Re: Measuring resonance from coax far end.
Hi, Calibration at the far cable end can be difficult at times, as not always you have easy access to your antenna feedpoint. An alternative way to get what you want, using easily measured rig side data and SimSmith, is described in this video: Simply measure your antenna's impedance response across the band of interest with NanoVNA at the rig side cable end and save it with NanoVNA saver as .s1p file. Import the file in SimSmith. The video explains how to obtain and save the actual feedpoit impedance response file. With this, SimSmith makes it easy to optimize your antenna and feed system without even leaving the shack. I described an application to this forum. You can find it under: broadbanding my W3DZZ on 80m Enjoy. 73, Hans DJ7BA -----Urspr¨¹ngliche Nachricht----- Von: [email protected] < [email protected]> Im Auftrag von Chris Keladis Gesendet: Dienstag, 7. Januar 2020 22:32 An: [email protected]Betreff: [nanovna-users] Measuring resonance from coax far end. Hi folks, This is more a generic VNA question, but thought i'd ask. I've seen a trick to "eliminate" the coax by calibrating via the OSL on the far end of the coax, rather than having OSL connected directly to the VNA. Essentially it's re-calibrated with the coax factored into the equation, virtually moving the feedpoint to inside the shack for accurate antenna feedpoint measurement. If i calibrate the far end, take resonance readings, then recalibrate with the OSL connected directly to the VNA, re-take resonance readings, would it be safe to assume if there is common-mode current interference on the coax it would account for differing results (if there are any)? If there's no common-mode currents, the coax-length should be invisible. Just want to make sure i've got my theory correct. :) Thanks & 73s, Chris.
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Re: errors of "error" models
And here is another introduction. Read section 2.1.3 This document from 1990 also mentions the ratio of cross ratios and describes various different approaches to calibration next to the well known SOLT -- NanoVNA Wiki: /g/nanovna-users/wiki/homeNanoVNA Files: /g/nanovna-users/filesErik, PD0EK
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Re: errors of "error" models
Attached document should give some answers The impedance model of the calibration load represents the actual impedance of the load either as it is calculated from theoretical modelling or from measurements like in attached document. -- NanoVNA Wiki: /g/nanovna-users/wiki/homeNanoVNA Files: /g/nanovna-users/filesErik, PD0EK
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Re: errors of "error" models
Interesting.....
My question still doesn't seem to be getting through... Yet it remains a simple one. :-) Let me try this again.
Question: What justifies characterizing the calibration standards?
Answer: Because it improves measurement accuracy.
Question: How does it do that?
Answer: It makes post calibration measurements of the standards plot with the profile of the standards and not plot as though the standards were perfect.
Question: How does doing this make the measurements more accurate?
Answer: Because HP says characterization of the standards improves accuracy, everybody agrees, this is the way it's always been done, and it actually works.
Question: How much more accurate are the measurements after calibrating with the characterized standards?
Answer: Close to absolute.
Question: How do you know the measurements are close to absolutely accurate?
Answer: Because they were characterized by a certified test lab, who provides us with the correction coefficients.
Question: Does the certification lab provide a tolerance on the accuracy of the coefficients they give you?
Answer: I don't know... Probably.
Question: So you are confident that your results are as accurate as you can make them?
Answer: Yes.
Question: Why?
Answer: Whatever do you mean???
Question: Isn't there some degree of uncertainty remaining?
Answer: Well sure...
Question: How much?
Answer: I don't know, but I know its not very much once I've calibrated appropriately with my Characterized calibration standards.
Question: What impedance does the open circuit standard represent?
Answer: Oh I don't know, but it's pretty high... maybe a few k ohms.
Question: I think I read something around 50 femptoFarads being used to compensate for the fringing capacitance of the open standard as a typical correction. Does that sound about right?
Answer: Yeah! I might have heard or read something like that. It's a very small number.
Question: I'll say... but 50 femptoFarads is about 32 ohms at 100 GHz. If the the open circuit impedance drops to 32 ohms, How far does that move the dot?
Answer: I don't know, but it isn't very far?
Question: But it shows that it has a noticeable span on the display. Why is that?
