I seem to recall commercial advertisement for single mode fibres that
involved a graduated refractive index rather than extremely small diameter.
One thing is certain, until the amount of work involved with polishing the
ends is reduced, it will never replace copper wire for low cost or casual
applications.

"Flounder" wrote in message
ink.net...
Absolutely the same thing as modes in a fibre optic cable. Except cables
are
cylindrical (field solutions described by Bessel functions) and the
water-air geometry is described by some form of Hankel functions.

Single mode fibres are very thin. The thick cables have very many modes,
all
traveling with different phase velocities and distorting pulses over the
length of the cable. Single mode fibres propagate only one mode and the
pulse retains its shape over very long distances.

The fibre optic cables act as waveguides. If the diameter of the cable is
too small (about less than 1/2 wavelength) a wave will not propagate in
the
cable. If the diameter is greater than about 1/2 wavelength up to roughly
..6
wavelengths then the first mode propagates called the HE11 mode. A little
larger diameter propagates the HE11 and the TE01 modes. As you increase
the
diameter the number of modes increases. There's also degenerate modes,
hybrid modes, transverse electric and magnetic modes. It gets fairly
complex. There's a formula used to calculate the number of modes in big
fibres. This is all for stepped index fibres.

Fibre cables are clad to keep dust of the cable. The dust disturbs the
evanascent modes on the outside of the cable (kills the signal) just like
dirty prisms cause binoculars to work poorly.

Feel free to ask more questions, I can use the review.



"Donal" wrote in message
...

"Flounder" wrote in message
ink.net...
There's a layer of air on the surface of the water that has a
difference
in
temperature in the air above it. The permitivitty (index of
refraction)
of
the two layers of air are different. The difference is in the
thousandth
decimal place. Because of the difference in the permitivitty a radio
wave
striking the upper layer at or below the Brewster angle experiences
total
internal reflection. The wave becomes trapped between the water and
the
second air layer. Part of the wave does travel in the second layer, at
the
boundary. That is the evanescent wave. That is also the type of wave
that
allows your prism binoculars to work. The wave also travels at two
different
velocities in the two air layers.
So, looking at the transmitting antenna as an isotropic radiator at
150
MHz,
the layer height at 30 meters how many modes propagate on the sea
surface?


You call *that* an explanation??

BTW, these "modes" that you refer to, - are they in any way similiar to
the
"modes" in fibre optic transmission systems? They sound like they might
be.

If they are, I would be very interested is a *plain* English
explanation!
I really should know the difference between multi- and single-mode fibre
optics.

BTW this isn't a trick question. I'm genuinely interested.



Regards


Donal
--