../s3_ae.gif 1940. Transmissions in the Maginot Line
Radio: principle
Document carried out starting from technical explanations provided by Lionel CIMA, Lecturer to the ENS Cachan. ©1998-2010 Autoédition Cima


The radio is a subject which annoys the French ego, each time one evokes it in connection with the French Army of 1940. It is obvious that the High command saw it of an evil eye, was hostile even there and, since its PC of the castle of Vincennes, the Gamelin general, chief of general staff of the French Army between 1935 and on May 19th, 1940, refused to communicate by radio (too much indiscreet).

In this file we wanted to point out the principles of the radio, so that one can have a clear idea of the possible problems which it was likely to generate as well on the level of its use as to that of its implementation technical.

Principles of the radio
General principles

General principles

Two stations

The principles of the radio impose the setting in cascade of a certain number of apparatuses installed in two stations:

- one, the transmitter, codes then sends information;

- the other, the receiver, receives information and decodes it.


The central element of the TSF is the antenna. In the transmitter it transforms electrical energy into electromagnetic energy. The latter is propagated without palpable support and can be intercepted by the antenna of a receiver able to carry out opposite conversion. Electromagnetic energy is then retransformée in electrical energy.

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It is to be noticed that, if the transmitter has electric energy requirement to emit, in theory the receiving set does not need any to receive. Indeed, its antenna collects transformable electromagnetic energy in electrical energy. This is why, in the general diagram above, we did not put an electric energy source at the receiver whose lamp lights all the same with each reception of electromagnetic waves.

In practice, this would be only to amplify the received signal, the receiver has also its own electric energy source.

Radio telegraphy - Radiotelefony

In radio telegraphy a switch varies the electric current of the transmitter, which results in the emission (then reception) of a more or less long beep. Language Morse.

Into radiotelefony a microphone converts the sound emitted into electrical energy and, in the receiver, a loudspeaker (or ear-phones) carries out opposite conversion. Phonic or musical language.

Amplifier of power

Amplifier of power

As the electromagnetic waves lower intensity with the distance of the transmitter, this last will be all the more powerful as its signal will have been amplified at the beginning.

For the same reasons, the reception will be also improved by an amplifier of power on arrival.


The amplifier has energy requirement, which requires the installation of an energy source for each of the two stations.

Modulator and demodulator

Modulator and demodulator of amplitude

If there were not that a transmitter on Earth the problem of the radio would quickly be regulated. In theory (for the radiotelefony for example), it would be enough to directly connect its microphone to an antenna (after having amplified its signal). Further, each receiver would need only one loudspeaker connected on an antenna and the turn would be played. As it of it is nothing, one sees oneself obliged to transmit, in the same space, several signals coming from several transmitters which it is necessary to differentiate with the reception.

In the transmitter a modulator relocates the audible signal (i.e. located in the audio waveband being spread out 20Hz at 20kHz) in an inaudible signal located around an arbitrarily selected frequency FP.

Each transmitter is thus characterized by its FP (frequency of carrying). One has carrying 100kHz, the other carrying 200kHz etc

Of sound with dimensions the receiver must thus be equipped with a demodulator which carries out the opposite operation of the modulation and brings back the band of the frequencies collected to its initial place, i.e. between 20Hz and 20kHz. The signal, transmitted to an ear-phone, is then become again audible.


This double translation complicates a little the fitting of each station and requires, of course, a complement of electrical energy.

Selective filter of reception

Selective filter of reception

The antenna, because inter alia its form and of its site, collects several wavelengths in a preferential way. A finer selection is ensured by a filter placed directly behind the antenna. This filter lets pass only the desired waveband. While operating this filter one is fixed thus on the transmitter of his choice.

Provided with the whole of the devices which we have just pointed out, the stations are then operational.

Note: it is noted that certain circuits and bodies (antenna) are found at the same time in the transmitter and the receiver. Some switchs, making it possible to use them in one or the other case, can then contribute to carry out at the same time transmitting and receiving stations. It is, inter alia, the case for station OTCF type 1939 for example.

To also know…
Without them, would there be a radio?

Without them, would there be a radio?

Heinrich Hertz


Heinrich Hertz (1857-1894), is a German physicist. In 1887 it highlights the existence of the electromagnetic waves imagined by James Maxwell in 1873.

Edouard Branly


Edouard Branly (1844-1940), is a French physicist pioneer of the radio. Without work of Branly on its coherer (1890), Guglielmo Marconi could not have carried out into 1895 the first radiotelegraphic connections. Branly also created the prototype of the radio controls currently used to operate our domestic apparatuses as well (television set, etc) as on the space probes.

Alexandre Popov


Alexandre Popov (1859-1906) is a Russian physicist. He studies the electromagnetic emissions of the storms when he has the idea to improve the sensitivity of the receiver equipped with a coherer with Branly by connecting the wire with a lightning conductor to it. He has just invented the antenna.

Nikola Tesla


Nikola Tesla (1856-1943) is a Serb physicist. In 1893, before even as Marconi the possibility did not show of transmitting telegraphic messages by Hertzian waves, Tesla written on the transmission of electrical energy without wire. It can be regarded as one of the pioneers of the radio.

Guglielmo Marconi


Guglielmo Marconi (1874-1937) is an Italian physicist. In 1895 it makes experiments on the Hertzian waves. It reproduces the material used by Hertz by improving it with a coherer of Branly and the antenna of Popov. In 1895 it carries out in the Swiss Alps a radio contact on 1,5km. In 1897 it carries out the first communication in Morse on more than 13 km between Lavernock (Wales) and Brean (England) over the Bristol-board Channel.

And well of others…



Space relaxation
Little game and advertisements

Space relaxation

Little game

Click: to charge the play


Closed switch. The transmitter is supplied with electrical energy (lit lamp) and starts to emit electromagnetic waves. The receiver does not receive anything yet because the wave propagation is not instantaneous.

The transmitter is supplied with electrical energy and emits. The receiver does not receive anything yet.

The transmitter always emits and the receiver does not receive anything yet.

The antenna of the receiver receives the electromagnetic waves and transforms them into electrical energy. This electrical energy lights the lamp of the receiver.

Open switch. The power supply of the transmitter is cut. This last does not emit any more but the receiver still receives the last emitted waves.

The transmitter does not emit any more but the receiver still receives the last emitted waves.

The transmitter does not emit. The receiver receives the last emitted waves.

The receiver does not receive any more waves and its lamp dies out.

Transmitter and receiver are extinct.