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Most authors explain how a wave is generated, then resort to reciprocity
to explain the reception process. But a clear and simple direct explanation appears in Bailey, _TV and Other Receiving Antennas_ (pp. 141-2), of what happens when an electromagnetic wave strikes a conductor: "The second, and equally important effect [the first being reflection of much of the incident energy] is that some energy /does/ enter the outer skin of the conductor. That part of the energy, which is not reflected, must enter the conductor. The conditions at the surface of the conductor, as we have already seen, give rise to a small resultant electric vector and a large resultant magnetic vector. The presence of these at the conductor is direct evidence that power is entering the conductor. The small electric vector acts on the internal electrons of the conductor and impresses a direction force, tending to drive the electrons along the skin of the conductor in the direction of the electric vector. But from experience we know that /no/ electrons can ever be caused to move without gradually establishing their own magnetic field, and this usually takes /time/. The motion of electrons (which is electric current by definition) never takes place without the magnetic field. How, then, is the electric vector from the electromagnetic wave going to put these electrons in motion? It can only do so because the electromagnetic wave /also supplies a magnetic vector/ as well as an electric vector. And the value of this magnetic vector is exactly proportioned to supply just the right amount of magnetic field energy which the electrons require for immediate motion. Thus the electrons do not have to establish their own magnetic field, since this field is supplied by the electromagnetic wave. Hence, electromagnetic wave energy entering the conductor establishes immediate motion of electrons /along/ the conductor, the direction of motion at any instant corresponding to the direction of the electric vector. If the electric vector changes direction, the electrons will follow suit." Other posters have correctly pointed out that an antenna doesn't and can't receive a signal solely due to the E field; a time-changing E field can't exist without an accompanying time-changing H field. Roy Lewallen, W7EL Paul Taylor wrote: Hi, I am looking for an explanation of how an antenna receives a signal due to the E-field of an electromagnetic wave. I have looked in some books, and can understand transmission, but the books I have looked in don't explain reception. I have found an explanation of how the H-field induces a signal in a loop antenna: a changing magnetic flux will induce a current. But what about the E-field and a dipole antenna? I guess that the E-field causes electrons to move in the antenna wire, because in a solid conductor, electrons will move until the E-field inside the solid is cancelled out? I have googled but having difficulty finding a good explanation. Any pointers? Thanks & regards, Paul. |
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