In my last article, I explained how to measure the resonant frequency of an LC circuit by simulating a grid dip meter using an antenna analyzer or a NanoVNA. This method is convenient when you only have an antenna analyzer. However, a NanoVNA has two ports, and you can use an S211 measurement to find the resonant frequency of your LC circuit. This brief article will show you how to do it.
We will measure the same trap I constructed for the previous experiment using a two-port S21 measurement. The resonant frequency measured with the RigExpert and the grid dip meter method was 35.62 MHz.
Making a S21 measurement
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Set the Frequency Range on the NanoVNA to cover the expected resonant frequency. If you do not know the resonant frequency, start with a broader range and then narrow it down.
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Calibrate the NanoVNA for the frequency span you are measuring. Follow the NanoVNA's user manual to perform a full calibration using calibration standards (open, short, load, isolation, and trough).
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Connect the device to test. The device under test should be connected between the CH-0 and CH-1 (Port-1 and Port-2)2.
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Look for a minimum in the S21 (LOGMAG) plot. The dip corresponds to the resonant frequency where the impedance is minimum. You can use the Search Minimum in the Marker menu to move the cursor to the minimum and read the frequency on the display.
Examples
The image below shows the small trap connected between Port-0 and Port-1. The display indicates a resonant frequency of 36.06MHz. Comparing the grid dip measurement method, which was discussed in the previous article and using an antenna analyzer with the NanoVNA's S21 measurement shows only a 1% difference. This suggests that using an antenna analyzer for this type of measurement is more than adequate.
In the example below, we measure a trap for a QRP trap dipole 40/20 meters. On the NanoVNA display, we can see a resonant frequency of 13.830MHz, just below 14MHz, the beginning of the 20m band.
More measurements
The NanoVNA allows you to display more information on the device under tests such as impedance, reactance, phase, inductance, and capacitance. Don't hesitate to tinker with the menus.
In the picture below, we can see a complex impedance of (50.2+0.124j).
Conclusion
Measuring the resonant frequency of an LC circuit using a NanoVNA is straightforward and provides valuable insights into the circuit's performance. By following the steps outlined above, you can accurately determine the resonant frequency and further analyze the characteristics of your LC circuit. The NanoVNA, with its portability and ease of use, is a powerful tool for ham radio operators working with RF circuits.