test for ~1uH air coil inductor? - Page 1
Forum section: electronics / rf-microwave

author: coppercone2
author_rank: Super Contributor
timestamp: July 29, 2025, 05:21:52 am
content: I want to do my first ceramic silver coil inductor. Basically I am at a limitation of my materials right now, because the best I can do because of what tape I can get as a thick film paste mask. (1mm hobby masking tape)What circuit or test would best show case it to compare it to other coils? It is going to be approx 2.5 inches long and 1/2 inch wide. I finally got the nice diamond blade for my proxxon saw so I can nicely cut this stock I got long ago. I also got some ceramic... v-block like things to rest a coil on so it does not roll around in my little ceramics oven so its practical to use.The first thing I thought of was to just use a LCR meter. But I only have a DE-5000 meter. My impedance analyzer is also an option, but it starts at 10MHz and it does not really have a good interface for such a coil (its too big!)I was thinking if there is another good option, perhaps to make a LC oscillator circuit of some kind. The idea would be to compare the performance of the circuit with other inductors of the same value and to do some crude tests like using a hot air pencil, trying to de-tune it with nearby objects or whatever else I can think of.I don't mind building a circuit because getting to the point of being able to possibly make this inductor has been a big process and some electronics work would be welcome rather then thinking about rather finicky mechanical behavior of things and also about temperature and goo.And I also thought a LC oscillator would give the option to run some juice through the coil.

author: iMo
author_rank: Super Contributor
timestamp: July 29, 2025, 07:23:43 am
content: Silver baked into ceramics coils were used around WWII for improved stability.. A good "because I can.." experiment, but without a practical sense, imho..

author: radiolistener
author_rank: Super Contributor
timestamp: July 29, 2025, 07:44:30 am
content: You can try building an LC tank with your coil and a known capacitor, then excite it with a sharp pulse and observe the damped oscillations on an oscilloscope triggered on the pulse edge. This lets you estimate the resonance frequency and Q-factor by measuring the decay rate. It’s a simple and effective way to compare coils without needing high-end equipment.

author: radiolistener
author_rank: Super Contributor
timestamp: July 29, 2025, 07:52:51 am
content: Quote from: iMo on July 29, 2025, 07:23:43 amI’d say silver-on-ceramic coils still have value - not mainly for thermal stability, but for their very high Q. That’s critical in sharp analog filters, RF front-ends, or narrowband applications. The reason they’re rare today isn’t because they’re obsolete, but because digital signal processing has largely replaced analog filtering in most systems. For some high-performance RF applications or hobbyist experiments, they still make perfect sense.However, silver-on-ceramic technology is still commonly used at microwave frequencies, where DSP is not feasible, and high-performance analog circuitry remains essential.It's also worth noting that not all ceramics are suitable for this purpose - high-grade porcelain is typically preferred due to its low dielectric loss and excellent thermal stability, which are critical at RF and microwave frequencies.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 29, 2025, 11:13:58 am
content: Quote from: coppercone2 on July 29, 2025, 05:21:52 amWhy are those not good? At what frequency do you want to measure?

author: coppercone2
author_rank: Super Contributor
timestamp: July 29, 2025, 01:29:22 pm
content: I wanted the experiment to have more.. result. Its very boring to see just a meter display change after alot of effort. And I thought maybe the thing can be scaled up to some high power.Scope display is a better conclusion.IDK how good these coils will be because you would need a 0.0005 inch thick tape to get the 'spec' of a thick film. These will likely have conductors that are too thick.

author: coppercone2
author_rank: Super Contributor
timestamp: July 30, 2025, 01:22:06 pm
content: and I figure the frequency these might be used at is like.... AM ?I assume less then VHF.I thought I might see a part like this in something that is maybe 500KHz-3MHz?

