by jacquesm | 2026-02-05 11:53:20+00:00
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