Will there be a significant difference in the performance of two shielded
loop antennas (used for receive only) if the only difference between them is
that one is made of copper and the other of aluminum? Does one "shield"
better than the other? By performance I'm looking at depth of null, SN
ratio, or overall gain. Any other comments on choice of material greatly
appreciated.

Thank you.
Al
KA5JGV

There is not the slightest detectable difference between copper and aluminum
shielded receiving loops except in their mechanical properties and the
effects on your bank balance.

If you use the loops outside in uncluttered surroundings you would very
likely notice no difference in performance even if you entirely removed both
shields to leave only the antenna inner wire or wires. The tuning
capacitors would need to be increased in value to compensate for the missing
shield capacitance.

For a given loop diameter, receiving sensitivity is increased with thicker
wire and fewer turns. A single turn of copper or aluminium self-supporting
tube has greatest efficiency just as for a transmitting magloop.
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Reg, G4FGQ