For radio, and especially AM radio, the most important thing is the voltage across the tuner input. For LF signals, that's best acheived with a long-wire antenna and a good ground.
Since the radio has external antenna terminals (which probably feed a coil on the radio's internal loopstick antenna), try connecting a piece of wire to one, and a good ground to the other.
The best extarnal (active and passive) AM antennas are probably those sold by the
C. Crane Co.