Richard Clark wrote:
"This must be a convention that is particular to only a very few hams.
The FCC database describes AM antennas in both electrical degrees and
physical height as follows."

It is the convention to describe AM broadcast towers in electrical
degrees. Harold Ennes reprints an RCA resistance chart for heights
between 50 and 200 degrees in "AM-FM Broadcast Maintenance".

Formula given is:
Height in electrical degrees = Height in feet X frequency in kc X 1.016
X 10 to the minus 6 power.

Example Towers:
50-degrees self-supporting: R=7. jx=-j100
50-degrees guyed mast: R=8, jx=-j222
90-degrees self-supporting: R=40, jx=+j35
90-degrees guyed mast: R=36, jx=j0
200-degrees self-supporting: R=23, jx=-j50
200-degrees guyed mast: R=80, jx=-400

There are values of R and X for 16 different heights. If you are
interested, look at the book.

Best regards, Richard Harrison, KB5WZI