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Andrew, my coil program might interest you. It calculates inductance and Q for solenoids made of solid or Litz wire. It models a number of wire alloys and form dielectrics, dielectric ribs or ridges, coil leads, and polygonal coil shapes. The program can automatically maximize coil Q while keeping inductance constant.

To validate accuracy, the documentation compares calculated values with hundreds of inductance and Q measurements made with an HP 4342A Q meter on dozens of coils of widely varying size and shape. For the coils from my own junk box, average error magnitude is 2% for inductance and 5% for Q.

The program runs in Windows and is free.



Brian

A useful article;

- I am guessing a “trap network” is a series resonant circuit, but I’m not sure.

YES, IT IS A SERIES TRAP NOT A PARALLEL TRAP BUT IT IS IN SHUNT WITH THE OUTPUT, CH1. SO THIS IS A S21 MEASUREMENT. IN AFFECT YOU ARE USING THE VNA AS A SPECTRUM ANALYZER-VOLTMETER AND TAKING A MEASUREMENT OF THE NOTCH DEPTH. IF THE SERIES R OF THE LC NETWORK UNDER TEST IS SMALL, SAY ON THE ORDER OF MILLI-OHMS, YOU WOULD BE SEEING Q's ON THE ORDER OF 300 OR GREATER.

THE MEASUREMENT PROCESS IS JUST THAT OF A VOLTAGE DIVIDER.

- I don’t know what constitutes a “high Q variable capacitor” which your method requires.

IF THE Q OF YOUR COIL IS ASSUMED TO BE 300 OR MORE... YOU ARE GOING TO NEED TO FIND A C LARGER THAN THAT! AIR VARIABLES LIKE BC BAND ARE PRETTY GOOD AND THEIR ARE SOME CERAMIC CAPS THAT HAVE Q'S GREATER THAN 500... SEE AVX CAPS

- When X is 0 at resonance, I get that Rs is all that is left of impedance… but why is that in parallel with the termination resistance?

AGAIN, RS IS IN SHUNT WITH THE LOAD, 50 OHMS, BUT ITS VALUE IS SO SMALL, THAT THE NOTCH DEPTH CREATED IS THAT OF A VERY SMALL SHUNT R. THE END RESULT IS THE NOTCH DEPTH IS DIRECTLY CORRELATED TO THE Rs OF THE LC NETWORK AT RESONANCE.

- Indeed, is “termination resistance” simply the built-in resistance of the meter?

YES, 50 OHMS.

- What is a “notch” and why is there an attenuation depth when series resonance provides a lack of attenuation?

SERIES RESONANCE PROVIDES A LACK OF ATTENUATION IN THE SERIES MODE. BUT THIS HAS IT CONFIGURED IN THE PARALLEL MODE!

- Couldn’t I simply measure the series resonant circuit with the VNA and get a Rs reading directly, but would it be any better than what I get off the coil alone?

YES, BUT THE NUMBERS ARE SOME ARE DIFFICULT TO DISCERN. THIS IS AN INDIRECT WAY TO GET TO A NUMBER WITH SOME IMPROVED ACCURACY. KEEP IN MIND, IF THE FIXTURE LOSSES OR R VALUES ARE NOT SMALL, YOU WILL NOT OBTAIN CORRECT ANSWER.

Parallel Q is the same as series Q at a single frequency. Google the concept of a series to parallel conversion. There you will find the proof and the math. It is not hard, but will require your familiarity of complex numbers.

There is some older firmware with quite large fonts (see the group wiki) but the latest firmware by DiSlord and Hugen do not have these large fonts. However the font face has been improved to make it more readable. I suggest you try Hugen's 1.0.69. He just released 1.0.70 (tracking DiSlords firmware development) but I don't know what happened with the fonts in this release.



Roger

All good questions Andrew. Standby while I compose some answers.

