"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 and physical height as follows. .... it would seem
odd to step out of this expectation to change to calling all
antennas 90° simply because they resonate.
_____________

The FCC data cited does not include the reduced velocity of propagation
along the radiator -- which means that an FCC "90 degree" radiator is not
resonant, it has some inductive reactance. A network is used at the
radiator feedpoint to transform the complex impedance there to properly
match the transmission line.

That "90 degree" radiator would need to be shortened by several percent in
order to be self-resonant. Kraus (3rd Ed, p 182) shows a feedpoint Z of 73
+ j42.5 ohms for a thin-wire, linear dipole that is a physical
1/2-wavelength, and that self-resonance occurs at a length a few percent
shorter, when the radiation resistance drops to about 65 ohms.

An unloaded 1/4-wave MW broadcast monopole working against the typical
broadcast radial ground system has about 1/2 the impedance that Kraus shows
for a dipole in free space.

RF