2001 Crystal Set Building Contest entry, class Freestyle/General
Cardboard and Soda Pop – It’s A Mystery To Me…
Charlie Cotterman KA8OQF
This variant on the Australian Mystery Crystal Set was a large-scale experiment in construction technique.
Everyone seemed to be raving about the Mystery Set when it made its appearance on the Web. This thing was considered to be the second coming of Marconi – numerous comments appeared about how/why how well/poorly it worked/couldn’t work – it was the radio equivalent of a bumblebee (aerodynamically speaking, a bumblebee shouldn’t be able to fly – but no one told the bee…). I had an idea that this may be the head-of-the-bed fall-asleep-to-talk-radio no-batteries special I had been looking for. With the Building Contest here, I had no more excuses.
For a long time, debate has raged on the subject of coil forms, their loss/stability, and other imponderables. Living in a small apartment that’s already an archaeologist’s paradise, there wasn’t a whole lot of room for various tubes/cylinders/pipes that can only be found in ten-foot lengths. After seeing the pop can capacitors, I was struck by a thought (and it was only mildly painful…). If the wire used to wrap a coil is sufficiently stable to hold its shape more or less, that means the coil form doesn’t have to be bulletproof. I did some measuring, and found my coil form material available as plastic soda bottles (the straight-sided ones). Depending on the size of the bottle (20 oz, 1 liter, or 2 liter), various sizes of sheet plastic (polyethylene, .015” thick) can be cut.
All that’s needed is the proper size non-compressible form to wrap the plastic around. (In the pictures of my Mystery, you may note what could be called excessive use of black tape to hold the coil in place. While some is needed, the form I used was slightly compressible, and the windings had to be secured to the plastic with more tape than usually would be called for.) Cut the sheet so it will be the proper length of coil (allow extra for mounting etc), and also for about 1” of overlap on the cylinder circumference. Tape the edges of the sheet so that a cylinder is formed (with a few minutes of thinking, you can figure out how to tape both the inside and outside of the overlap so it’s secure). Now, wind your coil on this non-compressible form. Tape the windings down firmly at the ends, and maybe run a long spiral around the length of the coil once for security. Don’t go overboard with this – if you have to wind a two-layer coil, you’ve introduced dielectric into the mix that may not have been considered.
When the winding and taping is done, gently slide the plastic cylinder off the form, and you have an almost-air-core low-loss coil. If you’ve made taps in the coil, use some sort of heat sink when soldering to the taps, otherwise you may very well melt a hole in your form.
To mount the coil in my set, I sliced off the bottom of a pop bottle larger than the coil diameter, slit the ends of the bottle bottom to allow it to compress and slide inside the coil, and punched appropriate holes in both coil and its new base (using a paper punch, no less). To hold the coil on the support, note the two caramel apple sticks through the coil and base. If I decide to strip this set down for rework, I can take the coil off easily and quickly.
The capacitor is of the famed pop can persuasion, using the previously-mentioned pop bottle plastic for the dielectric. The calculations for rotor size were found in a design program contained in Hamcalc. Again, there’s a caramel apple stick through the top of the plastic for a handle to move the rotor and tune things.
The frame of the set is just two cardboard boxes that were lying around and happened to fit together. A couple of 6-32 nutz’n’boltz hold the box halves together.
The antenna is a loop style patterned after the loops found on the back of old table model 5-tube superhet sets. Two 5 ½” semicircles are spaced 6 ¾” apart for the loop pattern. An odd number of slits were cut into the box, and 20 turns of 22ga insulated stranded wire (total 75 ft or so) was threaded to form an oblong spiderweb configuration.
Please note that there is no diode shown in the schematic. For rectification and audio, the MCFAD is used with this set. The MCFAD is not pictured here , but is fully detailed in the entry for the Signal Sucker MkII. The reader is encouraged to view everything there, insuring that (A) I don’t have to type my fingers to the bone reentering all that stuff (B) they see both my creations (“It’s alive! It’s alive!!” “Yes, master…”).
The Electric Kool-Aid Acid Test
Does the set work? To paraphrase a former Chief Executive, “…that depends on what the definition of work is…”. Yes, I can hear something, but it’s so faint as to be unrecognizable as human speech. I think my days of compact antenna design are just about over – this thing needs more signal input. There is still room on the antenna form for a couple of turns of pickup loop from an external antenna (c’mon, springtime, so I can climb that tree in the back yard…). This may be plenty to get enough RF into the set to hear the locals better. For experimentation purposes, I’ve got an old MFJ Active Antenna (wideband RF amp) that I could slide in line to see what happens. Also, the pop can cap needs additional work. I still haven’t got the parameters exactly right, and I may substitute a Russian mil-surplus variable capacitor that’s laying around somewhere (just gotta remember which box I stashed it in).
And In Conclusions…
Was the effort worth it, even though the radio doesn’t really work that well? Absotively posilutely yes – a lot was learned about construction technique and adaptation of materials on hand. On that basis alone, this set is a screaming success. Now, if I could just hear something recognizable out of it…
-- Charlie Cotterman KA8OQF