VNA calibration question about open options? - Page 1
Forum section: electronics / testgear

author: Battle
author_rank: Contributor
timestamp: April 28, 2025, 12:47:10 pm
content: Im always learning so forgive me if i ask stupid questions.VNA calibration kits. Specifically N type. I have bought a Anritsu female Tee & Keysight male set. Ive also bought some Narda loads & shorts of both male & female N. As well as some midwest microwave loads. I understand the loads are important to be accurate. My question is about the open option. Since its just an open no connection. Is it really that important what you use for an open? As long as its an open connection? Could you use a barrel connector? Or a new 9913 N connector with no center pin or coax on it? Or is there something special about these pricey calibration kits?

author: jjoonathan
author_rank: Frequent Contributor
timestamp: April 28, 2025, 01:27:42 pm
content: The open standard is notorious for being easy to approximate but difficult to accurately implement.Problems if there is no mated connector:Before reference plane: The fingers of the slotted female connector will have reduced outer diameter.At Reference plane: There is no section of reduced outer diameter between the end of the female conductor and shoulder of the male conductor.After reference plane: Fringing capacitance is uncontrolled, forcing any effort to characterize it to rely on assumptions in this region.Do these actually matter for your purposes? Dunno. I don't have a 7mm cal kit to check. I could probably check on 3.5mm though.

author: joeqsmith
author_rank: Super Contributor
timestamp: April 28, 2025, 01:39:29 pm
content: I wonder how much it would matter for all three standards. As long as they are 1) repeatable 2) characterized 3) the software supports the characterized values

author: jjoonathan
author_rank: Frequent Contributor
timestamp: April 28, 2025, 02:11:55 pm
content: I suspect it would work quite well if you characterized it, but that's the rub. The easiest way to characterize an "invisible open" standard is with an expensive characterized open standard, and the way the expensive characterized open standard was characterized was with an extremely expensive parade of better behaved offset short standards. Short standards are "the good standards" for metrology since the boundary condition is easy both in math and IRL.The only free lunch here is if you can afford to shrug your shoulders at the error terms in the open standard. To that end, I've measured a Maury 3.5mm slotted female connector using an 85052D cal kit. Or, rather, I have used the cal kit to calibrate the measurement and then with the calibration set I measured a "flush open" obtained by simply leaving the connector open and free of nearby dielectrics or reflectors and a "flush short" with construction that you will see shortly.Hopefully someone with an actual N cal kit can do a similar experiment and get you more pertinent results, but my point here is that relatively speaking open standards are squirrely and short standards are well behaved.

author: jjoonathan
author_rank: Frequent Contributor
timestamp: April 28, 2025, 02:13:04 pm
content: More photos

author: jjoonathan
author_rank: Frequent Contributor
timestamp: April 28, 2025, 02:21:30 pm
content: So fringing capacitance seems to be the dominant term, I wonder how repeatable it is for N connectors? There's probably a good 80/20 solution in the form of a rule of thumb "female N connectors have ? fF of fringing capacitance."

author: jjoonathan
author_rank: Frequent Contributor
timestamp: April 28, 2025, 02:33:54 pm
content: ChatGPT says:> If you want a 1-liner "engineer's back pocket" rule: Assume 45 fF of shunt capacitance at the end of a typical unpopulated female N connector unless you have a better measurement.I asked it about 3.5mm too since we have ground truth and it was in the ballpark:> For an open female 3.5 mm connector, the typical fringing capacitance is much lower than for an N connector: About 10–15 femtofarads (fF)I'd still love to hear from someone with an N cal kit of course (wink, nudge). Did the all-knowing machine gods get it right? Or are they trying to deceive us?

author: joeqsmith
author_rank: Super Contributor
timestamp: April 28, 2025, 03:34:08 pm
content: Quote from: jjoonathan on April 28, 2025, 02:11:55 pmQuoteSeems like your post answers the OPs question.To your point, is it needed. A little background from the OP:https://www.eevblog.com/forum/testgear/best-vna-for-around-or-under-$2000/The only time I have ever really tried to nail down the calibration on the low cost VNAs was my attempts to improve on the T-Check:https://www.eevblog.com/forum/rf-microwave/nanovna-custom-software/msg5685217/#msg5685217More just for the fun of it.

