Crystal Radios and Tesla

[ke5frf]

Last night I was reading up on the history of crystal radios. I was always intrigued by the WWII "foxhole reciever", which is a point-contact detector (diode) made from a rusty razor blade and a pencil lead. Allied soldiers built them to avoid detection from German troops who could detect the local oscillator in superhet recievers. Stories also abound from those early days when people picked up AM signals from lead fillings in their teeth.

Anyway, since the early days of electromagnetic experimentation, inventors have sought a way to wirelessly transmit (useful) power. I know most posters in this forum are familiar with Nikola Tesla and his obsession.

Well, it appears that cell phone companies have figured out how to harvest a bit of AM power to charge a battery, based on the same basic technology of the AM crystal set. AM radio is a very inefficient form of modulation, with the power behind the carrier frequency and one sideband being unused, wasted energy. The crystal set has no power source other than the radio waves it rectifies.

I find it interesting that such an old technology, one of the earliest useful electromagnetic devices, "MAY" finally vindicate Tesla's obsessions. It has only become feasable to harness that radio power in the modern age of micro-miniaturization and low-power digitalization. It is kind of like "radio recycling" if you think about it.

Still, it seems way more sensible, reliable, and efficient to use piezo generator or gyro generator to do the same thing.

Perhaps Tesla's fanatsy will always be one step behind current technology.

 

[tcmtech]

If your really good with building accurately tuned RF coils and related HF circuits and have a good sized radio transmitter within a reasonable distance its possible to make a RF energy collector that can easily power a few LEDs.

There are countless stories of people building much much higher powered RF energy collectors as well too!

 

[deepak george]

same here....even iv been so amazed by the whole concept of foxhole radios....lets hope that wireless electricity becomes a reality....and we use it in our day to day lives....

researchers are plannin to install these things at lamp posts in streets for people to charge their mobiles as they stand close to it.....its not much...but its definitely a start..

 

[mneary]

researchers are plannin to install these things at lamp posts in streets for people to charge their mobiles as they stand close to it.....its not much...but its definitely a start..

I liked the idea of wireless electricity until a friend got a pacemaker and they told him strong EM fields could kill him.

 

[Dean Huster]

There have been circuit designs for "dual" crystal receivers where you tune the first to the most powerful signal you can find. It rectifies (detects) this, sometimes using a voltage multiplier, charges a small capacitor and uses this DC to power a small amplifier on the second crystal receiver tuned to the station of interest.

Dean

 

[stevez]

I have to wonder if enough energy could be captured on a round the clock basis that could charge a battery for play at modest audio levels for short period of time. This might make for an interesting emergency receiver, transceiver or other powered device. I probably have enough information close by to figure this one out - just have to take the time to do it.

 

[mneary]

Even a strong radio signal with a large antenna gives you microwatts. To get a milliwatt to transmit for just a minute you would need to collect for days. Your voice will carry farther than a milliwatt transmitter, so nobody hears it. You use it up and then wait several more days to transmit another minute.

If it's a real emergency the radio signal will probably fail too.