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I came across the following paragraph while reading about the tuned circuit in a radio.

enter image description here

Only the resonant frequency oscillates efficiently in the LC tank circuit, so only it will appear at the output terminals-even though many waves set up currents in the antenna-ground circuit.

There are many radio signals that hit the antenna at the same time, Do all the signals set up currents in the antenna ground circuit at same time? Or Does it happen one at a time?

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    \$\begingroup\$ I have split up your single block of text. The now-second paragraph looks like a paste of a homework question with "So, " put in front of it. Is this just a homework question? If so, you need to edit your question and show all your own work and own findings so far. As it stands, it's just two pastes of existing text that expects a lot of work in return. \$\endgroup\$
    – TonyM
    Feb 9, 2023 at 11:32
  • \$\begingroup\$ It isn't a homework question . it is a thing that I am clearly confused about. \$\endgroup\$
    – Jack
    Feb 9, 2023 at 11:40
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    \$\begingroup\$ The quote in your question is problematic at an engineering level. It seems to be more targeted at the casual reader i.e. it's very simplistic and, its simplicity renders it wrong from an engineering standpoint. This is an engineering site so, your question is flawed in that it asserts something that is untrue. \$\endgroup\$
    – Andy aka
    Feb 9, 2023 at 12:03
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    \$\begingroup\$ My only advice is to remove the quote and simply ask how a tuned circuit works to promote one part of the spectrum whilst attenuating other parts of the spectrum. However, it's still quite broad and it's unclear what level any answer should start at (beginner, intermediary etc.). \$\endgroup\$
    – Andy aka
    Feb 9, 2023 at 12:30
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    \$\begingroup\$ When you ask about efficiency, you should think of power transfer from antenna to a resistive load on the tuned circuit. The diagram shown shows no antenna source resistance, and no tuned circuit load resistance. Adding these two components makes the circuit more complex, but helps show how antenna power is delivered to the resonator load resistance, and how that power maximizes at one frequency. \$\endgroup\$
    – glen_geek
    Feb 9, 2023 at 14:49

1 Answer 1

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What you're showing is a parallel tuned circuit. Qualitatively, it works like this.

As the frequency increases, the impedance (resistance) of an inductor increases, ignoring parasitic effects. Conversely, as the frequency increases, the impedance (resistance) of a capacitor decreases.

At the resonant frequency of the LC network, the impedance of both the inductor and the capacitor is the same. So this frequency produces the highest voltage across the LC network.

Added the clarification below

As was pointed out by periblepsis in the comments below, the impedance of a parallel, ideal LC network approaches infinity, because of the way the complex L & C reactances combine in a parallel circuit.

At frequencies below the resonant frequency, the currents are shunted to GND through the inductor. And at frequencies above the resonant frequency, current are shunted to GND through the capacitor.

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    \$\begingroup\$ This kind of misses the point about the ideal total impedance of a resonant tank approaching infinity at the resonant frequency. Could completely miss that detail from a reading. \$\endgroup\$ Feb 9, 2023 at 15:04
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    \$\begingroup\$ But it doesn't reach infinity. That would imply that both the inductive and capacitive reactances are infinite, which is not the case. \$\endgroup\$
    – SteveSh
    Feb 9, 2023 at 15:45
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    \$\begingroup\$ You're right, I stand corrected. I was forgetting about the sign difference in the ideal reactance between the inductor and the capacitor. \$\endgroup\$
    – SteveSh
    Feb 9, 2023 at 20:33
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    \$\begingroup\$ Hehe. All is again right with the world! The universe has been saved! ;) +1 (I did really like your easily read wording before.) \$\endgroup\$ Feb 9, 2023 at 21:15
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    \$\begingroup\$ The antenna picks up all signals within its design parameters. It captures signals more efficiently at the frequency for which it is designed, but also captures signals different from it's "tune" frequency. So all those captured signals are applied to the tuning circuit - the parallel LC in the diagram. But the maximum response - voltage - is at the frequency for which the LC network is tuned. \$\endgroup\$
    – SteveSh
    Feb 13, 2023 at 12:29

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