In 1887, the German physicist Heinrich Rudolf Hertz invented and built a radio transmitter and radio receiver, conducted experiments on the transmission and reception of radio waves,
which proved the existence of electromagnetic waves, investigated the basic properties of electromagnetic waves.
The first radio transmitters of the spark principle based on the Rumkorf coil were very simple in design — damped oscillations were excited in their oscillating circuit using a spark discharge, and the modulator was a telegraph key — it closed and opened the Rumkorf coil power circuit. With the help of such a radio transmitter, information was transmitted in a coded discrete form — for example, Morse code or another conventional set of signals. The power of spark transmitters reached hundreds of kilowatts. Their disadvantages were low efficiency, as well as a very wide range of radio waves emitted by them. As a result, the simultaneous operation of several closely spaced spark transmitters was practically impossible due to the interference of their signals, and the receivers were “blocked” by the signal of a nearby transmitter. The construction of spark transmitters ceased around 1916.
Since 1912, transmitters with an electric arc included in an oscillating circuit have been used. An arc transmitter, unlike a spark transmitter, generates continuous oscillations, that is, it allows transmitting a voice signal with amplitude modulation. The telegraph signal had to be transmitted by the method of frequency manipulation: when the key was pressed, the tuning of the oscillating circuit shifted, and the transmitter radiated at a different frequency; it was on this frequency that the receivers should be tuned. The arc was, for example, a 100-kilowatt transmitter of a radio station on Shabolovka in Moscow, which was put into operation in February 1920. Because of the properties of the arc discharge, the arc generators worked only on long waves, it is impossible to get a frequency of more than 400 kHz with their help. You can now purchase the 500W MF/HF Transmitter at a great price via the Internet.
Another direction was the use of an electric alternator in the transmitter (from about 1908). Such a generator made it possible to obtain sufficiently stable oscillations of a certain frequency, which can be changed by adjusting the frequency of rotation of the rotor of the generator. Power could reach tens and hundreds of kilowatts. The signal of such a generator can be modulated in amplitude, which makes it possible to transmit an audio signal by radio. However, an electric machine generator is practically suitable for generating frequencies no higher than tens of kilohertz, that is, the transmitter can operate only in the longest wavelength range. Until the 1950s, electromachine transmitters were used in broadcasting and radio communications. So, in 1925, at the October radio station in Leningrad, two generators with a capacity of 50 and 150 kW designed by V. P. Vologdin were installed. As a historical monument in Sweden, the Grimeton radio station (opened in 1925) is maintained in working condition with a 200 kW Alexandrsen generator, designed to operate at frequencies up to 40 kHz.