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 your parents 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-worl crystal radio performance
So any 10 kW stations are too far away, and the remaining stations are at 1kW due to the local nature of their broadcasts. At night the local and regional AM channels are mostly trashed here more than about 15 miles from the transmitter. The only clear reception one can count on throughout is on the clear channels from stations like WJR in Detroit, WSM in Nashville, and the like.
Thus, the signal (field strength) is too weak to make much in a crystal radio set that I got for the science fair, 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 for a science fair indoors with noisy fluorescent lights overhead.
In terms of my science fair project, I put explanations like this into the report, showing how it would work and how it worked during WWII with various interesting choices for detectors from cuprous oxide to coke or razor blade. I was using a 1N34 germanium diode for mine. Germanium diodes have a low threshold voltage and are often used as detectors in cheap (and moderate) superhet AM radios.
For now I don’t yet have the money or equipment to move further ahead aggressively, but I did find the local TV shop would give me some junk/old equipment they had, and from what I can piece together from garage sales and the trash pickup, I’ll get somewhere.