Wireless Wireless is rightly regarded as the greatest invention of the late XIX century, opened a new era in the history of human progress. Just like the old electric telegraph started the electrical engineering, the creation of wireless has served as the starting point for the development of radio, and then electronics, tremendous progress we now see everywhere. You can note another interesting parallel in the history of these two inventions: the creators of the telegraph Zemering and Schilling were the first inventors who tried to use human interest of a newly discovered rarity - an electric current, and the basis for action radiotelegraphy Popov and Marconi lay just discovered phenomenon of electromagnetic radiation. Both then and now the first communication equipment claimed and used the latest achievements of science.
The electric telegraph carrier signal is an electric current. In wireless as this support are the electromagnetic waves that propagate through space at high speed and do not require any wires yourself. The opening of the electric current and the discovery of electromagnetic waves are separated from each other by exactly one hundred years old, and their example can be seen some striking successes achieved during this century physics. If an electric current, as we recall, was discovered by Galvani accidentally, the electromagnetic waves for the first time showed themselves as a result of targeted experiments of Hertz, who knew what and how it should look for, because even twenty years before his remarkable discovery the existence of electromagnetic waves with mathematical precision was predicted by the great English physicist Maxwell.
To understand the principle of operation of wireless, remember that such an electric field and a magnetic field that is. Take a plastic ball and rub his woolen cloth - ball then acquire the ability to attract small pieces of paper and rubbish. He, as is usually said to have become charged, that is, get on its surface a certain electric charge. In an earlier chapter it was reported that the charge can be negative and positive, and the ball will be two equally charged repel each other with a certain force, and two balls with opposite charges are attracted. Why is this happening? At the time, Faraday suggested that each ball creates around itself an invisible indignation, which he called an electric field. Field of the charged bead acts on the other ball, and vice versa. Currently, Faraday hypothesis accepted by science, but about the nature of this field, that it from itself represents, as such, is not known. In addition, the electric field exists, evident only two of his undoubted properties: it is distributed in the space around any charged body with huge, though finite speed of 300,000 km / s, and acts on any other electrically charged body is caught up in this field, drawing or pushing it with a certain force. A variety of such effects can be considered current. As already mentioned, any electric current is directed movement of charged particles. For example, in metals, the movement of electrons, and electrolyte - the movement of ions. What makes these particles move ordering in the same direction? The answer is known: this force is an electric field. When contact is made in the conductor along its entire length from one pole of the power source to the other, an electric field which acts on the charged particles, causing them to move in a certain way (for example, in the electrolyte, positively charged ions are attracted toward the cathode and negatively charged - to anode) .
Much of what has been said about the electric field can be attributed to the magnetic field. All we dealt with permanent metallic magnets and are aware of their ability to attract and repel each other, depending on what the poles - like or different - they are directed towards each other. The interaction of the magnets is because any of them around a magnetic field, the field of the magnet acts on the other magnet and vice versa. It has been noted that the magnetic field arises in the space around each moving charge any electric current (which - to repeat it once again - is the direction of the flow of charged particles) generates a magnetic field around itself. It speaks about the reverse phenomenon - the phenomenon of electromagnetic induction, when the changing magnetic field induces an electric current in the conductors. But why there is this current and this only occurs when the magnetic field is changing? Let's try to figure it out. Take already considered above the transformer, which is two coils, worn on one core. Inserting a primary winding of the transformer to the network, we obtain the current in the secondary winding. This means that the electrons in the secondary winding come into directed motion, that is some kind of force the beginning of work on them. What is the nature of this force? For a long time scientists and electrical engineers grew to a standstill in front of this issue. Already using transformers, they could not fully understand the processes that occur in them. Apparently it was just that this phenomenon can not be explained only the influence of a magnetic field.
An interesting hypothesis to explain this and many other electrical phenomena, put forward in 1864 by the famous English physicist Maxwell. To understand it, we note that the process that occurs in the secondary winding of the transformer, is very similar to that observed in any conductor closed circuit - and here and there the electrons come to directional movement. But this is a conductor circuit under the influence of an electric field. Perhaps, in the secondary winding of the transformer is also an electric field? But where did it come from? In the closed loop electric field arises from the inclusion of the power source (battery or alternator). However, in the secondary circuit of the transformer, as is known, there is no external power sources. Maxwell suggested that the electric field arises here under the influence of a changing magnetic field. He went further and argue that these two fields are closely linked, that any changing magnetic field generates an electric, and any changing electric field generates a magnetic, and they can not exist without each other, presenting as it were a single electromagnetic field.
