[Greenkeys] crystal radio -- and fakin' out the spam filters

[gil smith]

Hey folks:

Here is the quick message I was sending before I lost 40 minutes of 
my life to spam filtering.  It is apparently ok to say "my 
medicine"  but you simply cannot say "t-h-e medicine" or it 
bounces.  Try it, and you can waste precious time too.

gil
-----------------------------

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


At 03:20 PM 10/7/2008, you wrote:
>toilet paper tubes work great
>
>as for the coil, you might wanna just put a 100 to 330 pf cap across it and
>calculate turns for inductance to resonate at 1 MHz with the cap... that
>puts center of tuning range of coil in center of broadcast band... I use #40
>enameled with my groupd but just cuz have a LOT of it on hand... you can
>skip the cap if ya wanna rely on 'stray' capacitance but a fixed value sure
>make it a heck of a lot easier to calculate necessary inductance  (and from
>there, number of turns, diameter, etc)...
>
>BEST earphone, if you can find them, are the super el cheapo 'crystal'
>earbuds so common many years ago... last ones I found were like 3 for a buck
>from some surplus place...
>
>the 1N34 and 1N34A seem to do fine for me... the 60 and 270 also work but
>are usually a little more expensive
>
>as long as you have some kind of slider for tuning the sets should work
>fairly well unless you are REALLY a long distance from the transmitter...
>with a crystal earbud, I get trememdous volume just touching the ant
>terminal of my medicine bottle crystal set to the metal window frame... same
>thing with the metal finger stop on my mom's ancient dial telephone...  with
>2K magnetic phones I dont get that kind of reception at all so the >20K
>impedance of those ear buds is quite helpful...
>
>my Physical Science kids sure get a kick out of (you can't say "the" 
>here -- gil-edit) medicine bottle crystal
>sets...  when we can find the 'crystal' ear buds they run all over campus
>probing metal ojbects to compare reception...  a 2"  6-32 screw out the
>bottom of the (f-ing gil-edit) medicine bottle is the 'probe' 
>...   10 ft of wire attached to
>it brings in KTIP (1450 KHx and 5 miles distant) loud and clear...
>
>have fun
>
>J



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