K3 Diversity

[Don Rasmussen]

http://tinyurl.com/5kgauh


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[Jim Brown-10]

See the ARRL Handbook. If it isn't a question on the current ham
exam, it should be. :)

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.

If the two RXs are synchronous (same osciallator) and the signal
paths have the same phase response, detected audio will be in
phase, so adding the output of the two RXs will result in coherent
addition on signal (6 dB increase if the signals are equal) but
only a 3 dB increase in random noise. Also, there are some nice
psychoacoustic things that the brain can do if you put one RX in
one ear and the other RX in the other. But this is ONLY true if
the RXs are synchronous (running on the same oscillator) and have
matched phase responses. THAT's why we care about matching the
filters -- the phase response of the filters is additive with the
phase response of the rest of the signal path.

73,

Jim K9YC

On Fri, 4 Jul 2008 14:36:25 -0500, Jim Miller wrote:

>OK, I have to ask.  WHAT is diversity?






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[Vic K2VCO]

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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