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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: 5, Interesting) by VLM on Sunday July 29 2018, @03:03PM (4 children)
Unless I missed something in the links there's no EE input, which would superficially seem important for a radio problem...
Now a days of course with EMI/EMC regulations and problems its pretty crazy not to filter the output of a transmitter. But what was it like in 1937? At some point in time, "dump all over your transmitter harmonics" went from stuff happens to forbidden. I tried and failed to find early FCC regulations online. Then I tried searching for transmitter schematics of the 30s; I am not old enough to really speak "vacuum tube" so I'm a little unclear how much the output matching networks would filter vs merely impedance transform; when I was a kid, rumors were unfiltered vacuum tube output was always cleaner than unfiltered transistor output, "tubes are just naturally more linear" and stuff like that, which admitted sounds like something a tube amp aficionado would tend to say regardless of any EE truth (or not).
Finally I found a 14th edition ARRL handbook from 1936 online and in the "Principles of transmitter design and operation" around page 156 to paraphrase and summarize not filtering transmitter output makes plate efficiency look inaccurately higher so beware of that, and known interference issues on non-harmonically related ham radio bands was pretty much a "tough shit" situation for those interfered with in the late 30s. Life was a bit different in those early days, kinda like everyone was the equivalent of a FCC Part 15 operator back in ye olden daze. Or perhaps my interpretation of ham radio publications does not match reality on the bands in the 30s; would have to find a ham licensed for 90 years and ask him; there probably are a handful still alive. I think they're all members of my local radio club but I digress.
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, then run that thru modern propagation analysis tools using historical sunspot records and a bunch of handwaving, with an antenna finite element analysis of the plane on the ground over a salt reef marsh, etc, to provide some engineering-reasonable estimates of likely SNR at various global locations. I mean, we (as in EEs) have the tools such that we could provide a pretty reasonable guess of the as heard SNR the supposed reporters heard of various transmissions would have gotten, EE techniques have advanced quite a bit beyond "aw shucks meebe it worked"
Note that salt water and crashes and high voltage transmitters and glass vacuum tubes and antenna insulators don't really mix very well; if you think the proposed paper scenario would be improbable using indestructible modern gear, its a whole nother level of difficulty in the late 30s. I find the theory implausible and suspect most of the reports of transmissions were the 1930s equivalent of "internet trolls weren't invented in 2010" or whatever crazy recent date claimed. "Lets pull a joke on Earhart fans" sounds weak in 2018 but its not like they had /b/ as a containment board in the late 30s.
(Score: 2, Interesting) by Ethanol-fueled on Sunday July 29 2018, @03:56PM (2 children)
Exactly. When I was studying for my FCC GROL we learned about all kinds of things such as tropospheric scatter, and other methods which would extend the range of comms. The article and linked articles need to provide details.
(Score: 0) by Anonymous Coward on Sunday July 29 2018, @04:32PM (1 child)
Being sober doesn't make you smart. You're just as stupid as 40oz of Kentucky piss can make you, but sober.
(Score: 0, Troll) by Ethanol-fueled on Sunday July 29 2018, @07:33PM
I may not be from Kentucky, but I can drink Kentucky piss with the best of them. I still think Scots Whisky sucks assholes, by the way.
(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.