Answer: Because it probably moves that far after it's been calibrated. The open circuit might be offset by the connector length... and it only happens at really high frequencies. In the GHz range maybe.
Question: I'm guessing it's an unavoidable shunt capacitance and maybe there's some angular displacement in play also?
Answer: Probably... Maybe... It has to be something, or else it would be just a dot.
Question; So that isn't an error in the calibration?
Answer: No... It shows that it has been calibrated because it's displaying what the open circuit really looks like.
Question: Then the open circuit doesn't look like a real open circuit?
Answer: Correct. It's not possible to make a perfect open circuit.
Question: So I've been told... That's about as close to an open circuit as we can manufacture though am I right?
Answer: I think so.
Question: Then why isn't the standard used to represent a precise open circuit after calibration? Isn't this a real open circuit in the real world?
Answer: Because then it wouldn't be accurate, and not all open circuits are the same. They might be at a slightly different offset.
Question: That's still just a length change though correct.
Answer: Yes, but now we can measure it and use the data to measure others like it.
Question: You can't manufacture a precise open circuit, but you need to measure them accurately?
Answer: Correct.
Question: Why?
Answer: Huh?
Question: Why? What's the point? How does it manifest its value?
--
73
Gary, N3GO
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Re: test fixture - how bad can it be? A non-rhetorical question
You would need to have in the ballpark of 80 nH of series inductance due to series parasitics.. Seems a little high but not out of the question.
Measure caps a low freq first to get values with minimum parasitic effects. Don't go below 2 MHz to stay over about 500-800 ohms of reactance as the accuracy gets poorer at higher value of reactance.
If you have access to a good accuracy 10KHz or 100KHz LCR bridge it would give you a good base to work from.
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Re: test fixture - how bad can it be? A non-rhetorical question
On 1/7/20 6:27 PM, hwalker wrote: " ..A 100pF silver mica then gives the attached which indicates 105pF at 7MHz, 109pF at 14MHz and 126pF at 28MHz .."
=======================================================
The above frequency dependent impedance effects are why manufacturers specify their component values at specific test frequencies. A dedicated LCR instrument, i.e. the well regarded DE-5000, has five test frequencies 100Hz/120Hz/1kHz/10kHz /100kHz one of which will generally match the manufacturers specified test frequency. ???? Also, they choose lower frequencies not only because it's easier to measure at low frequencies, but also because many components suck at higher frequencies, and by specifying low test frequencies, component manufacturers were able to hide the uselessness of their parts.? Several decades ago I had access to an HP network analyzer, and used it to evaluate components for use in the VHF/UHF radio equipment I was designing.? Among standard ceramic chip capacitors, I found that only Murata and Panasonic were suitable for RF designs.? Pretty much everybody else's "capacitors" were much closer to resistors at those frequencies. But at the specified test frequencies of a few kilohertz to **maybe** a few megahertz, they looked fine, and I assume they would work fine, IF you were only designing audio equipment.
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Re: Measuring resonance from coax far end.
Hi wb2uaq,
Yes - i define resonance as R+j0. I had read that if there are no
common-mode currents on the feedline then length of coax shouldn't matter
as far as swr/resonance is concerned.
If it does change, then chances are there are common-mode currents on the
line affecting the reading.
I understand the basics of smith-charts, i'm a new owner of a nanoVNA but
have an (older) RigExpert as well.
Also understand the 1/4wl or 1/2wl multiples along the length of
transmission-line. I guess i'm a little confused if these will be seen at
the shack-end of the coax, if there was not any common-mode current.
I've read that if there is no common-mode then no change will be seen for
swr or resonance at points along the transmission-line, if there was, there
will be changes at every 1/4wl or 1/2wl point along the transmission-line.
Trying to wrap my head around how coax changes what you see at the
feedpoint. The RigExpert (AntScope2) has a menu to add/subtract/do-nothing
with the transmission-line when taking readings with it and it's always
confused me when taking readings with it.
Anyway, i appreciate your taking the time to reply & best 73s!
Thanks,
Chris.
