Inductor Impedance measurments not lining up with what I expect - Page 1
Forum section: electronics / projects

author: Glenn0010
author_rank: Regular Contributor
timestamp: June 12, 2025, 12:42:37 pm
content: Hi all,I have wound 2 identical planar air core inductors. The only difference being between the two is that one uses solid core wire of 2.2 mm diameter, while the other uses a type 2 litz wire of 7 bundles of 14 strands each of 32 AWG wire. I have connected them to a bode 100 and am sweeping from 100 Hz to 50 MHz. What I expect to see is a difference in AC resistance after 1 kHz as skin effect in solid core wire starts to increase AC resistance. Furthermore I expect the inductance to be flatter with the Litz wire across frequency due to less skin effect and proximity effect. This however, is not what I'm measuring.Below is the resistance of the two coils, green is the solid core and red is litz. They seem to be pretty much identical. I expect the litz wire to have much lower res at HF.The inductance is a similar story. The inductance value is in the range of what I calculated/simulated. However the solid core wire seems to have a flatter measurement vs frequency than the litz wire. Further in a simulation using ansys Q3D of a similar coil of different dimensions, the results show that the inductance should drop vs Frequency is suspect due to proximity effect and skin effect. This is seen in the 2nd image below. So that doesn't line up either.Finally here is a plot of the measured magnitude Very similar againMy conclusion is that there is something I'm not understanding or my measurement setup is influencing the measurement which is possible since it's not ideal. So am happy to improve it based on recommendations. I have used a shunt-thru measurement with a 10 Hz RBW to reduce noise. I'm new to such measurements so am any feed back on improving it would be great. I did a thru calibration before the measurement was taken to cancel out the leads. I'm wondering if the grabbers are somehow impacting the resultsCheers

author: Vovk_Z
author_rank: Super Contributor
timestamp: June 12, 2025, 12:54:16 pm
content: IDK, possibly there is a real difference between them, but there is some systematic error in measurements. So, possibly you have to design a calibration tool first.Check if your setup measurements coincide with calculations. Possible, there is a great mistake and it shows the same result for any piece of wire.

author: Glenn0010
author_rank: Regular Contributor
timestamp: June 12, 2025, 01:05:34 pm
content: With regards to the difference with the solid and with the litz, simulation shows there should be a difference

author: mawyatt
author_rank: Super Contributor
timestamp: June 12, 2025, 01:18:07 pm
content: The setup could use shorter leads, at 10MHz and above likely causing slight differences in measurements.The inductance should be determined by the core characteristics and the number of turns, with a small effect by the wire diameter/characteristics.Think you've got a reasonable comparison considering the setup and frequency range involved, tighten the setup and this should improve. Also pay attention to the surroundings near the core and measurement, like a possible ground pad underneath.BTW nice instrument, the Bode 100Best

author: Glenn0010
author_rank: Regular Contributor
timestamp: June 12, 2025, 01:51:02 pm
content: Hi Wyatt,Thanks for the input, I'll order some shorter leads. and try to improve it as you saidForgot to mention that this inductor is an AIR CORE with 5 turns, with a 3d printed bobbing using PLA to wind it so there shouldn't be any core material to speak of at allAnd yes the bode 100 is a fantastic bit of kit

author: iMo
author_rank: Super Contributor
timestamp: June 12, 2025, 01:52:46 pm
content: Afaik the litz wires are "effective" up to a half of a MHz or so.. With a higher freqs the parasitic capacitancies between the particular wires creating the litz (they must be isolated from each other) eliminate the advantages..

author: ejeffrey
author_rank: Super Contributor
timestamp: June 12, 2025, 02:21:08 pm
content: How many turns and what diameter is the coil?What do you expect the RF resistance to be? I'm guessing the RF resistance is just so low that you can't see it, at least in the low frequency region.You would expect the RF resistance to significantly effect the self resonance. Are you sure that peak you see is the true self resonance of the coil? It might include the parasitic capacitance of the sense leads in which case they may be de-Q the resonance enough to obscure the skin effect.

