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From my understanding a ferrite rod antenna is simply a length of wire wrapped around a ferrite rod. A wire will sticks out of each side of the rod, one is ground and one is the main receiver.

I opened up a few radios recently and one has 4 wires coming out of the rod and one has 3. What are these extra wires for?

Here is an image of the rods: enter image description here

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These are antennas, and at the same time they are inductors intended to be tuned with a variable capacitor, allowing one station to be selected in favour of others.

These inductors often send their output to a transistor amplifier or transistor mixer, so impedance matching is beneficial. This matching can be done a few different ways:

  • a tapped inductor
  • a link winding having fewer turns than the main winding

schematic

simulate this circuit – Schematic created using CircuitLab

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