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50 to 75 Ohms
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 |
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 |
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. |
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:
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 |
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. |
Re: Phase of very high quality N short
David,
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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 On Mon, Aug 17, 2020 at 02:05 PM, Dr. David Kirkby, Kirkby Microwave Ltd wrote:
....... |
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 |
Re: O S L on antenna side of a balun / choke with stud terminals?
I shall contribute another nit:
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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.. On 8/17/20 1:44 PM, KENT BRITAIN wrote:
Interesting concept. |
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. |
Re: Poor Quality 50 Ohm Load - Where to get accurate ones?
Perhaps you bought the wrong Harbor Freight DMM.
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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:
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Re: O S L on antenna side of a balun / choke with stud terminals?
On Mon, Aug 17, 2020 at 11:44 AM, KENT BRITAIN wrote:
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. A thorough discussion of Electric, Magnetic and ElectroMagenetic fields and field impedance is written by W8JI, who has an extensive and well-researched web site. Rog |
Re: O S L on antenna side of a balun / choke with stud terminals?
On Mon, Aug 17, 2020 at 09:29 AM, David Eckhardt wrote:
This is only true if the current on the transmission line has the same magnitude (but opposite directions) on the two conductors. If not the transmission line will radiate. Take the case of a dipole (or doublet) , connected to ladder-line transmission line, where the two elements do not have the same impedance due to being different lengths or affected by nearby objects. The current imbalance will cause the feed-line to radiate. The same thing happens with coax where radiation will take place from the outer surface of the shield. Roger |
Re: Performance of my nanovna V2 clone - return loss of port 2 seems much too high ?
Peter,
I have attached a measurement of port 2 return loss from my NanoVNA V2, bought from tindie. This one seems to be meet the specs (-20.3dB @1.5GHz and -15.5dB @3GHz). There is also significant ripple on your measurement. It looks like your calibration is bad and you have significant residual source match error (or possibly your cable is bad, but less likely; since you are calibrating directly at the port 1 connector, you are measuring the cable as well). So I would take the results with a grain of salt. Mine is not perfect either, but it looks a lot less bad. |
Re: O S L on antenna side of a balun / choke with stud terminals?
Only on the input, the output is always 377 Ohm.
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Example would be Yagi's with 50 or 75 ohm inputs. Now, how well you match the 50/75 input to the 377 outputis determining Gain and SWR.??? Kent On Monday, August 17, 2020, 12:08:24 PM CDT, <namerati@...> wrote:
On Mon, Aug 17, 2020 at 12:34:21AM +0000, KENT BRITAIN wrote: Takes a bit to think of your Yagi antenna as a 50 to 377 Ohm impedance transformer.So you are saying a Yagi is a transformer with multiple taps? :) |
Re: Can the NanoVNA be used on 75 ohm cables/ antennas --- Part 2 #75 ohm measurements
On 8/17/20 10:33 AM, John Baines via groups.io wrote:
For a 2:1 impedance transformer, 14:10 turns ratio gives 1.96:1 impedance ratio which may be close enough, or 17:12 turns ratio gives 2.007:1 impedance ratio (<0.35% error). A few minutes playing around with a calculator will probably find even better approximations.yeah, I'd just go hunting through the Minicircuits or Johansen catalogs and look for something that does the function - I have no idea how they make them internally (well, I know it's some windings on a core) - but wideband transformer design is partly an art, partly engineering. That would also solve a "unbalanced test equipment to balanced DUT" issue. In the OP's case, measuring a filter, there's sort of two cases: 1) "what the heck is this unlabeled thing" and for that the "software correction in NanoVNA Saver" strikes me as the easy way 2) "adjusting a filter or doing manufacturing checkout" - it's over a narrow frequency range, so you can probably find a transformer that would work. Minicircuits ADT2-1T+ goes 400kHz to 450 Mhz and 2:1 as an SMT device, for $3.85. Stick it on a board, solder it to the SMA connector, however you fixture it. FOr a bit more ($5.85) you can get the T2-1-X65 in a 6 pin DIP, goes from 50kHz to 600 MHz. Bigger pins to solder to if that matters - the pins are 0.1" apart (just like the SMT device) and on a standard "solder in" SMA, the center post is about 0.14" from the corner posts. Minicircuits DOES sell a board for things (with the fascinating note that you have to use your PNA's "impedance conversion function" ) - but they cost >$100. 73 |
Re: Can the NanoVNA be used on 75 ohm cables/ antennas --- Part 2 #75 ohm measurements
Hmmmm,
I have a question. 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. One of those little circuits is a ceramic filter. I presume its input and output impedance is anything but 50 ohm, more like 200-500 ohm. What happens there? Or is the filter especially selected for 50 ohm termination? Steve L. G7PSZ |
Re: Can the NanoVNA be used on 75 ohm cables/ antennas --- Part 2 #75 ohm measurements
For a 2:1 impedance transformer, 14:10 turns ratio gives 1.96:1 impedance ratio which may be close enough, or 17:12 turns ratio gives 2.007:1 impedance ratio (<0.35% error). A few minutes playing around with a calculator will probably find even better approximations.
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73 John M0JBA On 17 Aug 2020, at 18:19, Jim Lux <jimlux@...> wrote: |
Re: Poor Quality 50 Ohm Load - Where to get accurate ones?
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-- *Dave - W?LEV* *Just Let Darwin Work* |
Re: O S L on antenna side of a balun / choke with stud terminals?
So - the Right-Hand-Rule applies to antennas as well....?
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On Monday, August 17, 2020, 1:11:36 p.m. EDT, Jerry Gaffke via groups.io <jgaffke@...> wrote:
Yes, I used to know that. But having radiation described as the result of an accelerating charge got me thinking about it in a new way. I guess that creates a magnetic field around the dipole wires, which then begets an electric field when it collapses and off it goes. Jerry, KE7ER On Mon, Aug 17, 2020 at 09:29 AM, David Eckhardt wrote:
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