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Question on quarter wave circuit?

  1. Dec 31, 2015 #1
    Hi all, first post here.

    I am studying 1/4 wave circuits and I've been wondering.

    Can you have a LC circuit in which the reactances of the L and C produce resonance, while at the same time having the circuit be a 1/4 wavelength?

    If so what would this circuits characteristics be?
  2. jcsd
  3. Dec 31, 2015 #2


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    Staff: Mentor

    Welcome to the PF.

    Can you post the Relevant Equations? Please show what you have been studying, and show the part that you are wondering about... :smile:
  4. Jan 1, 2016 #3

    You have a series LC circuit. The L value is 500uH while the C value is 10pF. So the resonant frequency is 2.25MHz. Now, lets say the electrical length of the inductor is exactly 1/4 wavelength at 2.25MHz. How would this circuit behave when driven at it's resonant frequency (2.25MHz) by an AC source?
  5. Jan 1, 2016 #4


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    Gold Member

    I presume that the inductor is actually a piece of shorted transmission line having a length of 1/4 wavelength at 2.25 MHz. In such case it will behave as an open circuit and cannot look like 500uH.
    If, however, the length of wire in the inductor is about 1/4 wavelength and the inductor looks like a shorted line 1/8 wavelength long, then it will have an inductive reactance numerically equal to its characteristic impedance. In this case the circuit appears the same as the LC series series circuit over a fairly wide range of frequencies centred on the resonant frequency.
    500uH has a reactance of 707 ohms at 2.25 MHz, so the line would require a Z0 of 707 ohms.
    If the circuit is driven by a low resistance generator in series with it, high voltages will appear across L and C.
    Last edited by a moderator: Jan 1, 2016
  6. Jan 1, 2016 #5

    Thanks for the explanation, It is appreciated.

    What about the current? How would you determine current flow in this circuit?
  7. Jan 1, 2016 #6


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    A series resonant circuit has a resistance near zero, so the current will be mainly decided by the generator.
    Last edited by a moderator: Jan 1, 2016
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