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On Wed, Jan 8, 2020 at 1:22 PM WB2UAQ <pschuch@...> wrote: What is your definition of resonance in this situation? If you measure
say R + j0 at the antenna, the Z you measure at the input end will be
dependent on the electrical length of the transmission line. Only when it
is multiples of quarter waves long or a multiple of half wave long will it
look like a resistance again. Any other electrical lengths will result in
a resistance plus a reactance. If you are familiar with the Smith Chart,
you can see this readily. If not familiar with the Smith Chart. The
presence of common mode currents could add more complications but may or
may result in a definitive explanation. 73
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Re: test fixture - how bad can it be? A non-rhetorical question
On Tue, Jan 7, 2020 at 03:43 PM, Brian wrote:
" ..A 100pF silver mica then gives the attached which indicates 105pF at 7MHz, 109pF at 14MHz and 126pF at 28MHz .."
=======================================================
The above frequency dependent impedance effects are why manufacturers specify their component values at specific test frequencies. A dedicated LCR instrument, i.e. the well regarded DE-5000, has five test frequencies 100Hz/120Hz/1kHz/10kHz /100kHz one of which will generally match the manufacturers specified test frequency.
You can judge the accuracy of the NanoVNA at 100 kHz by comparing its results with those of a dedicated instrument like the DE-5000. At HF, you're a bit out of luck unless you have access to something like a high dollar HP 4294A Precision Impedance Analyzer, 40 Hz to 110 MHz. You might start a collection of "golden" components whose impedance values at HF you have determined using other instruments and use those as transfer standards to check the accuracy of the NanoVNA.
For your 100 pf capacitor example, I would expect at 100 kHz for the NanoVNA to measure 100 pf +/- the specified manufacturer's tolerance. At higher frequencies, especially for leaded components, I would expect values similar to what you measured.
- Herb
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Re: Measuring resonance from coax far end.
What is your definition of resonance in this situation? If you measure say R + j0 at the antenna, the Z you measure at the input end will be dependent on the electrical length of the transmission line. Only when it is multiples of quarter waves long or a multiple of half wave long will it look like a resistance again. Any other electrical lengths will result in a resistance plus a reactance. If you are familiar with the Smith Chart, you can see this readily. If not familiar with the Smith Chart. The presence of common mode currents could add more complications but may or may result in a definitive explanation. 73
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I have often thought that there should be a law that all portable devices have a hard off switch or battery disconnect. Both issues can be fixed by the clever tinkerer. For the NVNA, add a battery disconnect jumper. For the Wouxon, move the voltage divider tap. 73 -Jim NU0C On Mon, 06 Jan 2020 20:41:55 -0800 Ismo V??n?nen OH2FTG <ismo.vaananen@...> wrote: On my nanovna clone the battery was slightly bulged from the factory.
It's mad that the known wouxun fault from 2010 still persists (they put the battery voltage measurement voltage divider BEFORE the power switch. So the 1600mAh battery lasts almost exactly a week in storage with that drain). Baofengs have been a bit hit & miss, one holds charge for months easily, while the other only for a few weeks.
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Re: Part Number for the Demokit Cable
On Tue, Jan 7, 2020 at 02:44 PM, Ron Bussiere wrote:
" I made a simple tool to install the board side connectors, but also doubt they will survive very long. .."
=========================================================
Ron,
Thanks for the suggestions. Both boards have been donated to students for their own use. I relegate this to the same pile as the other cheap Asian electronics I've purchased that didn't live up to their promise. The success of the NanoVNA and its derivatives caused me to let my usual guard down. I generally wait for user reviews from the first wave of buyers before recommending a purchase.
- Herb
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Re: test fixture - how bad can it be? A non-rhetorical question
Hi Brian,
Unfortunate but a 100 pF will NOT remain a 100 pF C independent of frequency. And in fact the C value will increase with an increase in frequency. In your case with the values you provided, the result you present is due to the presence of series inductance of ~ 67 nH. Part of this are leads on the mica C as well as L from the fixture. In any case ALL passive RLC components have a parasitic component and at the vary least, a C as well as L will show an increase in value with increased frequency as you have shown in your chart. Eventually the C will obtain series resonance and an L will obtain anti resonance or parallel resonate frequency.
