[TowerTalk] DB's, ad nauseam

[Jim Reid]

> An antenna with well known and predictable characteristics,
> compared to another lesser antenna(also with known and 
> predictable characteristics), producing results via 
> ionspheric propagation disproportionate to and far in 
> excess of any known  differences between the two. 
> Do you have your own stories to tell about this phenomonon...

The thread seems to be about reflections down from the
ionsosphere and possible non-linearities from the cloud
of charged ions up these being insufficient,   I think,  hi.

Anyway,   the most common "unusual" ionosphere propogation
mode is called "ducting".  When this occurs,  the RF is
"captured"  up there,  does not return to the ground for
the simple reflection,  but rather is guided (as in a microwave
waveguide) some great chordal distance above the planet
until the lower guiding region has changed in characteristics
such that the signal can return down to the planet surface
for another reflection or reception.

This results in much stronger signals at the receiving end
than would otherwise be detected since the losses from
ground reflections and multiple passages through the
signal absorbing D-layer have been reduced,  or even
eliminated on the particular path.

There are other unusual modes of propagation up there,  but
the above accounts for most of the greater than expected
signals at/from far away places.

BTW,  this can occur when the signal from the "lesser antenna",
as called by the questioner,  arrives at an angle at the
ionosphere which is not captured and injected into the "duct"
while the other antenna's signal is at just the right angle.

73,  Jim,  KH7M 


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