Hi all,

sorry for the basic question. I own a regular NanoVNA (small version) which I purchased via Amazon / AllAboutFun. Where can I get a recent firmware update? I am specifically interested in a firmware with bigger front as I need to measure max. 2 values per measurement.

Thanks!
Leo

Great, thanks! I can see that your article addresses the issue nicely, but it also uncovers so many questions a non-EE like me has:

- I am guessing a “trap network” is a series resonant circuit, but I’m not sure.
- I don’t know what constitutes a “high Q variable capacitor” which your method requires.
- When X is 0 at resonance, I get that Rs is all that is left of impedance… but why is that in parallel with the termination resistance?
- Indeed, is “termination resistance” simply the built-in resistance of the meter?
- What is a “notch” and why is there an attenuation depth when series resonance provides a lack of attenuation?
- Couldn’t I simply measure the series resonant circuit with the VNA and get a Rs reading directly, but would it be any better than what I get off the coil alone?
Indeed, possibly related to my third point above: I have never understood why X / Rs = Rp / X. I have played with hypothetical cases, and it seems to not work.

Andy

see msg #23812. Also type in Q measurement to the search box.

I am neither a programmer nor trained in electronics, but I am having a great time using my nanoVNA-H4 with my homemade crystal radio. I measure S11 using only CH0, and the results I get for Smith chart, inductance, and reactance seem about right and work well when I use them for things like predicting tuned frequency given a know capacitance.

However, I believe I have no way to estimate Q = X / R for my coils, because R is quite low and may vary between -2 and 5 ohms. That variability may not be all that bad for a $90 instrument, but it can’t be used for Q! I am hoping you EE types may suggest a workaround for getting a decent Q estimate.

Here is a totally made up example of what someone may suggest: "R gets quite high at frequencies way higher than those the coil will be used at. At those high R values, the error is less. Take that number, and “everyone knows” equation XYZ for skin effect is quite accurate, so use equation XYZ to back-calculate R at lower, practical frequencies. Now you can estimate Q reasonably well."

David Reichard:
That's not what I thought about, maybe it's a language barrier issue, I'm using a translator.The SWR meter designed for 50 Ohms is only able to measure SWR for a 50 Ohm antenna system, it cannot measure SWR for a 75 Ohm antenna system.
I have redrawn the measurement bridge from NanoVNA to LTspice to easily do sample calculations.
The bridge is powered from a V1 source of 1V. The NanoVNA measures the voltage across resistor R9 (I called nodes M1 and M2)
Let's calculate the voltage for values 1 50 2500 and their 25% higher values: 1,25 62,5 3125

testval V(M1)-V(M2)
1 -0,3161
1,25 -0,3120
diff -0.0041

50 0,0003
62,5 0,0273
diff -0.0270

2500 0,1937
3125 0,1949
diff -0.0013

As you can see, 0.0270 is 6.6 times larger than 0.0041, and as much as 20.7 times larger than 0.0013.
The NanoVNA measures the voltage and calculates the impedance. As you can see, for high impedances such as 2500 ohms, a very small change in voltage corresponds to a very large change in impedance, so that a small error in voltage measurement translates into a large error in impedance. At low impedances there is a similar problem, but not as large.
A neighboring thread relating to low impedances:
/g/nanovna-users/topic/nanovna_low_z_capabilities/86085049
Then there's the cable transformation, which can turn low impedance into high impedance and vice versa. Calibration tries to compensate for this, it is a software action, but the bridge measures what is connected to it, and this error cannot be avoided.

Btomek:

I think you are saying "A moderate change in impedance (resistance + reactance) has little effect on thee SWR/reflection coefficient when the SWR is close to 1:1 but a large effect when the SWR is high to start with." Is that about right?

I found this on Wikipedia: "SWR is usually measured using a dedicated instrument called an SWR meter. Since SWR is a measure of the load impedance relative to the characteristic impedance of the transmission line in use (which together determine the reflection coefficient...), a given SWR meter can interpret the impedance it sees in terms of SWR only if it has been designed for that particular characteristic impedance." Just as you stated.

