Stubblefieldâ€™s underground telephony VLF system was very simple. Two metal rods were used for each â€œtelephone setâ€, and were inserted into the ground several feet apart. Each set was outfitted with a large horn to act as a loudspeaker. A small carbon button microphone transmitted the voice messages thru the earth to the other receiving set. The most interesting thing about the device is that it was powered only by the natural currents of the earth, derived solely from his patented â€œearth battery.â€
Stubblefield never achieved any greater sending distance than one mile. While this may be considered a poor performance for a wireless communications system, one must remember that Stubblefield utilized only the natural power of the earth for these transmissions â€“ his earth battery being the operative power source. Stubblefield later suggested that he was developing a more powerful apparatus using automatic relays in which â€œdistance would be unlimited.â€ For reasons unknown, Stubblefield would later abandon â€œearth telephonyâ€ in favor of experiments with wireless radio thru the air. Either Stubblefield became disenchanted with the performance of his ground VLF system, or he simply left the idea to the many patent infringers to figure out.
The volume control on most any amplifier suffices. Simply connect the output leads for the speaker connection to the ground rods of one unit, and connect the grounds of the second unit to the microphone input. The units must be DC or battery powered though, as a typical alternating current supply will sufficiently destroy the signal by its introduction of 60 cycle noise and ground feedback loops. Small portable amplifying units such as the PA system can be easily obtained or built for the purpose. One must take care not to perform the experiment with any proximity to conventional power sources as these will interfere with the signals. Special filtering circuits can be designed to rid the units of most noise.
Now, specific attention may be focussed on the placement of the ground rods for the experiment. While geomantic considerations (such as a dowser might use) should be implemented, one must not forget conventional rules of propagation thru a restrictive medium such as the earth
Learn more about how to propagate VLF through the earth at The Journal of Borderland research or see also Teenager invents low-frequency radio for underground communications