Reg, G4FGQ wrote:
"Your ancient charts, which I think I have once seen but don`t now have
access to, apply only to LF. Permittivity was ignored when they were
calculated."

True, they are not for HF. My edition was reprinted by the Seabrooke
Printing Company, Inc. and covers the range ftom 540 KHz to 1600 KHz.
Dielectric constant (permittivity) is assumed to be 15 in all cases.

The reason there are graphs for frequency segments such as 1560 kc to
1640 kc is that loss increases with frequency. Skin effect is an
important faxtor. The higher the frequency, the less it penetrates the
earth, so the crust carrying the r-f is thinner. The decline of field
intensity versus distance from the transmitter is steeper at HF.

My set of curves has a page which gives the formulas used to construct
"surface wave field intensity versus numerical distance over plane
earth". It has separate sets of formulas for vertical and horizontal
polarizations. Curves in the book are for vertical polarization, the
only thing of interest to a broadcaster.

One option would be to construct your own set of curves. Another would
be to find some curves which have already been constructed. I don`t know
of any but expect that they exist.

Best regards, Richard Harrison, KB5WZI