NanoVNA and LPF?


Christopher Maness
 

I am ordered a nano VNA to use as an antenna analyzer, but I was thinking I might be able to use it to tune the LPF before installing it in my QCX-mini (20m) when it arrives.  Is this possible?  I would imagine I need to know what the impedance is looking back at the PA to accomplish this accurately (I am assuming it is not a purely restive  50ohm load).  If I did assume 50Ohms looking back towards the PA, would that get me close enough for horse shoes.  Not the biggest fan of putting it together then having to go back and desolder to prune and tune.  Would rather get it close enough that expanding/contracting the windings will peak it after installing it in the rig.  I am not shy about learning more about the VNA too.  I am comfortable with Smith charts and making matching networks.

TNX es 73 de KQ6UP SK ..


N3MNT
 

Here is my discussion on doing this.  There was concern by some it would not work, however after installing the tuned LPF, I have 4.5 W out.
https://groups.io/g/QRPLabs/topic/79504969#60473


JH
 

I have done this several times for the bandpass filter kits and it made life infinitely easier. 

For the QCX mini I just measured the inductance and capacitance of the circuit and made adjustments before soldering. It was nearly perfect the first time.

On Jun 23, 2021 06:44, Christopher Maness <christopher.maness@...> wrote:

I am ordered a nano VNA to use as an antenna analyzer, but I was
thinking I might be able to use it to tune the LPF before installing it
in my QCX-mini (20m) when it arrives.  Is this possible?  I would
imagine I need to know what the impedance is looking back at the PA to
accomplish this accurately (I am assuming it is not a purely restive 
50ohm load).  If I did assume 50Ohms looking back towards the PA, would
that get me close enough for horse shoes.  Not the biggest fan of
putting it together then having to go back and desolder to prune and
tune.  Would rather get it close enough that expanding/contracting the
windings will peak it after installing it in the rig.  I am not shy
about learning more about the VNA too.  I am comfortable with Smith
charts and making matching networks.

TNX es 73 de KQ6UP SK ..








Christopher Maness
 

Would you guys mind doing a little write up on doing this?  I am not new to Smith charts and antenna analyzers and understand the working principle of the thing, but I will just be getting my first Nano VNA today in the post.

TNX de Chris KQ6UP 


Christopher Maness
 

I also had a notion that a self calibrating text fixture is probably overkill at 14MHz.  The thing assumes that the rest of the network is purely resistive 50 Ohms in both directions.  That assumption sneaks in enough variation to probably cancel out being careful about calibrating the VNA with the test fixture included.  However the cross talk cancelation may still be of value.   I am not sure how critical that is at 14MHz though.

KQ6UP

On Wed, Jun 23, 2021 at 8:29 AM Christopher Maness <christopher.maness@...> wrote:
Would you guys mind doing a little write up on doing this?  I am not new to Smith charts and antenna analyzers and understand the working principle of the thing, but I will just be getting my first Nano VNA today in the post.

TNX de Chris KQ6UP 


N3MNT
 

Nothing special;  I used the adapter in the link below and installed wire sockets on an old LPF board from Hans.  The simply look at the filter cut off VS frequency and adjust the windings until the knee is at the lowest frequency you are operating.  I used the sockets so I would not have to solder and remove the torroids to adjust.  This method does induce some error, but it was negligible,
https://www.ph2lb.nl/blog/index.php?page=qrp-labs-filter-adapter-for-nanovna


Christopher Maness
 

Yeah.  I think I will try to build something out of perf board with some headers for the coils.  The caps are pretty easy to desolder from the perf board.

I guess I could find the VNA instructions for sweeping a LPF on YouTube.  I have not looked for one yet.

KQ6UP 

On Wed, Jun 23, 2021 at 9:08 AM N3MNT <bob@...> wrote:
Nothing special;  I used the adapter in the link below and installed wire sockets on an old LPF board from Hans.  The simply look at the filter cut off VS frequency and adjust the windings until the knee is at the lowest frequency you are operating.  I used the sockets so I would not have to solder and remove the torroids to adjust.  This method does induce some error, but it was negligible,
https://www.ph2lb.nl/blog/index.php?page=qrp-labs-filter-adapter-for-nanovna


Leland L. Bahr
 

Me thinks I will adjust my Low Pass Filters as done by Hans as you don't know all the parameters involved.

Lee, w0vt


howard winwood G4GPF
 

Thats how I did mine, just used some perf board and socket strips aligned with filter pins,
Surprising how easy it is and you can see the performance of the filter as you adjust.

