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I’ve been scouring the interweebs for some insight into how electrical signals travel in a wire. More specifically, I cannot wrap my head around how a receiving antenna works with hundreds if not thousands of electromagnetic waves interacting with it. How is one signal able to be usefully extracted? Surely the movement of electrons back and forth would be influenced by all the electromagnetic frequencies hitting the antenna that their back and forth movement would no longer reflect a single (useful) signal? My flawed understanding is based on thinking of the electrons in the receiving antenna as being analogous to a single conga line. Should I imagine the wire as many conga lines of electrons instead?

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    \$\begingroup\$ How familiar are you with the concept of "linear-time invariant systems" and/or "frequency domain"/"Fourier transform"? These concepts are probably the best description of why all the different frequencies can coexist happily on the antenna without interfering with each other. \$\endgroup\$
    – nanofarad
    Feb 5, 2021 at 20:17
  • \$\begingroup\$ I don't think your conga line works here, unless you can superimpose arbitrarily many rhythms in a single conga line... So, I'm going with nanofarad's point here: you might be missing too much background for a "short" answer. Generally, I'd recommend dropping the "material" image of charges; you're dealing with currents, induced by electric and/or magnetic fields, you can't map that very well to a particle model (an important aspect of modern physics is that matter, including electrons, sometimes behaves like waves, and sometimes like particles, because these are irreconcilable concepts) \$\endgroup\$ Feb 5, 2021 at 20:19
  • \$\begingroup\$ Thanks folks - I’ll start with the concepts mentioned by nanofarad. I was aware my understanding had to be wrong (or radios wouldn’t work!) but I didn’t know where to start. This is very useful. \$\endgroup\$
    – Chrispiddy
    Feb 5, 2021 at 20:34
  • \$\begingroup\$ No problem. The terms I mentioned are pretty mathematically heavy, but probably the most insightful direction to go that doesn't involve a lot of handwaving. \$\endgroup\$
    – nanofarad
    Feb 5, 2021 at 20:41
  • \$\begingroup\$ Keep in mind an antenna is also a filter... \$\endgroup\$
    – Voltage Spike
    Feb 9, 2021 at 23:02

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An antenna has a specified length. This length filters out all waves with wavelengths that do not match the length of the antenna.

Think of a tunnel with a complex cross section. Many cars with different cross sections can drive towards that tunnel simultaneously, but only the cars with matching cross sections can enter the tunnel.

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Surely the movement of electrons back and forth would be influenced by all the electromagnetic frequencies hitting the antenna that their back and forth movement would no longer reflect a single (useful) signal?

Yes. That's what happens. An antenna will pick up signals more strongly at frequencies at which it is resonant but in general an antenna will pick up lots of signals (to varying degrees) across a wide frequency range. Picking out a single wanted signal from the possibly vast range picked up by the antenna is the job of the receiver, not the antenna.

A tip: don't concentrate on what the electrons are doing. It seems to be a common beginners' trait to get tied up in knots trying to work out what electrons are up to. Learn to think in terms of currents and voltages first. You can worry about electron behaviour later, when necessary (which turns out not to be very often).

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Electrons moving back and forth is OK. They move because the electromagnetic wave is a traveling, oscillating electric field (paired with a traveling oscillating magnetic field). Electric fields make charges want to move. Oscillating fields make them want to move back and forth. In the case of copper wire, the charges are able to move back and forth because copper is a good conductor.

But antennas are selective. They do not receive all the signals in the world. They receive signals in certain frequency ranges they have been tuned to receive. They are resonant. They only receive signals that excite their resonance (signals that are on the right frequency).

After the antenna you go to a radio receiver. The antenna is somewhat selective. A good receiver is like a world champion at selectivity. It can ONLY hear the specific signal it is tuned to hear. Everything else is suppressed to a truly phenomenal extent using the magic of superheterodyne. Like being in a room so soundproof that a jet engine outside is just a quiet hiss.

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A moving element can hold in the same time numerous different motions which are all summed together. Think yourself having a bad hangover, every limb trembles uncontrollably say half an inch here and there. But you can still get up and walk to somewhere to get a remedy.

The trembling continues, but somehow you still succeed at the same time to walk to the wanted direction. Someone can notice from your complex combined motion only the trembling and think "Thanks God, I'm not in that condition!"

The barman in the place where you finally enter notices the familiar customer coming in and says "Welcome, Sir! Nice weather today! How can I help you? The usual?" He ignores your trembling totally, he is interested in only that part of your quite complex motion that brings the paying customer in. If he noticed the trembling he would add "With a straw, I presume!"

Different electromagnetic fields from different radio stations can cause to the same electrons in the same radio antenna several simultaneous movements (=currents) which all exist at the same time. The total current is the sum of the separate motion components. Cleverly designed circuits filter apart the wanted current generally based on the frequency. If you turn the tuning knob of a radio receiver you generally change the frequency that the receiver accepts. Rest is filtered out.

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