Re: Poor Quality 50 Ohm Load - Where to get accurate ones?
Despite that discussion is now starting to get carried away from any
relevance to NanoVNA (which will make this my very last post on that
thread)....
Apart from the skin effect - *any* load does have some connections to
the resistor. That means you *do* have some kind of maybe coaxial cable
and at it's end open wires going to the connection ports of your
Resistor. Even if it is just a very short piece. Right? Each Millimeter
of wire does have inductance. The 2 connection ports of your "perfect"
resistor - there are soldering caps at your SMD resistor or the
*connection ports* at your through-hole resistor where the connection
wires are attached which will form a *capacitor* element. The trace that
the carbon track on the ceramics will form kind of inductor...
So, guys, what do you think all of that L/C factors of your "perfect" 50
Ohms load would make out of it when not just seeing pure DC measurements
but you measuring at frequencies of round about 1 GHz or higher??? There
is no pure resistor existing in real world without having some virtual
components attached to it. (Virtual components means that Voltage and
Current are NOT 100% in phase but there's a component being at an angle
of +/-90 degrees off. This means that there is always either an
inductive or a capacitive component within the device). Even the
position of the device under test to your desk where you may work and
measure with it or your fingers itself will affect the results.
Wolfgang, DG7NEF
Am 17.08.20 um 22:43 schrieb Jerry Gaffke via groups.io:
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Show quoted text
Perhaps you bought the wrong Harbor Freight DMM.
I compared my free-with-purchase HF 69096 against my very expensive $10 UT10A.
The UT10A shows 0.0 ohms with the probes touching almost immediately.
The HF shows 2.0 ohms with probes touching if you give it a few seconds.
Correcting for this, when measuring my 50 ohm standard from Hugen,
I see 49.6 on the HF and 49.7 on the UT10A.
Makes sense the standard would be a bit low at DC,
at 1GHz the skin effect will likely raise that some.
I'm left to wonder what sorts of compensation are necessary in
a really good 50 ohm standard.
It is easy to cripple a HF DMM, perhaps forget it's on ohms
and go to measure a voltage. They are not well protected.
But given the price, it's a good pair for my $40 VNA.
I've got a dozen of the HF DMM's around here: One in the pickup, one in the shop area,
one in the kitchen, one by the ham gear, one in the pumphouse, one probably left up on the roof,
one that the dog ran off with, ...
Jerry, KE7ER
On Mon, Aug 17, 2020 at 10:28 AM, David Eckhardt wrote:
I have N, SMA, and OSM series precision cal. kits from HP. I made a DC
measurement of the 50-ohm standard loads for these three connector series
from these kits using the following instruments (what I have).
1) Bench DMM: HP 3478A
2) Hand-Held DMM from Harbor Freight: P37772
3) Hand-Held DMM from Sperry: DM6400
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Re: Poor Quality 50 Ohm Load - Where to get accurate ones?
Thanks Dave, that was exactly my point. We should take it as the wonderful and cheap tool, that opens up world of VNAs that many of us would never have been able to see otherwise. I don't think for most of us it would matter that much if impedance would be 50.00 Ohms or 51.4 Ohms. In terms of return loss that difference would not be a big deal and with real antennas it would be well in the range of influence of environment to an antenna. Even the factors like some stuff being located nearby the antenna, position at the roof, the ground heigth, the fact if you are touching the SMA port shield (and ground it more or less) and so on will have at least that much influence on measuring the DUT's, so let's not get carried away by hunting for superlatives but enjoy the usefulness. vy 73 Wolfgang (DG7NEF) Am 17.08.20 um 19:28 schrieb David Eckhardt: I have N, SMA, and OSM series precision cal. kits from HP. I made a DC
measurement of the 50-ohm standard loads for these three connector series
from these kits using the following instruments (what I have).
