EZNEC antenna modelling software
I have found this to be valuable and highly accurate in its predictions regarding
SWR.
"Elevation Angle Measurements During a Local Contest," QST,
January 2006, p28; Eric van Maanen PA3DES, Ben Witvliet PA5BW, Goos Visser PA0SIR,
Albert Westenberg PA0A.
This article gives a very solidly measured account of actual ground wave signals,
and shows that they don't travel as far as you might think.
NVIS newsgroup,
Yahoo
This is a good place to have your questions answered.