Quote:

I've always wondered "why doesn't someone make a small protable crystal radio? A diode as a detector, a High Z earplug, a small tuning cap, and a few other part - NEVER NEEDS power, and works with AM radios


Probably because they don't really work, and their performance is easily surpassed using a Ferranti ZN414 Tuned Radio Frequency Circuit Amplifier in a match box circuit.

http://www.mds975.co.uk/Content/trfradios.html

Even then the performance of the ZN414 was pretty woeful compared to superhet designs which are now standard in most AM radios.