Radio communication: period 1927 – 1935
This page briefly shows the state of the art at the start of the Measurement Building in 1927 in the field of radio communication and shows the radio communication capabilities that the Meetgebouw worked on in the 1930s.
Many forms of information transmission now rely on the use of electromagnetic waves. Since the 18th century, a large number of physicists investigated the phenomena of electricity and magnetism and searched for an appropriate theoretical explanation. In 1864, Maxwell postulated the relationship between the electric and the magnetic fields expressed in a new mathematical form based upon the previous work. He then introduced the much-debated concept of a “displacement current” for an open circuit. He derived a wave equation in free space similar to the one for light. In 1888, Hertz proved the validity of Maxwell’s theory. He discovered the advantage of tuning a receiver to the transmission frequency.
Marconi was the experimenter who provided the practical application, especially for ships at sea. In 1895, he started experimenting with transmissions over increasing distances until he successfully bridged the Atlantic from England in 1901. In 1904, Fleming, working for Marconi, reported the use of a light bulb with an added electrode as a means of detection to replace the awkward coherer. Two years later, De Forest added another electrode to Fleming’s diode giving birth to the triode.
In the early 20th century, only radio “wireless” telegraphy existed. Spark transmitters up to 2 kW on ships operated at wavelengths (as was the unit at that time) between 300 and 600 meters. Stations on the mainland up to 400 kW used increasingly longer wavelengths up to 18 km in accordance with the increasing distances to be travelled, thereby relying on the propagation path that follows the curved surface of the earth. These long-distance stations used heavy rotating generators and extensive tuned antenna systems.
This situation had changed drastically when the Measurement Building was founded in 1927. The development of the vacuum triode lamp had resulted in another type of wave generation. The transmitter modulation and amplification in the early 20;s ( “Transatlantic tests”) of the 20th century with relatively low power at wavelengths below 300 meters had shown the existence of reflective layers around the earth and day/night differences in transmission. Thus the “wireless” started with the development of both commercial and public (broadcasting) services, including voice and music transmission at short wavelengths up to several tens of meters. Those transmissions required smaller antenna systems and greatly reduced capabilities. Regarding receivers, the direct current anode battery was gradually replaced by rectified alternating voltage and the indirectly heated triode for alternating current supply appeared on the horizon. The first tetrodes were introduced which would be followed in the following years by more lattices and more function tubes that were adapted to more sophisticated receiver circuits.
Around 1930 wavelengths below ten meters (or frequencies above 30 MHz) began to attract professional attention to communication applications. It was known that these frequencies were not reflected by ionized layers and could easily be limited to a relatively small angle (beam). However, an object within this angle, in dimensions comparable to or larger than the bundle cross-section, prevented transmission or reception.
The first indications of interest by the armed forces on this topic are found in 1933. The Measurement Building was then asked to assist with the receipt of signals by a super-regenerative receiver of signals sent by a five-metre transmitter in an aircraft. In addition, the requirements for a future meteorograph for a weather balloon were discussed.
The Measurement Building proposed an investigation to use electromagnetic radiation below ten meters, caused by the engine ignition of an aircraft, for locating aircraft from the ground. With a super-regenerative receiver (3-10 meter band), aircraft could be detected up to 10 kilometres away. The investigations were, however, terminated in 1936 because electronic noise suppression was introduced in new aircraft. This was necessary for an undisturbed radio reception by the aircraft itself. Besides, German Junkers were equipped with diesel engines, which did not emit electronic noise anymore.
The receiver for the 5-metre aircraft transmitter; Von Weiler (left), Van Soest (right)The difficulties in designing practical circuits in the thirties of the last century should not be underestimated. Every discrete part and even the wiring consisted of a mixture of resistance, capacity, and self-induction. Skin effects and travel time of electrons in tubes played a role. In receivers, the lack of effective means for either amplification or frequency transformation before demodulation was solved by the so-called “super-regenerative” detection. This device served both for amplification and demodulation and contained a periodically (e.g. 30 kHz) self-interrupting circuit that oscillated violently at the frequency to be received. The main drawback was the high noise level in the absence of modulation. Transmitters even caused an even bigger problem to get the maximum radio frequency power on the antenna. This required careful high-frequency isolation between the generator circuit and the power supply required the prevention of radiation leaks. Temperature effects and frequency stability also play a role.
In 1934, Von Weiler experimented with two push-pull and single tube transmitter designs up to 1-metre wavelength using high power direct-heated triodes.
A spark transmitter comprises the first type of radio transmitter. Such transmitters were used, for example, for radiotelegraphy. A radio signal consists of electromagnetic and electrostatic fields in the atmosphere. A spark transmitter made those fields by spitting out electric sparks. Such a spark transmitter usually works in combination with a tuned circuit (also known as a resonant circuit). This circuit consists of a coil and a capacitor in series. The spark gap is still slightly conducting just after a spark has jumped over. This is because the air is ionised on the spot. At that moment an oscillation arises in the tuned circuit that can generate high frequency (HF) energy in the rhythm of the breaker for a short time. Below the image of the 1.5-metre spark transmitter developed at the Measurement Building in 1935.