The mechanical wave transfers. Mechanical waves: source, properties, formulas
Wave– the process of propagation of oscillations in an elastic medium.
mechanical wave– mechanical disturbances propagating in space and carrying energy.
Wave types:
longitudinal - particles of the medium oscillate in the direction of wave propagation - in all elastic media;
x
oscillation direction
points of the environment
transverse - particles of the medium oscillate perpendicular to the direction of wave propagation - on the surface of the liquid.
X
Types of mechanical waves:
elastic waves - propagation of elastic deformations;
waves on the surface of a liquid.
Wave characteristics:
Let A oscillate according to the law: 
.
Then B oscillates with a delay by an angle 
, where 
, i.e.
Wave energy.
is the total energy of one particle. If particlesN, then where 
- epsilon, V - volume.
Epsilon– energy per unit volume of the wave – volumetric energy density.
The wave energy flux is equal to the ratio of the energy transferred by waves through a certain surface to the time during which this transfer is carried out: 
, watt; 1 watt = 1J/s.
Energy Flux Density - Wave Intensity- energy flow through a unit area - a value equal to the average energy transferred by a wave per unit time per unit area of the cross section.
[W/m2]
.
Umov vector– vector I showing the direction of wave propagation and equal to the flow wave energy passing through a unit area perpendicular to this direction:
.
Physical characteristics of the wave:
amplitude
wavelength
wave speed
intensity
Vibrational:
Wave:
Complex oscillations (relaxation) - different from sinusoidal.
Fourier transform- any complex periodic function can be represented as the sum of several simple (harmonic) functions, the periods of which are multiples of the period of the complex function - this is harmonic analysis. Occurs in parsers. The result is the harmonic spectrum of a complex oscillation:
BUT
0 
Sound - vibrations and waves that act on the human ear and cause an auditory sensation.
Sound vibrations and waves are a special case of mechanical vibrations and waves. Types of sounds:
simple - harmonic - tuning fork
complex - anharmonic - speech, music
tones- sound, which is a periodic process:
A complex tone can be decomposed into simple ones. The lowest frequency of such decomposition is the fundamental tone, the remaining harmonics (overtones) have frequencies equal to 2 
and others. A set of frequencies indicating their relative intensity is the acoustic spectrum.
Noise - sound with a complex non-repeating time dependence (rustle, creak, applause). The spectrum is continuous.
Physical characteristics of sound:
Hearing sensation characteristics:
Height is determined by the frequency of the sound wave. The higher the frequency, the higher the tone. The sound of greater intensity is lower.
Timbre– determined by the acoustic spectrum. The more tones, the richer the spectrum.
Volume- characterizes the level of auditory sensation. Depends on sound intensity and frequency. Psychophysical Weber-Fechner law: if you increase irritation in geometric progression(in the same number of times), then the feeling of this irritation will increase in arithmetic progression(by the same amount).
, where E is loudness (measured in phons); 
- intensity level (measured in bels). 1 bel - change in intensity level, which corresponds to a change in sound intensity by 10 times. K - proportionality coefficient, depends on frequency and intensity.
The relationship between loudness and intensity of sound is equal loudness curves, built on experimental data (they create a sound with a frequency of 1 kHz, change the intensity until an auditory sensation arises similar to the sensation of the volume of the sound under study). Knowing the intensity and frequency, you can find the background.
Audiometry- a method for measuring hearing acuity. The instrument is an audiometer. The resulting curve is an audiogram. The threshold of hearing sensation at different frequencies is determined and compared.
Noise meter - noise level measurement.
In the clinic: auscultation - stethoscope / phonendoscope. A phonendoscope is a hollow capsule with a membrane and rubber tubes.
Phonocardiography - graphic registration of backgrounds and heart murmurs.
Percussion.
Ultrasound– mechanical vibrations and waves with a frequency above 20 kHz up to 20 MHz. Ultrasound emitters are electromechanical emitters based on the piezoelectric effect ( alternating current to the electrodes, between which - quartz).
The wavelength of ultrasound is less than the wavelength of sound: 1.4 m - sound in water (1 kHz), 1.4 mm - ultrasound in water (1 MHz). Ultrasound is well reflected at the border of the bone-periosteum-muscle. Ultrasound will not penetrate into the human body if it is not lubricated with oil (air layer). The speed of propagation of ultrasound depends on the environment. Physical processes: microvibrations, destruction of biomacromolecules, restructuring and damage of biological membranes, thermal effect, destruction of cells and microorganisms, cavitation. In the clinic: diagnostics (encephalograph, cardiograph, ultrasound), physiotherapy (800 kHz), ultrasonic scalpel, pharmaceutical industry, osteosynthesis, sterilization.
infrasound– waves with a frequency less than 20 Hz. Adverse action - resonance in the body.
vibrations. Beneficial and harmful action. Massage. vibration disease.
