[makomk]

I hope the spurious emissions aren't as bad as they look in the waterfall, because they're basically stomping all over the band and all the neighbouring bands and they're closely spaced enough that I'm not sure it'd be realistic to filter them all out. Is this even legal to use?

[morganvachon]

This is directly addressed in the article:

"Important Disclaimer: While the output power is very small, you should still take great care as the carrier is a square wave, and there is no filtering on the antenna output. So any transmissions will cause harmonics all across the spectrum – possibly interfering with life critical devices. A filter must be used if you actually plan on transmitting with any sort of range further than your room."

[makomk]

Harmonics are trivial to filter - they're at multiples of the fundamental frequency, so 100MHz, 150MHz, and so on. You can just stick a lowpass filter on the output and it'll block all the harmonics because they're so far away from the fundamental. Unfortunately, the waterfall appears to show strong spurs every 30 Hz, and those would be ridiculously difficult to filter out. I think the normal solution is to design your transmitter so that it doesn't do that.

[Kliment]

Given how regularly spaced they are they might be other channels (regular TV maybe?)

[GFK_of_xmaspast]

It looks like they're spaced at 30KHz, which seems to me to be a very tight grouping for any kind of transmission like that (and presumably if that band was that crowded to begin with, the author would have picked a different one).