James Kretzschmar NQHCJ
GENERAL BUILDING CATEGORY
General comment: I first read about the Crystal Radio building contest in the January 2001 issue of QST and for the last 2 months I do not know when I have had so much fun with amateur radio and building since the very first QRP rig I built in 1974. I have been to several libraries looking up how things were done in the early years of radio (fascinating by itself), searched ail over the internet, grown Rochelle salt crystals, used my scientific calculator more than it has ever been used before, salvaged every cardboard tube I could find, wound several coils, discovered shim stock (cheap, neat material for ail sorts of applications), and basically had a blast. Although my entry is probably nothing new, I'm proud of it and it works great. Here is the summary.
Detector: 1N34 Germanium diode ... hey it is 2001 might as well have a little bit of modern convenience. However I did tinker with different detectors, some worked and some didn't. The chunk of galena I had worked fine. Also the chunk of iron pyrite I had worked. In addition I also experimented with Copper oxide/copper which worked. This consisted of copper filings; 1/2 of the filings I heated on the stove so that they got coated with copper oxide (tumed reddish), other 1/2 left shiny. Filled a small glass tube with oxide coated filings on one side and shiny filings an the other. Plugged the ends and inserted a couple pins in each end. The junction needed some manipulation periodically but it did work. I will be refining this later ...... ran out of time to get this done before the contest deadline. Also experimented with other "junction" type detectors such as crushed galena/carbon rod, graphite/copper oxide, graphite/iron oxide. Have not explored all the possibilities as yet. One very simple detector system I did discover that works well consisted of heating a nail on the coil of an electric stove until it turns blue and then attach one wire to the nail and one to a carbon pencil lead. Search around on the blue part of the nail for the tight spot. Detecter: convenience.
Coil: The final coil configuration was discovered purely by accident one evening about 1130 pm just as I was heading oh to bed ...... you know the scenario ...... "let me try this before I cash it in for the evening". I had lots of wound coils laying on my desk leftover from the searching process involved in finding the perfect set up configuration. My coil (L1) consist of an 8" length of 4 1/2" diameter PCV pipe with 64 turns of #20 enameled wire spaced the diameter of the wire. This forms the tank circuit with the homemade capacitor. The antenna is attached to one end. At the opposite end inserted just inside the first couple of windings is the second coil (L2). The second coil consists of a 3 1/2" diameter cardboard tube about 3" long with 45 turns of #32 enameled wire close wound. This is glued and permanently positioned. I did experiment with positioning and this was the overall best spot. The inductance is roughly 250 uh.
Capacitor: This took me an entire Sunday afternoon to construct, however It is pretty neat and it does work reasonably well. It consist of 2 aluminum plates on a hacked "sled" moving into 3 aluminum plates with air spacing. To get it to move smoothly I put packaging tape on the frictional surfaces of the wood and this works well. I did calibrate a dial, but it is not too accurate. A little bump here or there throws it off pretty good. The capacitance works out to be about 250 pf and I am able to tune form about 600-1400 Khz. I do plan on reworking this because I think it adds a little uniqueness to the radio.
Earphone: When I first began this profect I had my mind made up that I was going "homebrew" all the way ...... to include the earphone. I spent a lot of hours on a lot of failed attempts to achieve the earphone part of this goal. The short summary: Many diaphragms of'.001, .002 .003 steel shim stock on cardboard/PVC tubes with nails, wires, and magnets positioned every way you can imagine, unfortunately ...... only successful with outputs from regular radios. I tried it all without success using my crystal radio. One neat thing I discovered during all these futile attempts was that a plastic 35mm film can with #32 enameled wire dose wound the entire length and a piece of .001 steel shim stock loosely lining the inside gave some impressive audio. Next up was growing Rochelle salt crystals and capitaliring on the piezoelectric effects of the crystalline stucture. I learned a lot here and got to reach back to my Chemisty days in graduate school and college. I grew some pretty good crystals (1.0' x 0.5') and the oscilloscope verified the piezo effect, but no success as earphones as yet. Actually once again ran out of time because of the entry deadline. There will be more tinkering with these crystals. timited success was ac\ieved with the piezo ignitor element from a Scripto BBQ lighter super-glued to an aluminum foil diaphragm on a 2" cardboard tube. The most powerful local stations were heard and were understandable but pretty weak. I did discover that the earphones from discarded telephones work well too, however for regular use I use a $2.00 crystal earphone.
Performance: With my 100' long wire antenna in the trees behind my house and no ground connected (things get worse with the ground wire connected) I have been astonished with what comes rolling in at night. Everything I read speaks of the battle between sensitivity and selectivity. In my radio both are good ...... probably because as a rookie h the "crystal radio" business f don't know any better. The two powerful local stations stay where they are supposed to be (690 KHz and 1230 KHz). One night I was impressed with the selectivity when all in a row I tuned in WWL 870 KHt (New Orleans), WABC 880 KHz (New York City), and WLS 890 KHz (Chicago). I couldn't believe all this from such a simple radio!!! The farthest catch so far has been WBAP 820 KHz (Fort Worth) ...... over 1000+ miles away.
Summanry: I have had a great time with this contest ...... THANKS FOR PUTTING ON ON. I would like to try some good magnetic headphones some day but have resisted buying any so far ...... next hamfest I will be looking for a pair.