On Aug 8, 2011, at 8:58 AM, John Brown wrote:
Probably need two antennas, two receivers and a multiplexer that can
switch between either receiver as the signal on each antenna fades
in and out. It was called "Diversity Reception" and was used for
commercial RRTY (among others uses)in order to maintain a constant
signal feed to the RTTY machines. The 2 antennas that were typically
used were 4 acre Rhombics or similar, so they were spread out over a
relatively broad area of land able to react to the ionospheric
changes causing the fading. Don't see how you can do it with just
two antennas without having them reasonably separated and some way
of detecting the change in signal strength to select the appropriate
antenna.
It was also used during WWII in Europe to hear the BBC over jamming.
The jamming signals varied in strength and noise from band to band, so
it was popular when they had them to use two receivers each on a
different frequency near each side of the head. The stereo effect put
the jammers off to the sides and the main signal in the middle,
reducing the effect of the jammers.
You can do something similar with modern receivers on the ham bands by
using a computer controlled set of receivers each with a different
antenna. For example if you use a computer controlled transceiver as
you tune it, the computer monitors the frequency and adjusts the
secondary receiver as needed. Then you listen to the two receivers
with stereo headphones one with each ear.
I don't know much about them, but one of the more famous tube
receivers used by the US military could be modifed for diversity
reception (it may have been a factory mod) combining two of them, but
that was for fixed frequency reception, you tuned them both, by hand,
to the same "channel".
This was probably what John wrote about above.
I believe MFJ has some sort of diversity reception antenna, but I know
nothing about it other than they sold/sell it.
Geoff.
--
Geoffrey S. Mendelson, N3OWJ/4X1GM
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