The antenna, regardless of its design, has the property of reversibility (it can work both for reception and for radiation). Often in microwave links, the same antenna can be connected to both the receiver and transmitter at the same time. This allows the signal to be emitted and received in the same direction at different frequencies.
Almost all parameters of the receiving antenna correspond to the parameters of the transmitting antenna, but sometimes they have a slightly different physical meaning.
Despite the fact that the receiving and transmitting antennas have the principle of duality, in terms of design, they can differ significantly. This is due to the fact that the transmitting antenna must pass significant powers through itself to transmit the electromagnetic signal over long (maximum possible) distances. If the antenna works for reception, then it interacts with fields of very low intensity. The type of the current-transmitting structure of the antenna often determines its final dimensions.
Perhaps the main characteristic of any antenna is the directional pattern. Many auxiliary parameters and such important energy characteristics as gain and directivity follow from it.
Directional pattern
The directional pattern (DP) is the dependence of the field strength created by the antenna at a sufficiently large distance from the observation angles in space. In volume, the directional antenna diagram may look like the one shown in Figure 1.
Picture 1
What is shown in the figure above is also called the spatial diagrammatic directivity, which is the surface of the volume and can have several maxima. The main maximum, highlighted in red in the figure, is called the main lobe of the diagram and corresponds to the direction of the main radiation (or reception). Accordingly, the first minimum or (less often) zero values of the field strength around the main lobe determine its boundary. All other maximum field values are called sidelobes.
In practice, there are various antennas that can have several directions of maximum radiation, or have no side lobes at all.
For the convenience of the image (and technical application), the MDs are usually considered in two perpendicular planes. As a rule, these are the planes of the electric vector E and the magnetic vector H (which are perpendicular to each other in most media), Figure 2.
Picture 2
In some cases, the BP is considered in the vertical and horizontal planes with respect to the plane of the Earth. Plane diagrams are depicted by polar or Cartesian (rectangular) coordinate systems. In polar coordinates, the diagram is more visual, and when superimposed on a map, you can get an idea of the coverage area of the radio station antenna, Figure 3.
Figure 3
The representation of the radiation pattern in a rectangular coordinate system is more convenient for engineering calculations; such a construction is more often used to study the structure of the diagram itself. For this, the diagrams are built normalized, with the main maximum reduced to one. The figure below shows a typical normalized reflector antenna pattern.
Figure 4
In the case where the intensity of the side radiation is rather low and it is difficult to measure the side radiation on a linear scale, a logarithmic scale is used. As you know, decibels make small values large and large values small, so the same diagram on a logarithmic scale looks like the following:
Figure 5
A fairly large number of characteristics that are important for practice can be pulled out of the radiation pattern alone. Let's examine in more detail the diagram shown above.
One of the most important parameters is the zero-emission main lobe θ 0 and the half-power main lobe θ 0.5. Half the power is 3 dB, or 0.707 in field strength.
Figure 6
Figure 6 shows that the width of the main lobe for zero radiation is θ 0 = 5.18 degrees, and the width at the half-power level is θ 0.5 = 2.15 degrees.
Also, the diagrams are evaluated by the intensity of the side and back radiation (the power of the side and back lobes), hence two more important parameters of the antenna follow - this is the coefficient of protection, and the level of the side lobes.
Coefficient of protection is the ratio of the field strength radiated by the antenna in the main direction to the field strength radiated in the opposite direction. If the orientation of the main lobe of the diagram is considered in the direction of 180 degrees, then the opposite one is at 0 degrees. Any other directions of radiation are also possible. Let's find the coefficient of protective action of the diagram under consideration. For clarity, we will depict it in a polar coordinate system (Figure 7):
Figure 7
On the diagram, markers m1, m2 represent the radiation levels in the reverse and forward directions, respectively. The protective action coefficient is defined as:
In relative units. Same dB value:
Side-lobe level (LBL) is usually specified in dB, thereby indicating how weak the side-lobe level is compared to the level of the main lobe, Figure 8.
Figure 8
These are two important parameters of any antenna system, which directly follow from the definition of the directional pattern. KND and KU are often confused with each other. Let's move on to considering them.
Directional factor
Directional action factor (CDI) is the ratio of the square of the field strength created in the main direction (E 0 2) to the mean value of the square of the field strength in all directions (E cf 2). As is clear from the definition, directivity characterizes the directional properties of the antenna. LPC does not take into account losses, as it is determined by the radiated power. From the above, you can indicate the formula for calculating the KND:
D = E 0 2 / E cf 2
If the antenna works for reception, then the directivity indicator shows how many times the signal-to-noise power ratio will improve when replacing a directional antenna with an omnidirectional one, if the interference comes uniformly from all directions.
For a transmitting antenna, the directivity figure shows how many times the radiation power must be reduced if the omnidirectional antenna is replaced with a directional one, while maintaining the same field strengths in the main direction.
The directivity of an absolutely omnidirectional antenna is obviously equal to one. Physically, the spatial radiation pattern of such an antenna looks like an ideal sphere:
Figure 9
Such an antenna radiates equally well in all directions, but in practice it is not feasible. Therefore, it is a kind of mathematical abstraction.
Gain
As mentioned above, the directivity does not take into account the antenna loss. The parameter that characterizes the directional properties of the antenna and takes into account the loss in it is called the gain.
The gain (KU) G is the ratio of the square of the field strength created by the antenna in the main direction (E 0 2) to the mean value of the square of the field strength (E oe 2) created by the reference antenna, when the powers supplied to the antennas are equal. We also note that when determining KU, the efficiency of the reference and measured antenna is taken into account.
The concept of a reference antenna is very important in understanding gain, and different types of reference antennas are used in different frequency bands. In the range of long / medium waves, a vertical monopole quarter-wavelength vibrator is taken as the standard (Figure 10).
Figure 10
For such a reference vibrator, D e = 3.28, therefore, the gain of the long-wave / medium-wave antenna is determined through the directivity as follows: G = D * ŋ / 3.28, where ŋ is the antenna efficiency.
In the range of short waves, a symmetrical half-wave vibrator is taken as a reference antenna, for which De = 1.64, then KU:
G = D * ŋ / 1.64
In the microwave range (and this is almost all modern Wi-Fi, LTE and other antennas), an isotropic emitter, giving D e = 1, and having a spatial diagram shown in Fig. 9, is taken as a reference emitter.
The gain is a determining parameter of transmitting antennas, since it shows how many times it is necessary to reduce the power supplied to the directional antenna, in comparison with the reference, so that the field strength in the main direction remains unchanged.
KND and KU are mainly expressed in decibels: 10lgD, 10lgG.
Conclusion
Thus, we have considered some of the field characteristics of the antenna resulting from the radiation pattern and power characteristics (directivity and control). The antenna gain is always less than the directional action, since the gain takes into account the antenna loss. Losses can arise due to the reflection of power back into the feed line of the feed, currents flowing through the walls (for example, a horn), shading of the diagram by the structural parts of the antenna, etc. In real antenna systems, the difference between the directivity and the KU can be 1.5-2 dB.
Providing a sufficiently low level of side lobes in the antenna pattern, as noted earlier, is one of the most important requirements for modern antennas.
When analyzing linear systems of continuously located emitters, the dependence of the level of side lobes on the AR law in the system was observed.
In principle, it is possible to choose such an AR law in the system, in which there are no side lobes in the DP.
Indeed, let there be an in-phase lattice of two isotropic
emitters located at a distance d= - from each other (fig. 4.36).
The amplitudes of the excitation of the emitters will be considered the same (uniform AR). In accordance with formula (4.73), the DN of the two-element lattice
When 0 changes from ± - the value of sin0 changes from 0 to ± 1, and the value of D0) - from 2 to 0. The DN has only one (main) petal (Fig. 4.36). The side lobes are absent.
Consider a linear lattice consisting of two elements, each of which is the lattice discussed above. The new lattice is still considered in-phase, the distance between the elements is X
d = -(fig. 4.37, a).
Rice. 4.36. In-phase array of two isotropic emitters
Rice. 4.37.
The AR law in the lattice takes the form 1; 2; 1 (fig.4.37, b).
In accordance with the rule of multiplication, the DN of the lattice has no side lobes (Fig. 4.37, v):
The next step is an in-phase linear system consisting of two
previous ones, shifted in a straight line at a distance - (Fig.4.38, a). We get a four-element lattice with AR 1; 3; 3; 1 (fig.4.38, b). The BP of this grating also has no side lobes (Fig. 4.38, c).
Continuing, according to the planned algorithm, the increase in the number of emitters in the system, for the DP of the in-phase array, consisting of eight elements, we obtain the formula
Rice. 4.38.
AR in such a lattice will be written accordingly in the following form: 1; 7; 21; 35; 35; 21; 7; 1. The written numbers are coefficients in the expansion of the Newton binomial (1 + x) 7 in a series, therefore the corresponding AR is called binomial.
In the presence of a linear discrete system P emitters binomial AR is determined by the coefficients in the expansion of the Newton binomial (1 + x) n ~ 1, and the MD of the system - by the expression
As we see from expression (4.93), the BP has no side lobes.
Thus, due to the use of binomial AA in the in-phase discrete system, it is possible to achieve complete exclusion of side lobes. However, this is achieved at the cost of a significant expansion (in comparison with a uniform AA) of the main lobe and a decrease in the directivity of the system. In addition, difficulties arise in the practical provision of in-phase excitation of the emitters and a sufficiently accurate binomial AA in the system.
A binomial AR system is very sensitive to changes in PRA. Small distortions in the PRA law cause the appearance of side lobes in the DN.
For these reasons, the binomial AA is practically not used in antennas.
The AR turns out to be more practical and expedient, at which the so-called optimal MD is obtained. The optimal one is understood to be such a DN, in which, for a given width of the main lobe, the level of side lobes is minimal, or at a given level of side lobes, the width of the main lobe is minimal. AR corresponding to the optimal DN can also be called optimal.
For a discrete in-phase system of isotropic emitters,
placed at a distance a> - from each other, optimal is
Dolph - Chebyshevskoe AR. However, in a number of cases (for a certain number of emitters and a certain level of side lobes), this AR is characterized by sharp "bursts" at the edges of the system (Fig. 4.39, a) and difficult to implement. In these cases, one passes to the so-called quasi-optimal AR with a smooth decay to the edges of the system (Fig. 4.39, b).
Rice. 4.39. Amplitude distributions: a- Dolph - Chebyshevskoe;
b - quasi-optimal
With a quasi-optimal AR, in comparison with the optimal level, the level of the side lobes slightly increases. However, it is much easier to implement a quasi-optimal AA.
The problem of finding the optimal and, accordingly, quasi-optimal AA was solved for systems of continuously located emitters. For such systems, the quasi-optimal AR is, for example, the Taylor distribution.
Relative (normalized to the BP maximum) radiation level of the antenna in the direction of the side lobes. As a rule, UBL is expressed in decibels, less often UBL is determined "By power" or "on the field".
An example of an antenna radiation pattern and antenna pattern parameters: width, directivity, UBL, relative level of back radiation
The antenna pattern of a real (finite size) antenna is an oscillating function in which a global maximum is distinguished, which is the center main lobe MDs, as well as other local maxima of the MDs and the corresponding so-called side petals DN. Term side should be understood as side, not literally (sideways petal). The petals of the DN are numbered in order starting from the main one, which is assigned the number zero. The diffractive (interference) lobe of the antenna pattern arising in a rarefied antenna array is not considered a lateral one. The BP minima separating the BP lobes are called zeros(the level of radiation in the directions of the AP zeros can be arbitrarily small, but in reality, radiation is always present). The lateral radiation region is divided into sub-regions: near sidelobe region(adjacent to the main lobe of the DN), intermediate area and posterior side lobe area(the entire rear hemisphere).
- UBL means relative level of the largest side lobe of the pattern... As a rule, the first (adjacent to the main) lateral lobe is the largest in size.
For antennas with high directivity, they also use average side emission(the BP normalized to its maximum is averaged in the sector of lateral radiation angles) and far side lobe level(relative level of the largest side lobe in the region of the posterior side lobes).
For longitudinal radiation antennas, the parameter relative backlight level(from the English. front / back, F / B- the forward / backward ratio), and this radiation is not taken into account when assessing the UBL. The parameter relative side emission(from the English. front / side, F / S- forward / sideways ratio).
UBL, like the width of the main lobe of the antenna pattern, are the parameters that determine the resolution and noise immunity of radio engineering systems. Therefore, in the technical specifications for the development of antennas, great importance is attached to these parameters. The beam width and UBL are monitored both during the commissioning of the antenna and during operation.
UBL reduction targets
- In the receive mode, an antenna with a low UBL is "more noise-immune", since it performs better selection in the space of the useful signal against the background of noise and interference, the sources of which are located in the directions of the side lobes
- Antenna with a low UBL provides the system with greater electromagnetic compatibility with other radio electronic means and high-frequency devices
- Low UBL antenna provides the system with more stealth
- In the antenna of the automatic target tracking system, erroneous tracking along the side lobes is possible
- A decrease in the UBL (with a fixed width of the main lobe of the pattern) leads to an increase in the radiation level in the direction of the main lobe of the pattern (to an increase in directivity): the radiation of the antenna in a direction other than the main one is an empty loss of energy. However, as a rule, with a fixed antenna dimensions, a decrease in the UBL leads to a decrease in the instrumentation, an expansion of the main lobe of the AP and a decrease in the directivity.
The price to pay for a lower UBL is the expansion of the main lobe of the antenna pattern (with fixed antenna dimensions), as well as, as a rule, a more complex design of the distribution system and lower efficiency (in PAA).
Ways to reduce UBL
Since the antenna pattern in the far zone and the amplitude-phase distribution (APD) of the currents along the antenna are related to each other by the Fourier transform, the UBL as a secondary parameter of the pattern is determined by the APR law. The main way lowering UBL when designing an antenna is the choice of a smoother (falling to the edges of the antenna) spatial distribution of the current amplitude. A measure of this "smoothness" is the surface utilization factor (UUF) of the antenna.
The level of the back and side lobes of the voltage radiation pattern γυ is defined as the ratio of the EMF at the antenna terminals when receiving - from the side of the maximum of the rear or side lobe to the EMF from the side of the maximum of the main lobe. When an antenna has several trailing and side lobes of different sizes, the level of the largest lobe is usually indicated. The level of the back and side lobes can also be determined from the power (γ Ρ) by squaring the level of the back and side lobes in terms of voltage. The radiation pattern shown in Fig. 16, the back and side lobes have the same level equal to 0.13 (13%) in terms of EMF or 0.017 (1.7%) in terms of power. The back and side lobes of directional receiving television antennas are usually in the range of 0.1 ..., 25 (voltage).
In the literature, when describing the directional properties of receiving television antennas, the level of the back and side lobes is often indicated, equal to the arithmetic mean of the levels of the lobes at the middle and extreme frequencies of the television channel. Let us assume that the level of lobes (in terms of EMF) of the directional diagram of the antenna of the 3rd channel (f = 76 ... 84 MHz) is: at frequencies of 75 MHz - 0.18; 80 MHz - 0.1; 84 MHz - 0.23. The average level of the petals will be (0.18 + 0.1 + 0.23) / 3, i.e. 0.17. Antenna noise immunity can be characterized by the average level of the lobes only if there are no sharp "spikes" of the level of the lobes in the frequency band of the television channel, significantly exceeding the average level.
An important note should be made regarding the immunity of a vertically polarized antenna. Let us refer to the directional diagram shown in Fig. 16. In this diagram, which is characteristic of horizontally polarized antennas in the horizontal plane, the main lobe is separated from the trailing and side lobes by direction of zero reception. Antennas with vertical polarization (for example, “wave channel” antennas with a vertical arrangement of vibrators) have no direction of zero reception in the horizontal plane. Therefore, the back and side lobes in this case are not uniquely determined and the noise immunity is determined in practice as the Ratio of the signal level received from the front direction to the signal level received from the rear direction.
Gain. The more directional the antenna, i.e. the smaller the opening angle of the main lobe and the lower the level of the back and side lobes of the radiation pattern, the more EMF at the antenna terminals.
Let us imagine that a symmetrical half-wave vibrator is placed at a certain point of the electromagnetic field, oriented to the maximum reception, that is, located so that its longitudinal axis is perpendicular to the direction of arrival of the radio wave. On the matched load connected to the vibrator, a certain voltage Ui develops, depending on the field strength at the receiving point. Let's put it further! to the same point of the field, instead of a half-wave vibrator, an antenna with a higher directivity oriented towards the maximum reception, for example, an antenna of the "wave channel" type, the directional diagram of which is shown in Fig. 16. We will assume that this antenna has the same load as the half-wave vibrator, and is also matched with it. Since the antenna "wave channel" is more directional than the half-wave vibrator, then the voltage across its load U2 will be higher. The voltage ratio U 2 / ’Ui is the voltage gain Ki of the four-element antenna, or, as it is otherwise called,“ field ”.
Thus, the antenna voltage or "field" gain can be defined as the ratio of the voltage developed by the antenna at a matched load to the voltage developed at the same load by a half-wave vibrator matched to it. Both antennas are considered to be located at the same point of the electromagnetic field and oriented to maximum reception. The concept of power gain Kp is also often used, which is equal to the square of the voltage gain (K P = Ki 2).
