[GreenKeys] off-topic -- crystal radio

[gil smith]

Hi folks:

This is obviously off-topic, but since there are lots of radio guys on 
greenkeys, I thought someone would have some input.

I want to have my cub scouts build crystal radios next week, and I need a 
bit of expertise to point me in the right direction.  Being an electrical 
engineer, you'd think that I would have played with a crystal set in my 
youth, but I never did.

So I picked up a cheesy little $10 kit yesterday and put it together -- 
actually works!  It is the simplest design, using a single coil wound on a 
cardboard tube, with a brass ball slider to tap it along a sanded section, 
and uses a 1N60 diode and a standard ceramic hi-z earphone.

I have eleven scouts, so I can't use up half my year's budget on $10 kits 
(plus they are a pain to put together).  So I want to use PVC or ABS pipe 
for the form, and pre-wind the coils for them myself.  I can make wood 
bases no problem, and just use screws for connecting hardware.  The most 
expense will be the earphones and diode, which look to be about $3 per kid 
plus shipping.

I think the basic one-coil, tapped design will be the best for us, since I 
likely can't afford to get eleven variable caps used in other crystal 
designs.  I have googled for hours, and have still not seen any good rules 
of thumb for designing these.

Does anyone have any guidelines on coil diameter, wire gauge, number of 
turns, etc?  I have seen coil winding calculators on some crystal web 
sites, but they are for getting a desired inductance to resonate with the 
(typical 365 pF) variable caps, targeting a general frequency of 
interest.  For the basic design without the cap, there must be some general 
coil parameters that change the tuning freq range, selectivity, or 
something, but I have not seen discussion of it.

Any thoughts on the diode?  1N34 and 1N60 Ge units are typical, but some 
folks say schottkys with work well.  I'd also like to try the 
razor-blade/pencil-lead thing, but just as a demo and not for the kids' units.

Antenna?  Just get a decent length up as high as practical?  I presume that 
the broad side should face the transmitters (which are all on one mountain 
here in Phoenix).

Any other ideas or thoughts?

thanks in advance,

gil





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