Fox-Hole Radio

We built a foxhole radio as another weekend project a few weekends ago. 

First we strung an antenna (22 AWG) out the window to a tree in the backyard. 

Then I set up a ground wire using an outlet and a child safety plug to cover it (and a lecture to the kids about the danger of doing this wrong and exactly what I was doing and why).

(DISCLAIMER - Do not do this at home if you are reading this.  There are safer ways to set up a ground.  Stick a piece of metal into the dirt outside in the yard and connect the wire to that.)

Then we made some coils of wire over cardboard tubes and sandpapered off the enamel coating to tune the frequencies, and hooked up a diode on the other side of the circuit (in parallel between the antenna and ground).  The diode only allows current to flow in one direction, which removes half of the wave from the AM signal and allows the signal to be played in a speaker as sound waves.  The sound output for the speaker is between the diode and ground in the circuit.  The circuit has a resonant frequency that is adjusted by the length of the connected coil to filter different stations based on frequency.  We experimented with several different diodes and coils.

In the picture you can see a zener diode and a green LED (light emitting diode) both of which worked fine when hooked up in the circuit.  The trick is making your own diode.  I tried a few treatments of razor blade surfaces to a graphite pencil point in the circuit.  One blade I rusted in salt water and air for a few days.  Another was heated with a candle, which put on a layer of black carbon on one side.  Another I heated directly on an electric stove eye until it oxidized to a deep blue color.  The third one worked the best and is shown hooked up (with thumb tacks and cork board, no soldering here).  The pencil is held up and connected to the rest of the circuit by a bent safety pin.  The contact point of the graphite to the oxidized metal surface can be adjusted until a good diode contact point is found where the electricity only moves across in one direction.  We also tried a few different tunable coils with different lengths and wire sizes.  The one that worked the best was an intermediate size shown here (30 AWG).  The circuit is connected at different points along the coil to tune different radio stations.  It was very touchy a slight movement to the contact and the station was gone.  It worked better to add a capacitor to the antenna input side.

Above is the capacitor.  The antenna is connected to a square of aluminum foil placed in a book.  This is separated by a single page from another square of foil that goes into the circuit.  The radio can be fine tuned by placing different weights on the book, which changes the distance between the foil sheets. 

Here is the setup connected to our amplifier and plastic plate speaker.  We also plugged it into the microphone input on a laptop to monitor and record the signal with audacity.   Using an empty frequency, we were able to detect our home built motor running on another table by recording, noise removal and amplification.  The brush contacts in the motor acted as weak spark gap transmitter.  We were also able to get several local AM stations tuned in.  I identified three of them by the call signs and looked up their antenna locations online; they were 10 to 15 miles away.