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Most early solid state portable MW AM radios used a ferrite rod antenna.

Some valve receivers like Phillips in the late 1950s used ferrite rods also. The early Germanium RF transistors were not too good by modern standards. Good sensitivity could be had with a large ferrite rod. As better RF transistors improved front end gain, the ferrite rod got smaller and performance was fine.

  • Sure a larger ferrite rod will pick up more signal. Is RF voltage proportional to core area?
  • What is the effect of rod length?
  • Is there an upper limit to all this when the rod is very large?
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    \$\begingroup\$ This strives to be very constructive feedback, in hopes you find it interesting: in English (and all other languages that use the Latin alphabet), punctuation is typeset "This is the last word of a sentence. "; i.e. the "." comes directly after the last word of a sentence, and a space after, not the other way around :) \$\endgroup\$ Nov 15, 2019 at 8:36
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    \$\begingroup\$ @MarcusMüller We have told him time and time again for years. \$\endgroup\$
    – winny
    Nov 15, 2019 at 8:45
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    \$\begingroup\$ @winny don't put him down like that; punctuation is easier for some than for others. \$\endgroup\$ Nov 15, 2019 at 9:59
  • \$\begingroup\$ @MarcusMüller That would be the case if he was inconsistent, which he isn't. \$\endgroup\$
    – winny
    Nov 15, 2019 at 10:07
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    \$\begingroup\$ @winny it might really be difficult to re-learn punctuation. I didn't know he had trouble with it before this very post! \$\endgroup\$ Nov 15, 2019 at 10:09

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They had used an auxiliary coil to provide a positive feedback. This means that due to ageing and drifting, the receiver had become a transmitter. The first use was introduced by Nikola Tesla with his patent.

It's a kind of weird functionality, like sending the received signal back into a coil antenna to get more received signal. I guess, more or less they had be projected with experimentation. The US submarines still use this antenna for VLF, the modern devices replaced this technology with hetherodyning to prevent transmission in case of malfunction.

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    \$\begingroup\$ I'm not quite sure which of OP's questions that answers? \$\endgroup\$ Nov 15, 2019 at 8:38
  • \$\begingroup\$ @MarcusMüller No one, but I wanted to tell him that is no a trivial answer, since it was lots of experimental work to make those receiving coils and people with this knowledge are probably not present on this forum and probably they are RIP. \$\endgroup\$ Nov 16, 2019 at 16:30
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Ferrite rod is used to lower the frequency response of the antenna. Using windings around a ferrite core it is possible to receive much lower frequency signals than with a metallic antenna alone. As technology has improved, we have made more use of high frequencies and so the ferrite core antenna is not necessary, but for picking up signals between 50 hz to 50khz or so, it can still be very useful.

In the case of the modern radio, most have an extendable antenna of around 75cm, tuned for the FM reception... Or they rely on the users headphones cable to act as an antenna.

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