Reg,G4FGQ wrote:
"---when a ferrite core is placed inside a loop the efficiency remains
the same. Its the same wire, the same coil dimensions, and hence the
same loss in resistance."

Not if the coil is doing the same job. Remember the permeability tuned
coils? You insert the core farther into the coil to increase its
inductance. The powdered-iron core has a much higher permeability than
air. The powdered-iron core gathers and concentrates lines of flux
inside the coil. If the iron-cored coil is to resonate at the same
frequency as the air-cored coil does, the iron-cored coil must have many
fewer turns, and thus it has much less loss. Good design requires a
ferrite with low-loss at the frequency.

For a ferrite loopstick antenna, sensitivity is proportional to the
length of the ferrite rod and best placement of the coil is squarely in
the middle of the ferrite rod.

Best regards, Richard Harrison, KB5WZI