Crystal Radios and Followups

[berkeman]

And, so people can mock me, I'd probably go from board to board with shielded ribbon cable.

At AM radio and audio frequencies, that should be fine. Just take care to choose your pinouts for the cables with crosstalk and loops in mind.

 

[Vanadium 50]

There are some very nice ribbon cables out there. You can get them with every 3d wire grounded, and you can get them as multiple twisted pairs in a common shield and jacket.

Getting them rated to 100 MHz is trivial, and 300 MHz if you spend some time with catalogs.

My point, is that sometimes the best choice is the one you will do the best with, and not the Theoretically Optimal Choice.

 

[tech99]

Here are some comments from my limited understanding. (I invite Baluncore and other experts to correct me if I go astray.)
1) Piezoelectric operation for the tan headphones seems right. See, e.g.
Amazon earphones
They have an impedance of a few thousand ohms. Why not 8 ohms, which is standard for HiFi/stereo speakers and headphones? Because of the need to match the load impedance to the crystal rectifier. The broadcast AM band is, for discussion purposes, pretty close to DC so we can look at the DC impedance of crystals, which is typically a few hundred ohms (see, for example, the center column in this table from Scaff and Ohl, Bell System Technical J., 1947).
View attachment 351422
These World War II microwave diodes literally used cat whiskers and semiconductor crystals inside of a cartridge.
Choosing the load impedance is a balance between best power transfer, which occurs with a matched load (500 ohms, say), and good selectivity (narrow bandwidth to capture just one station at a time) which requires high Q and therefore high impedance. I'm guessing that ~3000 ohms was empirically arrived at as a reasonable compromise.
EDIT: Another consideration is the need to forward bias the diode over at least part of the RF cycle in order to provide rectification. For a given power intercepted by the antenna, a higher circuit resistance will provide higher voltage across the diode. The forward drop sets the minimum voltage: 0.3V for germanium and 0.7V for silicon.

2) As a boy, I built a crystal set with enameled magnet wire wound on a toilet paper tube and held in place with airplane glue (toxic stuff, probably can't buy it anymore). The tube was nailed horizontally to a piece of wood and I used sandpaper to rub off the insulation along the top. A tin strip, nailed to the wood so it could pivot left to right and bent up to contact the coil, tuned the radio. I think this is easier than acquiring a ferrite antenna and variable capacitor, as you say.

3) I connected the RF input to a rain gutter downspout and the ground to a cold water pipe, and it worked! Gutters of old were steel with soldered connections, however, so this might not work so well with today's aluminum gutters and glued seams. Still, it's worth a try.

I think transistors are an obvious next step as the builder increases in ability to understand electronics (transistor action is not obvious...).

At the very small RF voltages being used the diode has high resistance. The highest possible headphone resistance is then the best. The piezo earpieces have infinite resistance so need a shunt resistor of say 100k in parallel. The traditional earphones for crystal sets were 4k; it is possible to use a transformer to allow lower impedance phones. In the UK, the AM broadcasting is being closed down now. Short wave broadcasting still seems to be used to some extent and crystal sets are easily made for short wave reception and need only a medium sized antenna.