NanoVNA

     Moving on up
: A couple of summers ago I experimented with a satellite dish antenna that was not receiving a signal where there should have been one. At the time I did not know how to test the antenna to determine whether it measured up to published specifications. It seemed possible that something could have shorted or failed in an inaccessible part of the structure. My antenna analyzer was of no use. The
maximum frequency that it could generate or scan is 230 MHz, far below the microwave range. Thus, I became curious about vector network analyzers, specifically those that operate at gigahertz frequencies. A cursory Internet search sufficed to dismiss that idea. Such instruments cost thousands of dollars. —I moved on to other projects.

    Not much had changed in regard to bench instruments and their cost
the next time I became curious about VNA’s. There was one outlier, however, a pocket-size ‘instrument’ that sold for less than $100 and claimed to work from 50 KHz to 900 MHz, and even in one model to 1.5 GHz. My thought was not that 1.5 GHz is far below 6 GHz, but rather that 1.5 GHz is a great deal higher than 230 MHz, and for an unbelievably low cost, compared to professional VNAs. That particular glass was half full.

NanoVNA in box

    Nearly everything in the box (photo) came with the unit, except the 3D printed touchscreen stylus (this one), and the large coax adapters stowed underneath the clear plastic tray. Accessories include three SMA male calibration connectors, one SMA female barrel connector, two short SMA jumpers, and a USB-C interface/charging cable. The kit also includes a single-sided fold-out menu diagram, without which it is easy to get lost, until the menu structure becomes familiar.

     Small steps: Having no previous experience with a VNA, I had trouble at first making sense of the display, even for calibration. There is no book. And the usefulness of relevant Internet resources varies. I found this manual by Gunthard Kraus DG8GB to be most helpful, although parts of it are beyond my present understanding.

Shakespeare Masthead Antenna    As if learning to perform and interpret NanoVNA measurements were not challenging enough, the instrument seemed to have a mind of its own, spontaneously jumping over submenus, sometimes reaching an obscure end branch of the menu tree. While the stylus worked much better than my finger, the touch screen had to be touched just right, not tapped or long-pressed. The multifunction switch also exhibited the jitters. I wanted to call it the ‘malfunction’ switch. It responded like a worn joystick. If precisely the right amount of pressure were applied in a magic direction the multifunction switch would sometimes do what was expected. More often the screen would jump to some unwanted option. —That is when the USB cable became a crucial accessory.

     Exercise 1 - VSWR: It is usually a good idea when exploring something new to start with a simple example. I chose VSWR for a first real-world measurement, after testing with the calibration connectors. By chance an antenna that I’d used in an unrelated project sat on the computer desk. It was a Shakespeare marine antenna, designed to be mounted on the masthead of a sailboat. I connected the antenna to CH0 of the NanoVNA using a 3-foot length of RG58 and a 6-inch SO-239 to SMA female adapter. The sweep range was centered on the marine VHF band. Later I used the RigExpert AA230 antenna analyzer to measure the same antenna, in the same physical place, and with the same RG58 jumper (but without the SMA adapter). These measurements are shown side-by-side below.


VSWR Example

    The y-axes
are scaled differently, because I did not realize when scaling the NanoVNA MOD v3 graph that the (RigExpert) AntScope2 graph could not be scaled in the same way. I cannot explain why the NanoVNA software label for the y-axis says dB—SWR is a ratio, not a logarithm. Also, the exact frequency of minimum VSWR is different between the two measurements. That could be due to the slightly different hookup (SO-239 to SMA part), or it could be a calibration issue. In any case the curve is quite flat (VSWR < 1.2 : 1) in that part of the range, so the difference is not significant.
Masthead antenna
    Prior to testing the Shakespeare antenna for this exercise I had supposed that marine masthead antennas are not much good, basically that they rely on height and possibly also on the mast itself (counterpoise) for their performance. However, VSWR measurements from both instruments indicate that this antenna is well matched at the marine VHF band. The moral is that a perfectly good marine VHF antenna should not be wasted on a non-floating piece of furniture.1

Crystal test jig     Exercise 2 - Parameters of a crystal: While an antenna analyzer has a single RF port, a vector network analyzer has two or more. In the NanoVNA, CH0 is the signal source and is also used in measuring reflections. CH1 is the input or receive port when the signal source is transmitted through an external circuit. The two jumpers supplied with the NanoVNA are used to connect the device under test with the NanoVNA source and receive ports. I thought a crystal would be the simplest thing to measure using the two ports. It could be plugged directly into the ports using a simple jig (photo)—the female SMA sockets on the device itself are a little more than 3 cm apart, too far to plug the crystal directly into the device.

