Gregory D. Durgin, originally posted on WaveRacer.com
18 June 2001
The world’s first digital radio system was actually the world’s first radio system. This article discusses the physics behind Marconi’s revolutionary communications scheme: coherer-based reception of Morse code.
Marconi’s Original Setup
Marconi’s first wireless transmission came in 1897. He soon commercialized his technology by installing wireless systems in cargo ships and luxury liners. These systems were initially used to send distress calls to other nearby boats or shoreline stations.
The original wireless system used a spark-gap transmitter. This was just a glorified spark plug that sprayed electromagnetic waves in all directions at all frequencies. In the days before communication regulatory agencies like the FCC, this was a perfectly acceptable thing to do.
The spark-gap transmitter could be wired to send simple Morse code sequences, but the real challenge of the system was to receive the radio signal.
When a radio wave arrives at a receiver, its power is incredibly diminished. Without solid state electronics or even vacuum tubes, the idea of detecting a radio wave seemed impossible. There was simply no way to amplify such a weak voltage at the terminals of an antenna. So Marconi used a receiving device that may mystify many of today’s wireless engineers: the coherer.
Physics of a Coherer
A coherer for radio reception consists of a glass tube with a bunch of loose metal filings resting in the bottom. Two contact wires are placed on opposite ends of the tube, allowing an external apparatus to measure the overall resistance to electrical current through the tube.
This measured resistance determines the presence or absence of a radio signal. If a radio wave passes across the filings, the overall resistance between the two contact wires changes.
In the presence of a strong electric field, the metal filings cohere and their overall resistance to an electric current drops.
To learn why the resistance changes, we must first understand the physical properties of metal at the atomic level. Metal consists of a free electron gas that clings tightly to a crystalline atomic structure. The presence of this free electron gas is why metal is such a good conductor.
However, when loose metal filings are resting upon one another in a tube, their overall resistance is actually quite high. The loose, jagged contact points between filings do not provide very good channels to conduct an electric current. In the absence of an electric field, the internal resistance of the chamber is high.
If a strong electromagnetic wave travels across the filings, they cohere — that is, the strong electric field of the wave causes the electron gas at the jagged metal interfaces to be joined. Electric current can now travel more freely from one end of the filings to another. To an outside observer, it looks like nothing has happened. The internal electrical resistance of the chamber, however, has dropped substantially.
So the appearance of an impinging radio wave can be detected by monitoring the resistance of the coherer. At this point, a problem arises.
How do you uncohere the metal filings so that you can make more measurements? The solution for Marconi’s radio was to shake up the tube by giving it a sharp blow. When the tube is shaken, the iron filings break apart electrically and settle again in an uncohered state.
Very simple and (from our modern solid-state point-of-view) very brutal.
The Necessity of Digital
At the turn of the century, digital communications was the only way to send radio signals. The coherer is, by nature, a binary detector. The device can only measure the presence or absence of radio waves.
The coherer was not the only limitation. There was still a need for signal amplification. These were the days before the invention of even the vacuum tube. Electromechanical amplification using relays was the best option. Because a relay is just an electrically-controlled switch, every weak signal had to be “amplified” to a state of ON or OFF — another digitally limiting factor.
Morse code — a digital representation of speech — became the standard method for communication across the early radio channels. Once the vacuum tube was invented, however, you could transmit a “real” human voice wirelessly across long distances. There was no desire to revisit the cumbersome Morse code systems.
Ironically, it has taken us a century to come full-circle in radio communications. We now live in an age where digital communications dominates the wireless radio industry. Let’s be humble by remembering the fact that we’ve been down this road before — just at a more leisurely pace.