Alan
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Re: test fixture - how bad can it be? A non-rhetorical question
Brian,
How much different are your results if you clip to the capacitor 0.25" away from the cap body vs 0.5" away from the cap body? The inductance of the leads in series with the capacitor will form a resonator. The measured capacitance will increase as the resonance is approached from below.
--John Gord
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On Tue, Jan 7, 2020 at 03:43 PM, Brian wrote:
I have also been looking to answer how accurate should the NanoVNA be at HF.
Most components I want to measure are wire ended so my test jig is a 60cm
RG316 with SMA on each end cut in half. Each cut end has 1cm of inner exposed
and an alligator clip put on. This is calibrated with open, short and a 50 ohm
made from 2 100ohm resistors with short wires soldered on. A 100pF silver mica
then gives the attached which indicates 105pF at 7MHz, 109pF at 14MHz and
126pF at 28MHz. I remove the alligator clips and repeat soldering the
connections (inconvenient but worth trying). This gives the same results.
Should the NanoVNA do better than this? Are there firmware versions which will
produce more accurate results? Is it my VNA hardware(made in China somewhere)
or test jig or test method?
I have found the VNA gives good results using S21 on filters but using a
single cable and S11, capacitors and inductors are difficult to measure. I
expect ferrite cored inductors to show a frequency variation but a 100pF
capacitor should still be 100pF at 30MHz. I've repeated calibration, tried
different leads but I don't have any confidence in the accuracy of reactance
measurement. From what I have read, I could manually add "fudge factors" to
the calibration and remove what looks like a systematic error but that is not
so easy for the mathematically challenged (like me).
My conclusion is that the accuracy depends on the accuracy of the standards
you have and the amount of effort you put into "improving" the calibration.
Some test jigs might be easier to get a good calibration but up to 30MHz the
test jig is not the most important factor.
I have done what Erik suggests and measured a 100 ohm resistor. It shows no
significant reactance at 30MHz. I don't have a record of it but certainly not
26pF.
73 Brian.
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On the nanoVNA(H) the spec on the IP5303 says its standby is less than 100 uA, assuming it's 'Smart load detector' detects the unit has been turned off and puts the chip in standby. For a 450 mAH battery that should be upwards of six months.
LiIon's have very low self discharge and should not be the issue. If the cell is physically puffing up like a party balloon it is being over discharged and the battery will go bad within several months unless you keep up on recharging it every.four or five days. The nanoVNA(H) has a diode from battery in series feeding the Vbat input to the STM processor. Firmware could be causing too much draw on the Vbatt line. I do suspect the IP5305 is charging the battery at too high a rate for the small 450 mAH battery.
The nanoVNA-F is a different animal. It has two 10k resistor divider across the battery providing half supply to one of STM's ADC input for battery voltage monitor. Having 20K across the battery all the time is not good. -F covers some of its problems by its brute force 5 AH battery. .
One thing I can add about STM ADC input Z. The STM spec claims a 100K ADC input Z with wide variation. This is not truly correct and gives designers the impression they need to have low enough divider resistor to prevent the 100k input Z from effecting reading. What the ADC input is really loading the input with is a switched capacitor of 10's of pF. You can fix the loading due to the switching SAR ADC load by placing about a 500 pF cap from ADC input to ground so you can then use higher value resistors in the battery monitor divider network and reduce the off drain loss through the resistors.
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Re: test fixture - how bad can it be? A non-rhetorical question
For flying wires and alligator clips to work, everything must be in the *exact
same position* as when the calibration was done. I use a couple of BNC
female connectors mounted on a piece of double sided FT-4 board. I've then
soldered alligator clips to the BNC teat which I use to mount the component
under test. This keeps everything in the same position as the cal.
Dave - W?LEV
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Show quoted text
On Tue, Jan 7, 2020 at 11:43 PM Brian <vk4bap@...> wrote: I have also been looking to answer how accurate should the NanoVNA be at
HF. Most components I want to measure are wire ended so my test jig is a
60cm RG316 with SMA on each end cut in half. Each cut end has 1cm of inner
exposed and an alligator clip put on. This is calibrated with open, short
and a 50 ohm made from 2 100ohm resistors with short wires soldered on. A
100pF silver mica then gives the attached which indicates 105pF at 7MHz,
109pF at 14MHz and 126pF at 28MHz. I remove the alligator clips and repeat
soldering the connections (inconvenient but worth trying). This gives the
same results. Should the NanoVNA do better than this? Are there firmware
versions which will produce more accurate results? Is it my VNA
hardware(made in China somewhere) or test jig or test method?
