For the development of the radio it was important to create modulated electromagnetic waves. This allowed to transmit music and voice signals via radio waves.
Damped Oscillation
The Vacuum Tube and the Dawn of Electronics
Continuous Oscillation with Triode Tubes
Amplitude Modulation
Superheterodyne Receiver

Development of modulated electromagnetic radiation

Radio telegraphy in the early 20th century was equivalent to wired telegraphy 40 years earlier. Morse code could be transmitted, but no speech or music. Spark gap transmitters were like drumbeats in the „ether“. Instead of tones, you only heard popping. This was because back then it was not possible generate continuous electromagnetic waves. For this you would need an oscillator that oscillates continuously with sufficient power at high frequencies.

Alexanderson Alternator

But there was, and still is, a technology that can generate high power electrical sine waves: electrical generators. However, the frequency of such generators is 50 or 60 Hz and not 50 kHz as needed. Nevertheless, attempts were actually made to design the generators in such a way that they ran extremely fast and had an enormous number of windings. This is how machine generators were created, which could generate up to 100 kHz.

Until now, experts have not believed that machine-generated oscillations could actually produce electromagnetic waves. It was believed that only the power of sparks could create waves. A researcher named Reginald Fessenden disagreed and had General Electric build a special machine transmitter. This Machin was designed by an engineer called Ernst Alexanderson and therefore called Alexanderson Alternator. In 1906 Fessenden connected this transmitter in Brant Rock (Massachusetts) with a 130 meter long antenna. He managed to modulate the power of the Alexanderson Alternator with an ordinary carbon microphone used for telephones. In doing so, he created the world’s first amplitude modulation.

When an amplitude modulated signal is received by a diode based receiver (i.e. crystal detectors), the signal is demodulated by the rectifier and becomes audible in the receiver circuit. Thus almost all telegraph receivers were able to „hear“ the signal.

In December, Fessenden announced a demonstration for Christmas. This was the world’s first radio show. Fessenden made announcements, played the violin, music was broadcast from a gramophone, even singing. In fact, the broadcast could be heard 18 km away.

This radio show was just a demonstration. Using channels for entertainment and broader information was not Fessenden’s vision. He wanted to make phone calls over the air. Real radio broadcasts didn’t appear until the 1920s.

Alexanderson Alternators proved impractical because they were expensive and heavy. They could not be installed on ships. They were also of little use on land, also because they had extremely long wavelengths. Fessenden’s 50 kHz transmitter had a wavelength of 6 km. Thus, its range was only 2-3 wavelengths away.

Damped Oszillation

How to generate continuous high frequency sine waves. The basic element for this was already known, the electrical oscillating circuit consisting of a capacitor and a coil. Let’s assume the capacitor is charged. The electrical energy is therefore stored in the capacitor. It will now discharge through the coil. This creates a magnetic field in the coil. At first it slows down the current, but after the magnetic field has built up, it strengthens it. Now the energy is in the coil. The capacitor then charges up again and the magnetic field is dissipated. The energy is back in the capacitor.

The whole thing is comparable to a children’s swing. Initially, the swing deflects, the child is on top, initially motionless and being attracted by gravity, has „potential energy“. If you let go, the child sinks down and starts to move. It converts potential energy into kinetic energy. Because of the inertia, the child does not stay down, but rises again, slows down and stops briefly again until the process repeats itself.

The problem with the children’s swing as well as with the electrical oscillating circuit is that the oscillation slows down. In the case of the children’s swing, this is due to the air resistance and in the case of the electrical oscillating circuit to the ohmic resistance of the cables. As a result, the vibration decreases with each cycle and finally comes to a standstill. This is called damped oscillation. How to prevent damping? Well, with the children’s swing, the child would soon be yelling „Push“. So an adult must constantly give the child a little push to compensate for the loss of energy.

But how is it possible to „push“ an electric LC Circuit?

The vacuum tube and dawn of electronics

It is well known that Thomas Alfa Edison invented the light bulb. Because he works a lot with filaments in a vacuum, he discovered that these emit negative electrical charges. The charges could be collected with a cathode and build a current through the tube. Naturally, this flowed just in one direction from the glowing anode to the cathode. This is called the Edison-Richardson effect.

The English physicist John Fleming in 1902 used the above effect and created a special tube which he used as a rectifier for telegraph detectors. This worked more reliably than the crystal detectors.

Two independently working physicists, the Austrian Robert von Lieben and the American Lee de Forest, soon discovered that the flow of electrons from anode to cathode could be influenced. Another cathode (the so-called grid) could be used to regulate the current. The application of this effect was the amplification of electrical signals. Using a weak electrical signal to control the flow of electrons could amplify the signal. This was a great invention, especially for the amplification of telephone signals. For the first time an „active element“ in electricity became available. Therefore, the year 1906 can be described as the dawn of electronics.
For details see: Triodes

With the invention of the Triode Tube starte the age of Electronics

The Bell Telephone Company recognized the value of triode tubes and acquired the patents from Lee de Forest. As early as 1913, amplifier valves were used in long-distance connections for the telephone network. In January 1915 they were able to establish a coast do coast telephone connection using repeaters based on triode tubes.

