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Homework Help: Orientation of Hertzian dipole and Plane wave approximation - when is it valid?

  1. Aug 13, 2012 #1
    1. The problem statement, all variables and given/known data
    I have a long EM question in which there is a Hertzian dipole at a point (0,0,-100), (unknown orientation) and I am told the equation of the physical magnetic field detected 100m away at the origin of Cartesian coordinates. $$(B_0 \sin (2 \pi f t)\mathbf{e}_x$$, and $$B_0 = 0.1 \mu T$$, $$f=30 MHz$$.
    I have to deduce the possible orientations of the Hertzian dipole and explain qualitatively why the plane wave approximation is valid at +/- 1m in any direction from the origin.
    2. Relevant equations

    Maxwell's equations.

    3. The attempt at a solution
    On the possible directions of the Hertzian dipole: using the Biot-Savart law, I think I can show that the vector product of the current element and the position vector have to produce positive e_x, which is the detected field. So I'm thinking that the dipole has to be pointing in the -e_y direction, because:
    $$\mathbf{e}_{?} \times -\mathbf{e}_z = \mathbf{e}_x$$ gives me the -y direction, although two problems: I don't know how to write that argument mathematically (can't divide by a vector) and it only gives me one possible direction, where the question suggests there are more. The question also has a lot of marks attributed to it, and I'm wondering whether I need to talk about possible angular dependence of the dipole.
    On the approximation part: I'm guessing that the plane wave approximation is only valid under certain conditions (maybe when the distance is much larger than the wavelength), but I'm unsure as to exactly how this comes in and whether or not the frequency plays a part.

    I would gratefully appreciate any pointers.
    Best wishes

    P.S. A thought just occurred to me that the Hertzian dipole must have a -y component to explain the e_x component of the magnetic field, but any z component could also exist and would simply disappear under the vector product operation...that gives me a new range of direction possibilities...
    Last edited: Aug 13, 2012
  2. jcsd
  3. Aug 13, 2013 #2
    Are you working in Cartesian coordinates or polar coordinates?

    When you have a dipole doesn't the magnetic field consist of concentric circles centered on the axis of the dipole. How then can a dipole produce a magnetic field that is only dependent on one axis? Is it a trick question? Or could the wave be polarized in some way?


  4. Aug 16, 2013 #3
    I'm not an expert - just passing some thoughts here.

    I guess, we need to talk about the magnetic field in spherical polar coordinates rather than Cartesian? I don't think this is a trick question but would you measure anything in the z-axis? My thought is that a EM wave is propagates perpendicularly so perhaps not

    I'd be interested though in getting some more thoughts on this as I've never truly understood the concept of Hertzian dipoles.

  5. Aug 19, 2013 #4
    I've had a think about this again over the weekend so here are my thoughts which I hope help:

    What does the Biot-Savart law tell us about the direction of the dipole in relation to the magnetic field?
    Given that result, and spherical coordinates, it should be possible to deduce possible dipole field patterns that should show what happens to the strength of the signal depending on where it is located.

    However, having said that; I still can't give any clue as to how it's well-approximated by the plane wave in the given region, as I'm not too sure about this one. Maybe someone else can help here

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