Aperture antenna. Horn antenna Horn antenna how to connect
The radiation comes from the open end of the waveguide. To channel electromagnetic energy, rectangular or round waveguides are used.
However, waveguides can be used not only to channel electromagnetic energy, but also to radiate it.
The open end of the waveguide can be considered as a simple microwave antenna.
The open end of the waveguide is a platform with an electromagnetic field.1
Features of the electromagnetic field at the open end of the waveguide.
1. The wave is not transverse type TEM. (has a more complex structure).
2. In addition to the incident wave, there is a reflected one.
3. Along with the main type of wave, higher types of waves are present at the end of the waveguide.
In addition, the field is present not only in the opening of the waveguide, but also on the outer surface due to the flow of currents onto this surface from the end of the waveguide.
Taking these factors into account greatly complicates the problem of determining the radiation field from the open end of the waveguide, and its rigorous mathematical solution encounters great difficulties. For this reason, approximate solution methods are usually used. For this solution, the problem is divided into two tasks: internal and external.
1) The internal task is to find the field in the opening of the waveguide.
2) The external task is to find the radiation field from the known field in the aperture.
Consider a rectangular waveguide.
Basic wave type.
Rice. 45. Rectangular waveguide (a) and the structure of the field in it for a wave of type: in the xOy plane (b); in the xOz plane (c); in the yOz (g) plane.
;
;
.
The intensity of the incident electromagnetic field in the middle of the waveguide opening.
Wavelength in the waveguide.
Wavelength in free space.
Complex reflectance.
Outfield field:
Characteristic impedance of the wave front at the open end of the waveguide.
The characteristic impedance of the medium is .
Taking into account the found field relations in the main planes
Waveguide opening area.
Radiation pattern of the open end of a rectangular waveguide.
Rice. 46. Radiation pattern from the open end of a rectangular waveguide at
As can be seen from the figures, the width of the radiation pattern is large. To obtain a sharper radiation pattern, the waveguide cross-section can be gradually increased, turning the waveguide into a horn. In this case, the field structure in the waveguide is basically preserved.
A smooth increase in the waveguide cross-section improves its coordination with free space.
Rice. 47. Main types of electromagnetic horns.
The most widespread are sectorial and pyramidal horns.
Consider the longitudinal section of a rectangular horn by plane E or H.
Rice. 48. Longitudinal section of a rectangular horn.
Opening the horn
Horn opening width.
Horn length.
The top of the horn.
Horn research is usually carried out using approximate methods due to mathematical difficulties.
Initially, the field in the opening is determined. When solving this problem, the horn is assumed to be infinitely long, and its walls are ideally conductive.
After solving the internal problem, the external problem is solved using the usual method, i.e. is the radiation field.
H – planar sectorial horn.
To find the field structure in the horn we use a cylindrical coordinate system.
The wave will have components.
Rice. 49. Cylindrical coordinate system for the analysis of sectorial horns.
Solving the system of Maxwell's equations and using asymptotic expressions of the Hankel functions for large values of the argument, we obtain the following values for the field components
(1)
.
Here the electric field strength at the horn point with coordinates and .
Formulas (1) show that at large components and the field in the horn represents a transverse electromagnetic cylindrical wave. Due to the fact that most of the horns used have a flat aperture and the wave in the horn is cylindrical, the field in the aperture will not be in phase.
To determine phase distortions in the aperture, consider the longitudinal section of the horn. A circular arc centered at the horn apex follows the wave front and is therefore a line of equal phases. At an arbitrary point having coordinate , the field phase lags behind the phase in the middle of the opening (at point ) by an angle
Rice. 50. Towards the determination of phase distortions in the horn aperture.
Since it is usually in horns, we can limit ourselves to the first term of the expansion
Formula (2) is approximate. They can be used when or. In the horns used, these conditions are usually met.
Sometimes it is convenient to determine the maximum phase errors in the aperture of a horn through its length and half the aperture angle.
The formula is true for any and .
