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You CAN put a crystal set or a regen in a metal CAN (or box, haha). In fact, a regen probably should be in one, as should just about anything with an oscillator or that is very sensitive to outside influences (temperature/RF/etc, although metal isn't necessarily the only/right material for temp issues). Luckily, most of the radiation from a local oscillator is, well, local. However, a regen/super regen CAN radiate out into the ether via the antenna. That's why sometimes you'll see an RF amp in front end for isolation.??There's a similar issue with backwave that happens if you leave a transmitter oscillator running all the time instead of keying it, which isn't a problem with most simple transmitters (everything gets keyed).?For harmonics, though, a good output filter is important for transmitters and shielding can help with some maladies, again, I suspect, with mostly local issues at QRP and QRPp power levels. Receivers benefit from shielding, although there may be a point of diminishing returns. I think most, if not all, of the QRPME stuff is fine without additional shielding as it works well within expectations of fun, easy-to-make, straightforward, simple, usable kits that can be mastered by the average Ham. Heck, even my beloved Elecraft K1 has its shortcomings, and it's orders of magnitude more complicated than most things I build these days.?

As for crystal sets, I rarely use a bcb loop antenna. I like FT-114 toroids. They make a nice coil with relatively few turns of magnet wire with good enough "working" Q (I have never measured it) that I have been able to separate 10kHz stations. Toroids don't need shielding, but that also works the other way-- it won't be influenced by sitting in an Altoids tin or too close to a metal front panel, either. The pic I have attached is my toroid version of the famous Australian "Mystery Set" built in a hobby store Halloween decoration?. It works pretty well. I have a couple of different experimental toroid crystal sets built on plastic clipboards that work reasonably well (I've logged something like 9 states so far), some oatmeal boxes, some Morgan copies, I've used all kinds of detectors--foundry coke, razor blades, old tools, chunks of Ge, Si, (probably oxidized), galena, pyrite, a jillion other things, including LEDs (which we have strong enough signals to light up and I can drive a homemade horn loudspeaker made from a piezo element to audible volume) and some diode junctions old credit card chips, and various ICs like a 555. I think think someone DID actually make a receiver out of 555, but I'm just using a PN junction available at a couple of pins. Another FUN circuit is the loop antenna (I've made them on "acquired" milk crates and ~2-ft quilting loops. Those work great on a regular RX with inductive coupling, but if you put a diode/earphone? on it, you can easily hear loud locals.??

Might be fun to see if Rex wants to delve back into XTAL sets with QRPME.? I know he's done a lot of XTAL set work in the past.

-HRS