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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

for just a few nanohenry you need to make your calibration very exact ... i would not try to exactly finetune it ... i would see if i am close .. then build my filter and tune the filter with all parts installed

dg9bfc sigi

Am 17.03.2025 um 20:22 schrieb KJ5FRJ via groups.io:

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

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

On Mon, Mar 17, 2025 at 02:51 PM, Roger Need wrote:

"apparent inductance" which is different than the actual inductance

The inductance you measure at the operating frequency is the value that influences the circuit. Some people reserve the word inductance for the value an inductor exhibits at low frequencies where self-resonance effects can be ignored. Some call this actual inductance or true inductance, but since it is not the value that affects the circuit at the operating frequency, these terms can be confusing.

self capacitance due to coupling between the turns

Ever wonder why a turn doesn't short out this inter-turn capacitance? Others have, and they have come up with modern theories of inductor self-resonance:




Brian

Good point on the upper frequency limit of home brew (and some
professional) fixtures. My home brew fixture which uses alligator clips
instead of binding posts has an upper limit of 100 MHz. I seriously doubt
your fixture is much good above 100 to 150 MHz. This is why the "big boys"
charge big bucks for their test fixtures.

I hate to think what the capacitance between just the two binding posts
is! That will be a major perturbation much above 300 MHz, let alone 900
MHz. A meticulous calibration is unable to cover all fixture "sins". The
best "amateur" fixture is to solder cal. standards and test items to the
butt end of an SMA female connector - WITH ABSOLUTELY NO LEADS.

Dave - W ?LEV

<>
Virus-free.www.avg.com
<>
<#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2>

On Mon, Mar 17, 2025 at 9:52?PM Roger Need via groups.io <sailtamarack=
[email protected]> wrote:

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

--

*Dave - W?LEV*


--
Dave - W?LEV

Hello

To perform this type of measurement, I use two BNC sockets bolted to a printed circuit board. I solder the components. This way, I can create a shunt or a series connection. I connect the SOL to the end of the cable that plugs into the BNCs.

The NanoVN is connected to a PC running nanovna-saver.

I've noticed that I get good results with capacitors and measurements in shunt mode. I suppose this is because the pF value is indicated on the board, and that's reassuring. For coils, the results seem more random; perhaps the "coil" components are more affected by stray components than capacitors.

Try it with capacitors; that will tell you if you're on target or not.
--
F1AMM
Fran?ois

I would calibrate the NanoVNA again with binding posts. I would use something like 30MHz-100MHz, 900MHz is surely too much. (Like others have said.) When the Vna asks "open" I would leave the binding posts empty, "short" mean shorted binding posts and "load" means a 50 ohms resistor(or two 100ohms in parallel).
Then I would check everything with capacitors. Values around 56pF are good, like 100pF and 10pF. Have you some coils too where you know the inductance. When all these show good values (+-tolerances), you'll know that everything works.

I have used crocodile clips up to 30MHz, your binding posts have shorter leads, so they should work with 50MHz. I have noticed that with crocodile clips, test leads matter a bit, your binding posts are better.

There's a lot of information here and I appreciate everyone's input- I reset and recalibrated the nanovna after finishing up the test rig according to Dave's build, i missed the copper tape on the edges. I'm sure the capacitance had quite an effect. I tested with a 10pf vishay hi Q cap and it's accurate along with my smith chart not running off the graph- I tested a coil and per the coil calculator I used it seems accurate. Now the fun part- trying to wind coils for the impedance I'm looking for at the frequency range I need?
If anyone sees anything here that looks amiss I'm all ears

Where did you get the filter design you are using? Playing around with my
filter design program, I found it appears you are using a 7th order
Chebyshev low pass filter, inductor input. Possibly a Butterworth, as the
values it gives are fairly close to those of the Chebyshev. This would
require two inductors of 72.8 nH and two of 214.8 nH. Is there any way you
can go to a capacitive input filter? That would use inductors with larger
values.

I usually don't measure the inductance directly for filters like that. I
wind the inductors according to formula (in this case simple wirewound
inductors), build the circuit, connect that to the NanoVNA and push the
inductors to make the windings closer together or farther apart with a
plastic diddle stick. I adjust until it gives a cutoff of the appropriate
frequency for a low pass filter.

Zack W9SZ

On Mon, Mar 17, 2025 at 2:29?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

<>
Virus-free.www.avg.com
<>
<#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2>

Zack,
Here's a link to the filter I'm building. It's not a chebyshev, there was an article in QEX about it comparing the two.

And the qex pdfs

When constructing filters, I ALWAYS measure every component before building
the filter! With that, I seldom have to do ANY tweaking with the completed
filter.

If your first image is without anything installed in your fixture - the
binding posts - after a good calibration, the marker over your sweep range
should be a single point at the extreme right and on the center horizontal
line with no capacitive trace below it.

If your first image is with a 10 pF capacitor, the S11 measurement at the
marker indicates a frequency of 3.9 MHz. What is the capacitance at 150
MHz? You want this capacitor to be "good" per your required value from
"DC" to at least 150 MHz, and preferably to 900 MHz (and above).