Maxwell's theory can be explained by the following simple example. Imagine that on a spring loaded ball suspended. If we pull it down, and then let go, the ball starts to oscillate around a certain equilibrium point. We assume that these fluctuations occur at a very high frequency (ie the ball manages to climb and descend a few hundred or even thousands of times per second). Now we measure the magnitude of the electric field at some point far away from the ball. Obviously, it is not constant: when the ball is closer, the tension will increase when it is removed - it will decrease. The period of these changes is likely to be equal to the period of oscillation of the ball. In other words, at this point, an alternating electric field. Following Maxwell's hypothesis, we must assume that it will generate a varying electric field around itself changing with the same frequency magnetic field, and the latter will cause the alternating electric field has a greater distance from the charge, and so on. Thus, in the space surrounding the ball there system periodically varying electric and magnetic fields. A so-called electromagnetic wave traveling in all directions from the oscillating charge at a speed of 300,000 km / s. With each swing of the ball in space electromagnetic wave emitted another. How much hesitation, so many waves. But no matter how many waves emitted per unit of time, the speed of their propagation is strictly constant. If we assume that the ball makes one oscillation per second, that means that the "head" of the wave will be at a distance of 300,000 km from the radiation source. If the frequency of 1000000 cycles per second, all the waves fill the space for 1 second, assuming a straight line away from the radiation source 300,000 km. The share of the each wave will have the way to 300 m. Thus, the length of each wave is directly related to the frequency of vibrations generated the its system.
Note that this wave as it has in itself all the conditions for its spread. Although each solid medium to some extent weakens its power, an electromagnetic wave, in principle, can be spread in the air and water to pass through the wood, glass, human flesh. However, the best environment for her is a vacuum. Now let's see what happens if the path of the electromagnetic wave will be the conductor. Obviously, the electric field of the wave will affect the electrons of the conductor, which in consequence will come directed movement, i.e. in the conductor occurs alternating electrical current having the same oscillation period and the same frequency as the electric field gave rise to it. Thus, it is possible to explain the phenomenon of electromagnetic induction, Faraday open.
It is clear that our example more perfect. Under real conditions the electromagnetic field emitted by an oscillating charged sphere, will be very weak, and its strength at a great distance is almost zero. The current induced in the secondary conductor will be so small that it did not register any instruments. For this reason, his theory of Maxwell's life did not receive experimental confirmation. Many scientists shared his views and looking for a way that would help to detect electromagnetic waves. Experiments in this direction have become the starting point for the development of radio.
Only in 1886 the German physicist Hertz conducted experiments confirming Maxwell's theory. To excite electromagnetic waves Hertz used a device he called a vibrator and detection - another device - cavity.
Dipole antenna consists of two rods of equal length, which were joined to the terminals of the secondary winding of the induction coil. On the facing ends of the rods stronger small metal balls. When the induced current passes through the secondary coil winding between the balls slipped a spark, and in the surrounding area radiates electromagnetic waves. Hertz resonator consisted of a wire bent in a ring, both ends of which also strengthened metal balls. An alternating magnetic field of an electromagnetic wave in the resonator tide began alternating electrical current, whereby discharge occurs between the balls. Thus, when the discharge vibrator overshoot observed sparks between the resonator balls. To explain this phenomenon was possible only on the basis of Maxwell's theory, so that through experience Hertz clearly proved the existence of electromagnetic waves.
Hertz was the first person to consciously control electromagnetic waves, but he never set out to create a device that allows you to establish a wireless radio. However, experiments of Hertz, the description of which appeared in 1888, physicists are interested in the whole world. Many scientists began to look for ways to improve the emitter and receiver of electromagnetic waves. Resonator device Hertz was very low sensitivity and therefore could catch vibrator emitted electromagnetic waves only within a room. First Hertz able to transfer by a distance of 5, and then - 18 m.
In 1891, French physicist Edouard Branly discovered that iron filings placed in a glass tube, by passing electric current through them is not always show the same resistance. When an electromagnetic wave near the tube, for example by sparks produced by the coil Ruhmkorff, sawdust resistance dropped rapidly and only recovered after a light shaking. Branly has indicated that it is their property can be used for the detection of electromagnetic waves.