I have found the VNA gives good results using S21 on filters but using a
single cable and S11, capacitors and inductors are difficult to measure. I
expect ferrite cored inductors to show a frequency variation but a 100pF
capacitor should still be 100pF at 30MHz. I've repeated calibration, tried
different leads but I don't have any confidence in the accuracy of
reactance measurement. From what I have read, I could manually add "fudge
factors" to the calibration and remove what looks like a systematic error
but that is not so easy for the mathematically challenged (like me).
My conclusion is that the accuracy depends on the accuracy of the
standards you have and the amount of effort you put into "improving" the
calibration. Some test jigs might be easier to get a good calibration but
up to 30MHz the test jig is not the most important factor.
I have done what Erik suggests and measured a 100 ohm resistor. It shows
no significant reactance at 30MHz. I don't have a record of it but
certainly not 26pF.
73 Brian.
--
*Dave - W?LEV*
*Just Let Darwin Work*
*Just Think*
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Re: errors of "error" models
Hi Gary,
Great questions. I have no idea what the answers are. But Erik¡¯s and John¡¯s replies seem reasonable.
Perhaps Dr. Kirby might know.
Best regards,
Jeff
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Re: test fixture - how bad can it be? A non-rhetorical question
I have also been looking to answer how accurate should the NanoVNA be at HF. Most components I want to measure are wire ended so my test jig is a 60cm RG316 with SMA on each end cut in half. Each cut end has 1cm of inner exposed and an alligator clip put on. This is calibrated with open, short and a 50 ohm made from 2 100ohm resistors with short wires soldered on. A 100pF silver mica then gives the attached which indicates 105pF at 7MHz, 109pF at 14MHz and 126pF at 28MHz. I remove the alligator clips and repeat soldering the connections (inconvenient but worth trying). This gives the same results. Should the NanoVNA do better than this? Are there firmware versions which will produce more accurate results? Is it my VNA hardware(made in China somewhere) or test jig or test method?
I have found the VNA gives good results using S21 on filters but using a single cable and S11, capacitors and inductors are difficult to measure. I expect ferrite cored inductors to show a frequency variation but a 100pF capacitor should still be 100pF at 30MHz. I've repeated calibration, tried different leads but I don't have any confidence in the accuracy of reactance measurement. From what I have read, I could manually add "fudge factors" to the calibration and remove what looks like a systematic error but that is not so easy for the mathematically challenged (like me).
My conclusion is that the accuracy depends on the accuracy of the standards you have and the amount of effort you put into "improving" the calibration. Some test jigs might be easier to get a good calibration but up to 30MHz the test jig is not the most important factor.
I have done what Erik suggests and measured a 100 ohm resistor. It shows no significant reactance at 30MHz. I don't have a record of it but certainly not 26pF.
73 Brian.
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Re: Part Number for the Demokit Cable
Hi Herb. I made a simple tool to install the board side connectors, but also doubt they will survive very long. My first thought was to solder them onto the board, but that would probably cause the small connector to lift off.
Perhaps some epoxy or super glue? I could make a 'dam' around the outside of the board and fill it 1/8" or so with clear fiberglass resin.....
ron
N4UE
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On Tue, Jan 7, 2020 at 03:06 PM, BruceN wrote: Lithium chemistry battery charges have a setting called "Storage Charge". Interesting, I'd never seen that parameter on data sheets before. That'll give me something new to study ;-) 73 de Andy
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Re: First PCB pictures of the V2
Mel, thanks for the great help with this project (and to Gabriel-san, of course).
You should change your username on groups.io to Vignette Tsukinose April.
Sorry about the inside joke, at least you can google it :)
73, Mike AF7KR
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Jim Allyn - N7JA
Jim Shorney