author: RoGeorge
author_rank: Super Contributor
timestamp: July 30, 2025, 04:36:00 pm
content: 1-3MHz? Try this #005 fromhttps://www.eevblog.com/forum/chat/secrets-from-the-ee-scrolls/msg5995029/#msg5995029(shameless plug).Build an LC tank with your 1uH and a good 10nF. Maybe a silver-mica 10nF, to stay with the silver. Calculate the expected resonance frequency (#003, another plug,https://www.eevblog.com/forum/chat/secrets-from-the-ee-scrolls/msg5983521/#msg5983521). #003 will tell the expected frequencyCode:$ qalc '1/(2*pi*sqrt(1uH*10nF)))'1 / (2 × pi × sqrt((1 microhenry) × (10 nanofarads))) ≈ 1.591549431 MHzand also the reactance at resonanceCode:$ qalc 'sqrt(1uH/10nF)'sqrt((1 microhenry) / (10 nanofarads)) = 10 ΩNow you can finally add the negative resistor from #005, and build an oscillator!But wait, there is more! See that following post, right after #005?That will let you to measure Q of your oscillator. In that simulation Q is considered 200, but I hope your Ag will give much higher Q. Replace the Rx with a potentiometer, and find the point where the oscillations start (or stop). Disconnect the potentiometer and measure it.That will let you deduce the Q of your tank, here the measured Rx/10 (for this particular X=10 ohms).But how to know when it stops and when it oscillates? Add this one transistor emitter detector from #002https://www.eevblog.com/forum/chat/secrets-from-the-ee-scrolls/msg5966723/#msg5966723, yes, yet another shameless plug!Alternatively, the #004 self powered BFT as detector can serve to tell if oscillations are present/absent: Or you can just use a self power transistor as a detectorhttps://www.eevblog.com/forum/chat/secrets-from-the-ee-scrolls/msg5989869/#msg5989869(you guessed, another shameless plug)Or if you have an oscilloscope, use this ring-down method to find Q, #001https://www.eevblog.com/forum/chat/secrets-from-the-ee-scrolls/msg5939188/#msg5939188(do I have to mention shameless plug?)Managed to plug them all, from #001 to #005.Another advice, never work with silver during a full moon!(Werewolfs might attack thinking you want to make silver bullets.)

author: coppercone2
author_rank: Super Contributor
timestamp: July 30, 2025, 06:14:49 pm
content: I meant a guess as to what kind of device you would find this inductor in.I don't know what they were used for. I just meant it 'looks' right for something working in that range for me when I imagine where it might be used.I thought it would be used for like AM radio. It sounds like some people here know what equipment it might have been used in.

author: coppercone2
author_rank: Super Contributor
timestamp: August 01, 2025, 02:11:18 am
content: I made the coil. Its a bit tricky because if you are slow the PVA soldifies and it tears with the tape. I bought methyl cellulose, which claims it can extend the cure time of PVA. Its also possible to add a bit of water.While its not fired yet, it does look similar to the one ceramic silver electronics thing I have.I made a 'holder' out of alumina v-block things that are inserted into a piece of soft firebrick segment that I shaped with a old file. I think this thing might actually workA photo-advantage brand metal 'spreader' for solder paste seems to work OK but I need to try plastic, it might be better for a round structure.Since it looks feasible, I bought even thinner tapes

author: RoGeorge
author_rank: Super Contributor
timestamp: August 01, 2025, 07:05:40 am
content: How does it looks like? Add a picture with the coil.

author: coppercone2
author_rank: Super Contributor
timestamp: August 01, 2025, 10:39:01 am
content: i might take a before baking picture but I still need to make some solder pads (more tape and goop work required)it might be delayed until I see the methylcellulose to see if it can make the paste more workable, its fine right now for a flat but for a 3d object its pot life is too low and my craftsman ship is rushed.basically if you are not what I call 'fast' then the it sticks to the tape and peels with the tape. it needs to be processed when still wetmy work procedures require taking maybe 80 pictures on how to cut a wire for step 1 so I am not fond of reaching for the camera

author: coppercone2
author_rank: Super Contributor
timestamp: August 02, 2025, 02:38:34 pm
content: it failed and its not worth taking a picture of yet. it must be something related to application because I got like line of electrical discontinuity across the axis of the coil (each turn is isolated). I think I need to rotate it in a drill and use something like thin spackle tool to press it in the groove between the tape like a lathe thread cutting operation. I think the squegee I used 'cut' the circuit when it was still wet at the point where I started. I thought it would just sit on the tape, but I guess the tape is more compressible then I thought and that doing it free handed is tricky (but I think possible).I will do a experiment to see if diamond abrasive on a buff is capable of cleaning the silver surface from the oxide or glass layer on it. I still need to find a way to 'process' round objects.