In 1894, the English physicist Lodge for the first time used the pipe Branly, which he called a "coherer" (from the Latin coheare - engage, communicate) to record the passage of electromagnetic waves. It is possible to increase the reception range up to several tens of meters. To restore the sensitivity coherer after the passage of electromagnetic waves Lodge set continuously operating clock mechanism that is constantly shaking it. In fact, Lodge had to do only a step to create a radio, but he did not do this step.
For the first time the idea of the possibility of the use of electromagnetic waves for the needs of communication was described by the Russian engineer Popov. He pointed out that the transmitted signal can be given a certain length (for example, some signals to make longer, others - shorter) and Morse code to transmit wirelessly dispatches. However, the device is made sense only if it were possible to achieve a stable broadcast over a long distance. After studying tube Branly and Lodge, Popov set to develop even more sensitive coherer. In the end he managed to create a very sensitive coherer with platinum electrodes, filled with iron filings.
The next problem was the improvement in the process of shaking sawdust after sticking caused by the passage of an electromagnetic wave. All mechanisms used to restore Lodge coherer sensitivity can not be guaranteed of the scheme: this shaking was erratic and could lead to missing signals. Popov was looking for an automatic method that would allow to restore the sensitivity coherer only after the signal has been received. Having done a lot of experiments, Popov invented a way of periodic shaking coherer using hammer electric bell and applied electrical switch to turn this call chain. The scheme, developed by Popov, is more sensitive, and already in 1894 he was able to use it to receive signals from a distance of several dozen meters. During these experiments, Popov pointed out that the range of the receiver increases markedly when attached to a vertical wire coherer. It was invented by a receiving antenna, using which Popov has made significant improvements in the working conditions of the receiver. By 1895, he created the device, which was a first in the history of radio.
This radio has been arranged as follows. Sensitive tube with metal filings (coherer) strengthened in a horizontal position; one conclusion tube joined wire segment representing a receive antenna, while the other end - the grounded wire. The electrical circuit is completed through the battery and coherer electromagnetic relay: due to the high resistance of sawdust in a tube (up to 100,000 ohm) current in the battery circuit was insufficient for attraction relay armature. But as soon as the tube is exposed to the electromagnetic waves, sawdust stuck together, and the resistance of the tube is significantly reduced. Increased current in the circuit, and the armature was attracted relay. This was accompanied by the closure of the second circuit, and the current is directed through a ringer relay coil, causing the call to come into action. Hammer struck the bell, and the circuit is opened. Hammer back to its original position by the spring and strikes the tube, shaking the dust. Thus, the tube again sensitized to the electromagnetic waves.
May 7, 1895 Popov demonstrated his radio work during the report at a meeting of the Russian Physico-Chemical Society. The source of electromagnetic oscillations in his experiments served as transmitting dipole antenna, transmitter only Popov spark gaps connected between the antenna and the ground. In January 1896 an article Popov describing his successor has been published in the journal of the society.
Popov then attached to his scheme Morse telegraph and introduced taping. The result was the world's first Wireless - transmitter and receiver with a record for Morse code signals.
Consider carefully his device. Between the battery and the primary winding of the coil Ruhmkorff was included Morse telegraph key. With the closure of this constant battery current key went through windings. Breaker with a high frequency of make and break the circuit, with the result that (see section "Transformer") DC converted to AC. Due to electromagnetic induction in the secondary coil Ruhmkorff tide began high-voltage alternating current. This coil snaps to the gap. Thus, each circuit keyer sparks spawned flows arrester. Shorter or longer closures made short and long streams of sparks, which corresponded to the dots and dashes of Morse code. One arrester pole is grounded, and the other is connected to an antenna that radiates electromagnetic waves generated by the discharger in the surrounding area.
