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.

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hasheddan 4 hours ago | parent [-]

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!

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JKCalhoun 4 minutes ago | parent | next [-]

(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.)

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jacquesm 4 hours ago | parent | prev [-]

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

	▲	pkolaczk 7 minutes ago \| parent \| next [-]
		If you can afford losing some 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.
	▲	hasheddan 3 hours ago \| parent \| prev [-]
		> 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!