[wrycoder]

If you cut back the antenna length so you can’t hear the fundamental 100 ft away, the harmonics shouldn’t be an issue. But, I’d have to build it to be sure. I might do that!

Here’s some info on suitable low pass filters for $5. You’d have to modify the 600 meter kit for 555 kHz, after reading up on the design at the linked technical pages.

https://qrp-labs.com/lpfkit.html

[mlyle]

Note that, for a given too-short antenna length, it will be better at emitting harmonics than the fundamental.

E.g. a 1m antenna is far below resonant for 1MHz AM, but may be pretty good at emitting any 37, 39, 41MHz ... content.

Of course, we expect that content to have less than 1/40th, of the fundamental, both because of Fourier series expansion of a square wave and because the frequency response of the system rolls off (edges off an ATmega aren't perfect, even before you get the reactance of the breadboard in the way.

A primary reason to add a filter, IMO, is to raise the voltage of the fundamental at the antenna, and therefore get a little more RF out on our desired frequencies.

[wrycoder]

Those are good points. One of the reasons I was thinking of building it was to put it against a tinySA and see what it's putting out.