Nanovna tutorials

work in progress...

The nanovna is a small inexpensive SDR based VNA (vector network analzyer) useful in measuring and analysis of electrical parameters in radio frequency applications.

calibration

VNA calibration is not specific to the VNA, but to the test leads you are using. The point of calibration is to remove effects caused by the test leads from the measurement. So when you calibrate, you should attach the reference loads to the same test leads at the same point you intend on connecting your device under test. The calibration procedure sets coefficients on equations used to interpret data from the VNA.

Note that some VNAs have a limited number of calibration points they can store in memory (some as few as 256), so for best results, set the desired frequency range before starting the calibration procedure.

Calibration typically runs through the following phases:

open

ideally, use the "open" load to reduce noise, but just leaving the test leads connected to the VNA with nothing on the ends works.

short

use a shorted (near zero resistance) load that shorts the shield and center conductor

load

use a 50 ohm coax terminator load

isolation

the S11 and S21 ports are not connected (only needed for some measurements)

through

connect the S11 and S21 test lead ends with a barrel connector (only needed for some measurements)

Note that above 3GHz, it is not possible to have "perfect" loads, and the reference loads themselves may have calibration offsets that need to be given to the calibration procedure. Also, calibration can only partially compensate for attenuation in the test leads.

Note that test leads are typically fragile and disposable. For SMA connectors, try not to spin the cable or adapter when connecting and disconnecting, as this wears the center pin and socket. Instead, turn the outside SMA collar if possible.

Uses

Uses for a nanovna:

• measure complex swr / impedance of antennas
• TDR
• filter measurement
• RLC meter https://www.ondrovo.com/a/20231105-nanovna-rlc/
  • nonlinearity of resistors and other components at HF frequencies
  • importance of frequency in RLC meters
• Dielectric and ESR measurements of random materials
  • what is ESR
  • relation of ESR, Dielectric constant, and breakdown voltage (none)

Single port coax based measurements

Items (such as antennas) can be measured from the S11 port.

• Calibrate for the appropriate frequency range
• Set up traces such as SWR, reactance, smith chart.
• Use the VNA to determine resonance and SWR of the device.

For example, when building a J-pole, with some experience, you can quickly determine if the radiating element is the right length, if the matching section is the right length, and if the feed point is at the correct location on the matching section.

Time Domain Reflectometer

A VNA can simulate a TDR but is actually more accurate than a TDR.

• Calibrate for frequency range from 0 to the maximum possible length of the coax (speed of light / distance * velocity factor)
• Enable trace (??) with velocity factor correction for distance

The length of the coax can be visually spotted on the graph as the first large discontinuity.

Smaller discontinuities may be seen sooner, such as adapters, filters, etc.

It should be possible to determine if there is an antenna at the end of the coax or if it is open.

It may be possible to determine if the coax is damaged (water intrusion, etc.)

Filter measurements

Filter measurements use both the S11 and S12 ports.

• Make sure you do the isolation and through calibration steps.
• Connect the filter to both ports; note that directionality is important for some filters, "radio" side of the filter should be on the S11 port.

Characteristics of note in filter measurement: (traces to enable)

• SWR seen by the radio at various frequencies seen on the S11 port
• pass bands of the filter seen on the S12 port
• Insertion loss

Devices that are interesting to measure:

• lightning arrestors
• attenuators
• pass band filters, low pass, high pass filters, cavity filters, coax

Baluns and ununs may also be measurable.

• ?? choke impedance
• ?? turns ratio

RLC measurement

Leads can be put through the holes in the side of the terminals or wrapped around the posts.

Resistance, Inductance, and Capacitance of devices (resistors, capacitors, inductors, coils, chokes) can be measured on a VNA using a coax to separate pole adapter on the S11 port.

Note that traditional RLC meters typically use frequencies below 20KHz so their readings are only valid at radio frequencies if the components are linear at the frequencies they will be used at. Wound resistors are NOT linear, and can become highly inductive in this range!! So a VNA is required to test them. Other components may also be suspect.

Calibrate for the frequency range you intend to use the components, especially if you suspect they may have nonlinearities. Some variation of the reference loads as follows:

Open

no load

Short

wrap an uninsulated wire (bus wire, solder braid, etc.) between the posts

Load

use a 50 ohm axial linear resistor or a pair of 100 ohm linear resistors in parallel

Enable traces for R+X (real and complex reactance), or if the meter supports it, capacitance and inductance.

ESR of capacitors should be visible as real resistance.

Dielectric constant of materials

Dielectric and ESR measurements of random materials should be possible with an appropriate test jig.

• ?? plates?
• mounting?
• calibration?
• S11 method
• S11 S12 (shunt) method?

Additional topics

• complex sdr
• smith chart
• ESR

Additional references

• https://www.ondrovo.com/a/20231105-nanovna-rlc/
• https://www.uni-due.de/imperia/md/content/hft/papers/measuring_and_understanding_rf_coils_using_a_vna.pdf