Doppler effect– change in the frequency of the waves perceived by the observer (wave receiver) due to the relative motion of the wave source and the observer.
Case 1: N approaches I.
Case 2: And approaches N.
Case 3: approach and distance of I and H from each other:
System: ultrasonic generator - receiver - is motionless relative to the medium. The object is moving. It receives ultrasound with a frequency 
, reflects it, sending it to the receiver, which receives an ultrasonic wave with a frequency 
. Frequency difference - doppler frequency shift:
. It is used to determine the speed of blood flow, the speed of movement of the valves.
The existence of a wave requires a source of oscillation and a material medium or field in which this wave propagates. Waves are of the most diverse nature, but they obey similar patterns.
By physical nature distinguish:
According to the orientation of disturbances distinguish:
|
Longitudinal waves -
The displacement of particles occurs along the direction of propagation; it is necessary to have an elastic force in the medium during compression; can be distributed in any environment. Examples: sound waves  |
Transverse waves -
The displacement of particles occurs across the direction of propagation; can propagate only in elastic media; it is necessary to have a shear elastic force in the medium; can propagate only in solid media (and at the boundary of two media). Examples: elastic waves in a string, waves on water
|
According to the nature of the dependence on time distinguish:
elastic waves - mechanical displacements (deformations) propagating in an elastic medium. The elastic wave is called harmonic(sinusoidal) if the vibrations of the medium corresponding to it are harmonic.
running waves - Waves that carry energy in space.
According to the shape of the wave surface : plane, spherical, cylindrical wave.
wave front is the locus of points to which oscillations have reached present moment time.
wave surface- locus of points oscillating in one phase.
Wave characteristics
Wavelength λ - the distance over which the wave propagates in a time equal to the period of oscillation
Wave amplitude A - amplitude of oscillations of particles in a wave
Wave speed v - speed of propagation of perturbations in the medium
Wave period T - oscillation period
Wave frequency ν - the reciprocal of the period
Traveling wave equation
During the propagation of a traveling wave, the disturbances of the medium reach the next points in space, while the wave transfers energy and momentum, but does not transfer matter (the particles of the medium continue to oscillate in the same place in space).
where v- speed , φ 0 - initial phase , ω – cyclic frequency , A– amplitude
Properties of mechanical waves
1. wave reflection – mechanical waves of any origin have the ability to be reflected from the interface between two media. If a mechanical wave propagating in a medium encounters some obstacle on its way, then it can dramatically change the nature of its behavior. For example, at the interface between two media with different mechanical properties the wave is partially reflected, and partially penetrates into the second medium.
2. Refraction of waves – during the propagation of mechanical waves, one can also observe the phenomenon of refraction: a change in the direction of propagation of mechanical waves during the transition from one medium to another.
3. Wave diffraction – wave deviation from rectilinear propagation, that is, they bend around obstacles.
4. Wave interference – addition of two waves. In a space where several waves propagate, their interference leads to the appearance of regions with the minimum and maximum values of the oscillation amplitude
Interference and diffraction of mechanical waves.
A wave running along a rubber band or string is reflected from a fixed end; this creates a wave traveling in the opposite direction.
When waves are superimposed, the phenomenon of interference can be observed. The phenomenon of interference occurs when coherent waves are superimposed.
coherent calledwaveshaving the same frequencies, a constant phase difference, and the oscillations occur in the same plane.
interference called the constant in time phenomenon of mutual amplification and weakening of oscillations in different points medium as a result of superposition of coherent waves.
The result of the superposition of waves depends on the phases in which the oscillations are superimposed on each other.
If waves from sources A and B arrive at point C in the same phases, then the oscillations will increase; if it is in opposite phases, then there is a weakening of the oscillations. As a result, a stable pattern of alternating regions of enhanced and weakened oscillations is formed in space.
Maximum and minimum conditions
If the oscillations of points A and B coincide in phase and have equal amplitudes, then it is obvious that the resulting displacement at point C depends on the difference between the paths of the two waves.
Maximum conditions
If the difference between the paths of these waves is equal to an integer number of waves (i.e., an even number of half-waves) Δd = kλ , where k= 0, 1, 2, ..., then an interference maximum is formed at the point of superposition of these waves.
Maximum condition
: 
A = 2x0.
Minimum condition
If the path difference of these waves is equal to an odd number of half-waves, then this means that the waves from points A and B will come to point C in antiphase and cancel each other out.
