Fascinating history of wireless communications, with lots of info and schematics to try reating your DIY wireless communication system on Ashish’s Programming Journal:
By Ashish Derhgawen, Logan, Utah, United States
In this modern age of high tech gadgets, it is easy to take the technology around us for granted. If we look at the world around us as if we have never seen it before, it would be impossible not to be filled with awe and wonder. In this article, I will tell you the story behind one of the most important technological inventions of modern times – wireless communication. I will also describe some of my own experiments with high voltage spark transmitters and coherers! But before I talk about those, a little background is necessary to appreciate how they work. We’ll have to step into the shoes of the early pioneers in the field of electricity and see the world in a completely different way!
Many years ago, when there were no cell phones or Internet, a great scientist by the name of Michael Faraday speculated the existence of electromagnetic waves when he observed the influence of magnetic fields on polarized light (Faraday Effect). Faraday speculated that light could be a form of electromagnetic disturbance propagating through space. However, this idea was received with considerable skepticism, and it was rejected by everyone until a mathematician by the name of James Clerk Maxwell proved it mathematically in his paper “The Dynamical Theory of the Electromagnetic Field” (1865). Maxwell predicted the existence of electromagnetic waves that could travel indefinitely at the speed of light until absorbed. He proved their existence mathematically without any experimental proof.
About thirteen years later, David Edward Hughes (an Anglo-American concertina player and inventor) observed something very bizarre. When working on his Induction Balance, a loose contact was creating sparks. Hughes noticed that a telephone circuit connected to his carbon microphone on the other side of the room was somehow picking up that noise. He took the telephone circuit outside, and he could still hear the clicks made from the induction coil up to 500 yards away! You could consider this the first mobile phone call in history. Hughes wrote –
“Further researches proved that an interrupted current in any coil gave out at each interruption such intense extra currents that the whole atmosphere of the room (or in several rooms distant) would have a momentary invisible charge, which became evident if a microphonic joint was used as a receiver with a telephone. This led me to experiment upon the best form of a receiver for these invisible electric waves, which evidently permeated great distances, and through all apparent obstacles such as walls &c. I found that all microphonic contacts or joints are extremely sensitive.”
Apparently, when Hughes showed this to members of the Royal Society, the scientists thought it was merely the result of induction. They did not realize that Hughes had accidentally discovered the electromagnetic waves that Maxwell and Faraday had predicted. Hughes never published his findings.
|Hughes wireless apparatus, a modified version of his carbon microphone detector (left), and a clockwork driven spark transmitter and battery (right) (photo source)|
In the 1880s, physicist Heinrich Hertz was trying to confirm the existence of Maxwell’s electromagnetic waves. After observing induced sparking in a Riess spiral, Hertz concluded that this phenomenon could be used to detect the waves. He set up a spark gap transmitter, and a receiver (which consisted of wire loop with a small spark gap). Hertz thought that if the spark gap transmitter created electromagnetic waves, the wire loop antenna would pick it up, and he would see a small spark at the gap in the antenna. Hertz would look at that gap in a dark room with a magnifying glass to see if any sparks appeared.
Around the same time, a scientist named Oliver Lodge was investigating some issues with lightning rods. To simulate lightning, he was using two Leyden jars to create high voltage sparks. He had two wires connected to the gap (as shown in the figure below) with multiple spark gaps between them. He observed that when the Leyden jars discharged, sparks appeared at all the spark gaps simultaneously. He also noticed that sparks at certain locations were more intense than others. In a darkened room, he could clearly see a visible glow between the wires at one-half wavelength intervals. When Lodge saw this pattern, he knew that he had discovered the electromagnetic waves predicted by Maxwell.
|Oliver Lodge’s experiment (source: “Oliver Lodge: Almost the Father of Radio” by James Rybak)|
Hertz also saw the spark he was hoping to see. Through the experiments of Lodge and Hertz, the existence of Maxwell’s elusive electromagnetic waves was finally confirmed.
Lodge went on to design better devices (detectors) to detect these waves. The detector that Lodge used was called a “coherer”. I think it was based on the idea of Hughe’s carbon microphone detector. A coherer consists of iron filings between two electrodes. Normally, the resistance between the two electrodes is very high. When a spark is created, the resistance between the electrodes drops.