10m is not 20m but I suspect the same effect applies:
"Using only the 45 degree azimuth, below is a plot of my takeoff angles for
all 3 antennas ( ), the bottom two ( ) and the bottom only ( ). I
often find that all 3 are best for early openings (11-14 UTC), then the
bottom two take over and occasionally, very late in the day, the bottom
antenna alone is best. Note that VOACAP propagation statistics ( )
indicate that 16 degrees is the maximum TOA to EU for my location."
http://users.vnet.net/btippett/terrain_&_toas.htm
Note that HFTA includes the VOACAP takeoff angle statistics mentioned
above. A little modeling will answer your question far better than
anecdotes. I discovered my statement above after many DX contests on 10m,
and HFTA confirms my experience.
73, Bill W4ZV
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