1) Bench DMM: HP 3478A
2) Hand-Held DMM from Harbor Freight: P37772
3) Hand-Held DMM from Sperry: DM6400
*Results follow:*
*HP** Harbour Freight Sperry*
50.375 45.50 50.20 Type-N
(HP 909C)
50.179 45.50 50.10 SMA
(HP 909D)
50.330 47.00 50.10 OSM
Connector (the expensive ones from HP)
AVERAGE OF ALL READINGS: 48.801 ¦¸ Clearly the Harbour Freight unit
is dragging down the average.
AVERAGE OF ALL READINGS (excluding the Harbour Freight measurements):
50.214 ¦¸ (0.43 % error against 50.0000 ¦¸)
These are precision HP cal. standards. Sure, they are better than what
comes with the NANOVNA's. But consider the cost!!!!!! Is it worth arguing
and expecting something less than 2.26 % error when the load reads 51.13
ohms? Really, now?
This is a hobby, not a metrology lab!!!!!!
Dave - W?LEV
On Mon, Aug 17, 2020 at 4:20 PM Wolfgang Wilde via groups.io <wwilde69=
[email protected]> wrote:
Fully agree with Dave. Are you not awaiting a little bit to much from
the NanoVNA? What precision can be expected regarding impedance, levels,
noise figures and so on? The FR4 material may not be precise enough to
guarantee 100% precise 50 Ohms impedance for the input/ output lines of
the NanoVNA itself!
Can it really work that precise? Over what frequency range? And in what
impedance range will it show the right numbers?
Would you really expect similar figures and precision from it than you
would expect from HP/Agilent/Keysight/Tektronix/Rohde&Schwarz labor
grade measurement equipment that costs 100 times as much or more? Don't
get me wrong, it is really a brilliant DIY project that helps me a lot
and is very handy tool for me to get a rough idea of what is going on.
But I would never expect the NanoVNA being absolute precise as I think
it never was target on that and I doubt it could deliver that precise
results. It already does almost magic, so let's adjust expectations a
little bit for reality.
vy 73,
Wolfgang, (DG7NEF)
Am 17.08.20 um 17:33 schrieb Dr. David Kirkby, Kirkby Microwave Ltd:
On Fri, 14 Aug 2020 at 01:52, Glen Jenkins WB4KTF <wb4ktf@...>
wrote:
The 50 OHM load (SMA-Male) that came with my nanoVNA-H4 measures 51.13+
ohms. Not a good start for calibration. Where is a good source for GOOD
loads that are accurate?
--
-----
Glen Jenkins, WB4KTF, Austin, TX Do you have a specification in mind? DC resistance doesn¡¯t tell you much,
other than it puts an *upper* limit on return loss. As soon as you go up
in frequency, the return loss will most probably decrease.
If you measure up to 3 GHz, it gives you no idea what it will be like at
6
GHz. Sometimes loads worsen dramatically with frequency, but others, from
the same batch will get better.
The last I looked, there was nothing in the NanoVNA firmware that properly
accounted for the open or short calibration standards. So there are limits
to what you accuracy you are going to achieve if you had a really expensive
load, as the phase errors will kill you.
Dave
--
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Re: We started selling SAA2 with N-type RF connector and 4-inch display
No, the APC7 coax cables are not necessarily connected to an APC7 equiped DUT. Typically, adapters (e.g. APC7-to-3.5) are connected to the cable ends to match the DUT's connectors, which can be N, SMA or BNC (the only people who obsess over phase accuracy are those who characterize either a single RF stage or a component, and their stamp-size evaluation PCBs cannot take the monstrous APC7). The setup is then calibrated using a cal kit similar the DUT's connector family (e.g. N, SMA or BNC). The key benefit is the sex at a reference plane can be changed AFTER calibration - e.g. by switching APC7-SMA(f) adapter to APC7-SMA(m) adapter. This technique known as "swap equal adapters" enables measurement of DUT with same sex connectors (e.g. a DUT with two N females) which is known as a "non-insertable device" in VNA parlance. I feel this ability to change sex at the measurement plane is a potential game-changer particularly for the nanoVna community because only male OSL standards are supplied, regardless of whether SMA or N variant is ordered.
