You're pretty damn likely to be inside the part 15 mask which is hundreds of microvolts/meter (30m away) up at UHF+. The total energy inside a single GPIO squa

You're pretty damn likely to be inside the part 15 mask which is hundreds of microvolts/meter (30m away) up at UHF+. The total energy inside a single GPIO squarewave is on the order of 30mW; harmonics are a tiny fraction of this, and most will not be radiated efficiently. (Not to mention that edge rates/stray capacitance will limit their energy in the first place). The big question is whether the fundamental frequency is compliant. A perfect squarewave has ~81% of its energy in the fundamental frequency.