Art wrote:
"Could you quote from the books that you are referring to the angle
relative to the radiator, that`s it?"

O.K.. but the angle is relative to the horizontal in the statement.
Termans 1955 opus on page 804 says the strength of radiation from a
short vertical antenna is proportional to the cosine of the elevation
angle. That`s right because at zero angle, radiation is maximumm as
cosine of zero degrees is 1, and cosine of 90 degrees is zero.

Best regards, Richard Harrison, KB5WZI