Dave - W?LEV

On Tue, Mar 18, 2025 at 7:59?PM KJ5FRJ via groups.io <coreyjenkins24=
[email protected]> wrote:

There's a lot of information here and I appreciate everyone's input- I
reset and recalibrated the nanovna after finishing up the test rig
according to Dave's build, i missed the copper tape on the edges. I'm sure
the capacitance had quite an effect. I tested with a 10pf vishay hi Q cap
and it's accurate along with my smith chart not running off the graph- I
tested a coil and per the coil calculator I used it seems accurate. Now the
fun part- trying to wind coils for the impedance I'm looking for at the
frequency range I need?
If anyone sees anything here that looks amiss I'm all ears

--

*Dave - W?LEV*


--
Dave - W?LEV

Dave, the first photo is with the 10pf cap and at 150mhz it measures 10pf.

Since you mention measuring all components, do you have any tips on getting a 50ohm match on the inductor at the desired frequency? I've tried a few different diameters and numbers of turns, I'm close to the inductance needed but can't seem to get the impedance right

On Mon, Mar 17, 2025 at 03:22 PM, W0LEV wrote:

best "amateur" fixture is to solder cal. standards and test items to the
butt end of an SMA female connector - WITH ABSOLUTELY NO LEADS.

Indeed, the HP16092A is the butt end of an APC-7 connector, with just 3mm of the centre conductor protruding beyond the insulator. It has an advertised max freq of 500 MHz.
I agree with Dave that a DIY fixture can be made using a female SMA connector with its centre conductor sawn to the same level as the dielectric. Then, a plastic bulldog clip (similar to one in the photo of the S21 fixture) can be used to clamp the chip component down.
I have measured a 89 nH inductor (6.5 turn, ID2.9mm) using the 1st type of fixture (fixt s11.jpg). The fixture is calibrated using a 1206 size 50ohm chip, and a brass shorting bar. Using the nanoVna V2+, the S11 from 10 to 1000 MHz (s11.jpg). The self-resonance is above 1000 MHz. The trace stays inside the chart. The Q can be calculated from the impedance = R+jX at different freqs.
According to HP/KS, the VNA has poor accuracy when the S11 trace is near the perimeter. Hence, I also measured the same inductor using the S21 method in a microstrip fixture (fixt s21.jpg). A plastic bulldog clip is used to clamp the inductor to the PCB. The fixture is calibrated bridging the microstrip gap with a brass bar (thru). The measured and modeled S21 points a parasitic cap of 0.16 pF (s21_gra.jpg)

On Fri, Mar 21, 2025 at 12:29 AM, <biastee@...> wrote:

I also measured the same inductor using the S21 method

Please post the .s2p file. I'd like to calculate results using the Y21 method that suppresses stray, shunt capacitance in the test fixture.

Brian

QUOTE: ....a DIY fixture can be made using a female SMA connector with its
centre conductor sawn to the same level as the dielectric. .....
***

You can buy such a connector with no need to clip off the tip. They are
known as launchers and instead of the cylindrical tip, contain a short
ribbon connector meant to be directly soldered to a microstrip trace.

Dave - W?LEV

On Fri, Mar 21, 2025 at 7:30?AM biastee via groups.io <biastee=
[email protected]> wrote:

On Mon, Mar 17, 2025 at 03:22 PM, W0LEV wrote:

best "amateur" fixture is to solder cal. standards and test items to the
butt end of an SMA female connector - WITH ABSOLUTELY NO LEADS.

Indeed, the HP16092A is the butt end of an APC-7 connector, with just 3mm
of the centre conductor protruding beyond the insulator. It has an
advertised max freq of 500 MHz.
I agree with Dave that a DIY fixture can be made using a female SMA
connector with its centre conductor sawn to the same level as the
dielectric. Then, a plastic bulldog clip (similar to one in the photo of
the S21 fixture) can be used to clamp the chip component down.
I have measured a 89 nH inductor (6.5 turn, ID2.9mm) using the 1st type of
fixture (fixt s11.jpg). The fixture is calibrated using a 1206 size 50ohm
chip, and a brass shorting bar. Using the nanoVna V2+, the S11 from 10 to
1000 MHz (s11.jpg). The self-resonance is above 1000 MHz. The trace stays
inside the chart. The Q can be calculated from the impedance = R+jX at
different freqs.
According to HP/KS, the VNA has poor accuracy when the S11 trace is near
the perimeter. Hence, I also measured the same inductor using the S21
method in a microstrip fixture (fixt s21.jpg). A plastic bulldog clip is
used to clamp the inductor to the PCB. The fixture is calibrated bridging
the microstrip gap with a brass bar (thru). The measured and modeled S21
points a parasitic cap of 0.16 pF (s21_gra.jpg)

--

*Dave - W?LEV*


--
Dave - W?LEV

for my saa2n i have cal standards with plugs (male connector) .. but i wanted also female (so i can calibrate at cable end with a plug without double female needed)

so i took 3 N sockets .. one with a short .. one with an open .. and third got 2x 100 Ohms SMD resistors in parallel (centre pin shorted on all three to exact same length)

works good to over 3 ghz :-)

dg9bfc sigi

adding a "keyring" to secure them against getting lost

Am 21.03.2025 um 18:21 schrieb W0LEV via groups.io: