CPL filter with circular polarization. Photography lessons. Polarization of electromagnetic waves
What is circular polarization?
Circular polarization is the rotation of the black vector E-electric field strength with a frequency of 4,000,000,000 revolutions per second (for the C-band).
Vector E of circular polarization can be represented as two orthogonal vectors, H and V, the magnitude of which constantly changes as the black vector rotates. It can be seen from the figure that if we take one of the orthogonal vectors instead of a rotating vector, then the signal magnitude will be half as large. Therefore, if a linear converter receives a signal with circular polarization, the loss will be 3 dB. Therefore, in order to receive the entire signal, it is necessary to convert circular polarization to linear, for this a depolarizer is used. A dielectric can be used as a depolarizer.
If the dielectric polarizer is located at an angle of 45 degrees, the vectors H and V at the output of the depolarizer are added in one phase due to the delay and acceleration of the components H and V in the dielectric. Thus, the magnitude of vector E is twice as large as vectors V and H. Depending on the angle of the dielectric polarizer to the converter electrode, circular polarization of right or left rotation will be assumed. Because A dielectric located perpendicular or longitudinal to the H and V vectors does not affect them, then using a mechanical or magnetic polarizer it is possible to create a converter that accepts all types of polarization. Such a converter will work on a satellite antenna fixedly aimed at one satellite, which, as a rule, makes no sense, or on an antenna with a polar suspension. The converter waveguide on a polar suspension antenna rotates depending on the direction of the antenna, and the angle of rotation of the converter is determined by the mechanical design of the antenna. Now, if you need to accept circular polarization, then you need to install the polarizer electrode at an angle of 45 degrees. to the dielectric, and if linear polarization, then parallel or perpendicular to the dielectric.
With this arrangement of the electrode, circular polarization will be assumed.
HellasSat
Angle: 39 East
Band: Ku
Frequency: 11630 MHz
Polarization: Horizontal
Symbol Rate: 20.500 Msps
NSS 6
Angle: 95 East
Band: Ku
Frequency: 11017 MHz
Polarization: Vertical
Symbol Rate: 10.500 Msps
Express AM1 NARROW
Angle: 40 East
Band: Ku
Frequency: 11656.75 MHz
Polarization: Vertical
Symbol Rate: 20.802 Msps
Express AM22
Angle: 53 East
Band: Ku
Frequency: 10974.4 MHz
Polarization: Vertical
Symbol Rate: 32.223 Msps
NSS 6
Angle: 95 East
Band: Ku
Frequency: 11017.4 MHz
Polarization: Vertical
Symbol Rate: 10.500 Msps
ABS1
Angle: 75 East
Band: Ku
Frequency: 12609 MHz
Polarization: Vertical
Symbol Rate: 22.000 Msps
HellasSat2
Angle: 39 East
Band: Ku
Frequency: 11512 MHz
Polarization: Horizontal
Symbol Rate: 30.000 Msps
Eutelsat W6
Angle: 21.5 East
Band: Ku
Frequency: 11435 MHz
Polarization: Horizontal
Symbol Rate: 28.782 Msps
Telstar 12
Angle: 15 W
Band: Ku
Frequency: 11000 MHz
Polarization: Vertical
Symbol Rate: 6.336 Msps
Yamal 200 90E
Page 2
Circular polarization corresponds to a constant value of emf, regardless of the angle of rotation of the antenna.
| Optical design for measuring CD. The radiation enters from the left, is deflected downward by mirrors M and M, is plane polarized by the composite prism P and passes through the Fresnel parallelepiped R, where it is subjected to two internal reflections, which leads to a phase shift of a quarter wavelength, i.e., to circular polarization. Using screen A, unwanted radiation is eliminated and necessary radiation is passed through. This entire circuit is placed in the cuvette compartment of standard spectrophotometers, the second circuit (with the opposite orientation is needed for comparison. The sample is placed at point b when measuring CD or at point a when studying the transmission of plane-polarized radiation. |
Circular polarization is carried out in two stages. First, the radiation flux must be made plane-polarized, and then the polarized flux must be passed through a device that decomposes it into components with right and left circular polarization. Then one of the components should be shifted in phase by one quarter of the wavelength. Three types of circular polarization devices are most important: the Fresnel parallelepiped, the electro-optical Pockels modulator, and the photoelastic modulator.
Circular polarization, and the reflected wave is circular polarization of the opposite sign, which is due to a change in the direction of its propagation to the opposite with the same direction of rotation of the vector E in space.
Circular polarization can be achieved by passing linearly polarized light through a quarter-wave plate so that the plane of polarization of the incident beam makes an angle of 45 with the principal directions in the plate. Therefore, a distinction is made between left and right elliptical (circular) polarization.
A circularly polarized wave can be defined as radiation in which the vector of an electric field of constant amplitude rotates around the direction of propagation, making one revolution per period of the oscillation frequency.
The circular polarization exciter is a section of a rectangular waveguide, on the wide wall of which a round waveguide is attached, connected to it by three coupling slots.
The direction of circular polarization can be reversed by changing the polarization of the incident light by 90.
The conversion of circular polarization to linear is achieved by introducing, using some device, an additional phase difference b l / 2 of two waves polarized in mutually perpendicular directions. Typically, a quarter-wavelength plate is used for this purpose (see Chapter. A Fresnel prism actually also serves as a device that introduces an additional phase difference between two waves polarized in mutually perpendicular directions. This method has the advantage that the achieved phase shift depends little on wavelength of incident light.
With circular polarization, the vector length does not change. The most common types of polarization are vertical and horizontal.
A circularly polarized wave is incident on a circularly polarized antenna.
A circularly polarized antenna can, of course, also be used to receive linearly polarized waves, just as a linearly polarized antenna can be used to receive circularly polarized waves.
The circular polarization exciter is a section of a rectangular waveguide, on the wide wall of which a round waveguide is attached, connected to it by three coupling slots. The arrangement of the slits is designed in such a way that it ensures the excitation of forward and backward waves of circular polarization, regardless of frequency, throughout the entire operating frequency range of the device. On the wide wall there is a communication probe with a transition to a coaxial connector.
§8. Producing light with elliptical or circular polarization
To prove this statement, consider the superposition of two waves of the same frequency, polarized in mutually perpendicular planes, which is equivalent to the decomposition of an arbitrary monochromatic wave into two mutually orthogonal components.
Wave equations
Where φ - phase shift between waves.
Equations (1) are the equation of an ellipse in parametric form. To verify this, let us exclude the time parameter from these equations t.
To do this, we write the equations in the form
By squaring equations (2) and (4) and using the identity, we obtain
From where after transformations
This is the equation of an ellipse inscribed in a rectangle with sides 2 A x and 2 A y(see picture)
At φ =π /2 and A x =A y =A the ellipse degenerates into a circle, and when φ =π m, Where m= 0, 1, 2, … -into a straight segment:
Thus, elliptical polarization is a general case of polarization of a monochromatic wave, of which circular and linear polarization of waves are special cases.
§9. Birefringence. Methods for producing linearly polarized light
In nature, there are isotropic and anisotropic crystals (uniaxial and biaxial). In an isotropic crystal, the speed of the light wave is the same in all directions. In an anisotropic uniaxial crystal, as experience shows, two waves arise: ordinary(o-wave) and extraordinary(e-wave). Two extraordinary waves arise in biaxial crystals.
In a uniaxial crystal the speed v o propagation of the o-wave is the same in different directions, and the speed of propagation of the e-wave v e-various. Therefore, the front of the o-wave is spherical, and the front of the e-wave is elliptical. Depending on the type of crystal it is possible v e >v o(negative crystal) or v e >v o(positive crystal).
There is a direction in the crystal in which the velocities v e And v o ordinary and extraordinary waves are the same. This direction is called optical axis crystal. In the direction of the optical axis, the fronts of o- and e-waves (sphere and ellipsoid) touch each other. Any plane parallel to the optical axis of the crystal is called main section crystal. If a light ray is applied to the boundary of a uniaxial crystal, then two refracted rays are formed at its boundary: ordinary (o-ray) and extraordinary (e-ray), corresponding to the o- and e-waves in the crystal. This phenomenon is called birefringence.
It turns out that o- and e-rays linearly polarized. Moreover, the o-ray is polarized in a plane perpendicular to the plane of the main section of the crystal, and the e-ray is parallel to the main section (see figure). The o-ray obeys the usual law of refraction: but the e-ray does not. Therefore, if a beam of light falls on a uniaxial crystal perpendicular to its boundary, then the resulting o-ray is not refracted, but the e-ray is refracted. If a shutter is placed in the path of the o- or e-ray at the output of the crystal, then a linearly polarized o-or e-ray will remain at its output.
If a crystal is cut so that its optical axis is parallel to the crystal boundary and a light ray falls on the crystal perpendicular to the boundary, then the o- and e-rays formed in the crystal are not refracted. In this case, two waves will propagate in the crystal in one direction, perpendicular to the optical axis, polarized in two mutually perpendicular planes.