author: Glenn0010
author_rank: Regular Contributor
timestamp: June 12, 2025, 03:22:10 pm
content: Quote from: iMo on June 12, 2025, 01:52:46 pmInteresting, do you have any references to any papers regarding this?Thanks

author: Glenn0010
author_rank: Regular Contributor
timestamp: June 12, 2025, 03:30:35 pm
content: Quote from: ejeffrey on June 12, 2025, 02:21:08 pmThe coil is 5 turns, rectengular with a 4 mm pitch. with a max dimension of 70 mm by 100 mm. I have a simulation running as we speak that should give me a final value of resistance.I'd agree that the RF resistance should effect the slef res. This is what im trying to figure out. Wether what im seeing is coil parameters or artificats in my setup. I've ordered some shorter sma cables sand extra SMA adapters to improve the setup

author: TimFox
author_rank: Super Contributor
timestamp: June 12, 2025, 03:57:48 pm
content: The classic discussion of RF resistance with litz wire is in F E TermanRadio Engineers' Handbook, McGraw-Hill (1943), pp 37, 74, and 80.The whole question of RF resistance in coils is complicated by the proximity of the next wire in the coil, as opposed to the resistance of an isolated wire.There are many pirated pdfs on line, e.g.https://electrooptical.net/static/oldsite/OldBooks/Terman-RadioEngineersHandbook_1943.pdf(Warning: whole file is 1,036 pages.)For these questions, the first edition of 1943 is better than later editions.The wire gauge for the individual strands makes a large difference at radio frequencies. Much of the litz wire on the market is for lower frequencies, such as winding transformers for switch-mode power supplies.I experimented with 1200 strands of AWG50 litz wire many years ago at 5 to 10 MHz, and was disappointed in the results.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 12, 2025, 04:17:58 pm
content: Quote from: Glenn0010 on June 12, 2025, 03:30:35 pmI get a RF resistance of the 2.3mm wire of ~60 milliohm at 1 MHz. So that's pretty low compared to the reactance and low on your measurement scale.The Q factor of the inductor is wL/R = 1500 at 15 MHz, although radiation will probably limit Q below this as well. You obviously aren't measuring anything near that. That's because your circuit Q is limited by the leads.The way I would do this is to first add a low loss capacitor to lower the resonance frequency to perhaps 1 MHz 250 kHz or so. Then, couple your network analyzer in through a low value capacitor. You should get a much higher Q circuit and may be able to clearly resolve the difference in ESR.

author: mtwieg
author_rank: Frequent Contributor
timestamp: June 12, 2025, 04:28:25 pm
content: Quote from: Glenn0010 on June 12, 2025, 12:42:37 pmOk so to start with these wires have different cross sectional area: 3.8mm2 for the solid wire and 3.14mm2 for the Litz. So even at low frequencies they're not directly comparable.QuoteThis is reasonable. But as frequency continues to increase the ESR of the litz wire is expected to catch up to the solid wire. For 32AWG stranded Litz I would expect this to happen below 1MHz. But your plot of Rs vs frequency makes the 1MHz data impossible to compare. Would probably be better to plot Q vs frequency instead of ESR.QuoteI wouldn't expect much difference in inductance (unless the dimensions of the coil are similar to the diameter of the wire, which would be a pretty terrible inductor in any case).

author: RoGeorge
author_rank: Super Contributor
timestamp: June 12, 2025, 04:49:25 pm
content: That coil wire in the pics doesn't look like litz wire for RF.Litz wire has to have very thin, parallel (not twisted together) isolated wire threads. The wire in the pics looks more like multistranded wire twisted together, as seen in the SMPS transformers of some power sources. Also, litz wire is usually to be found only in low frequency (couple of MHz max, in LW and MW radio). Above those frequencies, litz wire doesn't work well.Are the strands in the pics isolated from each other?Another thing, looks like there is an unintended 1:1 transformer (and an extra a 1 loop coupling) in the measuring setup. Keep the probe wires together, preferably shielded, twisted at least, or just run them such that the enclosed area between the wires is kept at a minimum.In the setup from the pics, put both coil ends on the same side (for example at the left side of the 3D print support), such that the exciting and measuring wires do not encircle the coil under test.