Thanks for correcting me on APC7 is no longer the default on <6 GHz Keysight VNAs. I guess there is no motivation for Keysight to continue supporting "swap equal adapters" because e-Cal and "adapter removal" are now standard on all their offerings, unlike the 90s till early 2000s, when "adapter removal" was only available on the higher end models like 8510x but NOT on the plain vanilla 8753x.
However, I feel it is moot to bring e-cal into this discussion because of any one of the following reasons:
1. the nanoVna has neither firmware nor hardware to control the e-cal module
2. the e-cal module's price is ...err, unmentionable in this group
As to "adapter removal", the big impediment is that it is not supported by firmware nanoVna, right? Additionally, the user will need to know the adapter's electrical length at each frequency and modify the cal-kit definition. Even an RF manufacturer has gone on record to express their frustrations with "adapter removal" and instead, recommended "swap equal adapters": .
As for deembedding the THRU "offline" on a PC post-calibration, I think this is for the math nerds in this group and not for simpleton hobbyists like me. :-)
Thanks for a good discussion. I've picked up much new knowledge from the group.
Leong, 9W2LC
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Re: Phase of very high quality N short
On Tue, 18 Aug 2020 at 00:01, <switchabl@...> wrote:
The SDR-Kits calkits try to fill this gap, but beyond 1.5GHz the
performance doesn't seem to be too great (at least for the female one).
This is maybe not surprising, as they are aimed primarily at the 1.5 GHz
VNWA. I suspect that the issue is mainly with the open and could be solved
if the fringing capacitance were included in the model. If anyone has
measured the SDR-Kits parts with a properly calibrated analyzer up to at
least 3GHz, I would be very interested in the touchstone files.
There¡¯s too much variation from one to the other. You really need the Touchstone files of the parts you have, which pushes up production costs significantly. However, I expect it will be much harder to make a flush open with low capacitance. And it is unnecessary to do so. And I suspect this is the reason why you usually don't find a flush short in a calkit. Indeed so, which is why I laugh when I see all these papers and videos explaining why the delay on the short needs to be so close to zero. If the difference in delay between the short and open is too large, the phase will eventually cross-over and the calibration equation becomes
singular. I think you mean if the difference in phase is too *small*. Yes, indeed they would, but in practice the kit would become unusable before the different in phase became zero. I believe about 20 phase degrees difference is the minimum needed. Below that the calibration would be unstable. Noise in the instrument would become more significant, as it tried to measure two devices very similar to each other. Dave -- Dr. David Kirkby, Kirkby Microwave Ltd, drkirkby@... Telephone 01621-680100./ +44 1621 680100 Registered in England & Wales, company number 08914892. Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Chelmsford, Essex, CM3 6DT, United Kingdom
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Re: Phase of very high quality N short
On Mon, 17 Aug 2020 at 22:50, Jerry Gaffke via groups.io <jgaffke= [email protected]> wrote: David,
Good stuff, thanks for posting.
Assuming the connectors physically stout enough to give repeatable results,
That particular short is very repeatable, as the female centre conductor has no slots in it, so it doesn¡¯t expand in diameter when the male pin enters.
would it be possible to calibrate out most of the errors encountered in a
cheap or homebrew calibration set?
Is this something that could be done through post-processing in
nanovna-saver?
