[BC] MW Space Diversity Reception

[Stanley Adams]

You do not put your diversity on the cross pol (i.e. H if the main is on V).
That is an auto 30 dB loss and is of no value.
You tie you lines together through either proper equipment designed for
diversity reception by the manufacturer (i.e. level switch, 
active combiner, and the like).  Active combining with properly delay
aligned (between the active and main antennas--we call it DADE) can actually
add about 6dB to the main down converter level.  But if you do not have
those means the only other possibility that I can think of is a 3dB
splitter. A level switch will also require some form of DADE'ing either of
the old manual type by adding extra coax to the 
diversity line to bring it up to the difference in the length of the main to
diversity antennas.  Or the auto type where you turn a screw on the
manufacturers receiver that has already accounted for delay lines.

You loose three dB on the main active path, or the backup for all that
matter.  However you keep up a path and a low path is better than a no path.
But in severe temperature inversions as we have this time of year, both
paths can just lay down and die for a while.
There is nothing 100%.  While doing the initial design engineering of the
path you will want at least a 45 dB fade margin between
where the main normal path level resides and where the 1 x 10-6 or 10-8
level occurs if we talk digital, if analogue you will want 
to switch at about -40 or so if the squelch is about a -60 or -65.  This
should help you keep up in the really high 99.9% area.
99.0 is worthless for a station like WLS or some of the other big (or
smaller) ones; it is the decimal side that really counts.

Use of the Bell MW Design manual is good on this as well as the old Air
Force MW set of manuals.  For the design engineers of Marti and others this
is a cake walk.  Andrew Corp has some downloadable software for path calc's
as do other of the manufacturer's sites.

Stan Adams
Memphus