Minimum condition:
The amplitude of the resulting oscillation A = 0.
If Δd is not equal to an integer number of half-waves, then 0< А < 2х 0 .
Diffraction of waves.
The phenomenon of deviation from rectilinear propagation and rounding of obstacles by waves is calleddiffraction.
The relationship between the wavelength (λ) and the size of the obstacle (L) determines the behavior of the wave. Diffraction is most pronounced if the incident wavelength more sizes obstacles. Experiments show that diffraction always exists, but becomes noticeable under the condition d<<λ , where d is the size of the obstacle.
Diffraction is a common property of waves of any nature, which always occurs, but the conditions for its observation are different.
A wave on the water surface propagates towards a sufficiently large obstacle, behind which a shadow is formed, i.e. no wave process is observed. This property is used in the construction of breakwaters in ports. If the size of the obstacle is comparable to the wavelength, then there will be a wave behind the obstacle. Behind him, the wave propagates as if there was no obstacle at all, i.e. wave diffraction is observed.
Examples of the manifestation of diffraction . Hearing a loud conversation around the corner of the house, sounds in the forest, waves on the surface of the water.
standing waves
standing waves are formed by adding the direct and reflected waves if they have the same frequency and amplitude.
In a string fixed at both ends, complex vibrations arise, which can be considered as the result of superposition ( superpositions) two waves propagating in opposite directions and experiencing reflections and re-reflections at the ends. Vibrations of strings fixed at both ends create the sounds of all stringed musical instruments. A very similar phenomenon occurs with the sound of wind instruments, including organ pipes.
string vibrations. In a stretched string fixed at both ends, when transverse vibrations are excited, standing waves , and knots should be located in the places where the string is fixed. Therefore, the string is excited with noticeable intensity only such vibrations, half of the wavelength of which fits on the length of the string an integer number of times.
This implies the condition 
Wavelengths correspond to frequencies 
n = 1, 2, 3...Frequencies vn called natural frequencies strings.
Harmonic vibrations with frequencies vn called own or normal vibrations . They are also called harmonics. In general, the vibration of a string is a superposition of various harmonics.
Standing wave equation :
At points where the coordinates satisfy the condition 
(n= 1, 2, 3, ...), the total amplitude is equal to the maximum value - this antinodes
standing wave. Antinode coordinates
: 
At points whose coordinates satisfy the condition
(n= 0, 1, 2,…), the total oscillation amplitude is equal to zero – this nodes standing wave. Node coordinates: 
The formation of standing waves is observed when the traveling and reflected waves interfere. At the boundary where the wave is reflected, an antinode is obtained if the medium from which the reflection occurs is less dense (a), and a knot is obtained if it is more dense (b).
If we consider traveling wave , then in the direction of its propagation energy is transferred oscillatory movement. When same there is no standing wave of energy transfer , because incident and reflected waves of the same amplitude carry the same energy in opposite directions.
Standing waves arise, for example, in a string stretched at both ends when transverse vibrations are excited in it. Moreover, in the places of fixings, there are nodes of a standing wave.
If a standing wave is established in an air column that is open at one end (sound wave), then an antinode is formed at the open end, and a knot is formed at the opposite end.
wave process- the process of energy transfer without the transfer of matter.
mechanical wave- perturbation propagating in an elastic medium.
The presence of an elastic medium is a necessary condition for the propagation of mechanical waves.
The transfer of energy and momentum in the medium occurs as a result of the interaction between neighboring particles of the medium.
Waves are longitudinal and transverse.
Longitudinal mechanical wave - a wave in which the movement of particles of the medium occurs in the direction of wave propagation. Transverse mechanical wave - a wave in which the particles of the medium move perpendicular to the direction of wave propagation.
Longitudinal waves can propagate in any medium. Transverse waves do not occur in gases and liquids, since they
there are no fixed positions of particles.
Periodic external action causes periodic waves.
harmonic wave- a wave generated by harmonic vibrations of the particles of the medium.
Wavelength- the distance over which the wave propagates during the period of oscillation of its source:
mechanical wave speed- velocity of perturbation propagation in the medium. Polarization is the ordering of the directions of oscillations of particles in a medium.
Plane of polarization- the plane in which the particles of the medium vibrate in the wave. A linearly polarized mechanical wave is a wave whose particles oscillate along a certain direction (line).
Polarizer- a device that emits a wave of a certain polarization.
standing wave- a wave formed as a result of the superposition of two harmonic waves propagating towards each other and having the same period, amplitude and polarization.
Antinodes of a standing wave- the position of the points with the maximum amplitude of oscillations.
Knots of a standing wave- non-moving points of the wave, the oscillation amplitude of which is equal to zero.