If a low-cost kit is designed properly, (and I am yet to see one that is), then yes, errors can be largely corrected in software, but it doesn¡¯t solve the current problems with the NanoVNA if you want to use it standalone, without a PC. You will not get the accuracy of a Keysight kit, as the female centre pin will always expand in diameter on SMA. It would be very expensive to make an SMA connector that avoids that problem
FYI, the two images in your "not-see-a-dot" FAQ don't show up
Yes, I noticed that after I posted it. I will resolve that today. It is weird what caused that problem,
Jerry, KE7ER
G8WRB -- Dr. David Kirkby, Kirkby Microwave Ltd, drkirkby@... Telephone 01621-680100./ +44 1621 680100 Registered in England & Wales, company number 08914892. Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Chelmsford, Essex, CM3 6DT, United Kingdom
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Re: Phase of very high quality N short
On Tue, 18 Aug 2020 at 03:06, Dragan Milivojevic <d.milivojevic@...> wrote: For flush open etc you might find this interesting:
It is technically flawed in several ways as are several other videos from the same author. *Every* video I have seen from him are wrong, although this is is not as bad as others. It¡¯s not that important that the phase of the open and short are exactly 180 degrees apart. The $11,000 18 GHz Keysight 85050B APC7 calkit is one example of a kit where no attempt is made to achieve 180 degrees, although it is technically possible to approach it. I don¡¯t know the figures off hand, and I can¡¯t be bothered to measure or calculate it, but I would expect that the phase probably varies from 170 to 180 degrees on the 85050B yet that kit is very accurate, with an error on the worst standard not exceeding 0.4 degrees to 8 GHz and 0.6 degrees up to 18 GHz He makes no attempt to explain why he is aiming to get 180 degrees, other than to say that his HP kit has 180 degrees. I doubt he has a clue why, as he never says why. For waveguide calibration kits you don¡¯t use opens for calibration. Leaving a waveguide open just makes an antenna with a return loss of about 13 dB. Instead two shorts of different delays are used. Since wavelength in waveguide is not inversely proportional to frequency, there¡¯s a massive difference between the phases in waveguide as the frequency is swept. 2) He has a totally flawed video on reference planes. His misunderstanding of that can be seen in this video too, but it more subtle. I find YouTube to be awash with people creating videos about things they know a little about. He is one such example. Dave. -- Dr. David Kirkby, Kirkby Microwave Ltd, drkirkby@... Telephone 01621-680100./ +44 1621 680100 Registered in England & Wales, company number 08914892. Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Chelmsford, Essex, CM3 6DT, United Kingdom
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Re: Can the NanoVNA be used on 75 ohm cables/ antennas --- Part 2 #75 ohm measurements
99:70 gets pretty close, but I wouldn't want to wind it.
73,
Gordon KF5JWL
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Re: Is the NanoVNA the right tool for the job?
Did not read the whole message , sorry, But I can tell you that as a hammer
it has really poor quality..
One have to ask himself if the job is good for the tool to!
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Le lun. 17 ao?t 2020 ¨¤ 19:20, Jim Lux <jimlux@...> a ¨¦crit : I have one, and it is the greatest thing for HF and VHF since sliced
bread - cheaper than a digital multimeter, plenty of accuracy for most
HF uses (at HF, whether you even bother putting the open on for cal
probably isn't an issue, and the "difference in measurement plane" is
negligible).
The other thing is that I think it is an invaluable teaching tool. EE
professors should be handing them out to their students. You learn a
lot more about "matching" and transmission lines and such from having a
VNA sitting in front of you and just "hooking stuff up".
Put that stub of coax on a T connector and watch it "suck out" the power
at the notch.
For that purpose, running it at 100-200 MHz is probably just about right
- the wavelength is short enough that "resonant things" aren't huge -
the wavelength is long enough that "precision of cal kit phase delays"
isn't an issue.
You can learn all about making microwave measurements, tuning a filter
(does anyone do that these days for new designs?), measurement
uncertainties.
Where I see a problem looming is in applying it to higher frequencies
(2GHz and up), because a) the underlying design and components are being
used at harmonics and well out of their nominal frequency range - it
works, but parts you buy tomorrow may not work the same as parts you buy
today and because b) calibration becomes more critical as wavelengths
get shorter.
