In pioneering experiments conducted 1887, Heinrich Hertz showed that a simple spark coil and a small antenna could be used to broadcast radio waves. Using a primitive radio receiver Hertz was able to detect these waves from a few feet away. Working in New York City around 1891, Nikola Tesla devised major improvements to the Hertz oscillator, creating the Tesla coil resonance transformer with its tuned primary and secondary circuits. When used as a wireless transmitter Tesla's oscillator was able to produce radio waves thousands of times more powerful than ever before achieved. By the mid 1890s Nikola Tesla was focused on the development of an independent, remotely-operated vehicle (ROV) called the "telautomaton." Using his newly developed wireless technology he was able to eliminate the physical connection that had previously been needed between the moving object and the stationary controller. Instead of depending on interconnecting electrical wires to carry the controlling signals, he used radio waves. On November 8, 1899 Tesla's ROV design received United States Patent No. 613,809, "Method of and Apparatus for Controlling Mechanism of Moving Vessels or Vehicles." Tesla's more powerful radio transmitter made his ROV a practical reality and opened up the age of wireless telecommunications.
An important feature of every system for wireless remote control is its immunity to interference. In order to achieve this Tesla developed a method for selectively activating the vehicle's wireless receiver. He called this "the art of individualization." In its most basic form he would transmit the control signals using only a single frequency. If greater security was needed, he used a wireless signal produced on a multiple of frequencies -- the transmitter worked at a number of separate wave lengths, like a pipe organ playing a musical chord or, alternatively, a specific sequence of notes. On the receiver side each one of the individual frequency components had to be tuned in, in order for the control circuitry to respond. In 1903 he was granted the "System of Signaling" and "Method of Signaling" patents covering these techniques. These patents describe the electronic AND-gate logic circuit, a fundamental element of all present day digital computers. They also lay out the basic principles of frequency-hopping and frequency-division multiplexing in wireless spread spectrum telecommunications.
From the transcript of U.S. patent interference "Nikola Tesla vs. Reginald A. Fessenden, Interference No. 21,701, Systems of Signaling"
46 Q. Please read to the Examiner the contents of the "Tesla Exhibit Colorado Sketch"?
A. "Colorado Springs, June 27, 1899.
"Arrangements of apparatus in telegraphy through the natural media aiming at exclusion of messages in accordance with matters experimented with in New York. This is not quite so good as method with condenser of commutating individual impulses, but a great safety can be secured nevertheless. The idea was to provide more than one synchronized circuit and to make the receiver in its operation dependent on more than one such circuit. Experiments have shown that a great degree of safety is reached with two circuits. I think with three it is almost impossible to disturb the receiver when the vibrations have no common harmonics very near to the fundamental tones. Several arrangements experimented with are illustrated below. These are to be followed up. Figures 1, 2 and 3 illustrate some arrangement of apparatus on the sending station, by means of which two vibrations of different pitch are attained. A great number is omitted to be shown for the sake of simplicity. In case one two sending circuits which should be some distance apart are provided and energized alternately by discharging condensers of suitable capacity through the corresponding primaries. In the diagram 2 and 3, one sending circuit is arranged so that its period is altered by inserting some inductance, as in 2, or by short-circuiting a part of circuit periodically by means of an automatic device. Such a device is not necessary to use, however; arrangements of this kind will be later illustrated. On the receiving station two synchronized circuits, responding to the vibrations, each to one of the sender. The receiver R responds only when both circuits I and II affect sensitive devices a a1. Diagrams are self explanatory."
- Nikola Tesla Guided Weapons & Computer Technology, pp.29-30.