On the length l of a string fixed at the ends, an integer n half-waves of transverse standing waves fit:
Such waves are called oscillation modes.
The oscillation mode for an arbitrary integer n > 1 is called the nth harmonic or the nth overtone. The oscillation mode for n = 1 is called the first harmonic or fundamental oscillation mode. sound waves- elastic waves in the medium that cause auditory sensations in a person.
The frequency of oscillations corresponding to sound waves lies in the range from 16 Hz to 20 kHz.
The speed of propagation of sound waves is determined by the rate of transfer of interaction between particles. The speed of sound in a solid v p, as a rule, is greater than the speed of sound in a liquid v l, which, in turn, exceeds the speed of sound in a gas v g.
Sound signals are classified by pitch, timbre and loudness. The pitch of the sound is determined by the frequency of the source of sound vibrations. The higher the oscillation frequency, the higher the sound; vibrations of low frequencies correspond to low sounds. The timbre of sound is determined by the form of sound vibrations. The difference in the shape of vibrations having the same period is associated with different relative amplitudes of the fundamental mode and overtone. Sound volume is characterized by the level of sound intensity. Sound intensity - the energy of sound waves incident on an area of 1 m 2 in 1 s.
Waves. General properties of waves.Wave - this is the phenomenon of propagation in space over time of a change (perturbation) of a physical quantity that carries energy with it.
Regardless of the nature of the wave, the transfer of energy occurs without the transfer of matter; the latter can only occur as a side effect. Energy transfer- the fundamental difference between waves and oscillations, in which only "local" energy transformations occur. Waves, as a rule, are able to travel considerable distances from their place of origin. For this reason, waves are sometimes referred to as " vibration detached from the emitter».
Waves can be classified
By it's nature:
Elastic waves - waves propagating in liquid, solid and gaseous media due to the action of elastic forces.
Electromagnetic waves- propagating in space perturbation (change of state) of the electromagnetic field.
Waves on the surface of a liquid- the conventional name for various waves that occur at the interface between a liquid and a gas or a liquid and a liquid. Waves on water differ in the fundamental mechanism of oscillation (capillary, gravitational, etc.), which leads to different dispersion laws and, as a result, to different behavior of these waves.
With respect to the direction of oscillation of the particles of the medium:
Longitudinal waves - the particles of the medium oscillate parallel in the direction of wave propagation (as, for example, in the case of sound propagation).
Transverse waves - the particles of the medium oscillate perpendicular the direction of wave propagation (electromagnetic waves, waves on media separation surfaces).
a - transverse; b - longitudinal.
mixed waves.
According to the geometry of the wave front:
The wave surface (wave front) is the locus of points to which the perturbation has reached a given moment in time. In a homogeneous isotropic medium, the wave propagation velocity is the same in all directions, which means that all points of the front oscillate in the same phase, the front is perpendicular to the direction of wave propagation, and the values of the oscillating quantity at all points of the front are the same.
flat wave - phase planes are perpendicular to the direction of wave propagation and parallel to each other.
spherical wave - the surface of equal phases is a sphere.
Cylindrical wave - the surface of the phases resembles a cylinder.
Spiral wave - is formed if a spherical or cylindrical source / sources of the wave in the process of radiation moves along a certain closed curve.
plane wave
A wave is called flat if its wave surfaces are planes parallel to each other, perpendicular to the phase velocity of the wave. = f(x, t)).
Let us consider a plane monochromatic (single frequency) sinusoidal wave propagating in a homogeneous medium without attenuation along the X axis.
,where
The phase velocity of a wave is the speed of the wave surface (front),
- wave amplitude - the module of the maximum deviation of the changing value from the equilibrium position,
– cyclic frequency, T – oscillation period, – wave frequency (similar to oscillations)
k - wave number, has the meaning of spatial frequency,
Another characteristic of the wave is the wavelength m, this is the distance over which the wave propagates during one oscillation period, it has the meaning of a spatial period, this is the shortest distance between points oscillating in one phase. 
y 
The wavelength is related to the wave number by the relation , which is similar to the time relation
The wave number is related to the cyclic frequency and wave propagation speed
x
y
y 
The figures show an oscillogram (a) and a snapshot (b) of a wave with the indicated time and space periods. Unlike stationary oscillations, waves have two main characteristics: temporal periodicity and spatial periodicity.
General properties of waves:
Waves carry energy.
2. Waves exert pressure on bodies (have momentum).
3. The speed of a wave in a medium depends on the frequency of the wave - dispersion. Thus, waves of different frequencies propagate in the same medium at different speeds (phase velocity). 