In determining the gain, two points must be emphasized. First, in order for antennas of different designs to be co-juxtaposed with each other, each of them is compared with the same antenna - a half-wave vibrator, which is considered a reference antenna. Secondly, in order to obtain in practice a gain in voltage or power, determined by the gain, it is necessary to orient the antenna to the maximum of the received signal, i.e. so that the maximum of the main lobe of the radiation pattern is oriented towards the arrival of the radio wave. The gain depends on the type and design of the antenna. Let us refer to an antenna of the "wave channel" type for explanation. The gain of this antenna increases with the number of directors. The four-element antenna (reflector, active vibrator and two directors) has a voltage gain of 2; seven-element (reflector, active vibrator and five directors) - 2.7. This means that if instead of half-wave
vibrator to use a four-element antenna), then the voltage at the input of the television receiver will increase 2 times (power 4 times), and seven-element - 2.7 times (power 7.3 times).
The value of the antenna gain is indicated in the literature either in relation to a half-wave vibrator, or in relation to a so-called isotropic emitter. An isotropic emitter is an imaginary antenna that completely lacks directional properties, and the spatial radiation pattern has, respectively, * the form of a sphere. In nature, isotropic emitters do not exist, and such a emitter is simply a convenient standard with which to compare the directional properties of various antennas. The calculated value of the voltage gain of the half-wave vibrator relative to the isotropic emitter is 1.28 (2.15 dB). Therefore, if the voltage gain of any antenna is known relative to the isotropic radiator, then dividing it by 1.28. we get the gain of this antenna relative to the half-wave vibrator. When the gain relative to the isotropic emitter is specified in decibels, then 2.15 dB must be subtracted to determine the gain relative to the half-wave vibrator. For example, the voltage gain of an antenna relative to an isotropic radiator is 2.5 (8 dB). Then the gain of the same antenna relative to the half-wave vibrator will be 2.5 / 1.28, i.e. 1.95 ^ and in decibels 8-2.15 = 5.85 dB.
Naturally, the real gain in signal level at the TV input, given by a particular antenna, does not depend on which reference antenna - half-wave vibrator or isotropic emitter - the gain is indicated. In this book, gain values are given in relation to a half-wave vibrator.
In the literature, the directional properties of antennas are often estimated by the directivity coefficient of the directivity of the directivity, which is a gain in signal power in the load, provided that the antenna has no losses. The directional action factor is related to the power gain Кр by the ratio
If you measure the voltage at the input of the receiver, you can use the same formula to determine the field strength at the receiving site.
- The side lobe level (SLL) of the antenna directional pattern (BP) is the relative (normalized to the BP maximum) radiation level of the antenna in the direction of the side lobes. As a rule, UBL is expressed in decibels, less often UBL is determined "by power" or "by field".
The antenna pattern of a real (finite size) antenna is an oscillating function in which a global maximum is distinguished, which is the center of the antenna pattern main lobe, as well as other local BP maxima and the so-called side lobes of the pattern corresponding to them. The term lateral should be understood as side, and not literally (petal directed "sideways"). The petals of the DN are numbered in order starting from the main one, which is assigned the number zero. The diffractive (interference) lobe of the antenna pattern arising in a rarefied antenna array is not considered a lateral one. The AP minima separating the AP lobes are called zeros (the radiation level in the directions of the AP zeros can be arbitrarily small, but in reality, radiation is always present). The area of lateral radiation is divided into sub-areas: the area of the near side lobes (adjacent to the main lobe of the antenna pattern), the intermediate area and the area of the rear side lobes (the entire rear hemisphere).
UBL is understood as the relative level of the largest DN sidelobe. As a rule, the largest side lobe is the first (adjacent to the main) side lobe. petal in the region of the posterior side lobes).
For longitudinal radiation antennas, to estimate the level of radiation in the “backward” direction (in the direction opposite to the direction of the main lobe of the antenna pattern), the parameter relative level of the back radiation (from the English front / back, F / B is the forward / backward ratio) is used, and when assessing UBL this radiation is not taken into account. Also, to estimate the level of radiation in the “lateral” direction (in the direction perpendicular to the main lobe of the antenna pattern), the relative side radiation parameter (from the English front / side, F / S is the forward / sideways ratio) is used.
UBL, like the width of the main lobe of the antenna pattern, are the parameters that determine the resolution and noise immunity of radio engineering systems. Therefore, in the technical specifications for the development of antennas, great importance is attached to these parameters. The beam width and UBL are monitored both during the commissioning of the antenna and during operation.
Related concepts
A photonic crystal is a solid-state structure with a periodically changing dielectric constant or inhomogeneity, the period of which is comparable to the wavelength of light.
A fiber Bragg grating (FBG) is a distributed Bragg reflector (a type of diffraction grating) formed in the light-carrying core of an optical fiber. FBGs have a narrow reflection spectrum, are used in fiber lasers, fiber-optic sensors, for stabilization and change of the wavelength of lasers and laser diodes, etc.
