Re: Zenneck Waves


Bill Miller wrote:
Zenneck waves are, apparently, EM waves that exist at the boundary of two
dis-similar dielectric materials. Air and ground would be one such
interface. An unusual aspect of this mode is that -- apparently -- the
radiated wave decays as 1/SQRT(r) rather than 1/r.

I don't know what "Zenneck" waves are, but clearly traveling waves can
and do follow dielectric interfaces of various geometries. The slower
decay rate has to do with the fact that energy is spreading out over a
2-D surface rather than into 3-D space. Normal traveling waves are
assumed transverse and that is usually more or less true at
frequencies where traveling wave structures are popular. In Tesla's
day radio waves were of very low frequency (we'll ignore Herr Doktor
Hertz for the moment). It was well known even then that there is a
hugging of the earth as low frequency waves follow around the earths
curvature. This is why all long-distance radio communications were
very low frequency until it was discovered that shortwaves were
reflected by layers in the upper atmosphere.

The question of longitudinal vs transverse waves has been around since
Euler and others. The true test is can the waves be polarized? Light
can be polarized therefore, the waves must be essentially transverse
in nature. That doesn't rule out a minor longitudinal component,
however.

My own observations of low frequency radio waves is they could well be
longitudinal while everyone simply pretends they are transverse
because those equations give more or less the "right" answers. Fact is
that polarization experiments at microwave frequencies are pretty
crappy and at low radio frequencies virtually impossible to
demonstrate. Maybe someone needs a trip on the space shuttle to place
some huge dipoles in space to settle the issue once and for all.
Mostly I've found low frequency "diversity" radio to be a joke. Yeah,
it sort of works but there is little evidence it's because of two
different polarizations being received.

That's about what I know.

Benj

.

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