[Greenkeys] crystal radio

[gil smith]

Hi John and gang:

Thanks for the info.  I didn't think of a fixed cap across the coil 
-- interesting idea;  was trying to avoid expense of a variable, but 
I have lots of 100pF caps lying around.  I need to experiment 
tomorrow.  I presume Q of the coil will be low enough that the cap 
could help sensitivity without losing too much selectivity.

I set up a little gear motor bolted to a plastic pipe cap so I can 
wind coils, and made a test coil pretty quickly on 1.5" ABS 
pipe.  Ordered a dozen hi-z ceramic earphones and 1N34A diodes from 
mouser.  Now I need to figure out the coil turns and possible cap, to 
get something workable.  Gotta make a dozen of these things, without 
spending a bunch.  Getting harder to find magnet wire these days too.

I have still never found, in all my googling, info on optimizing the 
coil for a basic (tapped/slide-tuned-coil with no-variable-cap) 
crystal design.  All I can say at this point is they all look like a 
hundred or so turns on a 1.5 to 2 inch form of some sort.  24 GA 
magnet wire seems to be a decent size, just to pull a number out of 
my ass.  Not a very scientific way to approach it, but it is all I 
have at the moment.  I have no data on wire gauge, or cardboard tubes 
versus PVC versus ABS plastic, but I presume they are all fine.  I 
plan to use ABS since it is cheap and black.  The 1.5" (ID) size 
seems to be about right for typical coils.

Local transmitters are about 25-30 miles away from here, but all in 
the same spot.  Am I correct in assuming that the (random-length 
longish-wire) antenna should be perpendicular to the path?  Seems 
that I recall that.  Found out that our home water faucets are indeed 
not grounds, since the copper pipe transitions to CPVC inside the 
wall.  I dare not tell the scouts to use the ground wire in an 
outlet, since I can't have them poking wires in a socket.  And if the 
traditional faucet is out, I guess they need to go sit out by the 
grounded electrical service box to clip on a ground lead.  Ah well.

Another odd question:  Since analog TV bites the big one this year, 
how long do you think it will be until digital radio replaces analog 
radio?  Then none of these crystal sets will work anymore.  Ah progress.

Anyway, thanks to everybody who offered insight on crystal radios -- 
I'll let you know what I finally come up with.

gil




Vaux Electronics, Inc.
480-354-5556
(fax: 480-354-5558)
www.vauxelectronics.com