It is clear from the formula that, for a given field in the aperture, the less the difference from the in-phase field will be, the greater the length of the horn. Dimensional restrictions require finding a compromise solution, i.e. determining the length of the horn at which the maximum phase shift in its opening will not exceed a certain permissible value. This value is usually determined by the largest directional gain that can be obtained from a horn of a given length. For a sectorial horn, the maximum permissible phase shift is , which corresponds to the following relationship between the optimal horn length, aperture size and wavelength:
To determine the distribution of field amplitudes in the horn aperture, we take
Thus, the field in the aperture of a sectorial horn can be finally represented by the expressions
In-plane radiation pattern
The characteristic dependences of the directivity coefficient on the relative horn aperture for various horn lengths are given below.
Rice. 51. KND dependence N – sectorial horn on the relative opening width
at different horn lengths.
In order to eliminate the dependence of the directional coefficient on the ordinate axis, the product is plotted. It is clear from the graphs that for each horn length there is a certain horn aperture at which the directivity coefficient is maximum. Its decrease with further increase is explained by a sharp increase in phase errors in the aperture.
The horn, which for a given length has the maximum directional coefficient, is called optimal. From the curves shown in Fig. 3 it is clear that at the maximum point of the curves corresponds to the equality
If the length of the horn is taken longer, then with the same opening area the directional coefficient increases, but not very much. The maximum points of the directional coefficient correspond to the coefficient of utilization of the opening area.
If the length of the horn is continuously increased, then in the limit at we will obtain an in-phase field in the horn aperture. The coefficient of utilization of the common-mode area with a cosine distribution of the field amplitude is equal to . Thus, increasing the length of the horn compared to its optimal length cannot increase the directivity coefficient by more than
Due to low losses, the efficiency of horn antennas can practically be taken as unity.
E-plane sectorial horn.
Field in the aperture of a planar sectorial horn
(1)
Here ; distance from the throat of the horn.
From formula (1) it is clear that the main difference between the field in a planar horn and the field in a waveguide is the cylindrical shape of the wave. As a result, there will be phase distortions in the horn aperture, similar to distortions in a planar horn.
If the opening angle of the horn is small, then you can put . In this case, the electric field strength in the opening can be represented by:
Radiation field of a sectorial horn in a plane
(2)
From this formula it follows that the radiation pattern in the plane of the planar horn is the same as that of the open end of the waveguide.
Field in plane:
(3) . . horns.
In this case, it is convenient to present the formula as:
the quantities in parentheses are directly plotted along the ordinate axes on the indicated graphs.
Horn antenna
The open end of the waveguide can be used as an emitter of electromagnetic energy.
The emission of waves from the open end of the waveguide is explained by the fact that an alternating electromagnetic field exists in the hole and the dimensions of this hole are comparable to the wavelength. Therefore, the waveguide hole can be considered as a multivibrator antenna. The directivity characteristics of such an emitter depend on the type of wave in the waveguide and the size of the hole.
If only one simple wave propagates in the waveguide, for example H 1O, then the directivity characteristic has approximately the same shape as shown in Figure 3.56:
The radiating waveguide is rarely used, because it has the following disadvantages:
There is no matching (i.e., incident waves are reflected from the open end of the waveguide), so a mixed wave regime exists in the waveguide, which leads to unnecessary losses;
The directivity characteristic turns out to be quite wide, because The dimensions of the emitting hole are small compared to the wavelength.
To narrow the directivity characteristic, it is necessary to increase the size of the emitting hole, while maintaining the in-phase field in it. This can be done by attaching a horn antenna to the open end of the waveguide (Fig. 3.57). In practice, three types of horns are used: sectoral, pyramidal, conical.
The first two horns are excited by rectangular waveguides, the third by a circular waveguide. In this case, the main types of waves are used in the waveguide.
The operating principle of a horn antenna is the same as that of a radiating waveguide. An approximately in-phase field is introduced into the horn aperture, and the aperture can be thought of as a multivibrator in-phase antenna. The horn creates a smooth transition from the waveguide to free space. Due to this, the reflection of waves from the radiating hole of the horn is eliminated and waveguide matching is achieved.
Directional characteristic horn antenna depends on its dimensions: length - l, width - d, heights - h, opening angle
Figure 3.58 shows an approximate shape of the directivity characteristic of a sectoral horn. From this figure it can be seen that the width of the main lobe of the characteristic will be smaller in the plane in which the horn size is larger.
Flaws horn antenna: bulky with a narrow directivity characteristic. This disadvantage can be eliminated if, to obtain a sharp response, several shorter horns are used, located nearby and excited in phase.
