Measure the loss of a coaxial cable with the NanoVNA

This guide shows how to measure how much signal a coaxial cable loses in the frequency band used by TVWS. It was with this type of measurement that TECER’s technical team confirmed, in the UFPA/LASSE laboratory, that the cables made in the field lost much more signal than those sent by the manufacturer.

The instrument used is the NanoVNA, a portable vector network analyzer: it sends a signal through one of its ports and measures how much reaches the other and how much comes back reflected.

What you need

• A NanoVNA with two ports (CH0 and CH1).
• The calibration kit that comes with the instrument: short-circuit (short), open-circuit (open) and 50 ohm load (load) pieces, plus a through connector (thru).
• Two short, good-quality cables to connect the instrument to the cable under test, plus adapters if the connectors are different.
• The cable to be measured.

Steps

1. Set the measurement range. For the TVWS equipment tested in the project, use 470 MHz to 698 MHz. For other equipment, use the range in its datasheet.
2. Calibrate the instrument with the short cables already connected, so that their loss is not included in the measurement. Follow the sequence in the calibration menu: open, short and load on port CH0, isolation and, finally, thru, joining the two ends with the through connector. Save the calibration.
3. Connect the cable to be measured between the two ends.
4. Display the S21 measurement on a logarithmic scale (LOGMAG). It shows how much of the signal sent by CH0 reached CH1, in decibels. The value will be negative: -1.5 dB means a loss of 1.5 dB.
5. Read the loss at the working frequency using a marker. Also write down the worst value in the range.
6. Compare with the cable’s datasheet. Manufacturers give the loss per meter, or per 100 meters, at each frequency. A loss far above what is expected for the length indicates a poor-quality cable, a badly mounted connector or damage.
7. Check the reflection by displaying the S11 measurement (SWR or LOGMAG) with the cable terminated in a 50 ohm load. High reflection usually indicates a problem with the connector.

Without the second port

If only one end of the cable can be connected to the instrument, leave the other end open or shorted and measure the return loss (S11). The signal travels along the cable twice, there and back, so the cable’s loss is approximately half the measured value.

See also

• Inspect and restore oxidized RF connections
• Why a radio link has a shorter range than predicted
• NanoVNA: project documentation