The nanoVNA is a mighty fine instrument considering you can buy one for less than $100. But like many instrumentation items, you have to know how to use it. In this blog post we will look at the illusion of error due to a failure to consider scaling.
Consider the example illustrated in the first photo. We are reading the vector impedance of an antenna at 3.530 MHz. We are seeing an SWR of 1.27:1, an absolute impedance of 49.2 Ohms, a +j 12.2 Ohms, and a Smith Chart reading of 49.2 +j551nH.
Just to sanity check the above, at 3.53 MHz, 551 nH should be:
XL = 2 . pi . f . L = 6.28 . 3.58e6 . 551e-9 = 12.2 Ohms
Thus the above calculation gives us a nice sanity check that the nanoVNA’s numbers are in agreement and that we properly understand what they are representing.
But there is something very curious apparent in the next photo. Since the real impedance is very nearly 50 Ohms we expect the blue marker triangle to lie near the resistance circle that intersects with home base. The green marker arrow points to this triangle so that you can readily identify where it is.
But here is the problem. There is significant inductive reactance–12 Ohms. Yet the blue triangle is resting (or appears to be resting) squarely and clearly on the real which indicates a net zero reactance. What the heck is going on? The blue triangle seems to indicate that we are at home base with an impedance of 50 +j0 Ohms, the Holy Grail!
The answer is that the nanoVNA has a scaling feature which is currently zoomed out so far that we are not seeing resolution sufficient to read what the numbers are telling us. The solution is to scale down the axes.
In the following steps we are going to be setting the nanoVNA’s scale factor but ONLY for the blue trace which is currently set for CH0 with a functionality of showing Smith Chart information. BE CAREFUL! It is very easy to not have the proper trace selected. In another blog post we go through the steps to identify and set this very tricky element of the nanoVNA. Before proceeding further, verify that the blue trace is both set to CH0 (or whatever the channel name is for your particular device) and that the blue trace is reverse highlighted. See the earlier blog post for a full explanation of this very tricky operation.
Call up the menu and look for “SCALE/DIV.” When you click this a mini key pad appears. This mini key pad allows you to input a scaling factor in terms of so many somethings per division. We are expecting to see something on the order of 10 to 20 Ohms of inductance but keep in mind that the nanoVNA Smith Chart is giving us data in terms of inductance or capacitance. So, we are expecting from 500 to 600 nano Henries. But the nanoVNA interprets in terms of micro Henries. Therefore, at a scaling of 1x, one division would represent 1 micro Henry.
In the example appearing at the bottom, we have set the scaling to 1x and the inductance is now resolvable as roughly 0.5 micro Henries. We could set the scale to 0.5x and the blue triangle would move to the first division for even better resolution. Setting the scaling would move the triangle to the line of the second division.