4. Waves bend around obstacles - diffraction.
Diffraction occurs when the size of the obstacle is comparable to the wavelength.
5. At the interface between two media, waves are reflected and refracted.
The angle of incidence is equal to the angle of reflection, and the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant value for these two media.
6. When coherent waves are superimposed (the phase difference of these waves at any point is constant in time), they interfere - a stable pattern of interference minima and maxima is formed. 
Waves and the sources that excite them are called coherent if the phase difference of the waves does not depend on time. Waves and the sources that excite them are called incoherent if the phase difference of the waves changes with time.
Only waves of the same frequency, in which oscillations occur along the same direction (i.e., coherent waves), can interfere. Interference can be either stationary or non-stationary. Only coherent waves can give a stationary interference pattern. For example, two spherical waves on the surface of water, propagating from two coherent point sources, will produce a resultant wave upon interference. The front of the resulting wave will be a sphere.
When waves interfere, their energies do not add up. The interference of waves leads to a redistribution of the energy of oscillations between various closely spaced particles of the medium. This does not contradict the law of conservation of energy because, on average, for a large region of space, the energy of the resulting wave is equal to the sum of the energies of the interfering waves.
When incoherent waves are superimposed, the average value of the squared amplitude of the resulting wave is equal to the sum of the squared amplitudes of the superimposed waves. The energy of the resulting oscillations of each point of the medium is equal to the sum of the energies of its oscillations, due to all incoherent waves separately.
7. Waves are absorbed by the medium. With distance from the source, the amplitude of the wave decreases, since the energy of the wave is partially transferred to the medium.
8. Waves are scattered in an inhomogeneous medium.
Scattering - perturbations of wave fields caused by inhomogeneities of the medium and scattering objects placed in this medium. The scattering intensity depends on the size of the inhomogeneities and the frequency of the wave.
mechanical waves. Sound. Sound characteristic .
Wave- perturbation propagating in space.
General properties of waves:
carry energy;
have momentum (put pressure on bodies);
at the boundary of two media they are reflected and refracted;
absorbed by the environment;
diffraction;
interference;
dispersion;
The speed of the waves depends on the medium through which the waves pass.
Mechanical (elastic) waves.
A special case of mechanical waves - waves on the surface of a liquid, waves that arise and propagate along the free surface of a liquid or at the interface between two immiscible liquids. They are formed under the influence of an external influence, as a result of which the surface of the liquid is removed from the equilibrium state. In this case, forces arise that restore balance: the forces of surface tension and gravity.
Mechanical waves are of two types
Longitudinal waves accompanied by tensile and compressive strains can propagate in any elastic media: gases, liquids and solids. Transverse waves propagate in those media where elastic forces appear during shear deformation, i.e., in solids.
Of considerable interest for practice are simple harmonic or sinusoidal waves. The plane sine wave equation is:
- the so-called wave number ,
– circular frequency ,
BUT - particle oscillation amplitude.
The figure shows "snapshots" of a transverse wave at two points in time: t and t + Δt. During the time Δt, the wave moved along the OX axis by a distance υΔt. Such waves are called traveling waves.
The wavelength λ is the distance between two adjacent points on the OX axis, oscillating in the same phases. A distance equal to the wavelength λ, the wave runs over a period T, therefore,
λ = υT, where υ is the wave propagation velocity.
For any chosen point on the graph of the wave process (for example, for point A), the x-coordinate of this point changes over time t, and the value of the expression ωt – kx does not change. After a time interval Δt, point A will move along the OX axis for a certain distance Δx = υΔt. Consequently: ωt – kx = ω(t + Δt) – k(x + Δx) = const or ωΔt = kΔx.
This implies:
Thus, a traveling sinusoidal wave has a double periodicity - in time and space. The time period is equal to the oscillation period T of the particles of the medium, the spatial period is equal to the wavelength λ. The wavenumber is the spatial analog of the circular frequency.
Sound.
Any oscillatory process is described by an equation. It was also derived for sound vibrations:
Basic characteristics of sound waves
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Subjective perception of sound (volume, pitch, timbre) |
Objective physical characteristics of sound (speed, intensity, spectrum) |
The speed of sound in any gaseous medium is calculated by the formula:
β - adiabatic compressibility of the medium,
ρ - density.
Applying sound
Sonars used underwater are called sonar or sonar (the name sonar is formed from the initial letters of three English words: sound - sound; navigation - navigation; range - range). Sonars are indispensable for studying the seabed (its profile, depth), for detecting and studying various objects moving deep under water. With their help, both individual large objects or animals, as well as flocks of small fish or mollusks, can be easily detected.