Advantages horn antenna: simplicity of design, small side lobes.
Properties
Horn antennas are very broadband and match the feed line very well - in fact, the antenna bandwidth is determined by the properties of the exciting waveguide. These antennas are characterized by a low level of the rear lobes of the radiation pattern (up to -40 dB) due to the fact that there is little flow of RF currents to the shadow side of the horn. Horn antennas with low gain are simple in design, but achieving high (>25 dB) gain requires the use of wave phase-aligning devices (lenses or mirrors) in the horn aperture. Without such devices, the antenna has to be made impractically long.
Application
Horn antennas are used both independently and as feeds for mirror and other antennas. A horn antenna, structurally combined with a parabolic reflector, is often called a horn-parabolic antenna. Horn antennas with low gain are often used as measurement antennas due to their favorable set of properties and good repeatability.
Characteristics and formulas
Pyramid horn antenna
The gain of a horn antenna is determined by its opening area and can be calculated using the formula:
Where is the opening area of the horn, is the coefficient of utilization of the horn surface, equal to 0.6 for the case when the difference in the path of the central and peripheral beams is less, but close to, and 0.8 when using wave phase-leveling devices.
Width of the main lobe of the beam according to zero radiation in the H plane:
Width of the main lobe of the beam according to zero radiation in plane E:
Since, if both are equal, the bottom in the H plane turns out to be 1.5 times wider, often, to obtain the same width of the petal in both planes, they choose
To keep phase distortions in the horn aperture within acceptable limits (no more than ) it is necessary that the following condition be met (for a pyramidal horn):
Where and are the heights of the faces of the pyramid forming the horn.
Types of Horn Antennas
Parabolic antenna feed in the form of a conical horn with grooves
- Pyramid horn - antennas in the shape of a tetrahedral pyramid, with a rectangular cross-section. They are the most widely used type of horn antenna. Emits linearly polarized waves.
- Sectoral horn - pyramidal horn with extension in only one plane E or H.
- Conical horn - opening in the shape of a cone with a circular cross-section. Used with cylindrical waveguides to produce circularly polarized waves.
- Corrugated horn - an aperture of horns with parallel slots or grooves that is small compared to the wavelength. Grooves cover the inner surface of the horn, across the axis.
Corrugated horns have wider bandwidth, lower sidelobes and less cross-polarization. They are widely used as feeds for satellite dish antennas and radio telescopes.
Horn parabolic antenna
Horn-parabolic antenna is a type of antenna in which a parabola and a horn are structurally connected. The advantage of this design compared to a horn is the low level of side lobes and a narrow directivity pattern. The disadvantage is that it weighs more than parabolic antennas. An example of use is the horn-parabolic antenna in the Mir space station, antennas for radio relay stations.
Antenna setup
The SWR of the antenna is adjusted in its waveguide part or in the KVP by selecting the position and size of the KVP power supply. The adjustment in the waveguide part is made using pins or diaphragms.
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See what a “horn antenna” is in other dictionaries:
An antenna in the form of a piece of radio waveguide that expands towards the open end. The shape of the horn opening is selected in accordance with the required radiation pattern (Fig.). Coordination of R. a. with an open right is determined by the size of the opening, shape and... ... Physical encyclopedia
horn antenna- Antenna in the form of a waveguide with a smoothly expanding cross-section towards the open end. [GOST 24375 80] Topics radio communications General terms antennas ... Technical Translator's Guide
It consists of a metal expanding socket (horn) and a waveguide connected to it. They are used for directed radiation and reception of ultra-high frequency radio waves, mainly as irradiators, for example. mirror antennas... Big Encyclopedic Dictionary
horn antenna- 3.9 horn antenna: An antenna formed by the expansion of the walls of the waveguide feeding it. Source … Dictionary-reference book of terms of normative and technical documentation
It consists of a metal expanding socket (horn) and a waveguide connected to it. They are used for directed radiation and reception of microwave radio waves, mainly as feeders, for example mirror antennas. * * * HORN… … encyclopedic Dictionary
horn antenna- ruporinė antena statusas T sritis fizika atitikmenys: engl. horn aerial; horn antenna; horn type antenna vok. Hornantenne, f; Trichterantenne, f rus. horn antenna, f pranc. antenne à cornet, f … Fizikos terminų žodynas
An antenna consisting of a metal expanding socket (horn) and a radio waveguide connected to it. R. a. used for directed emission and reception of radio waves (See Emission and reception of radio waves) in the microwave range as... ...
