[bars] NanoVNA - measure resonant freq of parallel L/C circuit?

[Andy Wallace]

Bammi, thank you for those!

It turned out I could use the S11 port, set for SWR, and set stimulus to sweep around 455 kHz. 

The item in question is the L/C BFO network out of a 1960 R-390A receiver. The symptom was I could adjust it for lower sideband but not upper. The variable inductor (ferrite slug) shaft was near end of travel. 

Using the NanoVNA like a grid dip osc with a coil on the end would work but since the circuit was output directly to pins outside the can I just clipped the VNA to the right leads. Bingo, nice trace. 

It shows the assembly resonates at 454 and 480 kHz at the ends of the travel - verifying my symptom. Two photos attached. 

It should be like 445-465 kHz. So I can probably fix it with a parallel cap. 

Thank you!
Andy
KA1GTT

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On Feb 25, 2026, at 2:35 PM, jbammi at mac.com wrote:

Hi Andy,
One way is do a S21  Logmag measurement and read off the resonant frequency of the dip.

Pls take a look at this Video

https://youtu.be/LLJBfWf2JVk?si=7Q68j7DckxUCStGC

To get more precision use nanoVNA saver , then narrow down the sweep range, and repeat to get a more accurate freq reading of the dip.

Above is probably the simplest, you could do it with just S11, by making a simple wire coil at the  S11 port, and then moving your circuit close to the coil and watching the S11 logmag for the dip. This is a close analog of the old grid dip meter, or a grid dip coil + an antenna analyzer like the one MFJ sold with their 259 and other meters.

Watch this very short clip at the end of a longer video
https://youtu.be/kuDkdrI4bBc?si=O3deEjqbwZQB6A89&t=398


Cheers,
Bammi


-----Original Message-----
From: bars <bars-bounces at w1hh.org> On Behalf Of Andy Wallace via bars
Sent: Wednesday, February 25, 2026 10:54 PM
To: W1hh <bars at w1hh.org>
Subject: [bars] NanoVNA - measure resonant freq of parallel L/C circuit?

Does anyone here know how I can use this to (like an old grid dip meter) measure the resonant frequency of a parallel inductor-capacitor circuit? I imagine it uses only the S11 upper port.

Tutorial link much appreciated. Other than doing the cal I have not used my NanoVNA.

Thanks
Andy
KA1GTT
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