NanoVNA

A NanoVNA is an inexpensive measuring device for all things electronic. VNA stands for "Vector Network Analyzer" and you can order one for about $30-50. It will fit in your shirt pocket and comes with a little stylus and/or guitar pick to select menu options on the touch screen. Here's some things it will do:

• Measure SWR - the SWR, or "Standing Wave Ratio", is a measurement of how well your antenna matches your transciever in impedance. The transciever typical has 50Ω of impedance. Getting your antenna at or near 50Ω of impedance means you won't overheat your transciever's amplifier (but doesn't necessarily mean that you have an efficient/good antenna)

• Measure Inductance - A current passing through a coil will create a magnetic field which opposes the current, and this is referred to as "Inductance". Inductance depends on lots of things like the number of coils, and the coil radius, and the coil spacing, but also the metal used, and the material of the core, and the frequency fed into the coil, etc. You can calculate a theoretical inductance with the formula L = (μ × N2 × A) / l, but that may miss some factors. Instead, using a known capacitor in series with your inductor, the VNA will measure the resonant frequency, f, of the LC circuit. You can then calculate the inductance with L = 1 / (4π2 f 2 C) 

• Measure Capacitance - The same process as described above for measuring inductance will work if you use a known inductor, measure the resonant frequency, and calculate the capacitance.

Measuring SWR

• Calibrate

  • Screw on the 

• 
  

Measuring Inductance - create an LC circuit by placing a known capacitor in series with the coil to be measured. Plug that into the top SMA connector of the NanoVNA (alternately called Port 0 or Port 1)

With only VNA

• Display/Trace: turn off Trace 1,2,3. Back

• Format: S11 Phase. Back. Back

• Stimulus: Start 1MHz, Stop 30 MHz (may need different depending on cap)

• plug the LC circuit into port 0

• Read resonant frequency of the jump/vertical asymptote

• Do the calculation

With VNA & PC/NanoVNA Saver Software

• open NanoVNA Saver software (https://github.com/NanoVNA-Saver/nanovna-saver)

• in NanoVNA, 

  • click "Display Setup",  

  • for "Displayed Charts" choose "S11 Phase", 

  • set "Sweep" between 1MHz & 30 MHz, or whatever

  • Sweep

  • Read resonant frequency of the jump/vertical asymptote

• Do the calculation

Video Demo: https://www.youtube.com/watch?v=3LOEhvFII6c

Measuring Inductance - Inductance depends on the number of coils, and the coil radius, and the coil spacing, but also the metal used, and the material of the core, and the frequency fed into the coil, etc.

Note: I use the program NanoVNASaver for this, as I find the device’s display too fiddly to use.

To keep it short, here’s a list of things to do to get accurate readings:

Preparations

• Connect the NanoVNA to the NanoVNASaver running on a PC. Make sure it detected it, then hit Connect.

• Some NanoVNA models let you choose the number of samples, between 101 and 401. The app does not remember this, I always have to set it again. You do that in the device settings dialog called “Manage”.

• Set your VNA to the frequency range you want to operate in. I usually use 1-30 MHz when working with ham radio stuff for the lower bands, then shrink the span when fine-tuning the circuit

• Calibrate your VNA. This is most important when working with small values, like nH inductors. Calibration compensates for stray reactance and losses in your test harness. NanoVNASaver has a calibration wizard for this.

  • Calibrate exactly as you will measure, including your test rig, any adapters, cables etc.

  • You don’t need the calibrations using port 2 (S12) for these measurements, just skip that part.

  • If you don’t have the calibration standards handy or compatible with your setup (and you probably don’t if you use e.g. screw terminals):

    • “short” can be a piece of wire

    • “open” is simply nothing

    • “load” is two 100 ohm resistors in parallel. This is not quite perfect, but will do the trick.

• Measure the part exactly as it will be used in the circuit. No long leads, for example. You can fine-tune an inductor and then ruin everything by trimming the legs. Also, note that spacing between windings on a coil alters the inductance. If you use a ferrite core, its position inside the coil matters. Some sources claim the position of windings on a toroid also make a difference, but I did not observe this, the effect is minuscule.

Test rig for SMD

Rig with screw terminals

Measurement

• Connect the component across the Port 1 (between the coax core and its shield)

• Set your VNA to any of the S11 modes, this means single-port measurement. There are handy presets for our measurement: Series L and Series C

• Run a sweep and move the cursor to the frequency you care about. You can read your component’s value in the values window on the top left (on the NanoVNA itself, it should be near the top of the screen).

• If you want to fine-tune an inductor, choose “continuous sweep”, then the plot redraws as you mess with the windings.

Here’s an example measurement of a 23uH inductor (seen above in the screw adapter) that I wanted to use on the 40m band. It looks fine up to about 15 MHz, the rolloff is probably caused by the material in the ferrite ring, parasitic capacitance between windings, etc.

NanoVNASaver interface showing inductor measurement