At one time telegraph was the principal method of rapid communications over long distances. A telegraph consisted of a battery keyed into a long wire, and on the other end of the wire a sensitive electromagnetic device would clack as the key at the remote end was pressed.
The reliance on a wire was the weakness of the telegraph. During a war, wires strung across enemy lines would be cut by the enemy. To get around this problem, something known as a ghost line was invented. A ghost line used the Earth as it’s wires. On one side of the enemy lines a set of ground stakes were placed in the ground as far apart as possible with the line connecting them perpendicular to the direction the signal was to be sent. On the other side of the enemy lines somewhere, a similar set of ground stakes parallel to the first set was also utilized.
On the transmitting end, battery was keyed into the ground stakes. On the receiving end, they were connected to a sensitive detector just as with a wired telegraph. This was not a particularly efficient system since only a small portion of the power keyed in at the sending end would reach the receiver, but it was a system that lacked any wires for the enemy to cut.
When I was in junior high and high school, I operated a pirate radio station and so did several of my friends. Near the end of high school, they all got busted by the enemy, the F.C.C., and I only narrowly escaped. After that we all started looking for legal ways to communicate a signal over distance.
One of my friends read about Ghost Lines. I thought it would be interesting to try it with audio. I drove two stakes into the ground at my parents house as far as I could get them apart in an east-west line, about a hundred feet.
I connected them to a 75 watt solid state amplifier using a PA line transformer backwards (hooking the eight ohm winding to the output of the amplifier and the 70 volt line to the ground stakes). I played music at close to the full power output of the amplifier.
I took a portable cassette recorder and connected a couple of portable ground stakes to the microphone input. With the receiving ground stakes not more than 8 feet or so apart and not more than a foot in the ground, I was able to detect a signal two miles away.
The limitation was not signal amplitude so much as power line hum amplitude which was abundant. I was surprised that the higher frequency audio components were not noticeably attenuated. It occurred to me that one could perhaps get around the hum problems by frequency modulating a carrier say around 40 Khz. I tried to build a little circuit to generate an FM modulated carrier in this range using a phase locked loop chip but I kept frying the chips and eventually gave up. This was late 70’s or very early 80’s, chips were primitive.
Back in those days digital signal processors also were not existent and as a result neither were more complex modulation schemes such as CODFM. With modern technology I wonder if it wouldn’t be possible to achieve high speed computer communications this way. This could serve all sorts of interesting purposes, data communications, voice, even pirate radio station STL with your FM transmitter up in a tree somewhere.