Not only are the optimum takeoff/arrival angles that are built into the
"Figure of Merit" calculation in HFTA based upon averages, they are
based upon monthly averages computed from VOACAP over an entire 11 year
sunspot cycle. On top of that, one of the original contributors to
VOACAP (I believe it was George Lane) has pointed out that of all the 20
or so parameters that VOACAP can address (signal strength, SNR, path
loss, etc), TANGLE (optimum takeoff angle) and RANGLE (optimum receive
angle) have the least rigorous data to substantiate them. I love HFTA
and I think it does a pretty fine job of computing takeoff angles over
varied terrain, but I take the optimum angle figures with a grain of salt.
Besides, as you say, the optimum angle at any moment is dependent upon
many factors. Generally speaking, very low angles will do a better job
when the band is opening/closing, but often have less of an advantage
(and for some paths even a disadvantage) during the midpoint of an
opening. Also, long path openings that depend upon gray line chordal
hop propagation can sometimes do just fine with surprisingly high
takeoff angles.
In general it's tough to beat raw height, but as you and others here
have said ... it all depends.
73,
Dave AB7E
On 7/31/2013 5:32 PM, Rick Kiessig wrote:
Regarding HFTA, while I think it's a wonderful tool, be aware that the
arrival angles it uses are based on VOACAP, which means they are averages
only. There will be (many) times when those numbers don't apply -- which is
another reason some flexibility is ideal.
73, Rick ZL2HAM (ZM1G/ZL1G)
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