Horn antenna- 1. Antenna in the form of a waveguide with a smoothly expanding cross-section towards the open end. Used in the document: GOST 24375 80 ... Telecommunications dictionary
biconical horn antenna- dvikūgė ruporinė antena statusas T sritis radioelektronika atitikmenys: engl. biconical horn antenna vok. Doppelkonushornantenne, f rus. biconical horn antenna, f pranc. cornet double, m... Radioelektronikos terminų žodynas
A device for emitting and receiving radio waves. The transmitting antenna converts the energy of high-frequency electromagnetic oscillations concentrated in the output oscillatory circuits of the radio transmitter into the energy of emitted radio waves. Transformation... ... Great Soviet Encyclopedia
Calculation of the director antenna……………………………………………3
Calculation of a horn antenna……………………………………………………………10
Calculation of a single-mirror parabolic antenna………………………17
Conclusions on the calculation work……………………………………………..24
List of references……………………………………………………….25
Vibrator antennas are used in the millimeter, centimeter, decimeter, meter and longer wavelength ranges and are straight conductors excited at certain points. Vibrator antennas, depending on the design, have a directivity factor from several units to tens of thousands and are used in radio communication systems, radio navigation, television, telemetry and other areas of radio engineering.
To increase the directivity, a vibrator with a reflector and one or more directors is used. Such an antenna is called a director antenna and is widely used in various fields of radio communications in the VHF range. The more directors, the greater the KND and already the main petal of the DN. Typically, the efficiency of director antennas is 10...30, but designs of director antennas with efficiency = 80...100 are known.
Drawing 1.1 - General view of the director antenna
The figure shows an active vibrator with a length of , a reflector with a length of , a director with a length of , a boom, a mast and an antenna mounting box, as well as the distances from the vibrator to the reflector, from the vibrator to the director, and the length of the antenna itself.
Theoretical calculation of antenna parameters.
In a director antenna, the length of the active vibrator is made equal to the resonant length:
With such a length, the input resistance has a reactive part close to zero. The reflector length must be longer than the resonant length:
The length of the directors is made less than the resonant length:
Moreover, the length of the directors decreases from the first to the last.
For a vibrator-reflector system, the optimal distance, from the point of view of maximum efficiency, is selected within the limits:
For the system, the vibrator is the first director:
The distance between neighboring directors is taken within the limits:
The wavelength is determined using the formula:
Where is the speed of light, and is the frequency of the channel. Because we are given 5 - 6 television channels, then we take the average frequency of the occupied frequency bands of these two channels: , then the wavelength from formula (1.7) will be equal to:
Let's calculate the lengths of the antenna vibrators and the distance between them using formulas (1.1 – 1.6):
We will take the total length of the antenna and its image in Figure 1.2 from the VIBRAT program.
Drawing 1.2 - General view of the calculated director antenna
To find the directional pattern of the director antenna in the plane, we use formula (1.8):
Where is the number of vibrators, k is the wave number, and is the average distance between the vibrators.
Substituting (1.9) and (1.10) into (1.8) and numerical values, we obtain an expression for finding the pattern of a given director antenna:
We will construct a normalized radiation pattern using the Mathcad package. Because it is symmetric about zero, then we will construct it for:
Drawing 1.3 - DN in plane
From the graph you can determine the width of the main lobe and the maximum level of the side lobes: .
The directivity factor and the width of the main lobe are determined by formulas (1.10-1.11):
Coefficients and are determined from the graph in Figure 1.4:
Drawing 1.4 - Odds chart
Let's determine the wavelength of the antenna:
Knowing the wavelength of the antenna and using Figure 1.4, we determine that . Then:
Let us compare the obtained calculation results with the results of the calculated director antenna modeled in the program. The results have a slight discrepancy due to the fact that the formulas used are approximate and do not take into account a number of factors.
Drawing 1.5 - Director antenna calculated in VIBRAT
Conclusion: we calculated the directivity factor, DP and DP parameters of the director antenna in a given frequency range. Using the VIBRAT program, we simulated this antenna and verified the validity of the obtained parameters.