author: mtwieg
author_rank: Frequent Contributor
timestamp: June 12, 2025, 05:47:49 pm
content: Quote from: RoGeorge on June 12, 2025, 04:49:25 pmIt's not clear what frequency the OP intends to use.For use in the 20-100kHz range 32 AWG strands can provide decent improvements over solid wire with similar crossectional area. Pretty common for inductive heating.

author: Tation
author_rank: Regular Contributor
timestamp: June 12, 2025, 09:50:01 pm
content: May I ask, from the ignorance, what's that?

author: Someone
author_rank: Super Contributor
timestamp: June 12, 2025, 10:15:27 pm
content: Quote from: Tation on June 12, 2025, 09:50:01 pmUSBA to USBC adaptor that is captive on the (cheap and crappy hub/dock) cable.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: June 12, 2025, 11:21:15 pm
content: I've done comparisons between solid wire and Litz. The difference is easier to see with an LCR meter in terms of Q at a given frequency, usually low, assuming you have the right Litz for the frequency. The difference might be large or small, depending on the size of the coil. Some good Litz design theory here-https://www.newenglandwire.com/traditional-litz-wire-theory/

author: ejeffrey
author_rank: Super Contributor
timestamp: June 13, 2025, 02:04:14 am
content: Quote from: RoGeorge on June 12, 2025, 04:49:25 pmHuh? Litz wire usually has multiple levels of twisting / braiding. It's important that each strand have approximately the same time at the outside and inside of the bundle otherwise skin effect will still prevent the core from carrying current.A single level of twisting won't do it because it won't mix up the inside and out. But if you have twisted bundles that are the twisted that works fine. Of course the strands do need to be individually insulated and not all twisted patterns are effective.The problem here really is just that the RF resistance is low and the measurement setup is not able to measure in the milliohms range.

author: Glenn0010
author_rank: Regular Contributor
timestamp: June 13, 2025, 09:00:16 am
content: So I'll give some context.Initially, I'm trying to design an LC low pass filter to see wether my Ansys simulations are somewhat accurate, The I will move onto a more complex filter arrangement that has additional anti resonant points to attenuate at certain frequencies.My goal is to filter out frequencies in the MHz range. On the more complex filter I'd want an anti resonant point in the MHz, so I though the Litz wire due to the lower AC res will give me better attenuation.I'm doing eddycurrent simulations using Ansys Maxwell. When I use solid core wire the inductance and and resistance are as belowWhen I set the wire to stranded (I cannnot chose a specific litz configuration, just stranded)

author: RoGeorge
author_rank: Super Contributor
timestamp: June 13, 2025, 09:11:33 am
content: From the last 2 plots, looks like 'stranded' means (for that simulator) to simply ignore any skin+proximity effects.

author: RoGeorge
author_rank: Super Contributor
timestamp: June 13, 2025, 10:23:24 am
content: Quote from: ejeffrey on June 13, 2025, 02:04:14 amIn my mind 'litz wire' summons a single group of very thin isolated wire threads, like this:Yes, that is a little twisted, but not like this heavily twisted multiple multi-stranded wire:https://litzwire.com/types-of-litz-wire/Apparently the name 'litz wire' is used not only in radio, but in power electronics, too, for thick multi stranded isolated wires like the ones in the former pic, so my bad I guess, sorry.

author: Glenn0010
author_rank: Regular Contributor
timestamp: June 13, 2025, 10:26:01 am
content: Quote from: RoGeorge on June 13, 2025, 09:11:33 amThis is correct. Wouldn't this be a fair assumption if the litz wire selected is of the right strand thickness so that skin effect and and proximity effect are negligible for the frequency of interest?

