hckrnws

Interesting, I'm right in the middle of this, so very timely, thanks for posting.

As a surprise to me, the price of variable capacitors (which you can pair with an inductor as described to get a particular resonance) has gone completely through the roof because apparently nobody uses them any more. So what used to be a 20 ct part is now $7,50 quantity one and quantity 50 as well!

You can see a couple in the picture of the regency radio, the circles with a screw in the middle. The ones in the other radio in the little metal cubes are adjustable coils, they have a split ferrite element that screws up and down to extend the length or contract it.

As for stripping the conductor of the enamel: the best and safest way is not to scrape or sand (which will affect the conductor by reducing its strength as well as its cross section) but to simply boil off some of the enamel in a bit of solder stuck to the tip of your soldering iron.

That way you only strip just as much as you need to and there is no mechanical damage. You also should not twist the two wires the way it is done in TFA because these coils will pick up LF noise as well and that will end up rubbing the wiring to the point that you can get a short. The easier way is to just put a drop of superglue on the paper that you're going to use as coil body and then to stick the winding end in it and wait for it to dry (which should be really quick). After that you can start winding.

Wind nice and tight and make sure to not leave any air between the coil body and the ferrite, this will result in 'microphony', your core will end up modulated by sound waves! That's why if you're going to wind around paper you should wind with the paper already around the ferrite as tight as you can make it. If the paper slides on the coil too easily then you may have to mechanically affix it to make sure that the microphony effect is minimized.

If you need to cut ferrite realize that the stuff is super hard. It is easier to score it using a grinder wheel (a big one, or one on a dremel) all the way around and then to snap it off. After that you will still need to dress the ending, the fragments are super sharp so be careful you don't cut yourself.

Finally, I hotmelt a little piece of perfboard onto the ferrite and then solder the coil wires onto that and exit using thin silicone wiring. That gives a very flexible interface which will result in fewer broken coils.

Author here -- thanks for the comments and feedback! I typically use the solder stripping technique as well when actually incorporating the inductor into a circuit. Interesting you should mention variable capacitors as I just went through the process of sourcing 5 of them (will be discussed in a future post) from Amazon, and not only were they expensive, but they also took significantly longer to ship than was originally estimated.

I am somewhat surprised that the small loops would pick up a meaningful amount of noise, though I suppose meaningful always depends on the use case and tolerances. Do you have any references for how significantly this can impact inductor performance? Or is the primary concern just the rubbing and potential shorts?

Despite ferrite core inductors and loopstick antennas being old, fairly well understood technology, I have found there to be very little in the way of hands-on experiments with tangible results available. I'm hoping to continue testing various techniques and posting about them -- you've already provided some great variations for me to test out!

(Aside: I saw those "PCBite" doo-hickeys in your photos and said to myself, "What are these magical scepters?" Picked up a small set just now.)

You will not be disappointed! Well worth the investment.

Hey there, fellow hardware hacker :)

> Interesting you should mention variable capacitors as I just went through the process of sourcing 5 of them (will be discussed in a future post) from Amazon, and not only were they expensive, but they also took significantly longer to ship than was originally estimated.

It really took me by surprise, I was ready to buy a bag of them and then I realized that the price was for single pieces! And those were the crappy foil kind, not even the cermic/silver versions which seem to be unobtanium.

> I am somewhat surprised that the small loops would pick up a meaningful amount of noise, though I suppose meaningful always depends on the use case and tolerances.

So was I :) But the spectrum is extremely polluted on the low end, just about everything contains a switching inverter these days and they're not exactly clean so you have to pay extra attention to this on the receiving end. Higher Q helps (more coil, less C), up to a point. Above 500 KHz or so it cleans up a bit but below that it is a real problem, because it is not just the fundamentals but also the harmonics which for a switched coil go up very high (in theory: to infinity, in practice, easily to 10x the switching frequency before they fall off to the point that you can ignore them).

> Do you have any references for how significantly this can impact inductor performance?

