The simplest and most generally used means of producing radio radiation is by passing through high frequency currents. It is the method which was used by Hertz in his famous experiments and it is still the method used today in all the transmitting stations.
In the minds of many technicians, high frequency radiation and current have become synonymous. However, this way of seeing is hardly admissible and the thing appears obvious if one wants to think about it.
High frequency current is not radiation nor is telephone current a sound vibration. To pass from one state to another, it is necessary to use an energy transformer which is, in the first case, a wave radiator or transmitting antenna, and in the second case, a loudspeaker.
An electrical circuit can be the seat of a very intense high frequency current without any trace of radiation. It is therefore interesting to try to understand how one can pass from a high frequency current to a radiated energy.
We must be careful not to want to build a real model of radiation. It couldn't make any sense. It is however in this error that one falls by tracing, as in Figure 10, the "components" of the radiation as two electric and magnetic fields orthogonal (perpendicular) of the same frequency and by assimilating this image to that of the radiation.
It is easy to point out the absurdity of this conception. First, according to Coulomb's laws, the intensity of each of the fields must decrease as the square of the distance while the intensity of the component radiated in absolute vacuum decreases as the distance.
On the other hand, by experimentally superimposing a magnetic field and an electric field, no radiation is obtained. Magnetic force, like electric force, are two absolutely independent manifestations which have no mutual reaction.
These fields must therefore be considered as two particular aspects of the radiation, as different from the radiation itself that a drawing constructed in two dimensions is different from a three-dimensional model.
We repeat : we must not try to build a radiation model. However, it is not forbidden to seek to understand how energy can pass from the electric form, to the radiated form. Radiation is energy detached from its support of matter. It is a question of explaining how such a detachment can occur.
Imagine that an electron, originally at rest, starts to move. In other words, the conductor who guides it is the seat of an electric current. The result, we know, is that a magnetic field will develop all around the conductor.
It has long been believed that this magnetic field suddenly invaded all space. However, we know from EINSTEIN that there can be no instant action. All phenomena that can serve as signals, whatever they are, propagate with a determined speed, the maximum possible of which is the speed of light. It is indeed at the speed of the latter (300 000 km / s) that the magnetic and electric fields propagate. Consequently, the magnetic field will gradually invade the entire space. It will appear at point P before appearing at point P' (Figure 11).
When we have reached a steady state, the field at point P, as at point P' will be constant and will only depend on the speed of the electron and the distance "d" which separates the point considered from the conductor.
When the current stops flowing, the magnetic field disappears but, in normal times, the total energy it represents appears in the form of an extra-current or a so-called self-induction voltage. This is how, when we cut certain electrical circuits, we see a spark of rupture appear.
We can imagine quite easily the phenomenon of self-induction at the time of the power cut. The lines of force (imaginary lines) gradually fold around the electron "e" and, by sweeping the conductor, supply the self-induction voltage. The manifestations will be all the more striking as the energy stored in the magnetic field will appear in a shorter time.
But suppose that the movement of the electron can be suddenly stopped. What would become of the energy stored in the magnetic field ? It would then be impossible for it to manifest itself in the circuit since we assume that the electron is immobilized. This device allows us, in a way, to completely detach the energy of the magnetic field from its material support. It then appears in the form of radiation.
When we suddenly cut an electrical circuit, we cause a spark to appear, but at the same time, we create an electromagnetic wave. Experience has taught us that the simple breaking of a lighting circuit in the vicinity of a radio receiver produces disturbing noise. The more abrupt the rupture, the relatively greater the radiated component.
This explains. If the rupture is relatively slow, the period of cessation of the current lasts long enough, thanks to the rupture spark, so that most of the lines of force can return to actuate the electron before it is constrained to it absolute stop. Thus, the magnetic field corresponding to point P will have time to return to "e", while that of P' may remain in space (Figure 11). The advantage of a sudden stop thus appears to us much better because the intensity of the magnetic field is inversely proportional to the square of the distance.
Let's go back to our previous assumption : an electron is at rest in a conductor. To produce a radiated component of maximum intensity, it is necessary :
1) Communicate to the electron as quickly as possible, as fast as possible.
Since the electron is supposed to start from a zero speed, we will translate the above into another form, by saying that the acceleration must be communicated to the electron as large as possible.
2) After which, it must be stopped in the shortest possible time, that is to say communicate to it a negative acceleration as high as possible in absolute value.