author: mtwieg
author_rank: Frequent Contributor
timestamp: June 13, 2025, 07:00:34 pm
content: Quote from: Glenn0010 on June 13, 2025, 10:26:01 amIn the MHz range even the best litz wire (i.e. a thousand strands, each of AWG 50 or more) will still have increased AC resistance (though still much lower than solid wire). In practice, modelling litz wire accurately is extremely difficult, empirical methods are preferred.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: June 13, 2025, 07:50:47 pm
content: I've no idea how you'd simulate Litz (Litzendraht) wire, but the link I gave above shows you how to calculate performance for any Litz construction.

author: uer166
author_rank: Super Contributor
timestamp: June 13, 2025, 07:57:49 pm
content: Quote from: Conrad Hoffman on June 13, 2025, 07:50:47 pmThe only real way that approximates reality is a 3D EM field solver that can also handle the correct termination/endpoints of the Litz.I'm pretty sure having the Litz strands exactly parallel to each-other, not twisted, and not somehow swapped with the inner strands periodically, will result in a AC resistance equivalent or worse than a solid wire. It acts the same due to a global circulating current in the strands that passes through the terminations, with the same kind of skin effect distribution that you'd see on a solid piece.Litz are twisted for a reason, and the geometry matters: the "ideal" geometry makes is toallstrands experience an identical magnetic field strength caused by themselves (skin effect) and their neighbors (proximity effect). All this stuff conks out at ~xx MHz in power electronics, though it's extremely layer dependent. Not sure of the applicability to modest RF where the skin depth is like 2 microns at 1GHz.

author: uer166
author_rank: Super Contributor
timestamp: June 13, 2025, 07:59:59 pm
content: Quote from: RoGeorge on June 12, 2025, 04:49:25 pmThis would achieve nothing at best and make the Rac actually worse in most conditions, assuming you short all the strands together at the input and output as you'd normally do. You HAVE to move the individual strands around in a specific way, it's what makes the magic of Litz actually work.

author: mtwieg
author_rank: Frequent Contributor
timestamp: June 13, 2025, 09:20:46 pm
content: Quote from: Conrad Hoffman on June 13, 2025, 07:50:47 pmThese equations and tables are all based on empirical methods though. One shouldn't expect it to apply to all sizes and twisting types.Also I'm assuming everything on that page refers to a straight wire or a single loop in a vacuum. Once you start making multi-turn coils, proximity effects will make things far more complicated.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: June 14, 2025, 03:07:18 am
content: Certainly true, though I'd bet the formulas cover a wide range of wire constructions. I've only seen Litz wire used to make multi-turn coils, though I suppose it could be used in a straight run as part of an antenna system. No idea if there's any benefit there, and only for low frequency systems if there is. I've only used it for air core coils. Once a core is involved, I'd think the number of turns drops and the need for Litz would be less. The OP appeared to be doing a planar coil, not the most optimum configuration for low losses/high Q.

author: TimFox
author_rank: Super Contributor
timestamp: June 14, 2025, 04:08:45 am
content: The reference I cited above (Terman'sRadio Engineers' Handbook) discusses straight litz wires and litz wires in coils, with detailed equations.

author: iMo
author_rank: Super Contributor
timestamp: June 14, 2025, 07:02:27 am
content: Quote from: TimFox on June 12, 2025, 03:57:48 pmFantastic book!

author: iMo
author_rank: Super Contributor
timestamp: June 14, 2025, 07:19:13 am
content: Quote from: Glenn0010 on June 12, 2025, 03:22:10 pmNope, I did not have researched for you, just a general info. As a teen I dismantled a lot of old radio coil assemblies and the litz wires were used for max 200-500kHz IFs.. Never seen litz used for higher frequencies (perhaps ~1.5MHz middle waves is the top freq considering some ferrite rod antennas made with litz I saw in past).. You may google or ask AI, my bet you will get the same info.. The very old high Q frame antennas (ie for the popular "crystal sets") were made from the thick litz, yes, but they operated at long waves usually (also below 500kHz)..We also experimented with the litz for our "modern" EFHW antenna's ununs (3.5-30MHz) and had not seen any difference against the simple enameled wire..

