A question about radio recievers

[AlexCaledin]

In principle, it's possible to hear AM radio with earphones connected directly to the antenna - if the permanent magnets in the earphones are replaced with non-magnetized ferrite cores. Then, the magnetic force is proportional to the square of the current so the membrane (and whatever else can vibrate) is vibrating with the audio frequency.

 

[sophiecentaur]

In principle, it's possible to hear AM radio with earphones connected directly to the antenna - if the permanent magnets in the earphones are replaced with non-magnetized ferrite cores. Then, the magnetic force is proportional to the square of the current so the membrane (and whatever else can vibrate) is vibrating with the audio frequency.

= a non linear element in the circuit. Tooth fillings can also account for people hearing radio transmissions but that's not a lot of use as a design feature of a receiver.

 

[AlexCaledin]

Still, the Marconi Magnetic Detector was the first really useful one . . .

https://en.wikipedia.org/wiki/Magnetic_detector

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As for the frequency selective element, it's not necessary when you are not far from one real powerful long wave transmitter! 30 years ago, there was such a transmitter here, it was strong enouh to use a loudspeaker instead of earphones, without any amplification.

 

[sophiecentaur]

Still, the Marconi Magnetic Detector was the first really useful one . . .

https://en.wikipedia.org/wiki/Magnetic_detector

View attachment 205963

Agreed but that and the 'coherer' were both just Detectors and did not demodulate AM. (Brilliant steps along the way, of course)

 

[marcusl]

In principle, it's possible to hear AM radio with earphones connected directly to the antenna - if the permanent magnets in the earphones are replaced with non-magnetized ferrite cores.

Not exactly. As stated previously, one needs a frequency selective element to isolate the desired broadcast from all others. This is called a "tuner" in the article you quote. Furthermore, Marconi's detector utilizes permanent magnets, and relies not on some purported square law force for detection but rather on the non-linearities in iron's hysteresis curve.

 

[Asymptotic]

From what I'd been reading, Reginald Fessenden devised the first audio detector circa 1902. As @sophiecentaur said, coherers and Marconi's magnetic detector aren't rectifiers, and are incapable of demodulation.
https://en.wikipedia.org/wiki/Electrolytic_detector

 

[marcusl]

Read the first paragraph in your link:
"It was one of the first rectifying detectors, able to receive AM (sound) transmissions. "

 

[Asymptotic]

Read the first paragraph in your link:
"It was one of the first rectifying detectors, able to receive AM (sound) transmissions. "

Perhaps I phrased it inelegantly. Giving it another go ....

Neither coherer nor Marconi's magnetic detector can demodulate audio. This milestone wasn't achieved until 1902 when Reginald Fessenden devised the electrolytic detector.

 

[marcusl]

Got it! Thanks for the clarification.