[Lowfer] MF Multipath Propagation

[Dexter McIntyre W4DEX]

Excellent explanation John!

Thanks,
Dex

John Davis wrote:

> 'Cloud' is actually a very good term for regions of e-layer ionization.
> They are roughly the same order of magnitude in size as water vapor or ice
> crystal clouds, and are pushed across the sky in the same way by the winds
> at those altitudes.
>
> Ionization clouds do not account well for phenomena observed at mid-HF and
> MF, though.
>
> Think instead of a swimming pool at night.  If the pool lights shine enough
> in an upward direction, waves on the surface reflect complex shifting
> patterns of brightness on the floor of the pool.  If we recognize that in
> lieu of a thin air/water interface being rippled, we're actually dealing
> with undulations of density within the tens or scores of miles thickness of
> the ionosphere, then the analogy is a pretty good one.  We illuminate the
> underside of this refracting "surface" with a broad front of radio waves,
> just as the pool lights shine up at the water's surface.  As the reflector
> tilts this way and that at different places, the light and the radio waves
> will arrive at any given point on the bottom from a number of slightly
> different directions.
>
> Let's say that an undulation in effective height of the reflector is
> propagating through the medium, and a portion of that undulation's virtual
> "surface" is at the proper angle to reflect electromagnetic waves from the
> source toward the receiver, at the same time a more-or less stationary
> region is also reflecting the transmitted signal toward the receiver.  The
> apparent motion of the reflective surface will represent a Doppler shifted
> copy of the original signal.  If repeated undulations of the same magnitude
> continue to propagate in the same direction, the result is a persistent
> frequency-shifted copy (with fuzz and minor variations as the individual
> undulations pass through the region where the angle is only approximately
> correct for the "moving" reflection to take place).
>
> Note that it is *not* necessary for the refractive medium itself to travel
> for this to occur! --just as water molecules are not transported anywhere,
> but are moved in relatively small looping oscillations by the waves on the
> pool's surface.
>
> If the Doppler shifts depended on lateral motion of regions of greater
> density in the ionosphere (clouds or quasi-clouds), at the speeds computed
> for the observed shifts, such clouds would move well out of range in a
> matter of minutes.  But we have seen Dopplerized traces that go on rather
> consistently for long periods of time, even hours.  Hence, ionized clouds
> are not as good an explanation for the observed phenomena as oscillations in
> the virtual height of the reflective layer.
>
> John D
>
>