Unexpected Smith chart behavior - Page 1
Forum section: electronics / rf-microwave

author: AJ528
author_rank: Regular Contributor
timestamp: September 20, 2024, 05:34:02 am
content: Hi All,I am trying to get a better understanding of Smith charts, so I set up a basic example today and got an unexpected result. I'm hoping someone can explain the behavior I saw.I learned recently that making your transmission line longer will cause a phase delay, which causes the measurement on the Smith chart to rotate about the center. A phase delay of 1/2 wavelength causes one full revolution around the Smith chart.I wanted to see this behavior in action, so I set up a simple example: I connected a 2 foot BNC cable to my nanoVNA, set it to display a Smith chart, and scan from 200 MHz to 300 MHz, then did OSL calibration. Next, I replaced the 2 foot BNC cable with a 3 foot BNC cable and measured the output shorted. I calculated that a signal somewhere around 250 MHz has a 4 foot wavelength, so by making my transmission line 1 foot longer, I expected to see the Smith chart measurement rotate 180 degrees and appear as an open. Instead, the measurement only moved 90 degrees. Anyone know why that is? Please see photos of my setup and weird result.

author: A.Z.
author_rank: Super Contributor
timestamp: September 20, 2024, 07:11:53 am
content: Let me try putting down some number, I'll use metric since that's what I'm familiar with and since wavelength is in metersLet's start by calculating Lambda for 250 Mhz, we have 300/250=1.2 meters, that's our wavelength, now, since our transmission line isn't perfect we should take in account the coax velocity factor, let's assume it's 0.66, in such a case 1.2*0.66=0.792 or 79.2 centimeters, this is our "corrected Lambda", now, starting with such a value we can calculate 1/4 wave as 79.2/4=19.8 centimeters; now that we have 1/4 Lambda (with correction) we can calculate our coax length, willing to start with 1/2 wave we just do 19.8*2=39.6 centimeters and then, willing to add 1/4 wave we'll have 59.4 centimeters, at this point we convert 39.6 and 59.4 from centimeters to feet obtaining 1.299 ft and 1.948 ft which will be the lengths we'll use for our coax cablesHTH

author: AJ528
author_rank: Regular Contributor
timestamp: September 20, 2024, 04:43:28 pm
content: Hi A.Z.,Thanks for the reply. The BNC cable I'm using is RG58 coax by Southwire, and their website says it has a velocity of propagation of 82% (https://www.southwire.com/wire-cable/low-voltage-cable/rg58-coax/p/ELC60). So calculating the corrected Lambda from that gives me a wavelength of 0.982 meters. 1/2 wavelength = 360 degrees, so making my cable 0.481 meters longer would cause a full rotation. I added 0.305 meters to the transmission line. 0.305/0.481 * 360 = 224 degrees (call it 225). So I would expect to see the measurement point rotated clockwise 225 degrees around the circle. But I'm seeing a 270 degree rotation. That seems like a big error to me. What could be causing that? Is my math wrong?

author: Randy222
author_rank: Frequent Contributor
timestamp: September 20, 2024, 04:55:56 pm
content: I always start with the basics. Is that VNA a legit "nano"? Is the nano running latest firmware and you sure it's calibrated at 250MHz ?After that, all the other basics for cable and what-not.

author: AJ528
author_rank: Regular Contributor
timestamp: September 20, 2024, 05:01:01 pm
content: I have no reason to believe it's a cheap clone. I bought it from Nooelec (https://www.nooelec.com/store/test-equipment/analyzers/nanovna-h4-bundle.html).As for frequency calibration, how would I go about verifying that? I have a 300 MHz oscilloscope I trust, can that be used here?Edit: I hooked up the VNA output to my oscilloscope and got 250 MHz on the nose. So that looks good to me.

author: wasedadoc
author_rank: Super Contributor
timestamp: September 20, 2024, 05:55:37 pm
content: BTW it is Smith (capital S) after the name of its creator. Nor is it Smitch.

author: AJ528
author_rank: Regular Contributor
timestamp: September 20, 2024, 06:01:24 pm
content: Quote from: wasedadoc on September 20, 2024, 05:55:37 pmCrap. I kept making that typo when I was writing the original post. I thought I caught them all, but apparently not. Is there any way to edit the post title?

