What handheld electronic devices existed in 1977? A flashlight would have been incandescent.

Handheld radios (walkie talkie) predate that by at least a decade.

They probably were mostly operating around 30 mhz in the CB band or 120-150 in the Ham or aircraft bands in that era, but harmonics are a thing

	▲	pyman 5 days ago \| parent [-]
		Or the government was planning to cut funding, and to stop that from happening someone deliberately created the interference. Statistically speaking, that's more likely than aliens going extinct after sending one single message.