A new version of Antscope2 has been released.
Online posters are excited that it supports some versions of nanoVNA, and one thread attempts to answer the questions:
The SWR image shows that the SWR minimum is at the center phase angle as you would expect. My question is:
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what are the other points that look like resonance,
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and should I trim my antenna based on phase?
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If so which one?
They are interesting questions which hint the ham obsession with resonance as an optimisation tartget.
Properly interpreting VNA or analyser measurements starts with truly understanding the statistic being interpreted.
In this case, the statistics being discussed are Phase and VSWR, and their relationship.
What is the Phase being discussed?
Above is an Antscope2 phase plot for an archived antenna measurement. The measurements are of a 146MHz quarter wave mobile antenna looking into about 4m of RG58C/U cable. We will come back to this.
Above is the SWR (VSWR) plot.
Let’s analyse both of these.
VSWR
What is VSWR?
Well you do need to understand the meaning of VSWR, the context, and expected values for the type of antenna being measured.
Minimum VSWR occurs at about 147.625MHz in this case, and at 1.3, it is quite within expectations for this type of antenna.
Phase
What is phase?
Let’s assume the reported complex Z at the cursor in the phase plot is correct, and calculate some interesting values from it.
Above, the phase of Γ is -160.6° which reconciles with the plotted phase… so it would seem that the plot of phase is actually a plot of the phase of the complex reflection coefficient Γ.
Is that useful in the context of the questions posed above about optimising VSWR and resonance?
No.
Above is the phase plot from Antscope (1) for the same file.
Phase is reported at 147.625 as -11.6°.
Can Antscope(1) and Antscope2 be plotting the same thing on their respective phase plots?
No. Antscope(1) plots the argument of Z and Antscope2 plots the argument of Γ.
Are either of them directly useful to determine if the antenna itself is resonant?
No. We are looking into a feed line, and without de-embedding the feed line from the measurements, neither phase directly indicates resonance of the antenna itself.
In fact, for a quarter wave ground plane antenna, minimum VSWR occurs very close to resonance, and feed line losses are least at minimum VSWR so it should be the optimisation target.