Waves of ultrasonic frequencies are widely used in medicine for diagnostic purposes. Ultrasound scanners allow you to examine the internal organs of a person. Ultrasonic radiation is less harmful to humans than x-rays.
Electromagnetic waves.
Their properties.
electromagnetic wave is an electromagnetic field propagating in space over time.
Electromagnetic waves can only be excited by rapidly moving charges.
The existence of electromagnetic waves was theoretically predicted by the great English physicist J. Maxwell in 1864. He proposed a new interpretation of Faraday's law of electromagnetic induction and developed his ideas further.
Any change in the magnetic field generates a vortex electric field in the surrounding space, a time-varying electric field generates a magnetic field in the surrounding space.
Figure 1. An alternating electric field generates an alternating magnetic field and vice versa
Properties of electromagnetic waves based on Maxwell's theory:
Electromagnetic waves transverse – vectors and are perpendicular to each other and lie in a plane perpendicular to the direction of propagation.
Figure 2. Propagation of an electromagnetic wave
The electric and magnetic fields in a traveling wave change in one phase.
The vectors in a traveling electromagnetic wave form the so-called right triplet of vectors.
Oscillations of the vectors and occur in phase: at the same moment of time, at one point in space, the projections of the strengths of the electric and magnetic fields reach a maximum, minimum, or zero.
Electromagnetic waves propagate in matter with final speed
Where - the dielectric and magnetic permeability of the medium (the speed of propagation of an electromagnetic wave in the medium depends on them),
Electric and magnetic constants.
The speed of electromagnetic waves in vacuum
Flux density of electromagnetic energy
orintensity
J
called the electromagnetic energy carried by a wave per unit of time through the surface of a unit area:
,
Substituting here the expressions for , and υ, and taking into account the equality of the volumetric energy densities of the electric and magnetic fields in an electromagnetic wave, we can obtain:
Electromagnetic waves can be polarized.
Likewise, electromagnetic waves have all the basic properties of waves : they carry energy, have momentum, they are reflected and refracted at the interface between two media, absorbed by the medium, exhibit the properties of dispersion, diffraction and interference.
Hertz experiments (experimental detection of electromagnetic waves)
For the first time, electromagnetic waves were experimentally studied
Hertz in 1888. He developed a successful design of an electromagnetic oscillation generator (Hertz vibrator) and a method for detecting them by the resonance method.
The vibrator consisted of two linear conductors, at the ends of which there were metal balls forming a spark gap. When a high voltage was applied from the induction to the carcass, a spark jumped in the gap, it shorted the gap. During its burning, a large number of oscillations took place in the circuit. The receiver (resonator) consisted of a wire with a spark gap. The presence of resonance was expressed in the appearance of sparks in the spark gap of the resonator in response to a spark arising in the vibrator.
Thus, Hertz's experiments provided a solid foundation for Maxwell's theory. The electromagnetic waves predicted by Maxwell turned out to be realized in practice.
PRINCIPLES OF RADIO COMMUNICATIONS
Radio communication transmission and reception of information using radio waves.
On March 24, 1896, at a meeting of the Physics Department of the Russian Physical and Chemical Society, Popov, using his instruments, clearly demonstrated the transmission of signals over a distance of 250 m, transmitting the world's first two-word radiogram "Heinrich Hertz".
SCHEME OF THE RECEIVER A.S. POPOV
Popov used radio telegraph communication (transmission of signals of different duration), such communication can only be carried out using a code. A spark transmitter with a Hertz vibrator was used as a source of radio waves, and a coherer served as a receiver, a glass tube with metal filings, the resistance of which, when an electromagnetic wave hits it, drops hundreds of times. To increase the sensitivity of the coherer, one of its ends was grounded, and the other was connected to a wire raised above the Earth, the total length of the antenna was a quarter of a wavelength. The spark transmitter signal decays quickly and cannot be transmitted over long distances.
Radiotelephone communications (speech and music) use a high-frequency modulated signal. A low (sound) frequency signal carries information, but is practically not emitted, and a high frequency signal is well emitted, but does not carry information. Modulation is used for radiotelephone communication.
Modulation - the process of establishing a correspondence between the parameters of the HF and LF signal.
In radio engineering, several types of modulations are used: amplitude, frequency, phase.
Amplitude modulation - change in the amplitude of oscillations (electrical, mechanical, etc.), occurring at a frequency much lower than the frequency of the oscillations themselves.
A high frequency harmonic oscillation ω is modulated in amplitude by a low frequency harmonic oscillation Ω (τ = 1/Ω is its period), t is time, A is the amplitude of the high frequency oscillation, T is its period.
