Re: Antenna reception theory

Richard,

Terman said no such thing, and your interpretation is clearly in error.

Magnetic fields cannot impart ANY energy to charges, such as electrons in a wire. This is because the force from a magnetic field on a charge is always perpendicular to the motion of the charge. No work can be done by the magnetic field, and the energy of the electrons does not change. Only electric fields can provide energy to an electron.

Fortunately, Faraday's Law saves the day. Changing magnetic flux is inextricably intertwined with electromotive force. Terman's comment on page 2 of the 1955 edition simply points out the operation of Faraday's Law. (Yes, I have this volume of Terman.)

Your conclusion statement is completely reversed. The magnetic field does nothing to induce current in the antenna, while the electric field does everything.

Again, however, the laws of physics save the day. Maxwell's equations link electric and magnetic fields in such a manner that the magnetic field you favor creates just enough electric field to drive the electrons in the wire.

As has been stated many times in this newsgroup, it is not possible to filter out one field component or the other. As long as there is some time dependence, i.e., other than purely static fields, both the electric and magnetic fields coexist.

73,
Gene
W4SZ

Richard Harrison wrote: 
Roy Lewallen wrote:
"You can find the explanation for why this is in any electromagnetic
text."

I found it in Terman.

As we all know, we place correctly polarized dipoles, for example,
parallel to the wavefront for maximum response. Terman confirms the
electric field in this instance induces no energy in the antenna. It all
comes from the magnetic field.

If antenna current flows, no matter where it comes from, loss resistance
causes a voltge drop. That`s why the wire needs to be perfect. The
electric field produces no voltage in the antenna because the wavefront
has the same voltage across its entire surface. That`s because it all
left the same point at the same time. So, a wire parallel to the front
has no difference of potential induced by the wavefront`s electric
field. It all must come from the mgnetic field.

On page 2 of his 1955 edition, Terman says:
"The strength of the wave measured in terms of microvolts per meter of
stress in space is also exactly the same voltage that the MAGNETIC FLUX
(my emphasis) of the wave induces in a conductor 1 m long when sweeping
across this conductor with the velocity of light."

From the above, it is seen that the electric field is not effective in
inducing current in a receiving antenna parallel to a wavefront. All the
energy intercepted by the antenna is induced by the magnetic field.

Best regards, Richard Harrison, KB5WZI

.

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