On 23 Jun 2021, at 17:12, Christopher Maness <christopher.maness@...> wrote:

Yeah.  I think I will try to build something out of perf board with some headers for the coils.  The caps are pretty easy to desolder from the perf board.

I guess I could find the VNA instructions for sweeping a LPF on YouTube.  I have not looked for one yet.

KQ6UP 

On Wed, Jun 23, 2021 at 9:08 AM N3MNT <bob@...> wrote:
Nothing special;  I used the adapter in the link below and installed wire sockets on an old LPF board from Hans.  The simply look at the filter cut off VS frequency and adjust the windings until the knee is at the lowest frequency you are operating.  I used the sockets so I would not have to solder and remove the torroids to adjust.  This method does induce some error, but it was negligible,
https://www.ph2lb.nl/blog/index.php?page=qrp-labs-filter-adapter-for-nanovna<20210105_150516.jpg><20210105_150415.jpg>




Yury Krasouski
 

Tune LPF is only half of the way.

You need to tune PA output by L4 to be 50 Ohms or at least close to this value and only than it makes sense to tune LPF with nano VNA.


Christopher Maness
 

He just does trial and error method in his video.  The only difference is doing it before you solder it in the board which I don’t like because the non zero chance of hosing up a trace during the desolder process.


KQ6UP 


On Wed, Jun 23, 2021 at 10:40 AM Yury Krasouski <krasoffski@...> wrote:

Tune LPF is only half of the way.

You need to tune PA output by L4 to be 50 Ohms or at least close to this value and only than it makes sense to tune LPF with nano VNA.


ON7DQ Luc
 

Chris,
Read these
Abt building QCX and tuning LPF before doing the rest
https://on7dq.blogspot.com/2020/12/some-notes-on-qcx-mini.html

And for NanoVna, study my slides
https://drive.google.com/open?id=1szJmKpQOCea_LoKyC5HImKART-fc8z9p
Put nanovna in CW mode (e.g. on 50 kHz), build my component adapter (see slide 55) and it makes a good L/C meter

73,
Luc  ON7DQ


Rod Smith
 

Hi Yury.  I think that once the LPF is good at 50 ohms we can tune L4 or C30(from memory) for max output and all is then sorted. 

Best wishes

Rod
G0VKX


Yury Krasouski
 

Rod,

Yes, you are right.

I did the same but a little backwards.

I tuned L4 for PA output for max amplitude using dummy load soldered to PA output.

Than just add L1-L3 and got about ~5.3W from 12V without any adjusting of L1-L3. =)

br,

Yury


alex.savochkin@...
 

As a former metrology engineer in the federal metrologic research institute, I assume you will not need any matching circuits for measuring your LPF using NanoVNA. Just calibrate it as usual and measure S21 in both directions. You will just need to determine where the transmission coefficient slope begins, and that's all. Of course, you can also precalculate both input and output impedances of the LPF and then build respective matching transformers, and even de-embed them from measurement results. Unless you're not a research scientist you don't need to do all this stuff. Just measure S21 and that's it.
--
Alex R2ATO
SKCC: 21618. RCWC: 1275
QTH: KO85OX


Christopher Maness
 

Thank you, I will take a look.

Chris KQ6UP

On 6/23/21 11:47 PM, ON7DQ Luc wrote:
Chris,
Read these
Abt building QCX and tuning LPF before doing the rest
https://on7dq.blogspot.com/2020/12/some-notes-on-qcx-mini.html

And for NanoVna, study my slides
https://drive.google.com/open?id=1szJmKpQOCea_LoKyC5HImKART-fc8z9p
Put nanovna in CW mode (e.g. on 50 kHz), build my component adapter (see slide 55) and it makes a good L/C meter

73,
Luc  ON7DQ


Christopher Maness
 

Yeah, I have another homebrew project I am working on that has very low output, I am going to try that first.

Chris KQ6UP

On 6/24/21 4:26 AM, alex.savochkin@... wrote:
As a former metrology engineer in the federal metrologic research institute, I assume you will not need any matching circuits for measuring your LPF using NanoVNA. Just calibrate it as usual and measure S21 in both directions. You will just need to determine where the transmission coefficient slope begins, and that's all. Of course, you can also precalculate both input and output impedances of the LPF and then build respective matching transformers, and even de-embed them from measurement results. Unless you're not a research scientist you don't need to do all this stuff. Just measure S21 and that's it.
--
Alex R2ATO
SKCC: 21618. RCWC: 1275
QTH: KO85OX