Some of these waves fall into the receiver antenna and induced in her a weak alternating current. Moreover, the duration of each current pulse received corresponded exactly to the duration of the signal arrester. receiver unit has been almost the same as in the previous model: coherer was connected to a battery and an electromagnet switch which drives a writing machine code, included in the circuit instead of a call using the local battery. While coherer not exposed to electromagnetic waves, the resistance was so high that the current coherer not flow in the circuit. When the coherer to produce effects of electromagnetic waves, the resistance is greatly decreased, and the current in the circuit increases so that the electromagnet attracted the anchor, including telegraph circuit. This attraction is not stopped until the electromagnetic waves act on the coherer. At the same time the circuit has come into action hammer that struck the coherer. recent increases resistance. However, if the waves continued to operate, the resistance once again decreased and the status of small resistance continued despite the concussion. All this time, the telegraph was drawing on the belt line. And just when the impact of electromagnetic waves stopped, manifested action tremors, and increased resistance to the old values - the device was turned off before the new wave. Thus on the telegraph tape were drawn dots and dashes, the corresponding signals forwarded mails. March 24, 1896 Popov demonstrated his instrument at the meeting of the Russian Physical and Chemical Society and has made the transmission of signals over a distance of 250 m. The world's first wireless message consisted of two words "Heinrich Hertz".
At the same time Popov his radiotelegraphy installation created young Italian Guglielmo Marconi. From childhood he was strongly interested in electricity, and then became interested in the idea of wireless telegraphy. In 1896, he assembled a transmitter and receiver that are very similar in structure to those invented by Popov. In the same year Marconi brought his invention in England. His mother was an Englishwoman, and thanks to her relations, he was well received in the British Isles. In 1896, Marconi received the British patent for his wireless (it was the first patent, taken on the wiring without wires, so that from a formal point of view, Marconi rightly considered to be the inventor of the radio, since the first was able to patent his invention). In June 1897 it was organized by the corporation for use Marconi's invention. In the 23 years he has shown amazing ingenuity and enterprise. From the first steps of his company received a solid financial footing. Whenever possible, Marconi tried to demonstrate what benefits afforded new wireless communication means. Thus, in June 1898 to be held traditional sailing race in the Dublin area. These races have always attracted attention. Marconi went to Dublin and an agreement with one of the major Irish newspapers that will transmit it by radio from the ship that was in the area of the race, all the information that may interest the public to place them in special paper. The experience was a success overall. Within a few hours Marconi hosted a program that accepts edited. The thus-obtained information ahead of all other, and newspaper circulation has increased significantly. For Marconi, it was also a great success: in the short term the share capital of the company it doubled to 200 thousand pounds. This gave him the opportunity to rapidly improve their wireless. A few years later he was already significantly ahead in their development Popova.
One of the main elements of the first radio was a coherer. It is therefore natural that the main efforts of inventors who wanted to strengthen the sensitivity of the receiving devices, were aimed at its improvement. Marconi first drew attention to the importance of the coherer property - namely, the dependence of its action on the applied voltage to it high-frequency oscillations. To the fullest extent possible to collect the energy of the magnetic field generated by induced in the antenna negligible current, it was necessary to strengthen it. Marconi found a simple and ingenious way to solve this problem. In 1898, he turned to his radio jigger (which means "sorter") - a high-frequency transformer, the primary winding of which is included in one circuit to the antenna, and the secondary - was applied to the coherer. In the same year, Marconi took out a patent for this scheme.
The conductors a and b are denoted antenna circuit in which the primary winding jig c was included. As a result of transformation of a weak antenna current voltage is significantly increased in the secondary circuit. With Jigger d signal is sent to coherer j, k which was connected battery b 'and the K relay, which included the telegraph, as it was in previous schemes. This simple innovation has allowed several times to increase the sensitivity of the first radio receiving station. Transmission distance immediately rose from 30 to 85 miles. In the same year, Marconi carried out the transfer of the English Channel.
Another extremely important step in the direction of increasing the sensitivity of the receiver was made in 1899, the closest assistant Popov Rybkin. In one of the experiments conducted by them, it appears that due to the distances involved devices are not operated. Not being sure of their good working Rybkin tried to include in the coherer circuit instead of the relay and telegraph ordinary telephone and learned that each bit of the station causes a slight crack in the phone, so that you can easily take on any rumor dispatch. The most striking here was that the coherer with such inclusion does not require shaking. The phenomenon is not entirely clear at the time, it was explained only a few years later. The fact is that if you normally coherer worked as a variable resistance, which is a result of the sintering of metal grains varied from almost infinity to a relatively small value, in this scheme, he acted on an entirely different basis and is nothing other than the detector in the modern sense of the word, that is, the device current is passed in one direction only, which had a one-way conductivity and turns (erectile) alternating current to pulsating DC. Straightened detector paltry antenna currents were quite insufficient to actuate the telegraph relay, but is able to operate at a very sensitive device - handset membrane, generating weak sound waves in the same way as in the ordinary phone. With a phone to your ear, you could hear the crackle of long and short, the corresponding dots and dashes of Morse code.
