Making VNA Measurements in a non-50 Ohm Universe (065)
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- Опубликовано: 11 дек 2023
- So you have some device which has an input impedance and, possibly, an output impedance which is not 50-Ohms. You have your nano-VNA, miniVNA tiny or other measurement device that you want to use to make measurements on this device AND maintain the proper impedance match with the device.
But these measurement instruments have an input and output impedance of 50-Ohms which is not an impedance match to you device.
In this video I am going to present two ways you can approach this problem.
WAY#1: The "Mechanical Impedance Conversion" Method
WAY#2: The PAD Method
I will compare the results from these two methods.
========= LINKS FOR YOU ===============
LINK to the PDF which has the formulas used in this video:
drive.google.com/file/d/10yI_...
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Time Markers for Your Convenience
----------------------------
00:05 Introductory Comments
01:24 Problem Definition
03:03 What exactly am I going to be doing?
04:17 WAY#1: "Mechanical Impedance Transformation"
05:52 Step#1 - Conversion
06:25 Step#2 - Calibration
09:55 WAY#2: PAD Method
10:21 What is it made of?
10:45 How do I design one?
14:00 How to use it...
14:56 Conversion: Moving from the VNA's 50 Ohm world to the non-50 Ohm World
17:18 The Comparison: WAY#1 vs WAY#2
17:59 SWR
19:16 Input Impedance
21:49 Thru (Frequency Response)
24:33 Multiple Measurements Averaged?
25:01 Thru (Frequency Response)
25:59 SWR & Input Impedance
26:57 Final comments and toodle-oots
-----------------------------
This popped up in my subscriptions and I wasn't familiar with why there would be a "50 ohm universe" but guessed it was some impedance topic for one of the many areas I don't usually stray into (my projects usually use DC stuff, so not too familiar with transmission line theory etc) but now I see why I couldn't guess the video topic - the video is about the occult teachings of RF.
Oh yes...there is a LOT of "black magic" in R.F.! You are right on that one. In part this is because stray inductance and capacitance is *everywhere* and this stuff throws all sorts of curve balls at us. 🙂
thank you for this video
You are very welcome! 🙂
Fine job
Thanks! 🙂
great video,thanks ralph
Thanks, man! 🙂
This wasn't actually the comment I was going to make; I'm still trying to digest all this but here are a few initial thoughts.
First, fantastic video Ralph; thanks for addressing this topic.
Second, I was told by (someone in authority) that those splitters do not have good low-frequency response (balun-core permeability issue?)
So, they are not really usable unless they just happen to do what you want.
The last one, may be showing my ignorance but in working with the VNA below 30 Mhz, I just don't seem to see any impedance anomalies that would warrant the differences of say two different N-connectors with minor geometry differences; and have concluded that these could only be instrumented way-up in frequency.
In fact, it would be interesting to compare the two N-connector geometries to see just what they did and at what frequency you could really notice.
I'm lucky in that most all of my interest is below 30 MHz, where things don't have to be on such a razor's edge.
Again Ralph, thanks for making this and going to all the trouble with your graphics and overlays; it does not go unnoticed!
73...
The video from that major manufacturer of high end VNAs, etc actually showed the ends of the two N connectors. To begin with, the center pin is a different size. Ya plug the bigger guy into the smaller guy and you damage the smaller guy's receptacle.
But, you are right ... lower frequencies are far, far more forgiving! There the stray capacitances and inductances, cable & wire lengths have far less affect on measurement outcomes. I, too, breath a sigh of relief when I can work down there.
Thank you for the kind compliments and encouragements. I am always looking for ways to "improve my serve." 🙂
@@eie_for_you Right. I guess I didn’t make clear when I was talking about the N-connector geometries; I was actually trying to say ‘electrical-geometries’ and how far up would you have to go in frequency, to actually see the minor mechanical changes they deemed necessary. In my mind, they probably should have made a different connector so that to the uninitiated, they look the different. I, myself have worked with N-connectors (even repairing commercial equipment) and never came across the 75-Ohm version!