BTW, I seldom contribute to this forum but I read most of the posts and find the discussions quite educational. Thank you all!

- David
Ham KD6DWR/GMRS WQYV533

Hello,

The calibration is done with the same cable and the same calibration set on both units.

Were you using "the same cal set file" between both units?

Seen this before where the new VNA s/w release changed cal data set format
thus breaking backward compatibility.? Opps.

73's wb9own

On Tue, Oct 5, 2021 at 10:44 AM, Walter Miller, AJ6T wrote:

I need to measure a very low impedance device for a non-ham
application.....probably less than 1 Ohm.? Is the NanoVNA able to make an
accurate measurement on such a load in the 14 MHz range?

73, Walt, AJ6T

You may be interested in this video where I demonstrate using the NanoVNA to read well below 1 ohm. I only ran it up to 2MHz but you should get the idea.

Wes!!

Thank you so much!! You’re a real life saver. I really do appreciate the time you took to respond. Nothing like having a quality group of guys helping each other.

73,
Ernie, W5NH

Ernie,
There are many to choose from on FleaBay; search, '*nanovna vector
network analyzer calibration kit*'.

Yeah, some C USB cables are *data only*. Again, FleaBay can be your friend
(or your worst nightmare;
it's all about perspective). Try this one:


Take care,

Wes KN4NPH

Gentlemen (and ladies too, of course).

I’ve had my NanoVNA since they first came out; I think a year or two. For several reasons I haven’t taken it out, nor even calibrated it yet. Now I have the time.

Upon opening the plastic box it came in, I notice that I’m missing one of the calibrating SMA threaded resistors. Does anyone know where I could purchase another set?

Second, I’ve tried multiple charging cables; the ones with USB “C” connectors, and none of them will pass current into the unit’s battery. My friend has the “sister” unit to what I have, and his USB cable works just fine. Are there differences in those cables, or do I have two that are bad? Where could I purchase another and be certain I’m getting one that works?

Any and all help in this regard would be most appreciated.

Ernie

Hi!
Thank you very much for your answer! :-) :-) I think this clears up my measurements and as I understand this is what could be expected.

Karl

Hi Roger!
I have installed a current choke in the feed point of the antenna.

I have both VNAs on a table, in my hand and on the ground, the measurements are close on each device, but the difference between the units are the same.

If I put the VNA on the table and touch the unit (or SMA) with my hand, the readings are steady.

Karl

The NanoVNA measuring bridge at the S11 connector is in equilibrium at a load of 50 Ohms, and works most accurately in this region. The further away from 50 Ohms the more error there is in the measurement per R and X. The NanoVNA measures the reflection coefficient, and for this 910+j1610 we have SWR=75.21, and for 1580+j1880 SWR = 76.36. This corresponds to a reflection coefficient of 0.9738 and 0.9741. As you can see, the difference is only 0.0003 of the reflection coefficient. For comparison, 46.5+j2.85 = SWR 1.098, 46.2+j2.48 = SWR 1.099. The corresponding reflection coefficients are 0.04671 and 0.04717, a difference of 0.00046.

You did not indicate if you have a current balun installed at the dipole. The dipole is a balanced antenna and the coax is an unbalanced transmission line. Without a current balun common mode current will flow on the outside shield of the coax and this will affect the feedpoint impedance. This will be most noticeable when you are off resonance with a higher feedpoint impedance.

The common mode current will depend on several factors including frequency, positioning of coax to antenna and ground and how the coax is terminated. In your case you are terminating with a NanoVNA-F or NanoVNA-H4. One is in a metal case and the other is plastic. Each will capacitively couple to your body in a slightly different manner and this will affect common mode current and the feedpoint impedance that you measure.

You can see if common mode current is affecting the results by placing the NanoVNA on the ground or on a table instead of holding it. Do the results change?

Roger