There's a whole bunch of cool stuff you could do in an antenna lab with
2.45 GHz antennas, measuring antenna patterns, learning how to do the
measurement at all, the traps for the unwary (reflections from
environment). But a lot of that fun stuff is more "qualitative" than
"quantitative" with a tool like the NanoVNA (or V2 or whatever. So it
would be incumbent on the professor to properly deal with the issues of
calibration, the increased uncertainty, and perhaps, coming up with some
lab exercises where "why good cal kits cost $20k" can be illustrated.
Another thing that would be fun with higher frequencies is making
interdigital filters and couplers on PCBs - send your design out to one
of the fast turnaround cheap protoboard places, hook it up to your VNA
and see if your filter turned out the way you thought it should.
For instance there's a whole lot of cool phased array components (MMICs)
becoming available at low prices for 5 GHz and up. And a cheap VNA
makes working with that kind of thing a lot easier. But I sort of worry
about someone getting frustrated trying to use something like the
NanoVNA (in its current form) to do that.
Maybe there *is* a market for a $500 VNA that does microwave stuff well
(enough). I was willing to pay that for the TenTec TAPR VNA back when
and that's basically a HF only box with a lot of limitations.
ANd there will always be applications where you *need* the high dollar
test set - 26 or 32 GHz has gotten cheaper, but is still a expensive
area to work in. Everything is more expensive - coax is more expensive,
connectors are more expensive, test equipment is more expensive, cal
kits are more expensive. About the only thing that's cheap is the ICs,
because, after all, it doesn't cost much more to make a GaAs amp at 6
GHz as at 30 GHz.
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Re: Phase of very high quality N short
For flush open etc you might find this interesting:
Concerning zero-delay/flush shorts, I think this is not really the issue.
toggle quoted message
Show quoted text
For 3.5mm it is quite possible to make male and female flush shorts (at
least you can order them from Maury Microwave). However, I expect it will
be much harder to make a flush open with low capacitance. And I suspect
this is the reason why you usually don't find a flush short in a calkit. If
the difference in delay between the short and open is too large, the phase
will eventually cross-over and the calibration equation becomes singular.
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The simple graphics didn't work out.
Here's a web page on various matching schemes, including the min-loss-pad you describe
plus some narrow band designs:
Jerry, KE7ER
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On Mon, Aug 17, 2020 at 06:27 PM, KENT BRITAIN wrote: So far I don't think the Min-Loss-Pad as been mentioned.A very simple way to
convert between 50 and 75 Ohm, yes ithas just over 6 dB loss, but very wide
bandwidth and simple.
50? ------ 47 --------? 75?????? i?????? i?????
91?????? i??? GND
Hopefully my simple graphics work out.
So you have a 91 Ohm resistor to ground from the 50 Ohm port,and a 47 Ohm
resistor in series with 75 Ohm port.These are the 5% resistor values for the
pad.
I find them handy for my TV work.?? MiniCircuits does sell50-75 Ohm
converters, a tapped inductor design.? But theyhave frequency limits.? Make
sure they covered you planned work.
Kent
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Re: Can the NanoVNA be used on 75 ohm cables/ antennas --- Part 2 #75 ohm measurements
Those little connectors on the VNA educational boards apparently rip off their pads way too easily.
This looks better to me, a proto board with 6 sma's for $8:
That plus some axial leaded R's and C's and L's should be fine for learning how things work
at moderate frequencies of 30mhz or so. And might actually be useful as a project board.
Jerry, KE7ER
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On Mon, Aug 17, 2020 at 10:52 AM, Stephen Laurence wrote: The test/ education board available for about ?15 has simple loads,
capacitors attenuators, etc with leads which have those inferal minute
connectors to select which circuit you wish to play with. Each little circuit
has the expected screen display overlaid on the circuit board. In my arrogance
(ignorance) I have resisted buying one.
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So far I don't think the Min-Loss-Pad as been mentioned.A very simple way to convert between 50 and 75 Ohm, yes ithas just over 6 dB loss, but very wide bandwidth and simple.
50? ------ 47 --------? 75?????? i?????? i????? 91?????? i??? GND
Hopefully my simple graphics work out.