author: A.Z.
author_rank: Super Contributor
timestamp: September 20, 2024, 06:49:30 pm
content: I'd start by using the VNA to measure the effective velocity and move on from thatalso, you may consider moving to 300Mhz that is a Lambda of 1 meter, not that important but may ease calcs a bit

author: AJ528
author_rank: Regular Contributor
timestamp: September 20, 2024, 07:28:25 pm
content: Good idea!I used the transform function on the nanoVNA, calibrated it with the 2ft BNC cable, then replaced that with the 3ft BNC cable. When my velocity factor was 82% (per the datasheet), it said the end of the cable was 0.360 meters away. I was expecting it to be 0.305 meters. So I adjusted the velocity factor until the end of the cable was 0.305 meters away. That corresponded to a velocity factor of 68%. 68% VF turns my effective wavelength into 0.816 meters, so adding 0.305 meters to that would cause the Smith chart measurement to rotate 270 degrees! I think that is a mystery solved!The only surprising thing here is the measured VF of 68% is much different than the advertised VF of 82%. Is that kind of VF deviation expected from affordable BNC cables?

author: A.Z.
author_rank: Super Contributor
timestamp: September 20, 2024, 07:48:24 pm
content: no, not for good quality, reliable ones, but 66% is common for standard RG58

author: Randy222
author_rank: Frequent Contributor
timestamp: September 20, 2024, 08:01:36 pm
content: Quote from: AJ528 on September 20, 2024, 05:01:01 pmGo online and watch all the vids for calibrating S11 port using the open,short, and 50ohm caps. The VNA must be calibrated 1st. I do cal before each use when doing certain tests that require real good accuracy.The cal will not change your funny looking Smith chart all that much, but in your test it's probably best to cal the device before doing your tests.

author: TimFox
author_rank: Super Contributor
timestamp: September 20, 2024, 08:06:25 pm
content: RG-58/U is a military designation for a 50 ohm cable with single braided shield, solid center conductor, and solid polyethylene insulation.That cable has a propagation constant of 0.66.Your cable is not actually RG-58/U, but is a quasi-RG58 with the parameters listed on the datasheet.There are other variations on RG-58/U, including 58A and 58C.

author: Randy222
author_rank: Frequent Contributor
timestamp: September 20, 2024, 08:56:41 pm
content: Was looking at the pics again.There's too many insertion points.At S11 you have 3, two adapters + S11.At the end you have 2, one adapter + sma cap.Get a BNC terminator for the end, and use a single BNC-sma adapter on S11.I bet ya with nano calibration and minimizing adapters, the expected number will get much closer to spec.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: September 20, 2024, 09:42:41 pm
content: The wavelength at 250MHz is c / 250 MHz * 0.66 (velocity factor) = 0.791m1 ft = 0.3048m1 ft is 0.385 wavelengths. Multiply that by 2 to get 0.77. You moved 0.77 of the way around the Smith chart, which puts you at about a 3/4 rotation, which is what you're getting.In your calculation you forgot to consider the velocity factor of the cable you're using.

author: AJ528
author_rank: Regular Contributor
timestamp: September 20, 2024, 09:53:05 pm
content: Quote from: TimFox on September 20, 2024, 08:06:25 pmWhat makes you so certain the propagation constant is 0.66? The link I found (https://www.southwire.com/wire-cable/low-voltage-cable/rg58-coax/p/ELC60) said it was 0.82. My measurements clearly indicate 0.82 is incorrect, but what makes you say 0.66? Is there something else in the datasheet that tells you the actual propagation constant?

author: AJ528
author_rank: Regular Contributor
timestamp: September 20, 2024, 09:55:28 pm
content: Quote from: EggertEnjoyer123 on September 20, 2024, 09:42:41 pmNo, I included the velocity factor of the cable I was using, but the datasheet I found for my BNC cable (https://www.southwire.com/wire-cable/low-voltage-cable/rg58-coax/p/ELC60) it says the velocity factor is 0.82. Where are you getting 0.66 from?Did I search the wrong information? Here's the 2 BNC cables I used

author: AJ528
author_rank: Regular Contributor
timestamp: September 20, 2024, 09:58:31 pm
content: Quote from: Randy222 on September 20, 2024, 08:56:41 pmI'm gonna respectfully disagree. I OSL'd the VNA with the 2ft BNC cable and adapters hooked up. That should have cancelled out the parasitics from the adapters. I then switched to the 3ft BNC cable and kept using all the same adapters.Why don't you think calibration would cancel out those effects?

