73,

Nandu.

 

Why Unconventional?
When we were kids we built them all the time, tricky playing with the Cats Whisker lump of Galena crystal!!
Max.

 

its just using the bse to collecter junction as a diode. the capacitor is just for smoothing the rectified rf, not for tuninng. with a 200 kw braodcast station nearby, almost anything will work for a "crystal radio", the trick is to make a dedesign that will let you hear farther stations, and for that, you need higher impedance headphones, for less load on the detector, a real paralell tuned tank with variable cap for tuning, and better coupling to antenna and detector for impedance matching.

 

How do you think the Low Z headphones might affect tuning?

 

there is no tuning, the coil is just an rf choke to develop rf across. the transistor acting as a diode keeps the capacitor from resonating the coil.

 

Thank you for the explanation and new information.

 

I suppose the variable, located after the diode, performs the resonance function while effectively bypassing the carrier and allowing only the envelope into the phones.

The problem with this explanation of the (undoubted) observation is that it doesn't tally with the 'expected' properties of a small signal diode or a diode substitute. One would expect to see only the diode junction capacitance linking the coil and the variable capacitor. The diode junction capacitance is presumably an order of magnitude or more less than the variable capacitor.

 

Hi,

I dont think it looks that unusual either. Low impedance headphones will probably take too much power to run so that might not be a good idea, but you try it and see.

More unconventional is a crystal radio set made with a home made diode. There are everyday materials that can be used to make diodes that work pretty well for these very low level signals, and that's because there is hardly any current flow.

 

the diode only passes half wave rectified rf, the "tuning capacitor isnt really connected to the coil when the diode is on the other half, so resonance isnt achieved. also, using a low impedance headphone might seem to work on a local 500kw station, but it prresents a high load on the inpur circuit, which by not tapping down on the coil for either antenna or detector, will not let the even if connected correctoly capacitor and coil combination work well. a paralell tuned lc tank has a very high impedance at resonance, and loading it with a low imedance load spoild the "Q" or quality factor of the tank, making the thing have low output and broad tuning.

 

For reference for your future crystal radio innovations, I recommend Crystal Radio Systems by Ben Tongue (of Blonder-Tongue). This is the definitive work on the subject. Food for thought.

Volume 1
www.lessmiths.com/~kjsmith/crystal/tongue1engineering.pdf

Volume 2
www.lessmiths.com/~kjsmith/crystal/tongue2engineering.pdf

 

Detector Diodes like the OA 85 or 1N34 will perform better than a Transistor Junction diode.

 

While at it, here is the mother load of crystal set wisdom:
http://www.bentongue.com/xtalset/xtalset.html

 

Using a Germanium diode and crystal earphone, the receiving signal may not enough to drive a low z phone.

 

trhe "crystal set socioty" has a lot of info on crystal sets and lots of different designs.