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Something is wrong. NOTHING should show outside of the Smith Chart for
S11, the green trace after a proper calibration is completed!

While the following has been mentioned many times:
1) Did you "RESET" the calibration before you started the intended cal.?
2) Once the cal was complete, did you store it in one of the registers for
convenience?
3) Once you completed the cal and before you installed your inductor, did
you go back and check for the proper position on the Smith Chart for the
open, short, and load? This is always a good practice.
Of course, all with the fixture in place and the OSL installed on the
binding posts.
***

Yes, the inductance should be measured at the frequency of application.

I presume the filter is intended for 50 to 55 MHz, and not 50 to 55
milliHertz?
***

If the filter is designed for a match at 50-ohms, it should measure close
to that when terminated with 50-ohms - both non-reactive.
***

I can not read the start frequency. But I must ask why you are sweeping to
900 MHz when the LPF design goal is a roll-off starting at 50 to 55 MHz.
Unless you are using a PC or laptop based application, you are losing
resolution as the number of measurement points, both for the cal. and the
measurement is severely limited (101 points?). As such, I do not see the 5
kHz dip you refer to.
**

In measuring the inductor (or capacitor where applicatle), I'd suggest
sweeping from 30 to 70 MHz since this brackets your intended roll-off
frequency for the LPF. Again, sweeping from "DC" to 900 MHz you will lose
a major amount of resolution where you need it.

Another caution: Most disc ceramic capacitors are pretty good below your
intended roll-off frequency. But they may or may not be good to 900 MHz.
With the VNA, verify they are still close to the intended values from your
roll-off frequency through 900 MHz.
***

Dave - W?LEV


On Mon, Mar 17, 2025 at 7:29?PM KJ5FRJ via groups.io <coreyjenkins24=
[email protected]> 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.

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.

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?

Any help is appreciated

--

*Dave - W?LEV*


--
Dave - W?LEV