Jim Brown wrote:
Each RX has its own antenna, separated by some distance. Most
fading is the result of cancellation between a direct wave and a
reflected wave. When the two signals are nearly in phase, they
add. When they are nearly 180 degrees out of phase, they cancel.
These peaks and dips are the result of time differences between
the direct and reflected waves, and these differences vary from
one point to another. So, when a signal is fading down
(cancelling) at one location, it is often fading UP (adding) at
another nearby location.
Another application is important to SWLs. There is as phenomenon called
'selective fading' where QSB attacks slightly different frequencies at
different times. So if you listening to an AM BC station, the carrier
might fade slightly after the LSB and before the USB. This really makes
mincemeat out of intelligibility; the signal changes from readable to
sounding like SSB without a BFO and back again (I remember it was a
problem with wide-shift RTTY too). This is more pronounced on the lower
bands where the difference between the carrier and sidebands is
proportionally greater.
Since diversity reception has the effect of 'filling in' the fades it is
capable of greatly increasing the intelligibility of an AM signal when
selective fading is present.
--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
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