A D V E N T U R E S in C Y B E R S O U N DA 'Selective' Razor Blade Crystal Radio from Gnome Technologies (see also a matching 'Sensitive' Set)
Introduction The use of two independent tuned circuits is main variation from the "classic" crystal set. While this means roughly twice the work and parts, the increase in selectivity was worth it for my signal-saturated urban location.
The antenna and detector inductors are identical - about 60 turns of #24 copper wire on a 2.25" outside diameter PVC water pipe. To increase selectivity, the inductors are NOT wound on the same form, but rather are loosely coupled sitting side-by-side. Taps every 5 turns allows experimentation with a wide range of detectors and antenna configurations. The total inductance is measured to be around 160 uH. To make coil winding easier, I covered the PVC pipe with double-sided tape. Variable capacitors are the old air-spaced metal type, cannibalized out of a busted tube-radio from the 1950's. Rather than drilling countersunk holes in the wood, the caps are epoxied to aluminum plates, which are then mounted on the wood. Both are the 35-365 pF type. The headphones are the classic old-style magnetics, with a coil resistance of around 2K. Modern headphones will not work with a crystal radio. I was lucky to find some vintage-1940 units tucked away in storage in our local physics department. The capacitor across them is .001 uF; it's probably not needed but I threw it in for good measure. Antenna and ground... suck. It's about -35 C where I'm at, and I don't have the heart to string a proper L-shaped long wire. About 20m of stranded copper wire strung along the fence (end fed) is the antenna for now. Ground is even worse- the screw that holds the plastic plate onto an electrical outlet. Better antenna and ground are coming once things thaw out.
This is where things get fun. I started with the typical 1N34 germanium diode. Very sensitive, nothing to tinker with. Boring. My general-purpose detector assembly is a safety pin mangled into a U shape, with a piece of 3H pencil bound to it by copper wire and soldered in place. The other contact is a big brass washer, which bits of rock and metal can use as a "solid" connection to the coil tap. It looks something like this:
XACTO BLADE Not having any razor blades handy, I tried an Xacto (tm) blade. It works, but takes a VERY light touch, the pencil just brushing against the surface of the blade. Sensitivity is good, but it seems kind of fickle and drifts due to the precarious nature of the contact. IRON RUST A more reliable detector substance is iron rust. We had some iron discs laying around the lab, so I just put some water on them and left them overnight. The resulting patches of orange are quite sensitive. These disc detectors need a fair bit of contact pressure from the pencil lead. Probably a rusty razor blade would do just about the same. MAGNETITE I also got some magnetite (Fe2O3, also called lodestone) - kind of a refined version of the rust detector. This is roughly the same performance; one could probably make a "fixed" detector using this. RAZOR BLADE I found a rusty razor blade; good sensitivity, and it's less picky about the junction contact than the Xacto blade. N-TYPE SILICON This is cheating, but a piece of N-type silicon (simply broken off a boule used to make IC wafers) makes an excellent detector, giving almost as strong a signal as with a germanium diode. FUSED SILICON Also located some fused silicon (Fisher Scientific S-164), which also a wonderful detector- very sensitive. IRON PYRITE A local toy store sells a wide range of minerals. Pricey (a small piece was $1.00), but they had iron pyrite. About the same sensitivity as magnetite. COPPER OXIDE Copper oxide, in the form of old vacuum sealing rings, is also a sensitive detector substance. This is a "renewable" detector- sanding the copper with some fine-grit sandpaper until it's shiny gives best results. Possibly the fresh, very thin oxide layer is most sensitive. Once again precarious nature of the contact makes using it tricky.
GALENA (lead sulfide) Recently read that it's possible to make your own Galena using lead and sulfur. This is worth looking into! CARBORUNDUM (silicon carbide) I've tried bits of grinding stones and wheels, but they seem to be carborundum laced with some kind of binding agent, and don't have any semiconductor properties. Also a bias voltage in series with the headphones will be needed to compensate for the large forward conduction potential of carborundum. ERUSSITE (lead carbonate) MOLYBDENITE (molybdenum disulfide) LEAD PEROXIDE ZINC PEROXIDE
Sensitivity and selectivity are always at odds in a crystal radio. This unit is designed for good selectivity, so the sensitivity leaves a lot to be desired. It can only pick up our 5 strongest local AM stations, and nothing else.
Selectivity is actually good. We have two CBC stations, one in French at 1050 kHz, and English at 990 kHz. I like both, and the French is the strongest signal on the whole band; the tuned antenna and loose coupling lets me pick out the English CBC station easily.
Tuning the radio is kind of tricky until you get used to it. With my antenna, using the bottom tap on the antenna inductor, the two capacitors roughly track to resonance. Turning both at the same rate will let you catch what stations are out there, then tweak each capacitor on it's own to increase signal strength. There is little mutual inductance so you can tune each circuit independently.
With the antenna tapped at the top of the inductor, the tuning capacitor C needs to be set to around halfway in order to receive a strong station at 680 kHz. This gives an antenna capacitance of around 160 pF.
In our age of instant point-to-point communication it's nice to go back to basics. My next project is going to be a crystal shortwave radio, but that will probably have to wait until spring when I can string a proper antenna.
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