Throughout the mid-late 80’s, the FCC worked to help out the AM band, which has been in decline since the late 70’s. One of the key actions was FCC Docket 87-267, which along with other moves simplified AM stations from ten classes down to four classes A-D.
Old Class I-A are the stations heard from half a continent away, 50 kW day and night with protected skywave coverage. The new Class A subsumes the clear channel and clear channel-like licenses and allows daytime Class D stations to tuck in at the edges of skywave coverage with as little as a few watts of power at night. No new Class D stations are authorized as of FCC Docket 87-131, but their ability to operate at low power at night is a big improvement over forced dusk to dawn shutdowns heard from “daytimer” AM stations only a few years ago.
Class B means in effect > 250 watts, and Class D can be 50 kW during the daytime, but less than 250 watts at night.
Real-world crystal radio performance: high power stations were too far away. At night the local and regional AM channels are difficult to hear more than about 15 miles from the transmitter. The only clear reception one can count on throughout is on the clear channel 50 kW stations.
Thus, the signal (field strength) is too weak to make much in a crystal radio set, when a superheterodyne receiver can hardly pull them out. The answer: better coil (higher Q), better antenna (capture more of the available field strength). The bigger antenna is not so practical indoors with noisy fluorescent lights overhead.
During WWII, improvised detectors ranged from cuprous oxide to coke or razor blade. Modern radios might use a 1N34 germanium diode, which have a low threshold voltage and are often used as detectors in cheap (and moderate) superhet AM radios.