author: Tation
author_rank: Regular Contributor
timestamp: June 14, 2025, 08:56:11 am
content: Quote from: RoGeorge on June 13, 2025, 10:23:24 amI remember having saw such wire in induction cookers, as the planar induction coil just below the pan. In fact I'm not sure if it was true Litz wire (i. e. isolated strands) or simply multi-stranded wire.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: June 14, 2025, 01:17:26 pm
content: A bit OT, but you can never go wrong with Terman. So many things that came later were just a rehash of what he had already done a better job of explaining. I understand that things sometimes have to be pared back to make way for newer material but, IMO, the 1930s Terman does a better job with resonance than the later versions.

author: G0HZU
author_rank: Super Contributor
timestamp: June 15, 2025, 06:36:47 pm
content: I don't know if this helps but back in the early 1990s I gained a lot of experience using Litz wire when designing high order elliptic LPF and HPF circuits down in the 200 kHz to maybe 2 MHz region. IIRC I mainly used Litz with something like 160 strands of 48G wire.This gave excellent results (compared to solid copper wire) up to about 2 MHz in various inductor types. I mainly used Litz with ferrite in order to get high Q. Back in those days the company had a large cupboard with maybe a dozen large reels (more like drums in some cases) of Litz wire of various combos of wire diameter and strand count.I do remember it was 160 x 48G Litz that was the most popular and I think this is about 0.75mm diameter (I don't think it was more than 1mm diameter).This worked well but I think I used thicker stranded Litz for the HPF sections down at maybe 200 kHz. It's so long ago I can't be certain. I can recall having trouble winding the thicker Litz on the former for the tiny ferrite material I used.Either way, Litz definitely can give good results up to maybe 2 MHz. By 3 or 4 MHz I recall the advantage that Litz offered had been lost.

author: G0HZU
author_rank: Super Contributor
timestamp: June 15, 2025, 07:12:34 pm
content: When I say Litz gave excellent results, I recall seeing a Q that was two or more times higher than using solid copper wire across maybe 200 kHz to over 1 MHz. Back then I used an old Marconi TF1245 Q meter and this was calibrated every year back in those days so I think my test results were valid and I achieved the expected (as in simulated) insertion loss performance at the cutoff point from the high order elliptic filters. Think in terms of 13th-15th order elliptic LP filters with 72dB stopbands and < 1dB insertion loss at the cutoff. These were very high performance filters and they had to be small in size with a very steep cutoff slope. Litz was the only game in town for this stuffMy inductors were typically in the 50uH range and were tiny at about 10mm x 10mm x 15mm in size with the Litz wound on a former along with a ferrite material. I had to find the highest performance from inductors no bigger than this as the equipment had strict size limits. I was typically achieving a Q peak of over 400 for the 50uH versions and this was measured somewhere around 400kHz to 800 kHz with a good Q over quite a bit of this range.

author: Glenn0010
author_rank: Regular Contributor
timestamp: June 17, 2025, 10:24:22 am
content: Hi All,Thanks for all the feedback. I Firstly I have improved my setup by using as short cables as possibleSecondly I have setup the inductor as a low pass filter with 2x 100 nF ceramic capacitors. I have taken 5 sweeps of each one and averaged it. From the bode plot I have extracted what I think is the resitance at~ the resonant point. Let me know if my maths is correct.First I start by calculating the Q factor from the bode plot. I start by calculating the fdelta (fres-f3dB). fdelta solid = 167 kHz, fdelta litz = 201 kHz. I then use Q = fres/fdelta. Qsolid = 289kHz/167kHz = 1.68. Qlitz = 362.5 kHz/201kHz - 1.8.Knowing f res and the size of the capacitor, I can calculate the inductance using fres = 1/2piSqrt(LC). fres solid = 280 kHz -> L = 1.61 uH, fres litz = 362 kHz -> L = 0.96 uH. This is slightly odd as I'm not expecting such a difference between the 2 inductors. They are identical, one is just 2.2 mm diamter solid, the other is 3 mm diamater litz.Anyhow from this I can use Q = R sqrt(C/L) and solve for R. This gives me a R of 4.77 R at approximately 300 kHz for solid and 3.89 R at approximatley 400 kHz for litz.Does this seem like a resonable approach.

