Al wrote:
"Is there a nominal impedance for a loop antenna?"

Kraus` 3rd edition of "antennas" is a good reference.

From page 207 we see the radiation resistance is proportional to the
square of the number of turns.
On page 209 Kraus gives a graph of radiation resistance versus loop
size. We know that radiarion resistance is less the smaller the loop
area. From the graph, a one-wavelength circumference loop slightly
exceeds 100 ohms.

Best regards, Richard Harrison, KB5WZI