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An AM radio that uses the Earth as the Antenna
I read an article on the Keelynet website that piqued my interest a couple of years ago: It was all about receiving and transmitting radio signals through the ground. The story goes that underground antennas were experimented with as far back as Tesla, and a 1912 patent by one James Harris Rodgers shows several ways of implementing an antenna which amounts to burying a dipole or helix in the ground, and, I assume, depending on the ground currents (induced by the radio wave being transmitted above the ground) to provide long distance, relatively static-free radio reception. This involved both transmitting and receiving; I decided to K.I.S.S. [keep it simple, stupid!] and play around with just the receiving part.
The schematic shows a simple TRF-like AM detector feeding a Radio Shack amplified speaker the little beige ones they've sold for many years at $11.95), whose "front end" is connected, not to a suspended antenna, but to a grounded rod, through a short piece of coaxial cable. Using two transistors in a Darlington configuration raises the impedance seen by the LC tuned circuit, giving less loading and more selectivity.
The 1 k ohm resistor in the base of the Darlington transistor pair helps prevent oscillation; the 330 pF to ground on the transistors was found to improve the overall gain (volume). Like any or all of these simple radios, you're not to expect superb performance; rather, the fascination is in how well they do work for such a small parts count. The "ground" radio did do better with a normal, above-ground antenna, but it did a surprisingly good job picking up AM radio stations "through the ground" also.
A variation on this setup would be to pound another ground rod some feet away, and connect the circuit's ground to that one. You would most likely need to readjust the inductance at the front end to accommodate the new impedance. You do hear some static and impulse noise, by the way, during a Summer night when thunderstorms are approaching. But in general this radio is quieter, static-wise, than a radio using "sky waves". |
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