Which one of these depict an EM wave accurately? For far-field EM radiation Electric and Magnetic Fields form loops as their divergence is 0. So how can the sine-wave representation depict an EM wave?
The first image shows a snapshot of the far field of a linearly polarized wave.
The second picture (movie) is wrong because in the immediate vicinity of the transmitting antenna, the far field is shown. Indeed, in the immediate vicinity of the antenna (to about 3 * lambda), there is only the near field with a completely different structure than the far field. In certain periods, the antenna emits energy into the near field, then it re-absorbes energy. The Poynting vectors then show back to the antenna. Outside the antenna, there are no charges and the divergence is always zero.
Only in the far field (distance more than about 10 * lambda), the Poynting vectors always point away from the antenna and there is no returning energy. In the near field, there are also radial field components that are missing in the film.
"The first image shows a snapshot of the far field of a linearly polarized wave."
But, in the far-field, aren't the Electric and Magnetic Fields supposed to make loops?
The first picture shows the fields for an infinite plane wave. In this case the fields do not form loops because they continue to infinity in straight lines (they meet at infinity and form an infinite loop?). An infinite plane wave has never existed, as it never starts in time, never stops in time and is infinitely wide, and contains infinite energy. The fields a long way away from a point radiator are not very different from those of an infinite plane wave, though, and locally are very nearly plane, but do curve to form very large loops, of radius equal to the distance from the radiator. For an electric dipole the magnetic field is constant around the entire circumference, whereas the electric field forms loops that close off before reaching the vertical axis, as shown on the video
The radiating field is not zero close to the antenna, but is much smaller than the reactive field. In the field equations the 1/r terms (radiation) are still there, but the 1/r squared and 1/r cubed terms (reactive field) grow proportionately much larger close to the antenna. The total energy in the reactive terms decreases with increasing radius, so swirls round the antenna every half-cycle alternating between electric and magnetic energy (there are no losses in this simple description), whereas the total energy in the radiation term is constant with radius so can keep going.
As Herbert says, the reactive energy swirls about the dipole, and doesn't appear to be shown in the video. It is possible that the video is only plotting the 1/r components, and not the near-field (reactive) parts.
WOW!
https://www.youtube.com/watch?v=7bDyA5t1ldU.
This video explains it beautifully!
Now I had certain epiphanies (oo my stupid brain) !
This video says that V and I are in quadrature phase but E and H are in time phase. Is this the reason that power radiated in the far field can be harnessed as P = (V/m)*(I/m) cosQ = VI = Active power. While, the power in the near field is stored as V and I are in quadrature phase; VI sin Q = VI = Reactive power ?
Regards,
Suhas. D.
- Badges
- Science topic
- Similar topics
- Physical Sciences
- Materials Science
- Materials Chemistry
- Polymer Chemistry