When an electromagnetic wave strike an antenna, a current starts to flow in it.

There are many EM waves (at different frequencies) around us. Does this mean that there is always current flowing in the antenna at each frequency?

If we want a particular frequency, do we just use a resonant circuit and take our signal?

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    \$\begingroup\$ I don't like to be this way, but you've asked several really basic questions today. Could you stop for a minute, and read up on the basics? This site can answer specific questions about things you don't understand, but it can't teach you the whole subject. Not understanding part of something you've read is normal - ask about the things that aren't clear. Asking very broad, basic questions makes it look like you haven't made any effort yourself - and that tends to get a bad reaction here. Folks like to help, but they prefer to "help you help yourself." \$\endgroup\$ – JRE Jun 7 at 11:28
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    \$\begingroup\$ Does the sun stop shining when you close your eyes? \$\endgroup\$ – Andy aka Jun 7 at 11:34
  • \$\begingroup\$ @JRE FWIW I think (after Marcus' edits to the tone) this is a great question. It's basic in the sense that it addresses an important fundamental principle, but it doesn't seem overly broad to me. \$\endgroup\$ – pericynthion Jun 7 at 16:51
  • \$\begingroup\$ @pericynthion: It isn't just this question. It was the cumulative effect of all the questions by this user in a very short time (some few hours.) \$\endgroup\$ – JRE Jun 7 at 16:57
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    \$\begingroup\$ @pericynthion: I did not down vote or vote to close. I just suggested a little more personal effort. \$\endgroup\$ – JRE Jun 7 at 17:48

This is a good question.

Yes, every changing magnetic field induces a current in a conductor going through that field. That's induction for you.

Think about the most elemental antenna you can think of: the dipole. Just two sticks of metal, end to end.

When these two metal sticks sum up to be half a wavelength, all the currents align in the antenna, sum up and reach a high amplitude at the middle point. That's good, because that's where your receiver is attached, so the receiver sees a high current at that frequency.

However, for wavelengths (which is just c/f, so different frequency -> different wavelength) that don't match the antenna size, you don't get this constructive overlay: for example, for a wave with a wavelength much smaller than the antenna, the induced current mostly flows within these metal rods, and not between.

So, antennas are naturally selective for frequencies, simply because of their geometry.

However, slight errors don't "break" reception alltogether. It just makes the antenna work slightly less well.

And if we want particular frequency of current, we just use resonance circuit and take our signal?

Most antennas and receivers these days aren't really resonant circuits, but yeah, that's basically how it works:

The receiver filters out the frequencies that it cares about. Just like the EM waves in the air superimpose and are still dividable to their individual frequencies, this is still the case for electric signals in a wire or in a chip.

  • \$\begingroup\$ that means any antenna active all the time, all the time there will be a current, current of any frequency, you are using or not ? \$\endgroup\$ – Prerna Jun 7 at 11:28
  • \$\begingroup\$ yeah! Within the antenna conductor, a changing EM field induces a current. My first sentence. \$\endgroup\$ – Marcus Müller Jun 7 at 11:31

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