"Jimmie D":
RF I had an electronics instructor (not sure what he knew about antennas)
say thet there was a 10/90 rule about antennas. That an antenna 10 % as
long as a 1/4 wl will radiate 90%as well, wile he didnt say it I assume
this means with all other sources of loss minimized. Is this anywhere near
true?
_____________

Theoretically yes, but not so much in practice.

Without losses, an "infinitesimally short" linear dipole has 91% of the
peak directivity of a self-resonant 1/2-wave dipole (1.5 vs 1.64). And
for a given applied power both of them would radiate the same total amount
of power, just with marginally different pattern shapes.

The problem is that system losses in a real, "loaded" short antenna can be
much higher than the radiation resistance, so a loaded short antenna may not
radiate much of the available power -- in some applications not even 1% of
it.

RF