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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

Today I have been measuring an antenna, and I got some strange results. Perhaps someone out there could give me an answer to the strange results? The antenna is a 2x19.3m dipole, about 6m above ground. I have used two NanoVNAs of different kind. One is the nanoVNA-F from deepelec.com, the NanoVNA-F from BH5HNU with the 1.0.3 firmware, and the other one is a NanoVNA-H 4 with the latest firmware 1.0.69.
Both NanoVNAs are calibrated for the frequency range 3MHz to 8MHz with the calibration plane at end of a coax cable, length of coax is about 7.35m. The calibration is done with the same cable and the same calibration set on both units. Calibration data is saved in slot 0 on both units. So far, so good.
Then, measuring the feedpoint impedance at 3.65MHz, this seems to be ok. 46.5+j2.85 and 46.2+j2.48 at the other unit.
Next measurement is done at 5.35MHz. The NanoVNA-F shows 910+j1610, while the NanoVNA-H4 shows 1.58k+j1.88k!
Has anyone got an idea why the measurents differs that much? Any opinion is would be nice!

Karl Jan - LA3FY

Good thing I don't enter them in beauty contests, I use them.

Kent,
Here is a case I designed for my Nano. It uses Neo magnets to hold the
accessories cover in place. The VNA is basically wedged (comfortably) in
the opening.
I use a 6" SMA to BNC coax cable from CablesOnline, to take the strain off
the VNA's connectors. The blocks 'pinch' the cable jacket to keep it from
moving. The blocks are glued; E6000, that can be removed if necessary.
I only use Channel 0. This may give you some ideas. Check my dimensions
against your VNA. YMMV

Wes KN4NPH

Roger,

On 9/10/21 12:26 am, Roger Need via groups.io wrote:

(CFR) 47 2.805
2013 update

That's precisely what I was after, many thanks!


- Roland 9V1RT

Andy,

I had noticed the duty cycle limitations in the datasheets for the Philips parts, so I figured that if I did use them, then I would limit output power. Perhaps the TACS part is the more sensible choice, as you state.

I was wondering about the attenuation outside of the stated frequency range of these parts. If it reduces external filtering requirements that would be great. I look forwards to testing this.

Thanks for the info!

On Thu, Oct 7, 2021 at 07:11 PM, Roland Turner wrote:

(I'm not subject to FCC jurisdiction, but am interested in understanding
the approach to various problems as background for discussions with our
own regulator.)

- Roland 9V1RT

Roland,

Since this is an off topic discussion I will keep it brief. Manufacturers of RF equipment in the USA are subject to the rules laid out in the code of federal regulations (CFR) 47 2.805 when testing their products prior to approval by the FCC. The rules changed in 2013 to be more restrictive fro certain types of products. Links below.

(CFR) 47 2.805
2013 update

In order to abide by the rules for this group this will be my last post on this subject.

Roger

To clarify, I intend to operate at a fixed frequency, no modulation, preferably somewhere in the 900MHz ISM band, driven by an amp operating at less than 5W. Each run of the experiment should only last a few minutes at most.

Probably due to me not knowing any better, I'm leaning towards making my own amp along with bypass filtering. The amp section would be based on modules such as:




--

Hello Mark,

The first amplifier you linked above would probably be your best choice. The two Philips modules are designed for the GSM cellular standard, which is pulsed with 12.5% duty cycle. They are not thermally capable of operating at high power with a continuous carrier. The Hitachi module was designed for the long ago obsolete TACS standard which is continuous carrier.

You wouldn't need to use a filter with these amplifiers. They narrowband 3 stage designs; the input, output, and interstage matching networks will have more than enough selectivity. You could verify this by measuring S21 of the amplifier with no filter. Be sure to use sufficient attenuation on the input and/or output to prevent damage to port 2 receiver in the VNA.

Andy - K?AF

Roger,

On 8/10/21 6:01 am, Roger Need via groups.io wrote:

The only legal way to transmit on this band is:
- by purchasing a device which has gone through the approval process above. described in FCC part 15.
- to be a manufacturer developing a product using good engineering practices to keep radiation to a minimum.
- to construct home-built experimental device defined under 47 CFR 15.23 and constructed with good engineering practice and Part 15 emission limits
- to be a licensed amateur radio operator using commercial or home-built equipment operating according to power and out-of-band emission limits.
- an approved ISM (industrial, scientific or medical) non-intentional radiator - FCC part 18.

I can trace 4 of those 5 to specific sections, but not the "manufacturer developing" case. Do you happen to know whether/where/how that's addressed in the regulations?

(I'm not subject to FCC jurisdiction, but am interested in understanding the approach to various problems as background for discussions with our own regulator.)

- Roland 9V1RT