The “Mystery” Crystal Set   After becoming involved in an
interesting discussion of the Mystery Crystal Set coil, and how
to go about measuring its inductance, I decided to wind one and
take some measurements. The details of the analysis are
discussed on the page: Calculation Methods Part 1d. I've
summarized some of the information from that page here.  I've
also created an Online Calculator for determining the coil
parameters.  After analysing the coil, I had no excuse not to
build it into a complete set. So the final result is pictured
below.  The Mystery Crystal Set, was first described in the July
3rd, 1932 Brisbane [Australia] Sunday Mail, and is unique in the
way that its main coil is wound. The original Mystery Set
circuit diagram is reproduced below:  One of the more
interesting characteristics of the Mystery Crystal Set is that
there is no apparent signal path between the antenna and ground.
The antenna is connected to the primary, while the circuit's
only ground point is connected to the secondary.