For me it made the difference between having something working and not being able to recover the signal at all. In the end I chose an extremely high impedance pre-amp and a coil/cap combination that is on the edge of what you can still make at home without having to worry about stray capacitance. It helps that I know exactly what the pattern of the signal is that I'm looking for as well, so maybe if you end up having problems you can use that kind of trick to raise your signal above the noise floor.

> Or is the primary concern just the rubbing and potential shorts?

That's the secondary concern. Also: beware of using metal in your fixtures and metal objects near to the receiver, that can have a massive effect (imaging, signal attenuation, or, if you're really lucky, signal amplification). The potential shorts issues usually only show up over the longer term, you can compensate for quite a bit of that by mechanically fixing your windings using either glue or by dipping the whole works into inductor resin and letting it dry.

> Despite ferrite core inductors and loopstick antennas being old, fairly well understood technology, I have found there to be very little in the way of hands-on experiments with tangible results available.

True, that's because these are the ways of the past, the modern way is to get out of the analog domain as fast as you can and then to do the rest digitally.

> I'm hoping to continue testing various techniques and posting about them -- you've already provided some great variations for me to test out!

Enjoy this, it is very rewarding to pick signals out of the air and there are a lot of interesting lessons to be learned by doing this the 'hard way'. Feel free to mail me if you want to take this off-line, email in profile.

best regards from NL,

Jacques

If you can afford losing a bit of frequency stability, you can use a varicap instead and control it by voltage. Precise multiturn potentiometers are much cheaper. Or just buy a programmable clock signal generator based on PLL and then make it into a superhet (you can still have analog filtering and detection, but digital frequency synthesis so it can look more like a modern radio with frequency display).

There are so many options and all are very cool to explore.

Yes, varicaps are good and there are even varicaps with very large capacitance swings exactly for this purpose.

But it does require a very steady voltage and many more parts than just a single passive to not accidentally load the resonant circuit to the point that it becomes ineffective. You need to de-couple considerably to make this work, especially without injecting (phase) noise. And frequency stability and phase noise can be extremely annoying in particular applications.

That’s why you probably want to use it in an oscillator rather than a filter. Then some of those problems go away. Although, some new ones appear like having to add a mixer … Some time ago I thought making a double superhet would be too complex but apparently it turned out quite a nice DYI project.

One of the problems I'm struggling with is that I'm trying to recover a signal with a known pattern from under the noise floor and every little bit helps. Varicaps will come into play once there is enough signal that the thermal noise and power supply influence would no longer drown out the signal. Interesting project but it is one of those where when you start you think 'how hard could this be?' only to find out that it is in fact pretty hard. We'll see if I can pull this off or not, I give it 10% chance at the moment.

Oh, and superhet wouldn't work, that would destroy the valuable part of the signal.

> There are so many options and all are very cool to explore.

Very much agree, controlling a varicap via an MCU is a nice intermediate step of "digital tuning" without going all the way to SDR.

> it is very rewarding to pick signals out of the air

I could not agree more :)

——

This is quite possibly the most positive interaction I’ve had on HN — thanks Jacques!

I'm currently going about it the other way, specifying custom magnetics to suppliers. Suffice it to say there are a lot more variables when you get in to manufactured designs, which may involve multiple coils, thermals, various component type options (SMD, THT), disparate wire types per coil, various footprint options, multiple core options, load modulation, etc.

I don't envy you at all. I'd ship a working prototype and say 'like this one' :) My ability to spec this would surely become an exercise in finding all of the ways in which I could mess it up.

Learning at industrial scale is sometimes temporally cheaper because it allows you to conclusively exhaust a design path through efficient and spectacularly memorable failure. ;)

HN should have a 'comment of the day' section. I'd nominate this one.

I love the symmetry of capacitors and inductors. It's just so elegant.

    Inductors: store energy in a magnetic field, pass DC, block AC

    Capacitors: store energy in an electric field, pass AC, block DC

Don't forget about ELI the ICE man!

You can use one of these to act as a local personal WWVB station amongst other things.

See https://github.com/hzeller/txtempus and https://github.com/GOTO-GOSUB/Txtempus-Passive-Antenna

I know there's a scientific explanation for it but the fact that the inductance changes just by shifting the rod baffles me.