Radio communication scheme using AM signal
AM oscillator
The amplitude of the RF signal changes according to the amplitude of the LF signal, then the modulated signal is emitted by the transmitting antenna.
In the radio receiver, the receiving antenna picks up radio waves, in the oscillatory circuit, due to resonance, the signal to which the circuit is tuned (the carrier frequency of the transmitting station) is selected and amplified, then the low-frequency component of the signal must be selected.
Detector radio
Detection – the process of converting a high-frequency signal into a low-frequency signal. The signal received after detection corresponds to the sound signal that acted on the transmitter microphone. After amplification, low frequency vibrations can be turned into sound.
Detector (demodulator)
The diode is used to rectify the alternating current
a) AM signal, b) detected signal
RADAR
The detection and precise determination of the location of objects and the speed of their movement using radio waves is called radar . The principle of radar is based on the property of reflection of electromagnetic waves from metals.
1 - rotating antenna; 2 - antenna switch; 3 - transmitter; 4 - receiver; 5 - scanner; 6 - distance indicator; 7 - direction indicator.
For radar, high-frequency radio waves (VHF) are used, with their help a directional beam is easily formed and the radiation power is high. In the meter and decimeter range - lattice systems of vibrators, in the centimeter and millimeter range - parabolic emitters. Location can be carried out both in continuous (to detect a target) and in a pulsed (to determine the speed of an object) mode.
Areas of application of radar:
Aviation, astronautics, navy: traffic safety of ships in any weather and at any time of the day, prevention of their collision, takeoff safety, etc. aircraft landings.
Warfare: timely detection of enemy aircraft or missiles, automatic adjustment of anti-aircraft fire.
Planetary radar: measuring the distance to them, specifying the parameters of their orbits, determining the period of rotation, observing the surface topography. In the former Soviet Union (1961) - radar of Venus, Mercury, Mars, Jupiter. In the USA and Hungary (1946) - an experiment on receiving a signal reflected from the surface of the moon.
The telecommunication scheme basically coincides with the radio communication scheme. The difference is that, in addition to the sound signal, an image and control signals (line change and frame change) are transmitted to synchronize the operation of the transmitter and receiver. In the transmitter, these signals are modulated and transmitted, in the receiver they are picked up by the antenna and go for processing, each in its own path.
Consider one of the possible schemes for converting an image into electromagnetic oscillations using an iconoscope:
With the help of an optical system, an image is projected onto the mosaic screen, due to the photoelectric effect, the screen cells acquire a different positive charge. The electron gun generates an electron beam that travels across the screen, discharging positively charged cells. Since each cell is a capacitor, a change in charge leads to the appearance of a changing voltage - an electromagnetic oscillation. The signal is then amplified and fed into the modulating device. In a kinescope, the video signal is converted back into an image (in different ways, depending on the principle of operation of the kinescope).
Since the television signal carries much more information than the radio, the work is carried out at high frequencies (meters, decimeters).
Propagation of radio waves.
Radio wave - is an electromagnetic wave in the range (10 4
Each section of this range is applied where its advantages can be best used. Radio waves of different ranges propagate at different distances. The propagation of radio waves depends on the properties of the atmosphere. The earth's surface, troposphere and ionosphere also have a strong influence on the propagation of radio waves.
Propagation of radio waves- this is the process of transmitting electromagnetic oscillations of the radio range in space from one place to another, in particular from a transmitter to a receiver.
Waves of different frequencies behave differently. Let us consider in more detail the features of the propagation of long, medium, short and ultrashort waves.
Propagation of long waves.
Long waves (>1000 m) propagate:
At distances up to 1-2 thousand km due to diffraction on the spherical surface of the Earth. Able to go around Earth(Figure 1). Then their propagation occurs due to the guiding action of the spherical waveguide, without being reflected.
Rice. one Connection quality:
reception stability. The quality of reception does not depend on the time of day, year, weather conditions.
Disadvantages:
Due to the strong absorption of the wave as it propagates over the earth's surface, a large antenna and a powerful transmitter are required.
Atmospheric discharges (lightning) interfere.
Usage:
The range is used for radio broadcasting, for radiotelegraphy, radio navigation services and for communications with submarines.
There are a small number of radio stations transmitting accurate time signals and meteorological reports.
Medium waves ( =100..1000 m) propagate:
Like long waves, they are able to bend around the earth's surface.
Like short waves, they can also be repeatedly reflected from the ionosphere.
Connection quality:
Short communication range. Medium wave stations are audible within a thousand kilometers. But there is a high level of atmospheric and industrial interference.
Used for official and amateur communications, as well as mainly for broadcasting.