Rice. Types of horn antennas: a) E-sectoral, b) N-sectoral, c) pyramidal, d) conical.
Properties:
Horn antennas are very broadband and match the feed line very well - in fact, the antenna bandwidth is determined by the properties of the exciting waveguide. These antennas are characterized by a low level of the rear lobes of the radiation pattern (up to -40 dB) due to the fact that there is little flow of RF currents to the shadow side of the horn. Horn antennas with low gain are simple in design, but achieving high (>25 dB) gain requires the use of wave phase-aligning devices (lenses or mirrors) in the horn aperture. Without such devices, the antenna has to be made impractically long.
Application:
Horn antennas are used both independently and as feeds for mirror and other antennas. A horn antenna, structurally combined with a parabolic reflector, is often called a horn-parabolic antenna. Horn antennas with low gain are often used as measurement antennas due to their favorable set of properties and good repeatability.
At the Holmdale radio telescope, which is a Dicke radiometer based on a horn-parabolic antenna, Arno Penzias and Robert Woodrow Wilson discovered the cosmic microwave background radiation in 1965.
Characteristics and formulas:
The gain of a horn antenna is determined by its opening area and can be calculated using the formula:
where: - horn opening area.
λ is the wavelength of the main radiation.
- 0,4....0,8 instrumentation(horn surface utilization factor), equal to 0.6 for the case when the path difference between the central and peripheral beams is less, but close to Pi/2, and 0.8 when wave phase-leveling devices are used.
Main lobe width DNA H:
Main lobe width DNA by zero radiation in the plane E:
Since with equality L E And L H
DNA in the plane N turns out to be 1.5 times wider; often, to obtain the same petal width in both planes, choose:
To keep phase distortions in the horn aperture within acceptable limits (no more than Pi/2), it is necessary that the following condition be met (for a pyramidal horn):
where and are the heights of the faces of the pyramid forming the horn.
From another source:
Where L H- opening width in plane N,
L E- opening width in plane E,
R E And R H- horn length.
For such an antenna KND in a simplified form it is calculated using the formula:
D RUR = 4piνS/λ 2
Where: S = L H * L E- horn opening area;
λ
- wavelength of the main radiation;
ν
= 0.4....0.8 - surface utilization coefficient ( instrumentation);
Depending on the type of horn, horn antennas are divided into N- And E- sectorial, pyramidal and conical. Horns whose dimensions correspond to the maximum value KND are called optimal. For optimal N-sectoral horn antennas horn length R H =L H 2 /3λ, for optimal E-sectoral horn antennas R E =L E 2 /2λ. instrumentation optimal N- And E-sectoral, pyramidal horns is 0.64. If we conditionally increase the length of the horn to infinity, then instrumentation antenna will increase to 0.81.
In a conical horn, optimal length R opt. con. depends on the diameter of its opening
d:
R opt. con. = d 2 /2.4λ + 0.15λ
instrumentation optimal conical horn v=0,5.
Table 1.2. Horn radiation pattern width with optimal length.
|
Horn type |
Radiation pattern width in the H plane |
Radiation pattern width in plane E |
|
E-sectoral |
2Θ 0.7 =68λ/L H |
2Θ 0.7 =53λ/L E |
|
H-sectoral |
2Θ 0.7 =80λ/L H |
2Θ 0.7 =51λ/L E |
|
Pyramidal |
2Θ 0.7 =80λ/L H |
2Θ 0.7 =53λ/L E |
|
Conical |
2Θ 0.7 =60λ/d |
2Θ 0.7 =70λ/d |
If we take an elliptical horn with an ellipse axial ratio of 1.25, then we can obtain approximately the same width of the radiation pattern in all sections passing through the horn axis.
The advantage of a horn antenna is its broadband, determined by the broadband of the feeding waveguide, efficiency. horn antenna is equal to unity.
The disadvantage of horn antennas is that the horn length must be too long to obtain highly directional radiation. The optimal horn length is proportional to the square of the aperture dimensions L H or L E, and the width of the radiation pattern is inversely proportional L H or L E in the first degree. Therefore, to narrow the radiation pattern of a horn antenna in N times, the opening width should be increased by N times, and the length of the horn is in N2 once. This circumstance imposes restrictions on the width of the radiation pattern of horn antennas.
