John Smith wrote:
"The big boys use 4 dishes for a link."

Yes. It is for space diversity which dramatically improves reliability
on many paths. Paths tend to fade independently when they are
significantly separated, say by 10 wavelengths.
More than just 2 antennas at a path end is required for diversity
reception. Separate receivers are required too.

Two dishes connected together just make an antenna array. If both
receive the same signal, it is probable a time will come when the signal
sum is zero even though either signal is usable.

Two receivers or more must be used for space diversity reception. A
special combiner must be used to select the best signal and reject all
others. Individual receivers are connected with individual antennas and
selection is made from among the receiver outputs. All other receiver
outputs are completely rejected.

The shortwave broadcaster I worked for in the 1950`s relayed its
programs via HF radio. There were 3 rhombic receiving antennas,
separated by about 10 wavelengths broadside, at the lowest frequency.
This was repeated for each reception direction. Each rhombic fed its own
multicoupler in the receiving station. A receiver for each frequency
being received was connected with an output from a multicoupler.The
receivers were all Hammarlund SP-600`s.

Three receiver outputs, representing signals from each of three antennas
were fed to a "triple diversity combiner" (Crosby or Pioneer). The
combiner selscted the best of the three signals and rejected all others.
This is a triple diversity reception, "TDR System".

In the 1960`s, I built a diversity microwave system into the Gulf of
Mexico from onshore. Water can often reflect well enough for complete
signal cancellation at microwave frequencies, so I used space diversity,
two dishes vertically separated on the towers, feeding separate
receivers at each path end. Only one transmitter at each end of a path
operates at a time., but both receivers work with a combiner which
functions like that used at HF for the TDR. Output is a video type
spectrum rather than audio output as from the TDR combiner. The video
combiner picks the better of the two received demodiulated microwave
signals and rejects the poorer signal. It works like Gangbusters.

Best regards, Richard Harrison, KB5WZI