author: A.Z.
author_rank: Super Contributor
timestamp: September 20, 2024, 10:17:08 pm
content: The 0.66 is what's reported in the mil specs for the standard cable

author: TimFox
author_rank: Super Contributor
timestamp: September 20, 2024, 10:30:09 pm
content: Quote from: AJ528 on September 20, 2024, 09:53:05 pmYour cable’s data sheet shows 0.82.That implies a foam dielectric, which has lower loss than solid polyethylene.The military spec for RG-58/U cable shows 0.66, since the dielectric is solid polyethylene for the “real stuff”.The actual term and spec are important.

author: showman
author_rank: Contributor
timestamp: September 21, 2024, 12:06:28 am
content: Here's a datasheet for Southwire 991043. Does not exactly match the jacket print, but I'd say close enough and the factor is 0.66. As suggested, just measure it, or alternatively contact the manufacturer for the correct factor for the specific cable.

author: AJ528
author_rank: Regular Contributor
timestamp: September 21, 2024, 02:46:10 am
content: Quote from: showman on September 21, 2024, 12:06:28 amInteresting....thank you for finding that datasheet! I wonder if I was looking at the wrong datasheet then. Maybe Southwire makes 2 different RG58 coax cables.

author: A.Z.
author_rank: Super Contributor
timestamp: September 21, 2024, 06:44:38 am
content: 1. calibrate the VNA using the cal kit2. connect the coax, terminate it and find the real VF3. cut the cable at proper length4. run the measurements

author: RoGeorge
author_rank: Super Contributor
timestamp: September 21, 2024, 06:59:32 am
content: 0. edit the first post, and fix the typo in the title

author: AJ528
author_rank: Regular Contributor
timestamp: September 21, 2024, 09:50:30 pm
content: Quote from: RoGeorge on September 21, 2024, 06:59:32 amThanks, fixed!

author: AJ528
author_rank: Regular Contributor
timestamp: September 21, 2024, 09:52:45 pm
content: Quote from: A.Z. on September 21, 2024, 06:44:38 amIs there a recommended way to find the real VF of a cable? I hooked my cable up the nanoVNA, put it in time domain mode, then tweak the VF in the settings until the VNA measured length equaled the actual cable length. Not sure if there's a better method...

author: wasedadoc
author_rank: Super Contributor
timestamp: September 21, 2024, 11:11:31 pm
content: Quote from: AJ528 on September 21, 2024, 09:50:30 pmYou did not fix all the instances of 'smith'.

author: Randy222
author_rank: Frequent Contributor
timestamp: September 22, 2024, 12:43:10 am
content: 1) you don't have a cable in the test, you have a transmission line.2) on that TL you have some uncharacterized adapters on the front side. Each insertion changes effective VF of the TL.I suggest you calibrate the nano using BNC open,short,load, leave all the S11 adapters there, calibrate with those attached to S11.And finally, as stated, the cable in pic is not a "cable", it's a TL (cable + connectors), aka cable assembly.

author: RoV
author_rank: Regular Contributor
timestamp: September 22, 2024, 05:10:18 pm
content: You can also establish vf by measuring the capacitance of a known lenght of cable of known impedance.Starting from well known equations Z0=sqrt(L/C) [cable impedance] and v= 1/sqrt(L*C) [propagation velocity],where L and C are inductance and capacitance for unit length ,you can use the known Z0 [50 ohm] to obtain L from the first equation and substitute it in the second, getting:v = 1/(C*Z0), or vf = 1/(3e8*C*Z0).With 24.8 pF/ft, or 81.36 pF/m, you get vf = 0.82.vf = 0.66 would require C = 101 pF/m.Clearly, a good measurement requires a few meters of cable, so you can ignore the effect of connectors, but you can get a reasonable idea of your cable characteristics with your existing cables, if you have a capacitance meter. Remember to compensate open probe capacitance, which generally ranges several pF.