Continuous oscillation with Triode Tubes

The major breakthrough for radio technology with continuous oscillations came from an engineer working for the German company Telefunken, Alexander Meissner. He connected an amplifier circuit to an LC circuit. Through inductive feedback, the capacitor was always (re)charged when no current was flowing through the coil.

Now for the first time it was possible to generate stable, continuous high-frequency oscillations. In 1913, Telefunken applied for a patent for this circuit. See: Meissner circuit.

As a result, it didn’t take long for the first voice radio connections to be tested. High frequency oscillation were generated with the Meissner circuit, amplitude modulation was used and the signals were amplified with amplifier tubes. First there were connections between Berlin and Nauen in 1913, followed by similar tests in England and America.

In 1914 World War One began. This had strong consequences for wireless technology around the world. No one was allowed to operate a radio transmitter without a license or permit. Only the military was allowed to use radio transmissions. So the German army got its own troops, which were in charge to exchange of information inside and outside the army. The technology progressed during the war. Above all, the production of tubes became better and cheaper. In addition, the variety of tubes grew. They became more efficient, especially when they could be cooled. This made it possible to create transmitters with sufficient power.

Bandwidth and Frequency Division

Radio transmission with amplitude modulation is fundamentally different from radio with spark gap transmitters used in radiotelegraphy. One major difference is the bandwidth of the signal.

If a radio signal is send out that origins from an amplified oscillator, it generates just one frequency. On a frequency scale, it would correspond to a line. On the other hand, if you excite a spark gap transmitter is used, strongly damped waves are created, i.e. the amplitude decreases quickly. The spectrum of such a radio wave would have the shape of a bell, at about the range of the oscillation frequency. This is no longer a line, but has a width that is called the „bandwidth“.

Modulated continuous waves made it possible to share frequency bands.

A high bandwidth is wasteful, at least if it doesn’t contain a lot of information. The actually free frequencies to the right and left of the „transmission frequencies“ are disturbed. That’s why there was practically only one band in marine radio that everyone had to share. For example, if one ship is transmitting, all other ships must remain silent. If they all radioed at the same time, there would be chaos. That’s why the SOS was introduced to silence all ships.

In 1912 the radio chaos led to a catastrophe. The luxury liner Titanic used its radio equipment to transmit information from ship to shore. As a result, the radio operator was distracted and did not hear messages warning of icebergs. A nearby ship, which had already stopped its journey and wanted to warn the Titanic, could not find a radio break in which the Titanic would have been ready to receive. After a long wait, the radio operator was tired, turned off the radio and went to sleep. As everyone knows, the Titanic struck an iceberg and sank.

Amplitude Modulation

We’ve talked about amplitude modulation many times now. It was already being used by a Alexanderson Alternator in 1906. Amplitude modulation is easy because a microphone can change and thus modulate the amplitude of a transmitter. It is also easy to demodulate on the receiving side. You only have to rectify the signal with a diode (e.g. a crystal detector) and you get a current that corresponds to the original signal that was used to modulate the carrier signal.

But what exactly does amplitude modulation do when you look at the spectrum of the transmitted signal. The pure carrier signal has only one frequency. But what happens when this signal is modulated?

Language and music has a certain bandwidth. This is defined by the frequency range that we can hear. Theoretically, this is from 50 Hz to 20,000 Hz. However, it is completely sufficient for telephoning to transmit only up to 4,000 Hz or for music up to 4,500 Hz. Before using speech or music for the modulation, the higher frequencies should be filtered out. This can be done electronically by a low pass filter or simply because old fashioned microphones can hardly transmit high frequencies anyway.

As can be calculated mathematically, amplitude modulation creates two frequency ranges to the right and left of the carrier frequency. The bandwidth of those ranges correspond to the bandwidth of the signal that is used for modulation. The resulting total bandwidth e.g. for music would be 2 x 4,500 Hz which is 9 kHz. Assuming a 10 kHz bandwidth, it is theoretically possible to transmit a radio signal every 10 kHz without the signals interfering with each other. For example, if you transmit at 1000 kHz, the signal goes from 995 kHz to 1005 kHz. Thus, there are 99 „channels“ in the 500 to 1500 kHz band.
See: amplitude modulation

Superheterodyne Receiver

It is possible to place several independent transmission next to each other. But there was another problem to be solved. If only one strong signal is transmitted, as with previous spark gap radiotelegraphy, reception was fairly easy. An oscillating circuit is used that is tuned to the corresponding frequency of the transmitter and can directly be demodulated. However, this is no longer possible if many transmitters are next to each other. it is required to „filter out“ the station you want to receive! But there weren’t any good filters that could filter out a narrow channel at around 1 MHz or so. This problem was solved by an American engineer named Edwin Howard Armstrong.

He invented the so-called superheterodyne receiver. With an electronic element called a mixer. A mixer essentially multiplies two signals with each other. With such a mixer, the desired, previously amplified carrier frequency can be transformed with its sidebands to a so-called intermediate frequency. A good bandpass filter lies at this intermediate frequency, i.e. a filter that suppresses frequencies to the right and left of the upper sideband. As a result, all other interfering transmitters are filtered out. If you amplify this intermediate signal, you can demodulate it as usual and hear the signal.
See: superheterodyne receiver

Soon there were receivers with the possibility to regulate the reception frequency with a rotary knob (by means of adjustable capacitors). As a result, even a layman could operate a receiver. The radio was born.