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How to find C and L in maximum current?

  1. Dec 8, 2013 #1
    In a RLC circuit, radio waves with different frequency, f, hit the antenna and exert oscillating electric forces on the electrons on the antenna . Thus they act via antenna as individual batteries, each oscillating at their particular frequency f. Suppose R=1000 ohm. What should L and C be so we get the largest current in the circuits at frequency in 90 MHz.However, at frequency 90.3 current is only 1/10 as large.

    Now I know that at Imax, Xl=Xc, but I don't know how to use the other given frequency to find C and L.

    Any help please?
  2. jcsd
  3. Dec 9, 2013 #2


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    Welcome to PF.

    I assume the R is in parallel with the L and C.
    Xl = 2*Pi*f*L, Xc = -1/ (2*Pi*f*C), when Xc + Xl = 0 you have resonance.

    The width of the resonance peak is determined by the Q which is the ratio of reactance to resistance. See https://en.wikipedia.org/wiki/Q_factor

    So find the Q required for 10% at 90.3MHz. Then calculate the reactance needed.
    Then solve for the L and C at 90MHz that have that reactance.
  4. Dec 9, 2013 #3


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    Then, if you want to build a suitable resonating circuit, you find that, at 90MHz, nothing behaves quite as you'd expect, because of parasitic effects. There can be a lot of 'suck it and see', when you bolt it into a circuit.
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