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Generating Circular Polarization II

  1. Jul 17, 2009 #1
    Statement:
    Consider two dipole antennas, oriented 90degrees apart [imagine the x-y plane, let "a" be the dipole oriented along the x-axis, and the "b" be the dipole oriented along the y-axis]. If "a" dipole radiates [tex]cos(\omega t)[/tex] and "b" dipole radiates [tex]sin(\omega t)[/tex], the field radiated by the two antennas will be circularly polarized:

    [tex]\vec{E}(z, t) = E_{0}[cos(\omega t - \beta z)\hat{x} + sin(\omega t - \beta z)\hat{y}][/tex] (#1)

    Relevant Question:
    Ok I almost get it now. Now in terms of a specific distance, say in the [tex]\hat{x}[/tex] direction, the cosine function has traveled a distance [tex]\omega t[/tex] (as did the sine function in it's respective axis). But I don't understand why To find electric field at a given location in the [tex]\hat{z}[/tex] direction, we subtract the distance traveled [tex]\omega t[/tex] by the wave number times distance in z, or [tex]\beta z[/tex] - for each component [tex]\hat{x}, \hat{y}[/tex]. The wave number is the wavelength of the sinusoid per unit distance. What happens when we take this wave number [tex]\beta[/tex] and multiply it by [tex]z[/tex]? What does that represent, I cannot see the relation between the two ([tex]\omega t[/tex] and [tex]\beta z[/tex])?

    Does one unit length of [tex]z = 1[/tex] for [tex]\beta z \hat{x}[/tex] and [tex]\beta z \hat{y}[/tex] correspond to a length of [tex]\frac{2\pi}{\lambda}[/tex] in the [tex]\hat{z}[/tex] direction?
     
    Last edited: Jul 17, 2009
  2. jcsd
  3. Jul 22, 2009 #2

    Redbelly98

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    I think we should talk about the simpler case of linear polarization, since your question is with the concept of traveling waves rather than about circular polarization.

    So ... let's represent an electromagnetic wave by

    Ex(z,t) = E0 cos(ωt - βz)

    and the electric field happens to be polarized in the x-direction.

    Now consider points in space such that

    ωt - βz = constant ≡ φ ​

    where we are calling this constant "φ".

    We can say 2 things about what this represents:
    • it represents points moving along the z-direction at constant velocity, which we can see by solving the above equation for z:
      z = (ω/β)t - (φ/β)​
    • it represents a fixed electric field strength of E0 cosφ

    Combining those two facts, it means that the entire electric field pattern moves along the z-direction at a speed (ω/β). And that's just what a traveling wave is.
     
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