Crystal Measurement Example

    It is possible to read measurements directly from the small VNA screen, but this is not easy—the NanoVNA is not a bench instrument. However, the computer applications that complement the NanoVNA not only graph data in user-selectable formats, but also perform analyses and calculate characteristic parameter values. The graphs above depicting selected measurement results for a 3.686 MHz crystal were produced by an application called NanoVNA-Saver. The three colored markers were placed automatically by the program. Each of these markers is associated with a block of measurement data. (not shown).

Analysis Example

    I also exercised the ‘Analysis
function of the NanoVNA-Saver application using this same crystal, and later with another higher frequency crystal (22.1184 MHz). The ‘Insufficient data’ warning (yellow) was puzzling at first. Wouldn’t 100 points be sufficient for a 20 KHz sweep? That number equates to 200 Hz between each pair of points, which makes the 3 dB bandwidth just 2 points wide, so perhaps not.

    Clearly, the observed center
frequency necessarily depends on the NanoVNA’s frequency calibration. Finally, given that crystals have a narrow bandwidth and high Q, these measurement results are not necessarily surprising or significant, but may serve to demonstrate some of the instrument and computer software capabilities.

Butterworth Filter (From QST magazine)

     Exercise 3 - Filter: There are no manufactured band-pass filters or IF transformers etc. in my old parts bin. So I thought maybe I could learn something by making one. This QST article from 1988 describes a 3-pole Butterworth filter, and includes a table of component values for each of the traditional HF ham bands 160 - 10 meters. The original article was aimed toward construction of high-power transmitter filters. And surely to make a useable RF band-pass filter it would be necessary to respect component values, construction techniques, shielding, etc. However, my goal was not to make a functional transmitter filter, but rather to use the canonic design and related component values as a bridge to another nanoVNA measurement exercise. I thought it should be possible to visualize the pass band graphically, and to exhibit properties of the filter that depend on L and C values, but substituting low-voltage components.

Band-pass filter

    I first assembled the circuit on a breadboard. However, the inevitable loose connections made it difficult to obtain reliable measurements, so I transferred everything to a perforated board (photo), substituting a small trimmer capacitor for C2 in the circuit diagram. Capacitance and inductance values were taken from the 7 MHz row of Table 1 in the QST article. However, as previously noted I did not have the same toroids as recommended in the article. Instead I used a small iron powder form that was on hand (T37-6), and a free application called mini Ring Core Calculator to compute the number of turns needed to produce tabled inductance values.

Calculating Number of Turns

Ferret toroids (joke)
     The illustration above shows an example calculation that computes the number of turns for L1 and L3, each 0.55 μH (Table 1 in the QST article). I should mention that I also experimented with ferrite toroids, but could not hit desired inductances dead-on with these. Overshooting the number of turns is not a big deal when winding inductors, because turns are easily removed. The opposite is not true, however. That is part of the reason why a 120 pF trimmer was substituted for C2 (100 pF in the table). I did not use high tolerance capacitors, and found that adjusting this capacitance would slide the pass band one way or the other, to yield a more nearly centered measurement result.2

 

Bandpass Filter - Return Loss and Gain

    Once again the results surprised me. Only one measurement indicated a narrow pass band that approximately coincided with the 7 MHz ham band. That specific measurement could not be reproduced, so I am omitting it. Most replications produced results resembling the diagrams above, where the pass band is roughly 2 MHz wide. And what is that wobble in the ‘Return Loss’ diagram? —Perhaps it has something to do with reflection (left graph) versus through-filter (right graph) measurements.

    Because of such dangling questions the exercise was not wholly satisfying. True, the test filter substantially attenuates frequencies above and below the pass band, which would suffice to provide a degree of isolation between bands. On the other hand I had pictured a narrower pass band, with cutoffs nearer the 40 meter band edges [colored markers]. Maybe that is where additional filter stages would be indicated. Clearly there is much to learn!

   
Demo video: [forthcoming, or maybe not]


 



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1. Of course, a satisfactory impedance match does not in itself imply that the antenna radiates efficiently.
2. Ferret image from Wikimedia Commons.



Project descriptions on this page are intended for entertainment only. The author makes no claim as to the accuracy or completeness of the information presented. In no event will the author be liable for any damages, lost effort, inability to carry out a similar project, or to reproduce a claimed result, or anything else relating to a decision to use the information on this page.