author: coppercone2
author_rank: Super Contributor
timestamp: August 03, 2025, 03:35:47 pm
content: it looks like a 1.5mm turn on a 1/2 inch coil has a resistance of approx 100 miliohms when its good (some of them go up to 180, because it what looks like.. spalling? perhaps it needs a slow bake out @ the recommended temperature).So I got approx 23 mili ohms per square resistance (theoretical calculation based on pi * 1/2 . It should be like 3 mili ohms per square (for 2% palladium). This composition might be heavy in palladium (5 to 20%), I forgot what it was. I don't know how thick it is. I will take a picture after I try diamond polishing and ultrasonic cleaning. I think the glass content is high too, maybe 9%. I think it should be 3% .My 19 turn coil has a resistance of approx 2 ohms, with a trace thickness of maybe 1.5mmI don't think this coil will be winning any contests. A copper trace of 2.5mils (copper tape) @ 29.849 inches and 1mm wide would have a resistance of 0.2 ohms.It might be a fine.. low Q coilI wonder if I can get results better then 'oven element'I will do some experiments on a flat with careful stenciling so I can get a nominal value for a trace.

author: jonpaul
author_rank: Super Contributor
timestamp: August 05, 2025, 08:17:14 pm
content: Hello Coppertone: Whats your objective/freq range/power?Why bother with silver?We use off the shelf 1 uH indrs from Coilcraft. Very well specs, avail in many SMD and TH sizes, with complete characterization.The formulas to design a PCB coil are well known.Measurement of 1 uH is easy with a RLC or VNA specified at Freq above resonance.We used Wayne-Kerr, HP , ESI bridges, inductance analyzers at 100 k-10 M and HP 4195A to 500 MHz with Z adapter.The low cost Chinese meters are not capable.Enjoy,Jon

author: coppercone2
author_rank: Super Contributor
timestamp: August 05, 2025, 08:40:51 pm
content: That is the value I happened to get with the stuff I had. I am waiting for 0.2mm tape to make smaller coils.Some people just don't want to be at the mercy of the coil dealersI am surprised no one else is irritated by all these proprietary ceramic objectsand you can paint spheres too, to make custom variometers. You don't want a little marble sized variometer/transformer?en.wikipedia.org/wiki/Transformer_types#Variometer_and_variocouplerperhaps connected to a high precision piezomotor?

author: k6sti
author_rank: Contributor
timestamp: August 05, 2025, 10:35:34 pm
content: Use this to refine your expectations for coil inductance and Q:https://k6sti.neocities.org/coil.htmBrian

author: coppercone2
author_rank: Super Contributor
timestamp: August 05, 2025, 10:52:10 pm
content: right now I need to get the paste conductivity right

author: coppercone2
author_rank: Super Contributor
timestamp: August 16, 2025, 04:38:53 am
content: ok with a trace width of 1mm and thickness of 0.003 inches (wet paste), I got a resistance of 80mohm vs 60mohm (with the new paste) for approx 2.3 inch long traces. The better one seems to come out to about 1miliohm/square.Does anyone know how that is? I assume the thickness is roughly 0.0018 inches after firing. Not sure what a commercial film resistance would be at the same dimensions. They are specced thinner.If thickness is doubled resistance is halved. So to me if a specced film is 4mOhm / square @ 15um, and my something around 45um thickness film is 1mohm/square, I think I am in the ball park for having decent performance?I wonder why it did so poor on the coil. I did the math wrong. If its ~30 inches, that is 760mm, and a square is 1.5mm, then that is 500 squares, or around 4miliohm per square. Still its alot higher then the trace on the board. I think the paste works better after it sat for a week btw. Smoother to apply. So I guess the 2 ohm coil is in the correct order of magnitude, but maybe I can get it down to a ~600 miliohm coil if I try harder. But even so, it would be approximately equal to winding it with 32 awg wire. Not that impressive. I would need to figure out a way to slap that shit on real heavy.An option might be to nanocoat a square or rectangular copper wire that can be pryed off a coil leaving the silver behind in the 'trench' it left.Failed again, you need a 'critical mass' of this stuff to make a shape more complex then a flat stencil. Its too hard to keep the consistancy as you are working on it. I figure you need at least a 25g jar so you can actually get a feel of how it works, and be able to dillute it in a repeatable fashion. I also got some more proper alumina rods, rather then trying to use whatever sharpening stones I had... my last coil is alot of broken circuits. 0wow this whole process is annoying.I finally found some figures after doing alot of searching, it says the critical mass of Pt-239 is 10kg. So I will extrapolate that I should mix up at least 10 grams of silver paste for the next experiment.