>At HF, the wave has enough area and conductivity is high
>enough that conductivity becomes a real issue. Lower
>frequency signals have the electric field "shorted" by the
>earth, and we all know (except perhaps the guy Bill
>mentioned who was talking about the coax and the magnetic
>field getting through the shield) when we take the electric
>field to zero the magnetic field goes to zero.
>
>We have a huge polarization filter at HF, the earth. The
>lower the frequency the better the filter. Ground wave by
>definition requires the electric field not parallel the
>earth.
>
>I can't work five miles on 80 meters on surface or ground
>wave with horizontal polarization. I can if I let the
>feedline radiate or if something near the antenna
>re-radiates a vertical component, but even that vertical
>component attenuates rapidly with distance on HF. 160 meters
>is the only band below 15 MHz or so with substantial
>non-skywave coverage. It might be 30 miles or more with big
>vertical antennas and reasonable power over soil.
>
>Of course groundwave can go a long distance at MF or HF over
>saltwater with vertical polarization (but no distance at all
>with true horizontal polarization).
Hmmm..
however, is it that the wave (launched by some mystery box) can't propagate
over the dielectric boundary as horizontally polarized,OR, is it that a
practical antenna can't launch a horizontally polarized wave because it has
no gain at the horizon.
Here's a way to contemplate it: (that might be wrong)
Consider an infinitely small horizontally polarized radiator (A Hertzian
dipole in the antenna lit) some distance above the earth (of any dielectric
properties). It's well known that for horizontally polarized waves the
reflection from the surface is almost total, particularly for low angles of
incidence. So, at low angles (i.e. propagating parallel to the surface),
the reflected wave and the direct wave will cancel (because of the phase
flip on reflection). But that's an artifact because the radiator is small
and radiates just as well in the direction of the reflection point as in
the direct path. (you can also do this by considering the reflection as
originating from an image)
However, it's also possible to launch a wave that travels along the
dielectric boundary. The dielectric boundary serves to "guide" the
wave. This might be a lossy process (if the dielectric is lossy). We're
not talking here about dielectric slab waveguides (like optical fibers)
where the propagation is inside the dielectric and there's total internal
reflection. This is an attached wave of some sort. (As it happens, it's
quite useful at millimeter wave frequencies, because most dielectrics are
pretty lossy at 100 GHz and the skin depth is pretty small, so resistive
losses are also high). It takes a fairly funky launcher to get the wave
propagating, and, as I recall, the wave can't turn corners (i.e. you can't
bend the dielectric).
There are things called "creeping waves"
And, there's some diffraction and refraction effects also in the mix in a
practical system (otherwise, you wouldn't see ANY signal, vertical or
horizontal, if you weren't line of sight to the antenna)
Ii realize that this is somewhat academic. For practical antennas, there's
very little propagation at the horizon of horizontally polarized waves.
_______________________________________________
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Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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