On another note, you were aware of my comment (from your other video) regarding common-mode measurements of a 1:1 balun. I did look at the reference you sent me, and (in his hand drawn test-schematic) showed yet another way of doing it. So, I think we’re up to about 4 ‘prescribed’ methods that simply cannot all be right! So, I thought I’d put this comment here since (to me) the CMR measurement across the balun would certainly be ‘another world’ impedance. Anyway, in all of those CMR measurement methodologies, they are trying to (with resistors) trying to interface it with VNA’s 50-Ohm system. And this to me defies logic in that a CMR measurement is just that; it is not a differential measurement, which would imply a certain input/output impedance (i.e., you are looking for the amount of parasitic conduction of what might be termed (in coax land) as the I3 common mode current. And these parasitic conduction is being mitigated my the core’s permeability.
So, the confusion for me, is the fact that we are looking at common mode, not differential, and therefore, given the individual 1:1 balun’s physical configuration, I don’t know that you could make any assumptions about its inherent common mode impedance; such that you could simply throw a few strategically placed resistors and you’re good!
So these are the things, that keep me up at night, and eating way too many cookies! 😂
73…
@SpinStar1956 Gotta watch out for those cookies!!!! LOL ;-D
Yeah, making measurements on BALUNs, including CMR is tricky. The REAL method for measuring CMR requires a VNA with more ports than anything I have (I think 3 is the minimum). So, we have to find ways to do this on a poor man's budget. The resistive divider is the answer which I choose. I can do that!
In the end, I ask the question, "How accurate do we **really** need to be?" We just have to have a reasonable expectation that we are getting an indication of the real CMR of the balun. Thankfully, we do not need laboratory quality measurements to determine if the BALUN is a bust or not. Not all do a good job of it. 🙂
Very interesting video, results were closer than I thought they would be. I guess now I need to play around and see how things look with and without a pad for a "ladderline" of reportedly 370 ohms and also some 300 and 450 ohm stuff I have and want to make into a zs6bkw antenna with an adjustable length ladder line feed. Thanks for doing the video!
Thank you and you are very welcome. I will be interested in hearing how this project works out. 🙂
👍Thank you sir.
You are very welcome! 🙂
Great video, thanks for making that. I'd be curious at the delta of the "measurement error" as you get further and further away from 50ohms, say at 1000 ohms. With antennnas, we use transformers to attempt to match the impedance, but those are very frequency sensitive. I guess the "pad" loss is calibrated out, during the cal process. Definately a video that makes you think about the input impedance. Thanks again, for the professionalism to your videos, you certainly dont waste my time with distractions, you stay on subject.... That is appreciated greatly by me when I invest time in watching your video from start to end, I don't even need the "1.25" speed up. (:-).... Best of Holidays..
It also depends on the limits of the VNA and its ability to "calibrate" outside of the 50 Ohm world. I am sure that with some situations you will have to use the pad as the VNA just cannot "reach that far."
You are right, the VNA will "calibrate out" the loss of the pad.
Thank you for the encouragement in the video quality. I am always trying to "improve my serve" so to speak. I, too, get frustrated with folks that never seem to get where they say their going.
Have a Merry Christmas! 🙂
A superb tutorial Ralph! Many thanks. I think the most important part of this video though is the line you are repeating, "is it close enough for you". This is spot on!
In my opinion, most students have lost the art of estimating and rely far too much on the measurement taken to the 1/10000th decimal point. They are needlessly taught this way in many math classes for accuracy, but in the "real world" this level of accuracy is not necessary and certainly does not constitute a failure of design. There is always wiggle room. A house framer does not need to measure the cut to 1mm, but a surgeon does. It all depends on the tolerance we expect and demand. Teaching students this concept is extremely important. Getting them to put down their calculators and "guesstimate" an answer first and THEN verify it with the instrument is the way to go. I think we did better with slide rulers vs calculators for this very reason. (tongue in cheek). 73 OM Merry Christmas
I'm a big one to carry WAY too many decimal points in the middle of my calculations and then round at the end. I've been burned by rounding too early in the process.