As soon as the electron is stopped, the radiated component will be launched into space. The conditions will be the same as at the beginning and everything can start again.
To obtain radiation in a continuous way, we will therefore have to launch our electron, stop it and then start again. The result will be exactly the same if, instead of advancing the electron always in the same direction, we launch it alternately in one direction, then in the other.
But when the electrons of a conductor go alternately in one direction, then in the other, always keeping the same average position, it should be said that the conductor is the seat of an alternating current. To obtain radiation, alternating current must be created.
The intensity of current in a conductor represents the amount of electricity, that is to say the number of electrons which crosses a section in one second. We conceive, from this that the conductor of Figure 10, this intensity will be proportional to the amplitude of the displacement of our supposedly unique electron. As it is an alternating current, we will be led to consider the maximum amplitude of the oscillation (corresponding to the maximum intensity).
It is clear that for the same amplitude, the accelerations transmitted to the electron will be all the greater the higher the frequency. Acceleration is indeed the increase in speed in the unit of time.
We specified above that the radiated component becomes more important when we increase the acceleration communicated to the electron. It is therefore certain that the radiation will be easier to highlight if high frequency currents are used.
With a relatively low frequency current, the radiation will be imperceptible at p'', because the energy of the magnetic field will be able, in a way, to reintegrate the circuit. If the frequency is large enough, the energy at point P' will remain in space, that is to say will appear in radiated form.
It follows from previous reasoning that the frequency of the radiation is necessarily equal to that of the current which gave it birth.
As the radiation propagates in space, we can make it correspond to the notion of wavelength.
We have seen in Physics what should be understood by period, frequency, wavelength (We will see these notions in the title "Electronics").
We learned there that the frequency is the number of period per second and that these two quantities whose symbols are respectively T and F are connected by the relations :
T = 1 / F
OR
F = 1 / T
We have defined frequency as the number of periods per second. It should now be specified that the word period, if it is didactic, is not the legal unit of the frequency. We will therefore now use this unit which is the hertz (symbol H), name of the German physicist HERTZ (1857-1894). The multiples are kilohertz (kHz), megahertz (MHz) and gigahertz (GHz) which are respectively 103 Hz, 106 Hz and 109 Hz.
Finally, we have seen that the wavelength is a distance. More precisely, it is the distance that the wave travels during a period (or one hertz). Its symbol is (letter "I" from the Greek alphabet which reads lambda).
The wavelength ,
frequency F and speed (v) of the electromagnetic waves are linked by the relationships :
Given that the propagation speed considered is 300 000 000 m / s, we can write while respecting the correspondences of the units :
If, always wishing to obtain the wavelength in meters, we have the frequency expressed in kilohertz, that is to say by a number 1 000 times smaller, we must also express the speed by a number 1 000 times smaller, that is, in this case, in kilometers, that is :
Finally, if we still want to express the wavelength in meters, we have the megahertz as a frequency unit, so a number 1 000 times smaller than the previous one, we must still divide the number expressing the speed by 1 000 and we then get :
The Figure 12 shows the spectrum of electromagnetic waves whose frequencies vary from a few tens of hertz to more than 5.105 MHz.
We can notice there that the waves corresponding to visible light and which are located between the ultraviolet and the infra-red occupy a relatively small space.
We draw your attention to the fact that in this figure the graduations are not proportional. Telephone frequencies ranging from 300 Hz to 3 000 Hz occupy the same footprint as the frequencies allocated to radars and satellites, which are between 10 MHz and 100 MHz. If in both cases the ratio between the extreme values varies from 1 to 10, the numbers expressing the frequency differences between these two extreme values are very far from each other.
Thus, for telephone frequencies and radar frequencies, we have :
3 000 / 300 = 100 / 10 = 10
But if the telephone frequency band is 3 000 - 300 = 2 700 Hz, it is 100 - 10 = 90 MHz for the radar band, although these two bands occupy roughly the same arc.
This type of representation is necessary because it would be practically impossible to use a linear scale on a sheet of paper of conventional format. Indeed, if in our example we had agreed to represent 1 kHz by 1 cm, we would have had to represent the telephone band by 2.7 cm, which is very feasible, and the radar band by 90 000 cm or 900 meters, which is much less achievable !
We thus finish our notions of physics which, we hope, will help you to better grasp the other parts of our work (or theoretical lessons from other programs) and, at the very least, to increase your knowledge.
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frequency F and speed (v) of the electromagnetic waves are linked by the relationships :
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