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Electric field in the vicinity of an antenna

  1. Mar 30, 2016 #1
    I don't know why I was persuaded that in the free space, the electric field of an EM wave is always orthogonal to the direction of propagation. I've recently read my old textbook, and found that this is true only when the wave is far from the emitting source. But if I've understood right the textbook, this need not to be true in the vicinity of the source (and actually is not true in general). I'm asking myself if I'm not doing a big mistake. Can anyone confirm this (or the contrary)? thx.
     
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  3. Mar 30, 2016 #2

    tech99

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    Near to most transmitting antennas there is a lot of stored energy in addition to the small amount that is radiated. The energy is stored by the inductance and capacitance of the antenna in the form of magnetic and electric induction fields. The total energy stored can be several times the radiated energy, and its field strengths can be very large when close to parts of the antenna. On the other hand, the radiated fields do not increase when the antenna is approached closely, when the inverse square law for intensity no longer applies. The shapes of the induction fields obey the ordinary rules for the fields of wires, magnets and charges, and these are different to the rules for EM radiation. So it is true that close to an antenna, the fields may not be at right angles to each other and to the direction of propagation.
     
  4. Mar 30, 2016 #3
    Thank you for your reply. Can I ask you one more question?: you seem to make a distinction between the radiated energy and the electric and induction EM field that I would like to understand. What do you call the radiated field and the electric and induction EM field ?
     
  5. Mar 30, 2016 #4

    Drakkith

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  6. Mar 31, 2016 #5
    Yes, fine. Thank you so many Drakkith.
     
  7. Mar 31, 2016 #6

    tech99

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    Briefly, the radiated field is the standard way we think of a ray of light, with electric and magnetic fields at right angles to each other and to the direction of propagation, and in-phase with each other.
    The induction fields are local effects caused by the inductance and capacitance of the antenna.
     
  8. Mar 31, 2016 #7

    sophiecentaur

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    The field patterns near the antenna contain so called 'evanescent modes' and no power flows via these modes. The Phase between Magnetic and Electric Field variations is nearly 90°, which means that the Dot Product is very small. The in-phase components are what account for the actually radiation and, as 'seen' by the transmitter, represent a Resistance (the Radiation Resistance), along with Reactance that is present, except when the antenna is resonant / tuned. In the far field, it's only the In Phase components that are present and the fields are then orthogonal to the direction of propagation.
    The reactive Fields around a physically small antenna (a thin wire) are a 'mechanism', in themselves that causes the antenna to have a larger effective cross section than just the wire it's build with. A receiving antenna is effectively a much bigger Energy Gathering area than just the wire because of these locally induced induced fields. Hard to get ones head around that one, I think. P21 of this link discusses that idea.
     
  9. Mar 31, 2016 #8

    tech99

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    Not sure the magnetic field is evanescent close to the antenna. I have noticed that the electric field remains roughly constant closer than 0.15 lambda in the meridian plane, and appears to be entirely that of the radiation field. On the other hand, B increases with 1/D right up to the conductor. So my supposition is that the impedance of the medium falls as the conductor is approached, requiring an increasing B, so that the power flow through every surrounding shell is equal. B must increase as the conductor is approached so that so that the dot product is constant. B must be in phase with E and so not evanescent. The E-field near the ends of the dipole do appear to be evanescent.
     
  10. Apr 1, 2016 #9

    tech99

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    Correction. Not meridian plane , the equatorial plane.
     
  11. Apr 1, 2016 #10
    It might be of interest that the far field is radiated as an effect of special relativity. As charges (electrons) accelerate in the antenna they change their reference frame and this change creates photons called the far field.
     
  12. Apr 1, 2016 #11

    tech99

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    Thank you. I am disquieted by descriptions, such as the one in the link given previously, which are not based around the acceleration of electrons.
    I am not sure if the high values of B near the antenna arise from the fluctuating radiated E-field, as per Maxwell, or from the velocity of the electrons after acceleration, or if these are the same thing.
     
  13. Apr 1, 2016 #12

    sophiecentaur

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    I was probably using the wrong term there. Does 'evanescent' imply exponential drop-off?
     
  14. Apr 2, 2016 #13
    A far field does not necessarily require E and H fields to be perpendicular to the direction of propagation. Example: TE or TM mode propagation in a waveguide.
     
  15. Apr 2, 2016 #14
    It is my understanding that waveguides work through nearfield effects. The field directly impacts/is impacted by the conductor/guide.

    A better example might be photon/field interactions with meta materials. But my understanding of such materials is weak. They might suffer the same issues.
     
  16. Apr 2, 2016 #15
    Never thought about it that way, you're right, there are lots of wiggling electrons in that waveguide wall.
     
  17. Apr 2, 2016 #16

    tech99

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    It is interesting that it seems impossible to influence an EM wave in any way (reflection, refraction, diffraction) without using a charge.
     
  18. Apr 2, 2016 #17

    sophiecentaur

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    Or a mass?
     
  19. Apr 3, 2016 #18
    That would be an interesting, but difficult experiment. I know it's hard to affect neutrons with electric charge. But would a group of neutrons affect an EM wave? They do have a magnetic spin and thus a magnetic dipole.

    Would an accelerating magnetic dipole count? (Would an accelerating electric dipole?) If so it would seem to be a 4th order effect and thus a nearfield effect.
     
  20. Apr 4, 2016 #19
    It just occurred to me you were talking about gravitational lensing. You are of course correct.

    (Depending on your model that might be considered a warp in space instead of course.)

    I still think the neutron experiment might be interesting though.
     
  21. Apr 4, 2016 #20

    sophiecentaur

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    I thought of very long wavelength EM being intercepted by a massive binary system. You could expect a sort of inverse Young's slits result. (Pretty damn big screen. :wink:
     
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