The speed of propagation of these waves v o And v e are different. Therefore, when passing through the crystal, these waves will shift relative to each other and a certain phase difference will arise between them φ , depending on the thickness of the crystal. As has been shown, the addition of two waves of the same frequency, polarized in two mutually perpendicular planes, generally gives an elliptically polarized wave of the same frequency.
In particular, a circularly or linearly polarized wave can be obtained at the output of the crystal. This issue will be considered in detail after studying the interference and diffraction of waves.
There are uniaxial crystals that absorb vibrations perpendicular to the optical axis of the crystal, i.e. absorbing ordinary waves. Such crystals are called polaroids (for example, Nicole[Nicolas prism]). The output of the polaroid will always be linearly polarized light in a plane parallel to the optical axis of the crystal.
Let in the direction of the axis OZ two electromagnetic waves propagate. The electric field strength of one wave oscillates in the direction of the axis OY in law EY(z, t)= Eosin(kz-wt), and the other - in the direction of the axis OX in law Ex(z, t)= Eocos(kz-wt).Phase of wave oscillations with an electric field oriented along the axis OX, lags behind p/2 from the phase of another wave. Let us find out the nature of the oscillations of the tension vector of the resulting wave.
You can simply make sure that the modulus of the resulting wave does not change over time and is always equal to Eo. Tangent of the angle between the axis OX and the vector of the electric field strength at the point z equals
tgj===tg(kz-wt). (1)
From (1) it follows that the angle between the vector of the electric field strength of the wave and the axis OX-j- changes over time according to the law j(t)=kz-wt.The electric field strength vector rotates uniformly with an angular velocity equal to w. The end of the electric field strength vector moves along a helix (see Figure 27). If you look at the change in the intensity vector from the origin in the direction of wave propagation, then the rotation occurs clockwise, i.e. in the direction of the magnetic induction vector. Such a wave is called right circularly polarized.
An electromagnetic wave with circular polarization, incident on a substance, transmits rotation to the electrons of the substance.
Result: right-polarized an electromagnetic wave has an angular momentum directed along the propagation of the wave, left-handed An electromagnetic wave has an angular momentum directed against the propagation of the wave. This result will be used in the study of quantum physics.
When adding plane waves of linear polarization with planes oriented at right angles and with an arbitrary phase shift a, the resulting change in the tension vector at a given point z can be rotation with simultaneous periodic change in module. The end of the electric field strength vector of the wave in this case moves along an ellipse. This type of polarization is called elliptical. It can be either left or right. Figure 29 shows the trajectories of the end of the strength vector of the resulting electric field of two waves of the same amplitude with horizontal and vertical planes of polarization at different values of the phase shift - from 0 before p. When the phase shift is equal to zero, the resulting wave is plane-polarized with the plane of polarization making an angle p/4 with a horizontal plane. With a phase shift equal to p/4, – elliptical polarization, at p/2– circular polarization, at 3p/4– elliptical polarization, at p– linear polarization.
In the case where the wave is a sum of randomly polarized components with a chaotic set of phase shifts, all polarization effects are lost. They say that the electromagnetic wave in this case is not polarized.
Any antenna, for example, "BOF-5xxx + Reflector" has a certain radiation sector. While spreading in this sector, part of the electromagnetic energy goes into space without reaching the receiver antenna. Some of the energy emitted below the horizon reaches the earth's surface. In this case, the energy is partially absorbed by the surface and partially reflected from the ground. This reflected signal also hits the receiving antenna. Summing up in the receiving antenna with a certain time delay and with a random phase in relation to the main signal, the reflected signal is a significant interference.
A feature of radio waves with elliptical polarization is that when the signal is reflected, its rotation vector changes to the opposite.
Fig.2. Change in direction of rotation upon reflection of an elliptically polarized wave.
The emitted signal with right-hand rotation will rotate to the left after reflection. With linear polarization, the signal retains its polarization vector upon reflection.
Fig.3. Change in the polarization vector upon reflection of a radio wave with elliptical polarization.
Circularly polarized antennas do not receive counter-rotating signals.
And therefore, at the receiving antenna, the reflected signal, now in the opposite polarization, will not induce E.M.F. The receiving antenna simply will not “see” this signal.
When constructing wireless communication channels on circularly polarized antennas, the peculiarity of signal reflection in mirror antennas should be taken into account. By using an active element in such an antenna that radiates with right-hand rotation of polarization (for example, a BOF-2xxx RHCP feed), you will receive a signal from the antenna with a left-hand rotation vector (LHCP).
