On Mon, Mar 17, 2025 at 12:29 PM, KJ5FRJ wrote:
Hello, new to this group. I've learned a lot from reading here, but i seem to
have run into some issues and could use some advice. I'm building a low pass
filter, and trying to learn to measure inductance using the nanovna. I've
built a test rig that I found on one of the threads here somewhere- three
pieces of double sided pcb for OSL, and I'm soldering my air core inductors to
that. I'm trying to use the S11 shunt method for a coil that needs to be
72.4nH.
I posted about my binding post and PCB OSL a few years ago. For best results you need to keep the slit in the middle as wide as possible and the same for all 3 boards. You also need to connect the top side to the bottom one with copper tape and then solder the tape to the board all the way around. The 50 ohm load should be an 0805 SMD.
Your upper frequency is set to 900 MHz. My jig gave reliable results to 150 MHz. so I suggest you cal with this as your upper frequency.
Ive got a couple things I'm not understanding- should my coil be adjusted for
the necessary inductance at the frequency for the filter of 50-55mhz? And does
it need to be adjusted for impedance at that band as well? I've attached
photos- my 90deg phase and 50 ohm mark is at 112mhz, but this inductor is for
the 6m filter.
A couple of things to mention here.
- First the S11 Phase is NOT the impedance phase. It is the phase of the reflection coefficient (gamma). Recent versions of DiSlord firmware allow you to select Z Phase for the trace.
- Yes you need to measure the inductance at the frequency of operation. With an air coil the inductance will be affected slightly by the "skin effect" and current not flowing as deep in the wire as the frequency increases. But the biggest factor is that the NanoVNA measures the "apparent inductance" which is different than the actual inductance. This requires some explanation. Any inductor will have some self capacitance due to coupling between the turns. This capacitance is in parallel with the inductor. So the measured Xm = Xl || Xc and the NanoVNA calculates L = 2*pi*F/Xm. The end result is that the apparent inductance will increase with frequency and at some point the self resonant frequency is reached. After this point it looks like a capacitor. The attached plot of an inductor I measured is attached.
Another thing I'm not understanding is the 5khz self resonance dip seen at the
beginning of the sweep, shouldn't it be reading capacitive since it's after
the phase reversal?
Not sure what you mean here and keep in mind you are not looking at impedance phase in your plot.
Roger
Roger Need
