NanoVNA measures Inducror made for 2M BPF always appears overly capacitive - Page 1
Forum section: electronics / rf-microwave

author: jut
author_rank: Newbie
timestamp: January 31, 2024, 12:24:06 pm
content: Hello,Forgive my various levels of stupid but I'm trying my hand at making a BPF for an amplified 4w SDR transmitter, for the first time. I'm aiming to build the filter with heavier passive components for higher powers later. Being made of passive components and various youtube vids making it look easy, i expected it to be a realistic possibility.I have a bunch of 21.5pf air gap variable caps and trying to make matching inductors out of solid 2mm rigid copper for 144mhz filtering. I haven't calculated specific windings yet but rather just taking a guess, measuring, making another try, measuring again, until i get close to 50nH-100nH at the 144mhz ide need for various filter designs. I've got them connected to the s11 coax port only like ive seen in examples.However trying to measure my inductors seems to result in surprisingly capacitive reactance across all required frequencies up to 144mhz, that looks nothing like what various demo videos show when they demonstrate similar measurements where a large frequency range sits at top of the smith chartI've tried various conditions (and always re-calibrated),- changing (removing entirely) the pigtail cable connector.- trying a directly soldered inductor to sma connector (rather than dodgy screw terminal)- 3 varieties of windings,( 3 thick winds, 5 thick winds, 11 thin winds) in case my inductance guestimations were way off.- extending my scan range to 1Mhz-1Ghz range in case im missing somethingNo matter what a try results seem to show high capacitance. I know the environment would make them all over the place but i expected something that resembles an rf inductor? Am i missing something really dumb like reading results wrong? or is there a possibility vna is busted?or maybe ive just discovered a great way to make solid RF capacitorsThanks!Edit: I've just seen that connecting NanoVNA to rooftop antenna without bleeding off static could cook it. and I've definingly done that in the past more than once :S but hopefully that's not the only possible explanation.

author: jut
author_rank: Newbie
timestamp: January 31, 2024, 02:30:42 pm
content: hmmm, actually there was one test that looked like a dead short with a tiny circle at the left center. But given the other results and the fact the circle was so small, i assumed it was an outlier rather than the norm. thanks for the observation, ill take a closer look!This is certainly a learning exercise so the link is much appreciated. ill give it a study, have many more tries in the coming weeks, and will surely brag if i figure it out.thanks

author: Miek
author_rank: Regular Contributor
timestamp: January 31, 2024, 02:47:59 pm
content: The plots you've posted don't make sense, you should never be seeing points outside the smith chart or S11 > 0 when measuring a passive device. I suspect there is something wrong with your calibration ormaybea hardware issue.

author: Bud
author_rank: Super Contributor
timestamp: January 31, 2024, 02:56:07 pm
content: i do not gave a nanovna but the Smith Charts you posted show absolute garbage. Could be the nanovna being a joke or something wrong with your setup.

author: jut
author_rank: Newbie
timestamp: January 31, 2024, 03:00:20 pm
content: yeah, this is my fear :/Its a worry the tests are repeatable too. Ill have a closer look at the calibration pieces and try some alternatives. if i have to get some new test gear, that's what ill have to do. :/But thanks for verifying the nonsense plots.

author: selcuk
author_rank: Frequent Contributor
timestamp: January 31, 2024, 03:09:44 pm
content: You are using nanovna saver. First calibrate the range with your VNA and store to calibration 0. Then calibrate the same range again from nanovna saver and preferably save it to a file. The software doesn't use the coefficients from nanovna. If you don't have two 50 ohm loads, skip the S21 calibration since you cannot complete it properly. Just finish the calibration for S11 when it asks and it will be fine.

author: Maurizio1957
author_rank: Contributor
timestamp: January 31, 2024, 03:19:14 pm
content: I think a filter should be measured in 2 port mode, S21.Greetings, Mauriziooops, you are measuring an inductor, not a filter...

author: LM21
author_rank: Regular Contributor
timestamp: January 31, 2024, 09:13:19 pm
content: Measuring capacitances or resistances should give you good results. Like 100-1000pF or 50-200ohms. I would use reasonable frequency, like 10-100Mhz. Values will change a bit when you change the frequency, you may even see sign change , a resonance. But if those do not work, you are in a trouble.

author: iMo
author_rank: Super Contributor
timestamp: January 31, 2024, 09:50:23 pm
content: While looking at the green marker in the first picture, the impedance seen by nanovna there is aprox 55ohm |Z|, and that is close to 50 I would use for calculation. That is at 264.7MHz, and that leads to aprox 33nH. The small inductor there could be around that value, imho..PS: talking the smallInductor2 picture..PPS: normally I would look at the nanovna screen, searching for |Z|=50ohm, picking up that frequency at 50ohm, and do the calculation of L=50/6.28/frequency..

author: jut
author_rank: Newbie
timestamp: February 04, 2024, 09:09:20 am
content: so first try today with the simplest suggestion from selcuk, seems to have done the trick.simply collaborating on the vna rather than in the software has given very different results that look much better. there was something very wrong with the procedure i was using the other night. but for now the results look great to me.im not home free yet since im still not seeing a dead short at low frequencies, but that might only be what one would expect with S11 to S22 connections. im just using s11 connector at the moment. ill just do some more tries on known inductors to confirm that im not just seeing bias results i want to see. good news is im more confident my hardware shouldn't be blown.thanks for all the help and suggestions. ill be spending a lot more time reading posts rather than writing noob questions as this forum looks like a fantastic resource.

author: jut
author_rank: Newbie
timestamp: February 04, 2024, 12:42:36 pm
content: sucsess!Update #2ok, last results were a change but not perfect either. and more with tests i started getting even crazier results. nanoVNA saver had been crashing, and the website currently doesn't provide binaries to get the latest version. given ive had more success on the VNA itself, i suspected VNASaver software might have corrupted settings on my device. even when performing the software reset option, i was still getting strange results and eventually the nanovna saver application couldn't connect without crashing at all.i found some tutorials for upgrading the firmware on my nanovna-h4 without having to open it up. and wham! i got exactly the results i expected. moreover, the latest firmware is substantially more feature filled and responsive than before.thanks againEdit again: confirmed that attempting to use nanovna saver config crashes and permenently messes up configuration again. could be a dodgy usb cable also but at this point i think ill just stick to the hand held.

author: selcuk
author_rank: Frequent Contributor
timestamp: February 04, 2024, 08:16:37 pm
content: It is good that you eventually have satisfactory results. NanoVNAs are quite capable and you don't have to use a software to have reliable measurements.If you want to inspect the results with a more capable smith chart running on your computer, then you may follow these steps:Make the calibration on NanoVNA. Run the measurements on NanoVNA. Store the results in s1p or s2p files on the microsd card. Turn off the VNA. Place the card into your computer and copy the files. Then use a smith chart software to inspect and calculate matching circuits etc. You don't have to calibrate under the software since you are loading already saved measurements.http://www.ae6ty.com/Smith_Charts.htmlhttps://www.atyune.comBy the way, I have nanovnasaver 0.5.5 and it is running fine under Ubuntu.