author: mtwieg
author_rank: Frequent Contributor
timestamp: June 17, 2025, 12:07:37 pm
content: A couple comments:1. I don't think you calculated your fdelta values correctly. It's clear that the 3dB bandwidth for both curves is <100kHz.2. The connections to the bode 100 might be killing your measured Q. I've never used the bode 100 myself but it's important that these effects are minimized or accounted for in some way. I'm not talking about cable resistance, but rather the terminating impedances of the bode 100 itself.3. Looks like you have some shield around the coils now. That is likely going to reduce the Q substantially (depending on the diameter of the coils), possibly to the point where the Litz wire's performance boost becomes negligible. It's also possible the the presence of the shield explains the difference in measured inductance. The closer the coil is to the shield, the more it's inductance (and Q) will be decreased by it.

author: Glenn0010
author_rank: Regular Contributor
timestamp: June 17, 2025, 03:48:46 pm
content: Quote from: mtwieg on June 17, 2025, 12:07:37 pmHi Thanks for this.Re 1 - I misunderstood how f delta is defined - full width at half maximum. as I understand it now, you look at the peak's magnitude, subtract 3 dB from this and find where these points occur either side of the peak. With this definition, now Q solid = 4.92, Q litz = 9.43. when I plug these in to the equations, R solid is 13.93 R, R litz is 20 R. The litz R is higher however this is at 362 kHz rather than 280 kHz.Re 2 - Blow is the image on the Bode 100 setup. Further I have performed a calibration with the cables which should account for themRe 3 -That shield is not actually a shield. That is copper tape the is used for the return path of the LC filter. The shield does not wrap around the entire coil. The coil is within the are marked by red square. The return path does no overlap it apart from the input and the output parts. The copper tape runs as you see it in the image. Some round the top and the side, there is no more copper tape underneath

author: ejeffrey
author_rank: Super Contributor
timestamp: June 17, 2025, 04:06:25 pm
content: I don't think you can eliminate the loading by the leads and by the generator itself in this configuration. My suggestion was to measure the circuit like this.The 1 nF capacitor will basically eliminate the loading effect of the leads. You might have to experiment with the value a bit. Maybe start a bit higher, say around 10 nF. But you should see a notch, and the depth of the notch directly tells you the Q of the circuit. The depth of the notch is basically the ratio of the loaded Q to the internal Q of the resonator. In this circuit, the loaded Q is set by the coupling capacitor and the internal Q by the ESR.

author: mtwieg
author_rank: Frequent Contributor
timestamp: June 17, 2025, 07:05:59 pm
content: Quote from: Glenn0010 on June 17, 2025, 03:48:46 pmJust in case you weren't aware, those R values are for equivalent parallel resistance, not series resistance. So higher value for Litz wire makes sense.QuoteOk I presume the software is properly accounting for the 50ohm source resistance.QuoteI see the gap in the shield, though the two capacitors apparently span that gap, meaning their is a closed current path there which will couple to the coil. It's likely that this affects the center frequency of the resonance and degrades its Q somewhat. Also possible that it creates other resonant modes....Also be aware that the cables introduce additional closed loop paths which will couple to the coil. Hard to say how significant they are though.Quote from: ejeffrey on June 17, 2025, 04:06:25 pmThe instrument uses CH1 to measure the actual voltage at the DUT, meaning it should account for the impedance on the OUTPUT port (including the internal 50ohm resistance and the cable resistance, but not any interconnect resistance after that point). The CH2 connection shouldn't affect Q significantly.