The Germans were using certain high frequency signals to communicate with each other during world war 1.
During that time, if the other countries wanted to know, well in advance as ' what the Germans are planning', they had to receive the signal and amplify it accordingly. The plan would be to design a tuned filter. Back in the days, it was easy to build a receiver circuit to track the signal, but there wasn't a proper Amplifer.
Why Amplifier?
Amplifier is a device which increases the voltage, current or power of the signal. In common terms, to increase the volume of the signal(Voice supposedly).
When United States entered world war 1, Armstrong joined Army signal corps and was sent to Paris, and was placed in charge of Research section. While travelling to France, Armstrong met captain H.J Round, an engineer with British Marconi Company. Armstrong learned that the British were ahead of Americans in development of Vaccum tables capable of handling frequency signals ranging from 500KHz to 3.5MHz, a frequency range that it was suspected, the Germans were using for communication. In that way, the British had kept track of many Germany ships and also had broken their codes and nearly read all the messages.
Germans continued to communicate with each other from different ships planning the destruction of allied countries. Americans tried to read those messages, but ultimately fell short to build a proper receiving circuit/Amplifier stage and were like
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They appointed Armstrong and assigned him to detect short wave enemy communication.
Armstrong realized that if radio direction-finding (RDF) receivers could be operated at a higher frequency, this would allow better detection of enemy shipping. However, at that time, no practical "short wave" (defined then as any frequency above 500 kHz) amplifier existed, due to the limitations of existing triodes.
It had been noticed some time before that if a regenerative receiver was allowed to go into oscillation, other receivers nearby would suddenly start picking up stations on frequencies different from those that the stations were actually transmitted on. Armstrong (and others) eventually deduced that this was caused by a "supersonic heterodyne" between the station's carrier frequency and the oscillator frequency. Thus if a station was transmitting on 300 kHz and the oscillating receiver was set to 400 kHz, the station would be heard not only at the original 300 kHz, but also at 100 kHz and 700 kHz.
Armstrong realized that this was a potential solution to the "short wave" amplification problem, since the beat frequency still retained its original modulation, but on a lower carrier frequency. To monitor a frequency of 1500 kHz for example, he could set up an oscillator at, for example, 1560 kHz, which would produce a heterodyne difference frequency of 60 kHz, a frequency that could then be more conveniently amplified by the triodes of the day. He termed this the "Intermediate Frequency" often abbreviated to "IF".
The signal with intermediate frequency had retained its original modulation(It means, "The secret message" was still encrypted in the carrier signal, albeit on a lower frequency).
But there was an advantage this time, the united states Army had proper equipment(Amplifier) to amplify the signals in the IF frequency range which was not the case in the short wave range.
The Military of US adopted the methodology and were able to increase the volume of signal(Amplify) and were able to hear 'what the Germans were planning'?.
If you are wondering how superheterodyne receiver looks like. Here you go.
One of the important advantages of Superheterodyne receiver is : Irrespective of the frequency transmitted by any station, the IF(Intermediate frequency) remains same, so the amplifier design remains robust and doesn't depend on the frequency of the station.
Almost all modern radio receivers employ this method and even today, Edwin Armstrong is regarded as one of the greats in Radio history.
