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Dipole antenna question

  1. Dec 22, 2012 #1
    I'm learning about an antenna theory and have a few question.

    According to the attached image.

    Can anybody explain what's going on in the image?

    I've done lots of calculation, but still have no idea what's going on.

    My question is

    1. How dipole produce a loop of electric field? (How a loop of electric field are made?)

    2. Why does the bigger loop of electric field produce high electric field

    and lower as the loop become smaller?

    3. Why does the smallest loop produce the highest magnetic field?

    (Isn't loop of higher electric field produce higher magnetic field?)

    Sorry for my bad English.

    Attached Files:

  2. jcsd
  3. Dec 23, 2012 #2


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    The images just look like EM waves being emitted from an antenna. The smaller loops closer to the antenna are just the part of the wave that is emitted later than the larger loops. The larger loops used to be smaller ones and have expanded during propagation.

    What did you base your calculations off of? The strength of the electric and magnetic fields should be smaller in the larger loops as far as I know.
  4. Dec 23, 2012 #3

    jim hardy

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    The image doesn't make sense to me.

    Electric field lines start and end on a charge, so i dont see what the loop represents.

    re: What's going on?
    Charge is cycling up and down in the antenna.
    The cyclic voltage gradient along the wire produces a cyclic electric field ,repesented by electric field lines connecting charges in the upper and lower sections as in that fig a.
    The cyclic movement of charge in the wire produces a cyclic magnetic field that'd be represented by circles, centered on, and in planes perpendicular to, the antenna.

    What the loops represent i dont know - perhaps the artist envisioned the e-field wafting away like smoke rings.

    In the second linked drawing it looks as if the artist tried to show two features:

    1. E and B fields are perpendicular. You already know that, e-field lines are in plane of antenna and B are perpendicular and concentric to it.
    2. Both are cyclic, as represented by their sinusoidal shape. (Remember - at speed of propagation, a cycle of time and a wavelength of distance are equivalent. Hence his ambiguous labelling of ordinate )

    hope this helps.

    ARRL Antenna Handbook is a good , practical reference for "What's going on".
    Last edited: Dec 23, 2012
  5. Dec 23, 2012 #4


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    Don't they represent the E field of the EM wave? They look like they are only looping because the antenna doesn't send out a perfectly spherical wavefront.
  6. Dec 23, 2012 #5
    It looks like the artist is drawing the field lines as they radiate outwards at 3 different phases of the sinusoidal potential difference being applied to the antenna. Heres an animation of the same thing from wikipedia. The color indicates the strength and direction of the magnetic component of the radiation, which is cylindrically symmetric about the axis of the dipole (up and down), with field lines running into or out of the image. The electric field lines lie in the plane of the image and should be proportional to the strength of the magnetic field. I dont think the axes of the plot mean anything..

    This shows that the transmitted signal is strongest in the disk perpendicular to the axis of the dipole
  7. Dec 24, 2012 #6


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    Those lines are Electric field lines - see that they join the two halves of the dipole , close in. The problem is that they can't show enough about what's going on in one simple diagram. When the polarity of the PD on the dipole changes, it takes time for the effect to propagate to a distant point and the lines can't just disappear but form the continuous loops on the diagram -and it all, of course, radiates outwards as the effect gradually reaches distant locations. Along the axis of the dipole wires there is no field (no lines on the diagram as the effectws of + and - on the wires cancel out at a distance) and at right angles, the field lines are closest together - giving a stronger E field the difference between effects of + and - on the wires is maximum. The 'holes' in the sausage shaped loops are where the field is zero. between max one way and max the other direction.
    Notice the strange direction that the electric field lines point in directions other than along the main beam (horizontal on the diagram).
  8. Dec 24, 2012 #7

    jim hardy

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    Gotcha, sophie, drak and gregu,,,, i think.

    If the 'sausages' are a half wave thick, then the directions of field on opposite sides of a sausage-loop would be upward on one and downward on other...

    It is quite a trick to represent in two dimensions a 3d time varying field.
    So i'll grant the artist some artistic license, and myself some slack for being befuddled at first.

    i'd guess he was trying to depict a cross section of this, from ARRL antenna handbook ?
    radiation pattern of a vertical simple dipole; E in vertical planes and B horizontal...

    Thanks, guys, and Merry Christmas !!

    old jim
    Last edited: Dec 24, 2012
  9. Dec 24, 2012 #8


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    I think the still pictures give you more time to think what's going on. You can then graduate to the flashy coloured moving, Pixar one.
  10. Dec 25, 2012 #9
    Charges are one source of electric field, changing currents are an other.
    E and B are perpendicular in far field but here we're near an antenna. here are additional constraints on the medium.
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