Detailed description of shaped antenna

Satellite communication has the characteristics of wide coverage, available frequency bandwidth, fast network construction and low cost, which makes it widely used in the field of communication. With the development of satellite communications, in order to meet the effective omnidirectional radiated power (EIRP) requirements of certain ground service areas, the communication antenna must be forced to use multi-feed forming or reflective surface shaped antennas, which greatly promotes multi-feed The development of source shaped or reflective shaped antennas. This can reduce the interference of the ground station outside the coverage area to the satellite system, improve the spectrum utilization rate and channel capacity of the system, increase the effective omnidirectional radiated power (EIRP) and the quality factor G / T value of the receiving system, and can The satellite ground station terminal equipment is simplified and the cost is reduced.
When the coverage area is a map of China's political area, considering that China's western region is vast, sparsely populated, and has low rainfall, while the eastern region is densely populated, economically developed, and heavy rainfall, rain attenuation is an important consideration for satellite communications. problem. Therefore, it is necessary to take into account that the country has appropriate power coverage, and it should respond to the eastern region of North China, so that it has a higher power distribution; and slightly reduce the western region, in order to make full use of satellite resources. In this way, the communication beam generated after shaping the antenna can cover the whole country and highlight the east. In order to prevent signal interference, the main and cross-polarization gains of antennas in the direction of neighboring countries should be sufficiently small. In short, for a constant area, as long as some major factors are taken into account, you can get a rough expected distribution.
1 Overview of shaped antennas Shaped antennas are divided into two categories according to whether the reflecting surface is variable: (1) single-shaped shaped antennas and reconstructed shaped antennas. (1) A single shaped antenna refers to an antenna whose purpose is single, and its use will not change after assembly and launch. The coverage area of ​​the antenna and the spatial position of the antenna no longer change, and the gain distribution of the target area covered by the antenna is determined and unchanged. The design of this type of antenna is usually based on the expected coverage area gain distribution analysis design reflection surface, once the reflection surface is determined, it will not change. (2) There are two cases of variable shaped antennas: one is to adjust the working system according to the change of the antenna track position, so as to obtain the corresponding shaped beam; the other is to adjust the system to produce corresponding shapes for different shapes of areas Beam coverage [1].
Shaped antennas are divided into two categories according to the number of feeds used: multi-feed antennas and single-feed antennas. In traditional satellite communications, an array-fed parabolic antenna (as shown in Figure 1 (a)) is usually used. The feed array is placed on the reflection plane or the focal plane of the microwave lens and is composed of feed antennas arranged in a certain way. The feed array is located on the focal plane, except for the feed at the center, each feed has a lateral offset with respect to the focal point, and the offset direction and offset are different, so that the beams generated by each feed After the reflection on the reflecting surface or the focusing of the lens, a group of mutually independent beam beams with approximately equal beam widths and uniform distribution will be formed in the far field area. The focus of this antenna shape design is to optimize the excitation coefficient and geometric arrangement of the feed. One of the important components is the beamforming network (BFN), which is used to adjust the excitation of the feed. But they have inherent shortcomings: a lot of antenna system overhead will be spent on designing and adjusting beamforming networks, and complex beamforming networks will cause radio frequency losses and reduce the overall gain of the antenna system. These defects will become more serious as the frequency increases, so multi-feed forming technology is generally used below Ka band (4 GHz ~ 7 GHz) [2-6].

Shaping a single reflecting surface (as shown in Figure 1 (b)) to obtain a shaped beam is a more feasible solution. In the case of beamforming a fixed area, the beamforming network can be used instead of the reflective surface forming design, and the single-feed forming reflective surface antenna is used. This shaped reflective surface antenna has simple machining and uncomplicated structure , And because there is no beamforming network, the loss is small, the advantage of higher gain [6-10].
According to the reflector type of the antenna, it can be divided into a single shaped reflection surface antenna and a multi-shaped reflection surface antenna (usually two reflection surfaces) antenna. In the design of shaped antennas, single-reflection antennas generally use offset-fed reflector antennas. Figure 2 (a) shows an offset-fed parabolic antenna. It consists of a conical surface with a certain deflection angle to cut a standard parabola. Get. Compared with other antenna types, it has the characteristics of simple structure and low centroid, and it also solves the problem of occlusion of the feed well. Based on this, the antenna is widely used in satellite communications. In the design of shaped reflector antennas, a common multi-reflector antenna is a double-bias reflector antenna. As shown in Figure 2 (b), the shape of the two reflecting surfaces of the offset Cassegrain or Gregorian antenna is shaped (for design and processing, generally only the secondary reflecting surface is shaped) , To achieve the purpose of shape design.