author: AJ528
author_rank: Regular Contributor
timestamp: September 22, 2024, 07:43:08 pm
content: Quote from: RoV on September 22, 2024, 05:10:18 pminteresting. What kind of tool would you use to measure the capacitance? I've never heard of a capacitance meter. Is that just the capacitance measuring function on an LCR meter?Quote from: RoV on September 22, 2024, 05:10:18 pmHow would you do that compensation on an LCR meter? On my nanoVNA, it's pretty easy to perform a calibration, but I don't think my LCR meter can be calibrated in the same way.

author: TimFox
author_rank: Super Contributor
timestamp: September 22, 2024, 07:58:25 pm
content: Measuring capacitance is a native function of an LCR meter.LCR meters (as opposed to the rough function in a DMM) need to calibrate the fixture by measuring the impedance with the fixture open-circuit and shorted.After removing the short, the measured capacitance includes anything connected where the short and open used to be.This is technically compensation for the leads between the box and the measurement connections; the actual calibration usually depends on a precision resistor in the box.Which LCR meter do you have?

author: RoV
author_rank: Regular Contributor
timestamp: September 22, 2024, 08:00:05 pm
content: Quote from: AJ528 on September 22, 2024, 07:43:08 pmThat would be fineQuote from: AJ528 on September 22, 2024, 07:43:08 pmMany RLC meters have a "zero" button to get a relative reading, otherwise you can read the open circuit capacitance and later subtract it from the measurement. Not perfect, but good enough.

author: AJ528
author_rank: Regular Contributor
timestamp: September 23, 2024, 04:12:08 am
content: I have an GW Instek LCR-916 (https://www.gwinstek.com/en-US/products/detail/LCR-900)Looking at the manual, it turns out there is a calibration button on it. Cool. That's good to know!

author: TimFox
author_rank: Super Contributor
timestamp: September 23, 2024, 02:37:44 pm
content: For LCR meters, it is mandatory to compensate the connections using the calibration procedure in the manual.I do it at the start of any measurement set, after connecting the test fixture or cables.For coaxial cable transmission lines, with normal construction, the velocity factor is a function of only the dielectric constant between center conductor and outer shield.Common cable dielectrics include air (vf = 1.0), solid polyethylene (vf = 0.66), foam polyethylene (vf = 0.80), ptfe (vf = 0.69), and foam ptfe (0.85 to 0.90)."Foam" means that there is a substantial amount of air within the solid, decreasing the dielectric constant.Also note in the RF literature that the velocity factor does change at high frequencies due to higher-order effects in the transmission line.

author: Randy222
author_rank: Frequent Contributor
timestamp: September 23, 2024, 03:51:56 pm
content: I would rethink the testing.Calibrate the VNA by itself.Attach all the adapters and cable.Obtain/calculate VF from the info the nano is showing you. Note this is not actual VF of the cable, it's a VF of the TL.Continue on to obtain measured TL length using the calculated VF. This is the measured or "effective" electrical length.Then compare those results to the Smith chart, it should make sense. The Smith chart should be matching the measurements of the TL because a TL has aneffective electrical length and VF, which are different than actual length and VF of just the RG cable.Get VF of the TL -->https://youtu.be/aWvPB299U60Get length of the TL -->https://youtu.be/9thbTC8-JtAAlso worth a read -->https://www.eevblog.com/forum/rf-microwave/vna-for-cable-characterization/Quote from: TimFox on September 23, 2024, 02:37:44 pmIt's also well documented how the changing dielectric materials impact VF, since insertions are often impedance mismatch to the next thing it connects to.In highly critical TL's, the connectors need to match the cable very closely.Once all those oddities are accounted for, the Smith chart will make sense. To see what an adapter does, just add another or two on the S11 side, see how that changes the Smith chart.And just for clarity in practical applications of the TL, nobody just measures a cable length and relates that to wavelength of the applied frequency. A TL is always an "effective" measurement, which can be tuned, but rarely is the math using just coax physical length.

author: Randy222
author_rank: Frequent Contributor
timestamp: September 30, 2024, 05:43:03 pm
content: sorry, wrong thread