Bill Bowden wrote:
"Does anyone know why the distributed winding capacitance of a loop
antenna, or any inductor, degrades the efficiency?"

I`ll speculate that current to build the magnetic field and the current
required to charge the stray capacitance of the inductor occur at
different times. The magnetic field is the source of self-inductance of
the coil, but the displacemnt current in the stray capactance is
gratuitous and only adds loss to the coil.

Best regards, Richard Harrison, KB5WZI