Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Coupling between tuned circuits

  1. Nov 24, 2005 #1

    I have two resonant circuits (tuned to the same frequency) that are magnetically coupled to each other via two coils placed paralell to each other. (energy and data is transferred from one circuit to another)

    Now, I know that the coupling between the two can be overcoupled, undercoupled or critically coupled, and that maximum powetransfer occurrs as the circuits are critically coupled.

    If I have understood correctly, the coupling type is determined exclusively by geometrical factors, i.e. distance between the two coils and their orientation and size with respect to each other. For two paralell, fixed sized coils, there should therefore be an optimal distance between the coils that gives two critically coupled circuits.

    I am not sure how to calculte this distance. Do anyone have a tip or maybe a link to some good material on this subject.

  2. jcsd
  3. Nov 24, 2005 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    I believe one is referring to Mutual inductance, which occurs in addition to self-inductance. The mutual inductance will affect the frequencies of the two circuits.

    Here some basic information - http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/indmut.html


    http://www.pact.cpes.sussex.ac.uk/~edmundjc/EMG/Lecture17/emg17.pdf [Broken] (save target as and then view)

    Course notes in Finnish only - Problems [in English and Finnish] - http://www.ct.tkk.fi/courses/ca1/opetusmoniste.html
    Last edited by a moderator: May 2, 2017
  4. Nov 24, 2005 #3


    User Avatar
    Science Advisor
    Gold Member

    I suspect that overcoupling refers to the selectivity of a tuned circuit. Two inductors that are both part of a bandpass filter for instance can be too closely coupled resulting in a bandpass that is too wide.
  5. Nov 25, 2005 #4


    User Avatar
    Staff Emeritus
    Science Advisor


    This definition may be a little misleading, though, because if the coils are very tightly coupled, they become a broad-band transformer.

    The mathematical model for a pair of coupled coils is


    /\I2 ....|

    V1 = L11 I1 + L12 I2
    V2 = L21 I1 + L22 I2

    L12 = L21

    the coefficient of coupling is L12 = k*sqrt(L11*L22)

    The rest is analyzing the problems in terms of differential equations. The problem of calculating the mutal inductance from geometry is hard enough that it's generally not attempted - it's easier to adjust experimentally.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook