Using the Nano-VNA to Find the ½ Wave Length Of Coax.

Sometimes a ½ wavelength of coax is needed. It can be figured by calculating, but it is better to be able to measure where it is.

The Nano-VNA has become quite popular. It is an excellent instrument and very reasonable on cost.

The coax will need to be 50 ohm to match the VNA. It is good to start a little longer so the coax can be cut to hit the ½ wavelength. A ½ wavelength in an antenna measured in feet is:

492¸frequency.

You will want to multiply this length times the “velocity factor” of the coax you are using. This will get you close, therefore cut the length a little longer.

To use the VNA you can start setting the scan at 1 to 30 MHz (for the HF spectrum).

Next go to “Display”, then “Format”, then either “LogMag” or “More”, then Linear. “Linear” seems to give a better display than “LogMag”. 

Note: LogMag is logarithmic amplitude.

          Linear is linear amplitude.

          The abscissa corresponds to the frequency. (What you are looking for.)

Connect one end of your coax to the VNA.

The other end of the coax, terminate it with a 50 ohm load.

The display will show a dip at the exact ½ wavelength frequency of the coax piece you are testing. Hopefully it will be lower than what you need.

If desired the “Start and “Stop” sweep may be narrowed if desired.

Trim your coax until you hit the ½ wavelength point.

A ¼ wavelength for that frequency will be half the length.

To find a ¼ wavelength section, double the frequency you want and measure for a ½ wavelength frequency at the double frequency. The formula for a ¼ wavelength is

246¸ frequency x velocity factor.

(This is for the frequency you are looking for)

This will get you close, then use the procedure described for the ½  wavelength.

It will take a little practice to get around on the VNA. However, it is worth the effort getting familiar with the VNA.

73,

Ralph WD0EJA

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