So you have a 91 Ohm resistor to ground from the 50 Ohm port,and a 47 Ohm resistor in series with 75 Ohm port.These are the 5% resistor values for the pad.??
I find them handy for my TV work.?? MiniCircuits does sell50-75 Ohm converters, a tapped inductor design.? But theyhave frequency limits.? Make sure they covered you planned work.
Kent
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Re: Can the NanoVNA be used on 75 ohm cables/ antennas --- Part 2 #75 ohm measurements
Here is an idea for those who only want to make VSWR and RL measurements at other than 50 ohms using their NanoVNA and are not overly concerned with accuracy.
The company RigExpert has a software package (AntScope2) that they recently modified to work with the 2.8" NanoVNA. It has an option in the settings to set the system impedance for the SWR and RL calculations to 75 ohms. I assume what they do is calculate the DUT complex impedance from the NanoVNA reflection coefficient and then compute a 75 ohm complex reflection coefficient using these complex impedance values. From there they compute the VSWR and RL in the usual manner. I ran a couple of tests using a 75 ohm system impedance. The first was with the 50 ohm cal load and the second with a 25 ohm load. The VSWR and RL results were pretty good and I put them in the attached file.
Roger
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Is the NanoVNA the right tool for the job?
I have one, and it is the greatest thing for HF and VHF since sliced bread - cheaper than a digital multimeter, plenty of accuracy for most HF uses (at HF, whether you even bother putting the open on for cal probably isn't an issue, and the "difference in measurement plane" is negligible).
The other thing is that I think it is an invaluable teaching tool. EE professors should be handing them out to their students. You learn a lot more about "matching" and transmission lines and such from having a VNA sitting in front of you and just "hooking stuff up".
Put that stub of coax on a T connector and watch it "suck out" the power at the notch.
For that purpose, running it at 100-200 MHz is probably just about right - the wavelength is short enough that "resonant things" aren't huge - the wavelength is long enough that "precision of cal kit phase delays" isn't an issue.
You can learn all about making microwave measurements, tuning a filter (does anyone do that these days for new designs?), measurement uncertainties.
Where I see a problem looming is in applying it to higher frequencies (2GHz and up), because a) the underlying design and components are being used at harmonics and well out of their nominal frequency range - it works, but parts you buy tomorrow may not work the same as parts you buy today and because b) calibration becomes more critical as wavelengths get shorter.
There's a whole bunch of cool stuff you could do in an antenna lab with 2.45 GHz antennas, measuring antenna patterns, learning how to do the measurement at all, the traps for the unwary (reflections from environment). But a lot of that fun stuff is more "qualitative" than "quantitative" with a tool like the NanoVNA (or V2 or whatever. So it would be incumbent on the professor to properly deal with the issues of calibration, the increased uncertainty, and perhaps, coming up with some lab exercises where "why good cal kits cost $20k" can be illustrated.
Another thing that would be fun with higher frequencies is making interdigital filters and couplers on PCBs - send your design out to one of the fast turnaround cheap protoboard places, hook it up to your VNA and see if your filter turned out the way you thought it should.
For instance there's a whole lot of cool phased array components (MMICs) becoming available at low prices for 5 GHz and up. And a cheap VNA makes working with that kind of thing a lot easier. But I sort of worry about someone getting frustrated trying to use something like the NanoVNA (in its current form) to do that.
Maybe there *is* a market for a $500 VNA that does microwave stuff well (enough). I was willing to pay that for the TenTec TAPR VNA back when and that's basically a HF only box with a lot of limitations.
ANd there will always be applications where you *need* the high dollar test set - 26 or 32 GHz has gotten cheaper, but is still a expensive area to work in. Everything is more expensive - coax is more expensive, connectors are more expensive, test equipment is more expensive, cal kits are more expensive. About the only thing that's cheap is the ICs, because, after all, it doesn't cost much more to make a GaAs amp at 6 GHz as at 30 GHz.