But you are right, is this measurement close enough for your application?? Let's not stress out over unnecessary precision or too many decimal points.
Thanks it was very informative,
I used transformer 75-50.
Yep, the transformer is a very viable solution. It is more frequency dependent that a well built resistive solution, but it is still a very good one! 🙂
@@eie_for_you Also, remember the L, PI, and T resistive pads are lossy. Note that the dynamic range of NanoVNA, for example is only 40dB advertised, in reality is no more than 35dB after the connection and the wires. So, if your pad has say 6dB loss, you can not measure say return loss of 29dB or higher.
@@aduedc I think you are mistaken about the specifications of the nanoVNA. Check out this page ... nanorfe.com/nanovna-v2.html#specifications
@@eie_for_you yes this is the newer one even this specify S11(return loss) at -40dB. the old one that does not go above 1GHz is even worse.
@@aduedc Aaaah...I guess I was looking at the S21 for measuring the frequency response of a filter or other device.
Besides the genuine article nanoVNA's S11 noise floor limitation, there are those cheapo knock off clones that do not perform anywhere near as well.
We still come back to the question ... How good is good enough for our application?
For many guys just looking to see the performance of an antenna ... these differences may not be significant enough to bother. Besides, for this group, they don't even need impedance conversion; they just need to calibrate their nanoVNA with a 75 Ohm load if they are working with a 75 Ohm system ("mechanical impedance conversion"). I'm not quite sure the limitation of the nanoVNA regarding what you can calibrate it with. Could someone calibrate it with a 450 Ohm load?
Thank you for this video. I would like to measure the impedance of an antenna using 450 Ohm ladder line. The only interface that I have found is 9:1 unin. Could use a 9:1 unun with the equations that you have given for the 75 Ohm pads? Is there a better way to do this that would account for possible mismatches in impedance? Thanks again.
You could use the equations I provided and create the resistive pad to interface between the 450 Ohm and the 50 Ohm for measurement purposes. Remember, too, the the impedance that you measure at the end of the ladder line is not necessarily going to be the actual impedance of the antenna unless your ladder line is exactly 1/2 wavelengths long, electrically speaking, because of transmission line effects (see the Smith Chart). With that said, you can estimate the impedance as measured using the procedure that I outline in this video:
ruclips.net/video/bPmtIVmlJok/видео.html
To do this, you will need to know the velocity factor of the ladder line. Hope this helps! 🙂
@@eie_for_youThank you. I will give it a try.
@@richardphillips2405 If you would please, let me know how this works out for you. 🙂
I recently read an article on NVIS antennas that mentioned this: "The impedance of a half wave dipole in free space is about 70 ohms. As it is lowered, the impedance drops. When it gets to 7-8’ high, the impedance is down to about 12 ohms. "
Can you explain why the impedance of an antenna changes with the height? Why doesn't it stay the same regardless of the height?
As a sanity check, I looked in every antenna book I have to try to verify this answer, but they all talk about impedance relative to length, not height above ground; this even my college textbook on the subject! 😞
Here is my educated guess ... the impedance of an antenna is a complex entity consisting of inductance, capacitance and resistance. This includes it proximity to nearby objects like buildings, other antennas and ground.
If we remember that capacitance is any two conductors (e.g. the antenna itself and ground) separated by an insulator (e.g. air) and is inversely proportional to the distance between said conductors (e.g. height of the antenna), then we can also see the capacitance of the antenna to ground increasing with reduced height above ground. Increased capacitance = decreased impedance.
Am I right on this one? Dunno, but that's my take on it. No one else is fessing up to the facts. 🙂
@@eie_for_you Thanks, that makes sense.
