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Brian Reay[_5_] · November 9th 14, 08:40 PM posted to rec.radio.amateur.antenna

On 09/11/2014 19:37, Ian Jackson wrote:
In message ,
writes
Jeff wrote:

You are not 'being dense', you are perfectly correct.

Waving a magnet will not generate an EM wave, it won't even induce a
current unless there is a conductor to hand.

Likewise, waving a battery around, won't generate an EM wave either.

Maxwell's equations come as a 'set' to generate an EM wave, you can't
start with just one. That was one of the flaws in the Cross Field
Antenna theory-or the original one, it varied as it was challenged. It
had other flaws, eg the idea that the Poynting vector was some 'extra'
physical phenomenon which could be 'synthesised', rather than just a
mathematical vector representation of the power in the E and M fields.

As I pointed out in a previous post, the differential term is zero in
the absence of one of the fields so the equations have no,
non-trivial,
solutions.

As I recall, this is one of the standard things you are taught when
you
attend a lecture on Maxwell's Equations. Perhaps someone missed a
lecture (or more),has lost some crucial pages from his notes,or
hasn't
got a clue.

Like all equations, if you apply them correctly, Maxwell's
equations do
work. However, if you can't understand them, you will mislead
yourself.

Thank you, Brian, I was beginning to doubt myself there. Not having
had the benefit of a University education like Gareth I tend to take
an empirical approach to this sort of thing.

Unfortunately this time Brian is not correct and a rotating magnet will
indeed produce an EM wave. The point that you have missed Brian is that
the differential term is zero for a static magnetic field, but when the
magnetic field is rotating analysis of the equations reveal that there
must be an E field produced in order to satisfy the full set of
equations.

Have a look at the link in my previous post.

Jeff

Even if true, the presence of an E field and an H field alone is NOT
the same thing as an electromagnetic field.

If a permanent magnet was oscillated inside a loop of wire, it would
produce an oscillating current in the loop. An oscillating current would
produce RF radiation (especially if it was tuned with a capacitor to the
frequency of the magnet's oscillation).

That is not the situation under discussion.

No one disputes that, if you CHANGED the situation to the one you
describe, then it would produce RF (ie EM waves).

BUT, there is no wire etc.

The question then is, "is the wire loop necessary to produce RF radiation?"

Or something similar, yes.

Essentially, the rotating field can cause a current to flow in a
conductor, that current will produce another magnetic field. The current
is charge movement, therefore you have an electric field. That
and the 'new' magnetic field can produce an EM wave. No different to any
other coil 'excited' with an AC current- perhaps the method of exciting
the coil is a little unconventional ;-)