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Arthur T Knackerbracket has found the following story:
Amelia Earhart's final moments may have been broadcast around the world days after her plane disappeared in 1937, according to a group that analyzed radio distress calls.
The International Group for Historic Aircraft Recovery (TIGHAR) believes the aviation pioneer waded out to her crashed Lockheed Electra on the reef at the then-uninhabited Gardner Island to call for help, it wrote in a research paper.
[...] The Electra's radio could only communicate within a few hundred miles, but the transmitter also put out harmonics that allowed the signal to reach beyond that.
"High harmonic frequencies 'skip' off the ionosphere and can carry great distances, but clear reception is unpredictable," the paper says.
As a result, the signal was heard by people using shortwave radios at home in locations like Texas, Kentucky, Wyoming, Florida and Toronto.
In St. Petersburg, Florida, a teenage girl transcribed phrases like: "waters high," "water's knee deep -- let me out" and "help us quick," the Washington Post notes.
-- submitted from IRC
(Score: 1) by Muad'Dave on Monday July 30 2018, @11:59AM
Very good points.
Interestingly, 1937 was quite near a sunspot peak [pbs.org], meaning ionospheric propagation at higher frequencies was quite likely, even at very low power levels that would result from harmonics.
> It would seem rather obvious to an EE to find the model of radio they had, fire that bad boy up into an attenuator, and plug it into my spectrum analyzer to measure how many dB down the harmonics were from the intended carrier,
You would also have to take into account any damage to the antenna or feedline since the output linearity (and therefore harmonic content) of early transmitters could be heavily influenced by high SWR/reflected power.
Re: regulations at the time - There was the Radio Act of 1927 [wikipedia.org] followed shortly by The Communications Act of 1934 [wikipedia.org] which remained the law of the land until it was amended in 1996. This paper [oswego.edu] is an interesting read.
Section 303(e) [criminalgovernment.com] of the 1934 Act allows the commission to "Regulate the kind of apparatus to be used with respect to its external effects and the purity and sharpness of the emissions from each station and from the apparatus therein".
This post [cornell.edu] tells you how to root around and find the source of any particular part of the rules. It would be interesting to see the purity standards back then, although we have no way of knowing if the exact setup on the plane met those requirements after the crash.