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Re: O S L on antenna side of a balun / choke with stud terminals?
On Mon, Aug 17, 2020 at 01:45 PM, KENT BRITAIN wrote:
Interesting concept.
Why does the impedance of the Universe change with distance???
If think you are confusing 'Far Field' with impedance.????? Kent
The "impedance of the Universe" does not change with distance and I never said it did. I am not "confusing Far Field with impedance". Did you actually read what I wrote and the theory at the link I provided? Basic antenna radiation theory was summarized in the link I provided but I will try and make it as simple as I can... The fields surrounding an antenna are divided into 3 main regions in order of occurrence. The distance of each from the antenna will depend on the frequency and the physical antenna dimensions. 1. Reactive Near Field. 2. Radiating Near Field (Fresnel region) The radiating near field or Fresnel region is the region between the reactive near and far field. ... 3. Far Field. The "field impedance" is = |E|/|H|, where |E| is the electric field strength and |H| is the magnetic field strength. The field impedance will vary in the different regions because E and H are different in each of the 3 fields and change with distance. They also depend on the type of antenna (loop, dipole etc.). This can be seen in the graphic which I have attached below. Jim Lux in his post provided the equation for the field impedance in free space that yields Zo = 376.730. The relative permeability of air is 1.00000037 so we can say that in air, far away from the antenna and other objects, the "far field impedance" is approximately 377 ohms which is what I posted earlier and you can clearly see in the graphic below at 300M. This discussion has gone way off topic so this will be my last post. Anyone interested in any further details on this subject will find it thoroughly discussed in the classic textbooks written by Kraus, Balanus or Jasik. Roger
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Re: Phase of very high quality N short
Well, I think for low-reflection DUTs the error might still be dominated by the residual directivity (and hence the return loss of the calibration load). But I agree that while the discussion about the loads is interesting, it will generally not be the limiting factor with the NanoVNA.
For the NanoVNA-V2 in particular, with frequencies of 3GHz and beyond, the firmware calibration routine and the included open and short standards are somewhat inadequate. By comparison, the load is fine... Apparently, there are plans for a 6GHz version, but unless calibration is sorted out, I'm not sure there is much of a point.
Not all is lost, with a PC connected you have a lot more options (of course for portable use that will not do you much good). NanoVNA-QT can use arbitrary touchstone files for the standards and NanoVNA-Saver can use a polynomial model. I usually use scikit-rf with Python instead, because it is more flexible for 2-port calibration. QT only does simple normalization for transmission and I suspect 2-port calibration in Saver is essentially broken.
Even if this is fixed, there doesn't seem to be a suitable low-cost option for a calkit at the moment (low-cost relative to the NanoVNA, I am aware of your products). At a minimum, it would need characterized offset delays. As you illustrate so nicely, the phase error can become signifcant and at some point you might be better off without a full SOL calibration.
The SDR-Kits calkits try to fill this gap, but beyond 1.5GHz the performance doesn't seem to be too great (at least for the female one). This is maybe not surprising, as they are aimed primarily at the 1.5 GHz VNWA. I suspect that the issue is mainly with the open and could be solved if the fringing capacitance were included in the model. If anyone has measured the SDR-Kits parts with a properly calibrated analyzer up to at least 3GHz, I would be very interested in the touchstone files.
Concerning zero-delay/flush shorts, I think this is not really the issue. For 3.5mm it is quite possible to make male and female flush shorts (at least you can order them from Maury Microwave). However, I expect it will be much harder to make a flush open with low capacitance. And I suspect this is the reason why you usually don't find a flush short in a calkit. If the difference in delay between the short and open is too large, the phase will eventually cross-over and the calibration equation becomes singular.
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Re: Phase of very high quality N short
David,
Good stuff, thanks for posting.
Assuming the connectors physically stout enough to give repeatable results,
would it be possible to calibrate out most of the errors encountered in a cheap or homebrew calibration set?
