioned, if you truly want to compare the two instruments, you must measure EXACTLY the same thing… and that means that when you disconnect the coax from the back of the radio and connect it to the analyser, you must RESTORE the common mode current path. I usually do this by holding the analyser coax connector (with antenna cable attached) firmly against the transceiver connector outer threads, and preferably isolating my fingers from the metal using an insulating sheet.

Above, an example of the nanoVNA with minimal adapters to UHF series, antenna patch lead attached and the shell of the connector held in good contact with the transceiver connector at far left. The white sheet is a silicone sheet to insulate my hand from the other stuff so that it is as close to the operating configuration as reasonably possible.

In this case, the minimum VSWR reconciles very well with the IC-7300 VSWR meter, provided the tx power is more than 60W.