Short waves (=10..100 m) propagate:
Repeatedly reflected from the ionosphere and the earth's surface (Fig. 2)
Connection quality:
The quality of reception at short waves depends very much on various processes in the ionosphere associated with the level of solar activity, the time of year and the time of day. No high power transmitters required. For communication between ground stations and spacecraft, they are unsuitable, since they do not pass through the ionosphere.
Usage:
For communication over long distances. For television, radio broadcasting and radio communication with moving objects. There are departmental telegraph and telephone radio stations. This range is the most "populated".
Ultrashort waves (
Sometimes they can be reflected from clouds, artificial satellites of the earth, or even from the moon. In this case, the communication range may increase slightly.
The reception of ultrashort waves is characterized by the constancy of audibility, the absence of fading, as well as the reduction of various interferences.
Communication on these waves is possible only at a distance of line of sight L(Fig. 7).
Since ultrashort waves do not propagate beyond the horizon, it becomes necessary to build many intermediate transmitters - repeaters.
Repeater- a device located at intermediate points of radio communication lines, amplifying the received signals and transmitting them further.
relay- reception of signals at an intermediate point, their amplification and transmission in the same or in another direction. Retransmission is designed to increase the communication range.
There are two ways of relaying: satellite and terrestrial.
Satellite:
An active relay satellite receives the ground station signal, amplifies it, and through a powerful directional transmitter sends the signal to Earth in the same direction or in a different direction.
Ground:
The signal is transmitted to a terrestrial analog or digital radio station or a network of such stations, and then sent further in the same direction or in a different direction.
1 - radio transmitter,
2 - transmitting antenna, 3 - receiving antenna, 4 - radio receiver.
Usage:
Contact artificial satellites Earth and
VHF are divided into the following ranges:
meter waves - from 10 to 1 meter, used for telephone communication between ships, ships and port services.
decimeter - from 1 meter to 10 cm, used for satellite communications.
centimeter - from 10 to 1 cm, used in radar.
millimeter - from 1cm to 1mm, used mainly in medicine.
Mechanicalwave in physics, this is the phenomenon of the propagation of perturbations, accompanied by the transfer of energy of an oscillating body from one point to another without transporting matter, in some elastic medium.
A medium in which there is an elastic interaction between molecules (liquid, gas or solid) is a prerequisite for the occurrence of mechanical disturbances. They are possible only when the molecules of a substance collide with each other, transferring energy. One example of such perturbations is sound (acoustic wave). Sound can travel through air, water, or solid body but not in a vacuum.
To create a mechanical wave, some initial energy is needed, which will bring the medium out of equilibrium. This energy will then be transmitted by the wave. For example, a stone thrown into a small amount of water creates a wave on the surface. A loud scream creates an acoustic wave.
The main types of mechanical waves:
- Sound;
- On the surface of the water;
- Earthquakes;
- seismic waves.
Mechanical waves have peaks and troughs like all oscillatory movements. Their main characteristics are:
- Frequency. This is the number of oscillations per second. Units of measurement in SI: [ν] = [Hz] = [s -1].
- Wavelength. The distance between adjacent peaks or troughs. [λ] = [m].
- Amplitude. The greatest deviation of the medium point from the equilibrium position. [X max] = [m].
- Speed. This is the distance that a wave travels in a second. [V] = [m/s].
Wavelength
The wavelength is the distance between points closest to each other, oscillating in the same phases.
Waves propagate in space. The direction of their propagation is called beam and denoted by a line perpendicular to the wave surface. And their speed is calculated by the formula:
The boundary of the wave surface, which separates the part of the medium in which oscillations are already occurring, from the part of the medium in which oscillations have not yet begun, - wavefront.
Longitudinal and transverse waves
One of the ways to classify the mechanical type of waves is to determine the direction of movement of individual particles of the medium in a wave in relation to the direction of its propagation.
Depending on the direction of movement of particles in waves, there are:
- transversewaves. The particles of the medium in this type of waves oscillate at right angles to the wave beam. A ripple in a pond or the vibrating strings of a guitar can help visualize transverse waves. This type of oscillation cannot propagate in a liquid or gaseous medium, because the particles of these media move randomly and it is impossible to organize their movement perpendicular to the direction of wave propagation. The transverse type of waves moves much more slowly than the longitudinal.
- Longitudinalwaves. The particles of the medium oscillate in the same direction as the wave propagates. Some waves of this type are called compression or compression waves. Longitudinal vibrations springs - periodic compressions and extensions - provide a good visualization of such waves. Longitudinal waves are the fastest waves of the mechanical type. Sound waves in air, tsunamis and ultrasound are longitudinal. These include a certain type of seismic waves propagating underground and in water.
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