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the horizontal plane.  

So if, given your geographical situation, you need to pick up signals from
significantly varying compass bearings, and you don't want to use an
antenna rotator (which have become increasingly impractical in this age of
DVRs and such), one thing you can do to increase gain *without* narrowing
the beam pattern too much is to go for an antenna with broad horizontal
directivity, but very tight vertical directivity.  Vertical stacking of
(identical) minimally-directional antennas can accomplish the same thing;
but it MUST be done correctly, or it will likely hurt more than it helps.