A reader of NanoVNA-H4 – VSWR only contended that “surely a Smith chart is superior for antenna system tuning, a universal display.”
Let’s explore that using both VSWR and Smith displays to make tuning adjustments to a real antenna system.
Baseline
Above is a sweep of the antenna system showing both VSWR and Smith chart traces.
The desired matched frequency is at the marker at 7.1MHz, and minimum VSWR is about 70kHz higher.
VSWR curve interpretation
The VSWR curve is easy to interpret, it is a cartesian plot with VSWR on the Y axis and frequency on the X axis. We can easily interpret the plot to say that the antenna system is tuned to around 7.17MHz, about 70kHz too high.
Essentially, we need to adjust the antenna system to slide the VSWR curve left so that its minimum is close to 7.1MHz, ie marker at approximately curve minimum.
Smith chart interpretation
The Smith chart is in the shape of a shallow arc concave down, and the objective is to move / rotate it so that the marker is close to chart centre. The position and rotational orientation of the arc are dependent on transmission line effects.
Since VSWR at a point is a function of distance from the chart centre, we can see that since that distance is least some clockwise distance along the curve, we can interpret the curve to mean that minimum VSWR is at a higher frequency but unless you play with markers, the frequency for minimum VSWR cannot be read from the Smith chart.
Realise that the Smith chart display is wrt some reference plane. If you calibrate your NanoVNA with cal parts screwed onto the VNA jack, the sending port becomes the Port 1 reference plane and the Smith chart is wrt that plane. The extra facility of the Smith chart to read R and X for example shows R and X at the reference plane which is fine if that is what is needed, but a source of significant error if it is interpreted as R and X at some other point.
In these examples, the NanoVNA reference plane is the sending jack on the VNA, and the cable that would attach to the transmitter is connected to that jack so the Smith chart is rendered looking into the entire antenna system as seen by the transmitter.
Try this for yourself
The task is to tune the antenna system so that minimum VSWR is close to 7.1MHz. Most hams will be familiar with performing this using a traditional inline VSWR meter, or one integrated into a transceiver.
You can try this for yourself by hooking up an ATU to an antenna system, and using the ATU to adjust the system tuning.
Set your VNA and ATU up for the displays shown in the previous figure, and then set about readjusting the ATU to shift the frequency of minimum VSWR to some chosen target frequency.
Above is the display of the re-tuned antenna system.
It is much more intuitive to make adjustments to ‘walk’ the VSWR curve leftwards whilst keeping the minimum low that it is to achieve the same outcome with the Smith chart display which moves in two dimensions and rotates about some point other than chart centre.
Conclusions
Online posters often talk about Smith charts like they understand them, but in most cases their confidence in themselves is misplaced. It is fashionable to appear Smith chart ‘enabled.’
For many if not most simple antenna systems, the optimisation objective should minimum VSWR.
The simple VSWR plot directly shows the relationship between VSWR and frequency and is the most direct presentation of the information needed for simple retuning of an antenna system for minimum VSWR. To an experience ham accustomed to using a transmitter and VSWR meter to tune an antenna system, this is a quite small extension requiring about the same skill set but producing a scan display with improved productivity.
Display the Smith chart if you want, but do not hide the VSWR plot.
Don’t overlook the value of a simple direct reading display of the optimisation metric, especially when working under adverse conditions like outdoors in bright light.