Maybe check ON4UN's book -- "Low Band DXing"
He has info on this
Too lengthy to quote here

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"John" wrote in message
...
Can someone tell me please an easy way to calculate the optimum angle of
radiation from a transmitting antenna over a given path on the HF bands
(160m - 10m)?

OK, I guess its all to do with the height of the reflective layer in play
and the distance of the QSO but I'd really appreciate some clues as to how
to work this out.

Thanks

John