Characteristic impedance

Discussion in 'General Electronics Chat' started by mghg13, Sep 17, 2013.

  1. mghg13

    Thread Starter Member

    Jul 17, 2013
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    what is Characteristic impedance? What is its physical meaning? How is it useful?

    I've got some explanation about it on this link: http://www.allaboutcircuits.com/vol_2/chpt_14/3.html

    but it does not explain the practical implications!! How is this data useful to the engineer or technician, etc...
     
  2. bountyhunter

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    Sep 7, 2009
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    For example: the characteristic impedance of free space is something like 376 Ohms so that explains why antennas were designed with 300 Ohm impedance to match it.
     
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  3. t_n_k

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    I believe that's incorrect. No essential relationship exists between antenna radiation resistance and free space resistance.
     
  4. Brownout

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    A network will have a characteristic impedance that is determined by the network's resistance, inductance and capacitance. The characteristic impedance does not change. It can be determined by measurement or analysis.
     
  5. crutschow

    Expert

    Mar 14, 2008
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    A practical example is a transmission line such as a coax cable. It has a characteristic impedance as determined by the distributed inductance and capacitance of the cable. It order to transmit a high speed signal along that cable and have all the energy absorbed in the load, the load must have a resistance equal to the cable characteristic impedance. If the load resistance is higher or lower than the characteristic impedance then some of the energy will not be absorbed in the load and will be reflected back down the cable towards the signal source. This causes standing waves if the signal is a sine-wave, or pulse distortion if the signal is digital.
     
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  6. bountyhunter

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    I was taught that receiving antennas have 300 ohm imp to get good power transfer from the signal in free space to the antenna. Then you go through a balun transformer to change the impedance to 75 Ohms (unbalanced) to match the coax.
     
    Last edited: Sep 18, 2013
  7. t_n_k

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    You are probably referring to a half wave folded dipole antenna which has a nominal (resonant) impedance of 300Ω. An attractive feature of the folded dipole is the convenience of matching it to a 300Ω balanced transmission line.

    If it were true that one gets optimum performance when the antenna impedance matches the free space impedance then every antenna would be logically designed with that optimum parameter as the design goal. That is not the case. The ubiquitous half wave open dipole is a case in point, with its radiation resistance being about 73Ω.

    In practice antennas present a complex driving point impedance which varies depending upon the useful operating bandwidth.
     
  8. bountyhunter

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    I am thinking of antennas in general. My recollection is like this:

    That is what I was taught in school. The transfer of energy from free space to antenna also has to be matched closely to get best matching and minimum reflected energy. That was the answer to the question: why don't they just make TV antennas 75 Ohm so we don't have to keep using the stupid balun transformers?

    Another input:

    http://www.setileague.org/askdr/matching.htm
     
    Last edited: Sep 18, 2013
  9. t_n_k

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    Consider - In the case of RF transmission in the region near the antenna the antenna will not "see" the free space impedance. There is a transition from near field to far field over a significant portion of a wavelength of the radiated signal. The free space impedance is only representative in the far field region.

    You didn't comment on why the half wave open dipole works perfectly well with a radiation resistance very much less than the free space impedance.
     
    Last edited: Sep 19, 2013
  10. mghg13

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    Jul 17, 2013
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    well guys, I thank you all for your explanation. It is now clearer in my mind.

    Recently I have done a lab experiment in order to determine the variation of characteristic impedance of a transmission line with frequency of the applied signal.
    The transmission line is modelled by a network of resistors, capacitors and inductors. The characteristic impedance was determined from short-circuit and open-circuit measurements.

    An inverse relationship was obtained, i.e. as the frequency of the signal is increased (from 100Hz to 8KHz), the characteristic impedance decreases non-linearly but at frequencies greater than 8 KHz, it increases. See the attached picture.

    Can any one explain this relationship?
    I have got an idea about it but I need some more clarifications!!!
     
  11. bountyhunter

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    Nobody ever said antennas must match free space to work. I just said that I was taught you get better energy transfer if the impedances match or come close to matching.
     
  12. Brownout

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    Might be clearer if you plotted it on a log scale. You netwok looks to be capacitive at low frequencies, resistive at mid freq's and inductive at high freq's.
     
  13. t_n_k

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    How does one then explain that a resonant half wave open dipole radiates just as efficiently as its half wave folded dipole cousin?
     
  14. mghg13

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    Jul 17, 2013
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    How can you deduce this by just looking at the graph??? I mean only the magnitudes of the characteristic impedance are obtained, no phase angle is available!!!
    What equations have you considered?
     
  15. Brownout

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    Why not? It should be included.
     
  16. mghg13

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    Jul 17, 2013
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    No only the magnitude is obtained. In the expt, the p.d V across the transmission line model is measured and the current I at the line input is also obtained. Thus the short-circuit (or open circuit) impedance is obtained as follows:

    Zoc = V/I

    similarly, Zsc = V/I

    then Characteristic impedance, Zc = sqrt(Zoc*Zsc)

    How can you get phase angle with this???

    BTW, it is mentioned in the experiment that only the magnitude is being measured?
     
  17. Brownout

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    Well ask yourself, what impedance goes up with frequency? What impedance goes down with frequency? What impedacne goes neither up nor down?
     
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  18. mghg13

    Thread Starter Member

    Jul 17, 2013
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    Ohhhh yeah!!!!

    Thanks for helping me in understanding!!!

    But there is still one thing...
    According to theory, the characteristic impedance should remain constant for high frequencies since it obeys this equation:

    Zc = sqrt(L/C)

    whereby L = inductance and C = capacitance and

    L and C are independent of frequency

    So why does Zc increase at high frequencies?? It should have been constant, right?

    I was thinking that L might be increasing with frequency but theoretically this is not the case!!!! So what possible explanation can there be for this behaviour????

    I don't think this is experimental error...



    P.S. I was referring to this site:
    http://home.mira.net/~marcop/ciocahalf.htm
     
    Last edited: Sep 19, 2013
  19. crutschow

    Expert

    Mar 14, 2008
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    The characteristic impedance of an ideal transmission line (zero resistance and perfect inductance and capacitance) appears as a constant resistance with frequency. Any change in Zc with frequency is due to the non-ideal characteristics of the line and depends upon how the particular line is constructed.
     
  20. MrChips

    Moderator

    Oct 2, 2009
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    This should be a sticky. It comes up time and again.
    Someone should go over this thread and summarize the questions and answers, re

    characteristic impedance
    transmission lines
    coax cables
    mismatch
    reflections
    freespace
    centre fed, dipoles
    end fed antennas
     
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