Therefore, such an antenna (note: “Dish+BOF-2xxx RHCP”) will only work with LHCP-polarized antennas. And, accordingly, vice versa.
Fig.4. A circularly polarized wave changes its directional vector when reflected from a parabolic reflector.
Please note that you cannot change the polarization by simply rotating the antennas 90°, as you could do with linearly polarized antennas. The polarization vector is set during the antenna production process and cannot be changed by the user.
Therefore, consider the configuration of your network and its possible further development (expansion) before ordering equipment.
If you find it difficult to decide what equipment you need, contact us. We will select for you only the necessary equipment that works with each other. A minimum set of optimal products, without “rubbing in” unnecessary rubbish.
Another advantage of using circularly polarized antennas
Under ideal conditions, when the signal propagates without obstacles, there is no difference in how the polarization vector of the signal is oriented in space.
In a real situation, there are a lot of obstacles and barriers to the propagation of a radio signal. Some obstacles allow the signal to pass freely, some are partially attenuated, others are completely or partially reflected or irretrievably absorbed.
Figure 5 clearly shows the propagation of linearly polarized radio waves, in the path of which there are obstacles in the form of a series of parallel metal rods located vertically and horizontally.
Fig.5. The passage of a linearly polarized signal through a series of parallel metal barriers.
Radio waves with vertical polarization are completely reflected from vertically oriented conductive obstacles. But at the same time, a signal with horizontal polarization overcomes this obstacle practically without attenuation.
On the contrary, a horizontally polarized radio wave penetrates unhindered through a series of vertical metal barriers.
Just one obstacle located at an angle of 45 degrees reduces the signal level by half. Moreover, this is true for both vertical and horizontal polarization. (See Fig.6)
Rice. 6. The influence of interference located at an angle of 45 degrees on signal propagation.
In real practice, a linearly polarized wave cannot overcome a number of vertically and horizontally oriented obstacles.
Although the situation seems “laboratory”, artificially created, in practice it is the most common. Moreover, these same obstacles are often not strictly orthogonal, but on the contrary have a range of variations.
Figure 6 clearly illustrates the changes in a linearly polarized signal after passing through just one tree:
Fig.6. The passage of a linearly polarized signal through the crown of just one tree.
Pay attention to the receiving party. The signal to the antenna arrives weakened; At the same time, a reflected signal arrives, and not in phase with the main signal
There are not only multiple reflections of the signal, and in different directions, its dispersion in space, but also distortion of the polarization vector during reflection.
As a result, the receiving antenna receives a multi-beam signal that is heterogeneous in signal level and polarization; having random phase and delay time due to different distance traveled.
All signals that reach the receiving antenna late from the main signal become interference (noise).
Often in such cases, with a very high level of the received signal, a low channel speed is set. This is due to the fact that only simple types of modulation can be accurately detected in conditions of multi-beam interference reception.
Is it possible to somehow combat this?
The only thing that really works in such conditions is antennas with elliptical polarization.
Their “range and penetration” is explained by the peculiarity of the passage of radio waves with a rotating polarization vector through obstacles.
Fig.7. Passage of an elliptical polarization signal through a number of barriers.Our "laboratory" example.
We see that when passing parallel oriented obstacles, the elliptical polarization signal loses only half of its energy to reflection, and absolutely regardless of the location of these obstacles. In practice, an elliptical polarization signal, like a corkscrew through a traffic jam, penetrates “complex” obstacles where linear polarization is powerless.
Let's look at an example of how a signal with elliptical polarization will pass through the same tree (as in the example above). And how this signal will be perceived by the receiving antenna.
It is obvious that, regardless of the polarization vector, the signal will be reflected in the same way.
Those. at the exit from the crown we will see approximately the same picture, both in the case of linear polarization (see Fig. 6) and in the case of elliptical polarization.
In the propagation of elliptically polarized radio waves, exactly the same signal interference is observed as in the case of a linearly polarized signal. However, reflected signals of elliptical polarization arrive at the antenna in the opposite polarization, having practically no effect on the level of the main signal, because cannot be combined with it.
And all signals arriving in the same polarization as the main one are summed up, increasing the overall level of the received signal. They have different time delays, i.e. phase (the angle at which the signal enters the antenna). At the antenna output, one signal will be recorded with a delay determined by vector addition. Moreover, this output signal will “vary” only in level and time delay.
These features are responsible for such a high “penetration” of the elliptically polarized signal.
In real conditions, MIMO systems “KNOW” BETTER to decouple channels precisely at elliptical polarization. This means that in such systems, when working on antennas with circular polarization, the speed is higher and the connection is more stable.