The receiving device to transfer to your phone much easier. Do not become a mechanism recording telegraphic signs, decreased battery, eliminating the need for constant shaking of the metal powder. If the same receiver, works on the recording deck, interference from lightning often leads to false alarms relay and distorted records, the reception at the hearing at a certain skill telegraph gave more opportunity to highlight the correct alternating telegraph characters on the background of a chaotic noise cod. But the most significant advantage of the new receiver has more than its considerable sensitivity.
The next step in improving the radio has been associated with an increase in their selectivity, since the first attempts to move from experiment to practical use electromagnetic waves to transmit signals over a distance of acutely demonstrated that the further development of this new form of communication and its widespread use would be possible only if found to be effective means that enable simultaneously transmitting multiple broadcast stations.
This problem was solved then it is very simple for the case of a wired connection. Enough was each of the receiving apparatuses located at any point, to connect individual wires to their respective transmit unit. But as it should do in the case of wireless transmission? Experience of the first stations Popov and Marconi immediately opened all the imperfections in this respect applies if equipment. Receiving signals within range of two simultaneous stations turned out to be due to interference absolutely impossible. The solution was found in the transmission waves of a wireless telegraphy signals using different lengths for their selection in the receiver of the resonance phenomenon.
To understand the essence of this method, consider the properties of more inductors and capacitors. Imagine a coil with a large number of turns through which alternating current passes. The changing electric current, as mentioned above, generates in the environment changing magnetic field, which in turn creates a changing electric field. This electric field induces in the windings of the coil electric current is directed towards the main - there is a phenomenon called self-induction. Externally this effect, in particular, that the closure of the current circuit at any coil reaches its maximum value immediately, but with some delay compared to, for example, with a conventional straight conductor. At the opening of the network changing electric field induces a current in the coil, which coincides with the direction of the main, and therefore the current in the coil is maintained for some time after power-off. This coil detain property, and how to save yourself some time to talk without any external influences characterized by a particular value, called inductance. Each coil has the inductance value of which depends on the conductor sizes and shapes, but is independent of the current flowing.
With regard to the condenser, it usually consists of two plates located very close next to each other but separated by a dielectric material that is not permeable electric current. The plates of the capacitor plates called him. If you connect the capacitor plates to the poles of a DC source (such as an electric battery), then they will accumulate electric charge, which will continue after the battery is disconnected. The ability of a capacitor to store the charge is determined by its electric capacity. Each capacitor has its electrical capacitance, and its value depends on the area of the plates, the distance between them and the dielectric properties of separating. If the capacitor plates to connect a piece of wire, it will discharge its fast - electrons with the plate, where they are in excess, will flow to the other, where they did not have enough, then the charge of each of the plates is zero.
Well, if you discharge the capacitor is not on itself, but through the induction coil? In this case, there is a very interesting phenomenon. Imagine a charged capacitor to the plates which are attached coil. Obviously, the capacitor begins to discharge, and the electric current will be in the chain, but its strength once it reaches the maximum value, and will increase gradually due to the phenomenon of self-inductance of the coil. At that moment, when the capacitor is fully discharged, the current in the coil reaches a maximum value. What happens? Despite the fact that both the plate capacitor will already have zero charge, current flow through the coil continue, as the same result of self-induction current in the coil can not stop instantaneously. The coil turns as if for a few moments in the current source and a capacitor will be charged in the same way as did the electric battery. Only now the charges are reversed plates - one that, until this has been negatively charged, becomes positive, and vice versa. As a result, when the current in the coil is equal to zero, the capacitor will be charged again. It is, however, at the same moment again begin to discharge through the coil, and the entire process will be repeated in the opposite direction. If it were not for the inevitable power losses, such recharging could take place as long as desired.
