Re: O S L on antenna side of a balun / choke with stud terminals?
We should clarify something here regarding very short (as a function of free space wavelength) radiators. A short radiator (a short monopole or small open ended 'wires' of a transmission line) exhibits an extremely *low real part *of the feed impedance or radiation resistance (neglecting ohmic losses, for the time being). The impedance is predominantly capacitive. It forms a capacitive probe to free space and is typically a very inefficient radiator due to resulting small currents whose fields (generated by those moving or oscillating charge carriers) open onto free space. A large series inductor is required to 'resonate' with this capacitance which boosts the generated fields due to increased currents at resonance. Remember, resonance is defined by the absence of the complex term in the feed or radiation impedance. Most of the current in such a structure exists below the inductor (loaded mobile whip) and on the structure to which the loaded "antenna' is attached. What's left is the radiation resistance and the ohmic resistance of the coil support and the wire of the inductor, itself. Typically, the sum of the last two greatly exceeds the radiation resistance - that which produces the energy radiated into free space - by a good amount. Small radiating structures exhibit a low real part and a large complex (capacitive) portion of the feed impedance. Dave - W?LEV On Tue, Aug 18, 2020 at 5:48 PM Jerry Gaffke via groups.io <jgaffke= [email protected]> wrote: Jim's response is making some sense, I think.
Ideal transmission lines and antenna wires, and ideal caps and inductors
in a matching network,
they should all be lossless.
The primary issue with very small out-of-tune dipoles is likely in
building an impedance matching network
using real components with resistance and dielectric losses to go from the
50 ohms of a transmitter
to the ridiculously high input impedance of that diminutive dipole.
Though the 50 to 3000 ohm impedance transformation needed for an EFHW can
be efficient enough.
No, I haven't read that book yet.
Jerry, KE7ER
On Sun, Aug 16, 2020 at 06:48 PM, Jerry Gaffke wrote:
Jim wrote:
You would have problems in other senses, some of which manifest in what
you might call efficiency.
The radiation resistance gets very low, so the current, for a given
radiated power, gets very high.
So, for a "real antenna" that has ohmic loss, that becomes a big
fraction
of the "power at the terminals" A transmission line with an open at the end has zero current and maximum
voltage there.
A very short dipole would be a minor step away from that case,
Seems the impedance into such a short dipole should be very high and the
antenna current thus very low.
Though Jim used the phrase "for a given radiated power", which
complicates
things some.
Guess I better read that book.
-- *Dave - W?LEV* *Just Let Darwin Work*
|
Re: Poor Quality 50 Ohm Load - Where to get accurate ones?
Thank you Wofgang and Dave for pointing out the obvious I have two Bird type meters and slugs that have been considered to be the standard in commercial radio service for decades. They are rated at 5%. I have several hundred dollars invested and happily use them. I would suggest that you who are designing this product put a tolerance of 5% on it. Then when these discussions get started everyone has a quick answer. I am old enough to understand what ¡°don¡¯t try to make a silk purse out of a sows ear¡± means. Sorry to you who are designing this. It is hardly a ¡°sows ear.¡± Actually I find this to be one of the finest products I have seen in years. I already have two and put them to good use. Thank you!
Actually I have enjoyed the discussions. There is a lot to learn here. But, ladies and gentleman, let¡¯s understand that you cannot make these more accurate then what they were intended to be. From what I understand they were intended to be used by amateurs for that purpose. They could also be used for commercial radio for 95% of all applications. That is a great deal for $60! Enjoy. For those of you who work in laboratories I don¡¯t understand where you get the budget to purchase tens (or more likely hundreds) of thousands of dollars worth of equipment. I¡¯m a little jealous.
This reminds me of a joke I heard in the 1960¡¯s. A scientist and an engineer were standing at one end of a gymnasium. At the other end were a couple of beautiful girls. They were told that at the sound of each buzzer they could go on half of the way to them. At the end of three buzzers the scientist was still standing where they started. He was asked what the problem was. He said there was no use because he would never get there. The engineer shouted back that he would get close enough for all practical purposes. I know. Rather sexist, but, I hope you all get the point. This nanoVNA was built for engineers, actually technicians, not scientists.
I will use these primarily for HF and VHF plus 70cm. Mostly I use it for antenna work and maybe lo check out a filter. I might use them for 1296 or 2.4 GHz but rarely. Someday I will figure out how to get them to work on my computer.
Thanks for those who develope the hardware and software.