Is this something that could be done through post-processing in nanovna-saver?
FYI, the two images in your "not-see-a-dot" FAQ don't show up
in either my Chrome browser or in Firefox.
Jerry, KE7ER
toggle quoted message
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On Mon, Aug 17, 2020 at 02:05 PM, Dr. David Kirkby, Kirkby Microwave Ltd wrote: .......
This link might help an understanding of what is happening on those plots.
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Phase of very high quality N short
With the recent discussion about high return loss loads, I made the point
that it is a bit pointless worrying about it unless there¡¯s firmware
support for calibration kits in NanoVNA, which as far as I am aware there¡¯s
not.
The following is the the phase of a *female N calibration standard* from an
HP 85054B calibration kit, costing more than $20,000
The photographs show the VNA calibrated to 6 GHz - I happened to VNA warmed
up and calibrated up to 6 GHz.
On the far left of the Smith chart is the theoretically ideal short. On the
right is the theoretical ideal open.
This VNA covers 50 MHz to 20 MHz, so with a minimum frequency of 50 MHz,
the phase would not be the idealised 180 degrees, but it would be quite
close to 180 degrees. Hence the trace starts not quite on the far left.
At 6 GHz the phase has changed from 180 to 59.34 degrees. For simplicity,
assume that the phase is 60 degrees. So the phase has changed 180-60=120
degrees over 6 GHz. Assuming a linear phase change with frequency, which is
approximately true, that¡¯s a phase shift of 120/6= 20 degrees per GHz.
So if this $20,000 kit was used to calibrate a NanoVNA using the NanoVNA
firmware, a 20 degree phase error would exist at 1 GHz. With such a large
phase error, it would be pointless worrying about the loads.
I would add it is possible to make an N female short with virtually no
delay, but HP chose not to do it.
When it comes to a male N calibration standard, it is technically
impossible to make it with zero offset.
This link might help an understanding of what is happening on those plots.
Dave
--
Dr. David Kirkby,
Kirkby Microwave Ltd,
drkirkby@...
Telephone 01621-680100./ +44 1621 680100
Registered in England & Wales, company number 08914892.
Registered office:
Stokes Hall Lodge, Burnham Rd, Althorne, Chelmsford, Essex, CM3 6DT, United
Kingdom
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Re: O S L on antenna side of a balun / choke with stud terminals?
I shall contribute another nit:
it's not 377 ohms, 376.730... ohms (approximately 120*pi, but I think that's a coincidence) - and yes it's the ratio of E field (V/m) to H field (A/m) in a vacuum "far" from any other object that is not "a vacuum".
And that's only if you're using a measurement system that is Volts and Amps - It's also the square root of the ratio of the permeability (in H/m) and permittivity (in F/m), and it related to the speed of propagation as well.
E/H or mu0 * c or sqrt(mu0/epsilon0) or 1/(c0*epsilon0)
The *precise* (as in past the 4th decimal place) values depend on the current definition for things like the speed of light and the ampere, which does change over time.
Does it matter, most of the time? Nope..
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On 8/17/20 1:44 PM, KENT BRITAIN wrote: Interesting concept.
Why does the impedance of the Universe change with distance???
If think you are confusing 'Far Field' with impedance.????? Kent
Just to clarify.? 377 ohms is? a physical constant relating the magnitudes of the electric and magnetic fields of electromagnetic radiation travelling through Free Space.? This only occurs when you are a considerable distance from the? transmitting antenna in the far field.
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Re: O S L on antenna side of a balun / choke with stud terminals?
Interesting concept.
Why does the impedance of the Universe change with distance???
If think you are confusing 'Far Field' with impedance.????? Kent
Just to clarify.? 377 ohms is? a physical constant relating the magnitudes of the electric and magnetic fields of electromagnetic radiation travelling through Free Space.? This only occurs when you are a considerable distance from the? transmitting antenna in the far field.
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Dr. David Kirkby, Kirkby Microwave Ltd
Roger Need