This phenomenon is called electrical oscillations, and system condenser - a coil in which these vibrations - an oscillatory circuit. Depending on the number of times per second capacitor recharge time to speak of varying the oscillation frequency. The oscillation frequency is directly related to the properties of the oscillating circuit, especially, inductance and capacitance of the condenser coil. It is noticed that the smaller the value, the greater the frequency of oscillation in the circuit, the capacitor has time to be recharged more times per second.
As with any fluctuations (eg pendulum), fluctuations in the system condenser - a coil, if not support from the outside, eventually cease, as the initial energy is consumed for heating wires and electromagnetic radiation. This means that with the maximum value of each oscillation of the coil current and the maximum voltage on the capacitor plates will be less and less. But just as the pendulum swings in mechanical watches, electrical oscillations can be maintained, if, for example, to connect the capacitor to the external AC power. But the alternating current, as we remember, too, is changing its value with a certain frequency, or, in other words, has a natural frequency. Any oscillation circuit is not indifferent to what is the frequency of oscillation of its current feed. If, for example, the current is too large or too small frequency fluctuations as compared to the frequency of oscillation of the circuit, the current and the voltage in the resonant circuit will never be large (as it is the external action will have more to interfere with his own volatile than help them). However, in cases where the frequency of the external current waveform is close to the natural frequency of the oscillation circuit, current and voltage loop current begin to increase and reach their maximum at the frequency of complete coincidence. In this case we say that the oscillatory circuit is at resonance. Especially pronounced resonance in the circuits with little resistance. In this case, the voltage on the capacitor and the coil may exceed the external voltage supply current many times. There is a sort of spike or surge voltage.
The phenomenon of electrical resonance and has been used for election radio. Marconi became one of the first to configure oscillatory circuits of the transmitting and receiving stations on the same frequency. For this purpose, inter alia, using a jig including parallel to the capacitor and the secondary winding thereby obtaining an oscillation circuit. transmitter circuit also been changed in the antenna switching circuit of inductive coils and capacitors, so that each transmission station can transmit signals at a certain frequency oscillation wave. Since it is now several radio stations broadcast messages each with its own frequency, the waves emitted by them excited in receiving antenna alternating currents of different frequencies. But the receiver to select only those signals whose frequency coincides with the natural frequency of its oscillation circuit, it is only in this case the resonance phenomenon observed. Jigger in this scheme has worked as a filter and not intensified any antenna current (as before), and singled out among them that the current frequency to which the receiver is tuned. Since that time, the resonant circuits have become an integral part of both receiving and transmitting devices.
At the beginning of the XX century, several dozen scientists in many countries enthusiastically engaged in wireless telegraphy. However, the greatest success is still connected with the name of Marconi, who was undoubtedly one of the most outstanding radio technicians that time. After a series of experiments, transmission over long distances Marconi made a startling discovery - it turned out that the convexity of the globe does not prevent the movement of electromagnetic waves. This prompted him to experiment on telegraphing across the ocean. Already in 1901 the first ever transatlantic radio transmission, during which the assistant Marconi, Fleming gave to the English station in Poldi Morse code letter "S", and Marconi, who was on the other side of the Atlantic Ocean, on the island of Newfoundland, took it at a distance 1,800 miles.
The next thing to be improved was the creation of new receivers volnouloviteley (detectors). Coherer Branly has played an important role in the early years of radio. However, he was too moody and difficult to handle. In addition, it had to constantly shake to restore the ability to respond to regular radio. One of the main problems was the creation of "self-tuning" coherer. The first attempt in this direction was made in 1899 Popov with the phone. Second Marconi constructed in the early XX century, a magnetic detector.
The operating principle of the magnetic detector is based on the phenomenon of the so-called hysteresis. The fact that iron is usually magnetized with a certain time delay. However, the magnetization can be strengthened when the external magnetic field cause a noticeable tremor iron molecules. This can be done by mechanical shock or other short pulse magnetic field. This phenomenon was used by Marconi.
Its magnetic detector on two roller drive stretched an endless belt made of soft iron wire, moves at a speed of five inches per second, and held under the poles of two permanent magnets inside a small glass tube. On this tube wound primary and secondary windings, the primary winding is included in the antenna circuit was attached to a secondary telephone. Passing under the poles of a magnet, metal tape magnetized first in one and then in the opposite direction. Needless reversal occurred under medium twin poles of the same name, but Mr.