@@AL-kn4yx 🙂
Good antenna analyzers offer selectable input impedance. My RigExpert and Rhode & Schwarz VNA antenna analyzer both offer selectable input impedance. Mike KC3OSD
Yes, they do! Man, they do make fantastic stuff. 😍 As I understand it, these are only switchable between 50 and 75 Ohms. This still leaves the guy who is trying to measure his 450 Ohm antenna grasping for answers. So, while I am using a 75 Ohm environment for this video demonstration, the aim was far more general than this. 🙂
@@eie_for_you My RigExpert antenna analyzers can do 25, 50, 75, 100, 150, 200, 300, 450, 600 Ohm system impedance.
@@mikesradiorepair WOW! Now THAT is quite impressive! Those RigExpert analyzers are awesome units! I am wondering, though, are they actually changing the output impedance of the unit or just changing the math (and calibration numbers) inside the device? As you saw in my video, you only really have to change the math for impedance measurements for many applications (SWR, Return Loss, Reflection Coefficient stuff all remains the same). The only place where the actual output impedance makes a difference is when doing the thru measurement. Because it doesn't do through measurements ... just thinking and wondering.
@@eie_for_you I believe it is probably done in software. And setting here thinking about it, it would almost have to be. Since it only has one RF connector and it's a 50 Ohm N female. They don't have a different connector for each system impedance.
I think you are right. Gotta LOVE those N connectors. MUCH more expensive than the UHF (SO/PL) connectors, but worth every penny.
I have over 2 dozen 50 Ohm to 75 Ohm and 75 Ohm to 50 Ohm adapters in numerous connector types. Pretty much every major connector adapter manufacturer makes them. My Rohde & Schwarz adapters even come with a calibration certificate. You must not have looked very hard for the 50 Ohm N male connector to 75 Ohm female BNC adapter. I have 6 from 3 different manufacturers. Mike KC3OSD
And I will bet good money that all of this cost a LOT of $$$$$. I looked at Pasternack, primarily. Also DigiKey, Mouser, Newark and a couple of other R.F. places. I do not remember finding anything. If I *did* find one, the price was probably WAY more than I cared to pay (admittedly, I am quite "frugal"). 🙂
This is what I was looking for a few months ago. I have one NanoVNA V2 and would like to make a transformation like this. I'm doing a PCB Board for ESP32 ICs and I found in the datasheet that its RF output has a 35Ohm + 0j output impedance and I only have a 50Ohm chip antenna. Please, tell me how to calculate and measure the CLC matching circuit using the NanoVNA. I need to calibrate it to behave like a 35Ohm output to measure the CLC matching circuit to 50Ohm. I want to use this method to tune the antenna ruclips.net/video/rbXq0ZwjETo/видео.html to make its Return Loss near or better then -20db. Thank you very very much.
The return loss currently sits at -15.1 dB without doing anything but connecting the two together. Also note that the physical traces on the PCB need to be impedance matched. See this for that: ruclips.net/video/J8VT0Qxn31k/видео.html
You could just use a resistive matching L network like I speak about in this video (see the "go along with the video sheet...link in the description) OR you could use impedance transformation using a stub transform similar to what I talk about in this video: ruclips.net/video/VVEjtGL7dsM/видео.html. Only you would be using PCB features to accomplish this.🙂
I tried the formula convertion but it is getting a negative square root if I use R2 = sqrt(35² - (50*35)) unless I use the module.
@@anlpereira Aaaaaahhhh...this is because you swapped hi Z and low Z
R2 = SQRT(Zhigh^2-(Zlo*Zhi))=SQRT(50^2-(35*50)) = 27.386 Ohms.
Easy to make this mistake. 🙂
@@eie_for_you Hehehe. I used pcb trace impedance for 50ohm from JLCPB. How did you calculate that I would have a -15db return loss without any matching components? I’m a little confused yet about the use of my NanoVNA. Your videos are very very good for me. Thanks
Where I can find your email to send the pictures of my circuit? Thanks