The NanoVNA can measure and display “phase”, is it useful for antenna optimisation?
Some authors pitch it as the magic metric, the thing they lacked with an ordinary SWR meter.
In a context where it seems most hams do not really have a sound understanding of complex numbers (and phase is one ‘dimension’ of a complex quantity like voltage, current, S parameters, impedance, admittance etc), lets look at it from the outside without getting into complex values (as much as possible).
The modern NanoVNA can display three phase quantities, only two are applicable to one port measurements as would commonly be done on an antenna system:
- s11 phase; and
- s11 Z phase.
Let’s look at a sweep of a real antenna system from the connector that would attach to the transmitter (this is the reference plane), plotting the two phase quantities s11 phase and s11 Z phase, and SWR (VSWR) and a Smith chart presentation of the s11 measurement.
Above is the measurement of the antenna system.
Like most simple antenna systems (this is a dipole, feedline, ATU), the most appropriate optimisation target is SWR, and minimum SWR well above 7.1MHz.
The SWR is 2.568 at the desired frequency, it is poor.
Do either or both of the phase plots give useful information on the problem, and leads to fix it?
s11 phase
s11 phase is -179.62° at the desired frequency (the marker).
Some authors insist optimal s11 phase is zero, some with a little more (and only a little more) knowledgeable insist it should be either 0° or 180°, take your pick. In fact the latter criteria essentially means the load impedance is purely resistive… but let’s deal with that under the more direct measurement s11 phase of Z.
Phase of -179.62° is approximately -180°=180°.
This metric is not very useful in this case.
s11 phase of Z
s11 phase of Z is -0.4°, approximately zero, which means the load impedance is almost purely resistive.
Of itself, s11 phase of Z does not identify the shortcoming.
So, what is the shortcoming?
If SWR is the optimisation target as proposed for this type of antenna, the SWR is poor, and the minimum is at a significantly higher frequency.
The SWR plot is revealing.
For more information, the value of Z is reported for the Smith chart marker as 19.47-j0.140Ω.
The reason that SWR is not 1.0 is that the feed point impedance is not exactly 50+j0Ω, and the main reason is that the real component is quite low at 19.47 and less importantly there is some very small reactance.
So, this provides information that to improve the match, the real component needs to increase significantly, and some minor trimming of the imaginary component.
Let’s make some matching adjustments
The sweep above is after some adjustment seeking to optimise the match.
Overall, the SWR plot shows that SWR is now fairly good at 7.1MHz, the Smith chart shows the marker just left of the prime centre so R is a little low and X is close to zero, the marker detail shows that Z is 45.57-j0.426Ω, so a little more information than the SWR curve, and with more resolution than reading the Smith chart graphically, R is a little low, X is close to zero. This is good information to guide the next matching steps if one wanted to refine the match.
The phase plots are of almost no value.
Conclusions
- Neither of the available s11 derived phase plots are of much use for this matching task.
- The SWR plot gives the best high level indication of the match.
- Knowledge of R and X components of Z can be helpful in understanding more detail of the match and guiding matching adjustments.
- This article has not explained the Smith chart in detail, it requires an understanding of complex quantities, so outside the scope and prerequisite knowledge set out for this article. In fact the Smith chart provides insight well beyond any and all of the other plots.
