nanoVNA – VSWR in terms of 400Ω

I was asked whether the nanoVNA can display VSWR in terms of 400Ω or some other arbitrary impedance.

Some antenna analysers and VNAs support display of results in terms of some specified impedance other than 50Ω, sometimes only a limited fixed set.

The direct answer to the question is “probably no, not directly on the ‘original’ nanoVNA today”, there are many firmware forks and many PC clients for nanovna, and now many significantly hardware versions appearing. Things may change.

However, if you can save a set of impedance measurements, they can be converted to VSWR relative to some other impedance reference.

An example

At Implementation of G5RV inverted V using high strength aluminium MIG wire – impedance measurements a set of impedance measurements made with a Rigexpert AA600 is presented.

A similar set of measurements could be made with a standalone nanoVNA and saved, but given that it is such a clumsy device and its USB socket has become so unreliable, I will not repeat the measurement but use the data used for the article above.

So we have a table of frequency and (R,X) measured looking into a ‘real’ 450Ω feed line, so we will calculate wrt 450Ω but we could use any reference.

Above is a plot or (R,X) looking into the feed line.

From each (R,X) pair we can calculate the complex reflection coefficient Γ, take its magnitude and calculate VSWR.

Above is a plot of the calculated VSWR(450).

Now the bigger question is, what is it good for?

My Rigexpert AA-600 supports presentation of measurements wrt 25, 50, 75 and 100Ω directly on the instrument. Apart from 50Ω I can only recall having used it with 75Ω on a few occasions for some specific applications where VSWR(75) was an optimisation target.

Antscope (v1) allows setting an arbitrary real “system impedance” and that is certainly a highly desirable feature (but not found in most PC clients).

Because two wire open lines are so often operated at high to extreme VSWR, directly visibility of metrics in terms of the nominal Zo is not usually very important.