Resonance Problems

Discussion in 'Electronics Resources' started by nelsonys, Jun 6, 2013.

  1. nelsonys

    Thread Starter New Member

    Apr 6, 2011
    I am now studying about resonance on transmission line.

    Is there any recommendation on good study materials (books or any online materials), or even drop some explanation here about the mechanism of resonance and the relationship between reflection (standing wave) as well as lambda/2? Further topics about resonance such as radiation(antenna theory), wave mode, etc...

    What I knew about resonance is just about the LC elements that cause resonance. Would like to further my understanding by relating VSWR, wavelength, multiples of half wavelength into resonance as well as other advanced topics.

    Besides, I'm now trying to learn about antenna. For an open-ended patch antenna, waves propagate to the edge of the antenna reflect back and create standing wave. According to the allaboutcircuits material, open-end creates full reflection (Vmax) at the edge, but what I don't understand is why is it have to be of the length of lambda/2?

    Besides fundamental resonant frequency, there are higher order resonant frequencies related to n multiples of lambda/2. Does this mean that in a certain transmission line, there will be infinite numbers of resonant frequency (ignore losses)?

    We know that it's the resonance that creates strong radiation in the antenna, but radiation in the antenna is narrow bandwidth despite there are n multiples of lambda/2 exist in the transmission line. Why is it so?

    I'm sorry for my vague description, but I really hope anyone of you could provide me insights on this matter.

    Thanks for your help in advance.
  2. w2aew

    Active Member

    Jan 3, 2012
    A couple of thoughts that might help your understanding. If a transmission line is exactly a half wavelength long (lambda/2), then it can also be thought if as 180 degrees in length (delay). Therefore, any reflection that bounces off the end travels an additional half wavelength, and experiences an additional 180 degrees of phase shift. Thus, for a half wavelength line, reflection are in-phase with the source at the input of the line. The implication here is that a half wavelength line with therefore present a load impedance that is exactly equal to the impedance at the far end of the line, regardless of the line impedance.

    A quarter wavelength line is more interesting. Since the round trip delay is 180 degrees, the impedance at the input of the line will be the conjugate of the load impedance at the far end. A quarter wavelength line that is open at the far end will look like a short circuit, and vice versa.

    As for understanding line impedance and reflections, you might find this video helpful