Joe
toggle quoted message
Show quoted text
On Aug 18, 2020, at 2:34 AM, Wolfgang Wilde via groups.io <wwilde69@...> wrote:
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 <>:
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
|
Re: Phase of very high quality N short
Every video on the topic or all of them? What is wrong with it? As I see it he is just trying to make his homemade set behave similar to the HP one. Could you point to a better article/video about the topic (making your own cal standards). On Tue, 18 Aug 2020 at 06:59, Dr. David Kirkby, Kirkby Microwave Ltd < drkirkby@...> wrote: 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
|
Re: O S L on antenna side of a balun / choke with stud terminals?
Jim's response is making some sense, I think.
Ideal transmission lines and antenna wires, and ideal caps and inductors in a matching network,
they should all be lossless.
The primary issue with very small out-of-tune dipoles is likely in building an impedance matching network
using real components with resistance and dielectric losses to go from the 50 ohms of a transmitter
to the ridiculously high input impedance of that diminutive dipole.
Though the 50 to 3000 ohm impedance transformation needed for an EFHW can be efficient enough.
No, I haven't read that book yet.
Jerry, KE7ER
toggle quoted message
Show quoted text
On Sun, Aug 16, 2020 at 06:48 PM, Jerry Gaffke wrote: Jim wrote:
You would have problems in other senses, some of which manifest in what
you might call efficiency.
The radiation resistance gets very low, so the current, for a given
radiated power, gets very high.
So, for a "real antenna" that has ohmic loss, that becomes a big fraction
of the "power at the terminals"
A transmission line with an open at the end has zero current and maximum
voltage there.
A very short dipole would be a minor step away from that case,
Seems the impedance into such a short dipole should be very high and the
antenna current thus very low.
Though Jim used the phrase "for a given radiated power", which complicates
things some.
Guess I better read that book.
|
Re: Phase of very high quality N short
Links within the FAQ now work. thanks!
I could understand the results shown in that FAQ
if the VNA is calibrated using one set of O-S-L standards
and the VNA is then used to measure the open and short standard
from some other calibration kit.
But if the VNA is measuring the standards to which it was calibrated,
should it not show the Smith chart at the top of the FAQ, with three nice clean dots?
Jerry, KE7ER
toggle quoted message
Show quoted text
On Tue, Aug 18, 2020 at 04:52 AM, Dr. David Kirkby, Kirkby Microwave Ltd wrote: This link might help an understanding of what is happening on those plots.
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Re: Phase of very high quality N short
David,
Having O-S-L correction factors only available in host software like nanovna-saver
seems a fine solution. Though there are larger ARM processors that should have
no trouble doing this sort of thing in a standalone unit.
In what ways are current low cost calibration kits not "designed properly"?
Is it mostly this issue of the SMA female center pin?
I'm guessing that variations in capacitance, inductance, and resistance
with frequency can be compensated for in software.
Are there reasonably cheap connectors that would be better than our SMA's
with regard to this center pin issue?
Would it be possible to determine parameters for this compensation
without access to more accurate equipment?
Seems doubtful.
Though once somebody with a $20k VNA creates a few O-S-L kits
with compensation parameters, perhaps those could be used with
a nanoVNA to create more of the same with somewhat less accuracy.
For me, this is mostly academic, but fun to think about.
I'd be quite happy piddling around down at 30mhz, learning about
Smith charts and how to design a matching network,
I have no real need for better accuracy.
The nanoVNA is a fantastic tool right out of the box.
Jerry, KE7ER
toggle quoted message
Show quoted text
On Mon, Aug 17, 2020 at 10:14 PM, Dr. David Kirkby, Kirkby Microwave Ltd wrote: 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
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Re: Phase of very high quality N short
On Tue, Aug 18, 2020 at 07:36 AM, Dr. David Kirkby, Kirkby Microwave Ltd wrote: 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.
For now I'm just hoping it will be close enough to at least somewhat improve the results. I wouldn't use them for a data-based calibration either, the idea is to maybe just fit delay and a single capacitance parameter (for the open). Ideally someone could supply data for a whole batch, so the variation could be quantified as well. But that seems rather unlikely (and maybe not worth thte effort). I am very much aware of the limitations of this approach. Compared to the kit that comes with the NanoVNA-V2 it is at least usable. The supplied short has a good chance of eventually damaging the test port and it is probably better to just leave the test port open rather than use the supplied dust cap. Ironically, the load is mostly fine...
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.
Indeed, I apologize for the somewhat ambiguous wording. What I meant to say is that when difference in offset delay is too large, there will be frequencies where the phase difference becomes zero. And of course anything even close to zero is a problem, since even if the calibration equation is not technically singular, it will still be quite ill-conditioned and any uncertainties (including noise) will be greatly amplified. In this respect, 180¡ã is theoretically ideal from a mathematical point of view. But as you remarked, as long as the deviations don't become too large, the effect will be very small and 170¡ã should be just as good from a practical perspective. It certainly seems ill-advised to trade off any other properties in order to maintain an exact 180¡ã phase difference. I wonder if this is a relic from scalar network analyzers where phase deviations cannot be corrected.
