The resourcefulness of respondents concerning the simple radio wave demo is
interesting. Electromagnetic (EM) waves are a crucial part of physics, and
demonstrating them in a class is important.
To truly demonstrate EM waves, we must distinguish them from near-field
induction. A source of EM waves has a near-field and a far-field region,
demarcated by the condition kr=1, where k is the wave number and r is the
distance from the source. The result is that for distances less than
lambda/2Pi, the fields have a complicated local dependence, and beyond that
the fields have the typical 1/r far-field behavior which we associate with
a radio wave.
We don't normally think of the near field as representing an EM wave, since
there is a lot of local reactive energy which is not propagated. For a
short Hertzian dipole, there are 1/r^2 and 1/r^3 terms in the fields E and
H which are far larger than the truly radiative transverse 1/r term. The
1/r^3 term is called electrostatic, and the 1/r^2 term inductive, for
obvious reasons.
In some of the demonstrations cited by respondents, the value of kr is
considerably less than 1, so that the near-field terms dominate. It is
likely that some of the experiments of the very early radio pioneers
actually demonstrated inductive, not radiative effects, since the
frequencies and separations did not satisfy the kr>1 criterion. For
example, a spark transmitter which oscillates at 10MHz has a wavelength of
30m, so that the radiative field only dominates for distances beyond about
5m. It would be difficult to obtain a small detector spark at such a
distance. For separations of say 0.5 meter, one is really observing the
inductive effect, essentially that of a rather inefficient transformer.
At 100MHz, the required distance becomes 0.5 meter, and real EM radiation
becomes more practical. But it is clear that some of the AM radio demos do
not satisfy the truly radiant EM wave condition.
John V
===================================
Dr. John Vanderkooy Audio Research Group http://audiolab.uwaterloo.ca/graphic.html
Tel: 519-885-1211 x 2223 Fax: 519-746-8115
Department of Physics, University of Waterloo
Waterloo ON Canada N2L 3G1