author: Battle
author_rank: Contributor
timestamp: April 28, 2025, 04:04:59 pm
content: I made that post thread when I was looking for a used VNA.I started out I had a Lite VNA 64. Small pocket VNA. I bought for amateur radio coax & antenna work. I used it to check a set of duplexers to see if they would tune to amateur frequencies. Basically looking if the tuning rods were long enough initially.Then decided I was going to be tuning them myself for a learning experience. So to have better equipment for that than a pocket VNA. I made that post about VNAS under 2k. Quickly found out i can buy used stuff from the 90s era with floppy drives & CRT displays for that price with only 100db of dynamic range. With worries of device age old equipment having a possibly short life span due to its aging components.So I opted to buy a LibreVNA that's new with 100db of dynamic range. It still lacked the range I needed due to the cans I have are 6 cavity and go down 120db. I knew this when I bought the LibreVNA. I just wanted it because it's better than the pocket one I had by far.I also bought a Siglent SSA3021X Plus. So I used the Spectrum Analyzer with tracking generator to tune the rest of the way down. I've also purchased a Measall KC901J VNA recently. Just because I wanted something with its own display. The LibreVNA has to be hooked to a PC & that's limiting sometimes.But to add to what I have I've been investing in some good name brand RF test cable sets. Then buying good name brand calibration kits vs the cheap Chinese ones. As well as loads. But I couldn't help but wonder, what's so special about the open sets? So I figured I'd ask & see what the guys or gals that know more than I do have to say.I'm a self taught person on most everything. So I ask questions alot. My first time replacing a transmission in a car was when I was a teen. Dad said go out there in the shop & do it. Ask me if you need help. I didn't need help. Figured it out as I went along. Same thing for building a PC. I worked at a controls company building hvac control panels from schematics. The PC guy said let's see if you can build a PC. I had never even really used PCs back then. So he said here's the parts let's see what you do. I managed to do it. But hey that's really not as hard as it sounds though. I take great pride in self learning things. I don't make much for money now days due to health issues keep me from working as much as I wish I could. But I still make efforts to put up repeater equipment to promote amateur radio good will & fellowship. So I can't afford super expensive VNAs & such. I can only hope people who have that stuff don't look down upon the little guy who enjoys learning & doing things to help others with what little he can afford.I really appreciate all the help here. So thank you.

author: joeqsmith
author_rank: Super Contributor
timestamp: April 28, 2025, 04:11:37 pm
content: I was very surprised by "theblinkingman"'s response about their LibreVNA in that thread of yours.QuoteHave you repeated their results?

author: Battle
author_rank: Contributor
timestamp: April 28, 2025, 04:14:21 pm
content: I have not. I have only used it for VHF 144/148Mhz. Used it to initially tune the duplexer. Then used the SA & TG to finish it down to 120db.The Measall KC901J I just recently got. I haven't compared the two against each other.Another reason I decided to get somethings for work like this was I couldn't find anyone local to tune the cans. So I figured if I got some equipment to use & learn how to do things I can in return help others around me. Not for profit though. I've never charged for helping fellow hams. Even risking your life on radio towers etc.

author: joeqsmith
author_rank: Super Contributor
timestamp: April 28, 2025, 04:25:18 pm
content: With you buying 6GHz standards, seems like you could run that test fairly easy. I would be interested in seeing your results only because after some careful work I was able to get that pocket LiteVNA to perform well to about 3GHz. More than double what they got with the LibreVNA. Something seems wrong.

author: Battle
author_rank: Contributor
timestamp: April 28, 2025, 04:28:32 pm
content: Have a look at the LibreVNA yahoo groups. The person who designed the LibreVNA is very active there. He can answer any questions about that as well. I may mess with it more this coming weekend. Now you have me wondering.

author: Battle
author_rank: Contributor
timestamp: April 28, 2025, 04:37:33 pm
content: When I just had my Lite VNA 64 I went a bit nuts designing some 3d prints for it. 😀This one has a small tool tray/calibration holder with battery pack & fan.

author: joeqsmith
author_rank: Super Contributor
timestamp: April 28, 2025, 04:42:49 pm
content: It's an easy enough experiment to run. I think someone in the groups.io also ran it and results were poor. That's been a while so don't quote me. Last time I was there, they were introducing an E-Cal product for the LibreVNA. I would think this would greatly improve things.Consider the LiteVNA is a 2-port 1-path system. So I am using an external transfer relay to run that test. The LibreVNA is a full 2-port system. For the LiteVNA, that means more cables, connection points, sources of errors.Then, we are talking about something that cost me $150 vs say $800. I would expect that LibreVNA to out perform the low cost LiteVNA in every measurement.