This is the kind of stuff that still proves Arthur C Clarke " any sufficiently advanced technology is indistinguishable from magic".

If you place the coil asymmetrically you are getting into the 'end effect' of the ferrite rod, so some of the magnetic field lines will escape and that will marginally reduce your inductance. It's as if you cut off a few windings from your coil. So the coil exactly in the middle has maximum inductance, moving it off to the side will reduce it very slowly until you hit the end, after that it falls off a cliff.

hth.

I thought it was going to be about making the powdered iron at home. Fun article nonetheless.

You can, but the results will be messy and sub-optimal compared to industrially produced cores.

I've done it for an inductor for which there wasn't a ready made core. It left me with something that sort-of worked but the grade was much lower than even the poorest commercial grade. To improve on that I would have had to invest a lot of money.

Here is an overview of the steps in commercial ferrite core production:

https://www.powerelectronicstalks.com/2018/11/ferrite-core-m...

Edit: Oh, and the way I did it: take a thermally resistant resin with low shrinkage on curing and load it with as much iron powder as it will take before it starts clumping, pour into mold, let it set and then press it out of the mold.

That was an interesting read. However it doesn't look like small scale DIY at a high quality level would be particularly expensive in terms of tooling? Only in terms of time and personal experience.

Ball mills are trivial to DIY - electric motor, belt drive, couple pieces of copper tubing, drill some holes in a wooden frame, use large metal coffee cans, throw some ball bearings in.

Calcination is also easy. 1000 C is not a difficult temperature to deal with. You can hit that using plain old nichrome wire. You will need to purchase a few ceramic bricks of course.

The only slightly tricky step I'm seeing is sintering. The nickle-zinc option mentioned in the article will be substantially easier (lower temperature, oxygen atmosphere) but the required temperature still pushes up against the limits of nichrome wire. Given that propane can get you to the vicinity of 1900 C and tanks of that are sold at the grocery store (at least where I live) I'm going to say this step is also cheap and easy to DIY.

The real issue is that you can't do this at home if you live in high density housing. You will need a shop or a small back yard.

I've done worse than that in my garage but I also know that if I burn it down (and the neighbors garage with it) that the insurance company will not look at me kindly.

Regardless it is not a process that you're going to make work on your kitchen sink :)

I wonder if you can do that in a microwave...

Hehe, well played.

> However it doesn't look like small scale DIY at a high quality level would be particularly expensive in terms of tooling?

Ahh, the lure of a DIY project that makes you go “How hard could it be?”

It’s an all too familiar feeling for me.

Is whole-word dyslexia a thing? I thought I saw "Making Ferrite Inductor Cores", and got excited to learn DIY techniques for making custom shapes/blend cores. Still a good article, though.

When I was a boy into electronics the one thing that always stymied me designing circuits was how to make an inductor of the right value. I can't recall these many decades why but I needed to make ones of a value and you couldn't specify "xuH" to a component supplier, like resistors or capacitors, but nor was there any resource to say "x turns of y gauge wire" etc. It was all a black art to a 12 year old in the early 70s.

Funny, I had the exact same experience as a kid in the 70's. Up to coils it all seemed pretty easy.

So yes, coils (or, to be more correct, inductors, in Dutch we use 'spoelen' which is closer related to 'coils' so I tend to make that mistake all the time) are 'different' in that sense, as are the equations that govern them. And the theory is sufficiently complex that you have a hard time hitting the right value right off the bat if you put something together that you think will have a particular inductance unless you've done it many times before. Slight variations can make big differences. On the plus side: the values are critical, even so, there are usually plenty of ways to compensate if you got it wrong. One trick is to overwind and then to remove windings until you hit the right value. Another is to hook up a scope through a very high impedance probe and to couple your coil magnetically to an oscillator with known frequency. You can then tune for the required response without ever knowing the exact inductance that you're looking for.

In the higher frequency domain (when you start using air coils of silver plated copper) you can usually achieve the same effect by slightly opening up or compressing the coil windings.

Resistors are easy, capacitors are bit harder, coils are 'magic', but with a bit of practice that magic becomes ritual and ritual should be at least reproducible to the point that the part becomes a manageable quantity.

[dead]