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Why E & B fields 90 degrees?

Discussion in 'Electronic Basics' started by Marvin Lukis, Jan 28, 2005.

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  1. Marvin Lukis

    Marvin Lukis Guest

    Can someone please explain why the E and B fields from the same
    emitter have a 90 degree phase difference?

    How can this be best visualized?

    Thanks

    Marv
     
  2. Bob Myers

    Bob Myers Guest

    This may not be easy, but we'll give it a go.

    I'm assuming that you mean with respect to an electromagnetic (or "EM")
    wave, and that by "phase difference" you mean the polarization
    of the electric and magnetic field components; if this isn't
    the case, then I'm not sure what you're talking about, because
    the the *phase* of these components (in terms of their
    amplitude vs. time) is NOT 90 deg. apart (instead, they
    are perfectly in phase).

    Now, to figure out all of this through a rigorous
    mathematical treatment takes some doing, and is typically
    the subject of the better part of an introductory course in
    electromagnetics. So we're not going to do that here. But
    to visualize the situation, consider the plain ol' dipole antenna
    - overall, a half-wavelength long, with the feedpoint in the
    middle (which splits the antenna into two quarter-wave
    elements which lie along the same line).

    Such an antenna can be viewed as an open-ended
    quarter-wave section of transmission line, "opened up"
    (i.e., the two conductors of the line separated, forming the
    two quarter-wave elements). At the feedpoint (one-quarter
    wave back from the "open" end - or an "infinite impedance
    load" - in the original transmission line section), we have
    a very low-impedance point; this is, on that same original
    line, a point of current maximum and voltage minimum.
    The open end of the line - which, when "opened up" into the
    dipole antenna - is a voltage maximum and current minimum
    (it better be, since the load impedance there is really high,
    remember!).

    Now visualize what this means in terms of the "standing wave"
    pattern on the antenna. You have a current maxmimum point
    right in the middle (at the feedpoint), and the current is moving
    along the elements, right? You also have a voltage maximum
    at the end points - the tips of the elements - and since the
    two ends are opposing one another, what this means is a large
    potential (voltage) ACROSS the antenna, end-to-end. Got it?

    So think about the fields that result from this. The electric field
    generated by the big potential across the antenna is aligned
    with the elements of the antenna - i.e., the "field lines" would
    be visualized as connecting the tips of the elements. On the
    other hand, you know that magnetic fields which result from
    current in a conductor are visualized SURROUNDING
    that conductor - so view the magnetic field as encircling the
    antenna elements.

    And voila, there you are - crossed electric and magnetic
    fields! Which, in this case, are being produced in such a
    way that the result is an EM wave being launched from the
    antenna, which was the whole idea in the first place.

    Bob M.
     
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