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Re: Phase of very high quality N short
Dr David Kirkby Ph.D C.Eng MIET Email: drkirkby@... Web: Kirkby Microwave Ltd (Tel 01621-680100 / +44 1621-680100) Stokes Hall Lodge, Burnham Rd, Chelmsford, Essex, CM3 6DT. On Mon, 17 Aug 2020 at 22:04, Dr. David Kirkby < drkirkby@...> wrote: 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
<snip> This link might help an understanding of what is happening on those plots.
Dave
As someone pointed out, the link was broken. I know what caused it to break. I recently changed the server and have not enabled the .htaccess file which was performing some redirects. /FAQ has now been moved to /Support/FAQ, so the link broke. I will find any more and fix them, as the "permanent redirects have been in place for almost a year now.". I should fix the remove the redirects and sort out whats missing. If anyone can find any more broken links on please let me know. Dave
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Re: We started selling SAA2 with N-type RF connector and 4-inch display
On Tue, Aug 18, 2020 at 08:20 AM, <biastee@...> wrote: 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. Ah, I see. I was thinking about mentioning swap-equal adapters, but didn't because they are mostly obsolete and expensive. They are not exclusive to APC7 though. The nicer 3.5mm calkits e.g. tend to come with a matched set of f-f, m-f and m-m adapters. Even used these adapters still go for a pretty penny though. I suppose that if you go with APC7 you might have better chances of getting a bargain...
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.
Essentially, yes. I didn't realize the older 8753 didn't support adapter removal. I think 8753D/E do though. In any case, adapter removal calibration is cumbersome and the newer analyzers probably support unknown-thru, which is both more convenient and more accurate (see below). 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. Well, yes and no. If you know the electrical length (and potentially loss), you wouldn't need adapter removal calibration, you can just modify calkit parameters accordingly ("defined through"). This could just be implemented in firmware. Right now, as far as I am aware, there is no support for calkit parameters whatsoever, so if you use the internal calibration (rather than some PC software), I would be more worried about the open and short definitions. For proper adapter removal, you don't need to know the exact electrical length (it is enough to know it within a quarter-wavelength), it is determined during calibration. Unfortunately, like some of the other more advanced options, it needs a full 2-port analyzer. In general there are three differnent types of VNAs: Four-receivers, full 2-port (=source can be switched between ports) These can do unknown-thru (or reciprocal-thru, SOLR, UOSM, depending on where you buy your VNA). For this you measure S/O/L on each port as usual and then connect an "unknown" thru adapter between the ports. The analyzer can calculate the full S-Parameters of the adapter from this and remove any effect of the adapter. You may even be able to use the DUT itself as the unknown thru, no need to even reconnect the cables. This is very accurate and very convenient (it might even be a better option than flush thru for some insertable DUTs!). Classical architecture (like the 8753) As I mentioned, these cannot do the internal measurements needed for unknown-thru. It is still possible with an additional calibration step (two-tier calibration), but it won't be as accurate and you will probably need to do it "offline". Otherwise you need to do adapter removal calibration. You do a normal 2-port SOLT calibration, but _twice_. Once with adapter connected to port 1 and once with the adapter connected to port 2. The VNA compares both calibrations and can remove the effects of the adapter. But you need calibration kits for both sides of the adapter. If you have a DUT with e.g. female SMA on both sides, you need both male and female SMA kits. You also need to make a total of 8 calibration measurements. Transmission/reflection "1.5 port" with source fixed to port 1 (like 8714, NanoVNA) - 1-port adapter removal: you do a one-port calibration at the test-port, then put on the adapter and then do another one-port calibration with the adapter. From the difference between these calibrations you can characterize the adapter and then later de-embed it (with some reasonable assumptions). I don't think this is commonly implemented on commercial analyzers, but it is just a matter of software. You need calkits for both sides (so e.g. male and female). - defined thru: If you the length of your thru (specified by the manufacturer, measured using one of the other methods once or estimated from the mechanical dimensions), you're good and need only one calkit. If you don't care about phase accuracy you can also just set the length to 0. On a full 2-port this may degrade error correction significantly, on a T/R VNA it doesn't really matter. - swap-equal adapters are indeed still a practical option if you have them (and don't need high accuracy), no software needed, only need one calkit - ecal is, of course, accurate and convenient (if you have it and it is supported...)
|
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:
toggle quoted message
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
|
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
--
|
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.
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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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Dr. David Kirkby, Kirkby Microwave Ltd