The Australian "Mystery Crystal Radio" has been praised since its original publication in 1932, but I don't think anyone has ever mentioned its capability for FM reception. To clarify somewhat, it is the Mystery Plus set, rather than the original circuit to which I am here referring. The original Mystery set had two coils wound in tandem, whereas the Plus set has three coils -- an extra one for antenna tuning. The only other substantive change I have made is an additional variable capacitor on the ground circuit, an outboard rock detector jack, and an audio output jack for playing through a sound system (optional). The capacitors cannot be ganged. Here is the circuit.
This set is an amazing performer.
It has crystal clear reception, good sensitivity, and excellent
selectivity. To my surprise, in addition to the expected panoply of AM
stations, I heard a faint signal that I could not tune out. At first I
could not identify it. To my surprise I heard the announcement "KiiM
FM, 99.5"! This is a country music station here in Tucson. It was
all over the dial, untunable, but the much louder AM signals masked it
when they were tuned in.
I set myself the task of trying to
improve the FM signal. I tried some simple circuit modifications, which
did not improve anything. Then I connected a dipole antenna instead of
the AM antenna I normally use. Suddenly, the FM signal was much
although still weak. By using the audio output and sound system
amplifier, I was even more amazed that four different FM stations came
in loud and clear. I found that changing the telescoping antenna length
and position I could tune the stations in and out. They were KRQ, KLPX,
KiiM, and KHYT all local FM stations with transmitters nearby.
Their reception was also affected by the length and position of the
audio output cable.
The "rock" is a beautiful "twinned"
pyrite from Spain. I have found that many metallic rock crystals will
work as detectors, including galena, molybdenite, various copper ores
such as "Peacock" copper, hematite, and zincite. Watch for a future
webpage on crystal detectors.