2 Design methods commonly used in shaping From the perspective of shaping methods, they can be divided into direct methods and indirect methods. As early as 1975, KATAGI T and TAKEICHI Y proposed a design method for shaping the reflective surface, namely the wavefront analysis method. Subsequently, researchers in North America and Europe based on this, based on geometric optics (GO) and physical optics (GO PO), Geometric Diffraction Theory (GTD) and Physical Diffraction Theory (PTD) and other methods, direct and indirect synthesis methods for forming reflective surfaces are proposed. The optimization object of the direct method [11-12] is the shape of the reflection surface itself. The optimization object of the direct method is the shape of the reflection surface itself. The expansion surface of the function is used to represent the reflection surface. Perform reflection surface synthesis. Generally speaking, it is very difficult to find such basis functions according to the requirements. This method is mostly expressed in the form of series. The optimization objects of the indirect method are some characteristic parameters of the shaped reflective antenna, such as wavefront and aperture surface field distribution. By optimizing these parameters to meet the shaping requirements, determine some nodes of the reflective surface, so as to fit and determine the reflection Face shape. Whether it is a direct method or an indirect method, it is just an optimization process. In this way, the search for an optimal optimization method is the key problem, and the optimization results of a certain method can be obtained from the subsequent error analysis. Whether the test method is feasible in practice must also be verified by strict physical methods.
2.1 Wavefront method As early as 1975, KATAGI T and TAKEICHI Y proposed a design method for shaping the reflective surface, namely the wavefront analysis method. This method assumes that the wavefront of the far-field radiation pattern is composed of two parts: the internal is Spherical waves confined within an angular range of the desired radiation pattern; the exterior is a control surface that uses the internal boundary contour as a guideline. The feed wavefront is assumed to be a spherical wave, so that the geometrical optical method can determine the reflection surface based on the incident and reflected wavefronts. In terms of geometric optics, the wavefront corresponds to the beam profile, and the wavefront determines the shape of the reflecting surface. Calculate the antenna pattern according to the surface current distribution generated by the initial feed on the reflective surface, and compare the calculated value of the antenna pattern with the expected value. If the result of the calculated value approaching the expected value is not ideal, readjust the wavefront and reflection. Calculate the antenna pattern until it is satisfactory [5,13].
The principle of the wavefront method is: the parabolic antenna converts the spherical wavefront formed by the feed source into a plane wavefront. When a point on the plane wavefront, feed and reflection surface is given, all points on the reflection surface can be determined by the law of optical path; similarly, when a point on the shaped wavefront, feed and reflection surface is given, the reflection surface is shaped Can also be determined by the optical path theorem. This method is relatively rough, and can shape the coverage area where the boundary terrain is not very complicated, but some of the far-field characteristics of the antenna cannot be determined. This method cannot solve the problem of geometric optical caustics related to the outside of the reflected wavefront. Therefore, this method has been rarely used in the design of modern shaped reflector antennas.
2.2 Optimization method of oral and facial field [13-17]
This method obtains a specific far-field coverage pattern by optimizing the distribution of the aperture surface field. In the optimization process, it is assumed that the radiance distribution of the caliber surface field is unchanged, and the phase distribution is expanded using a trigonometric function as a basis function. The optimization objects are the coefficients of these trigonometric functions or other basis functions. The least square method or other nonlinear optimization methods (such as Minmax method) can be used to establish the objective function, so that the far-field gain approaches the target value. According to the phase distribution of the optimized surface field, the surface shape of the reflecting surface can be calculated through the principle of geometric optics.
JORENSEN R proposed a stricter caliber phase synthesis technique in 1980. In this method, the phase distribution of the aperture is directly optimized from the far-field pattern, and the shape of the reflective surface is determined by the geometric optical method. The caliber phase synthesis technology eliminates the problem of caustics and can more easily control the characteristics of the pattern, but this method cannot simultaneously optimize the caliber amplitude distribution. This is one of the reasons why the electric field intensity of the reflective surface is assumed to be constant in the improved technology. JORENSEN R assumed a fixed Gaussian amplitude distribution in the analysis, which is not practical for complex pattern design. Represents the reflection surface, and the reflection surface is synthesized by the coefficient of the optimization function. The optimization objects of the indirect method are some characteristic parameters of the shaped reflector antenna, such as wavefront and aperture field distribution. By optimizing these parameters to meet the shaping requirements, the shape of the reflector is determined.
This method can achieve better graphics effect, and the main lobe and side lobe can be controlled according to the gain distribution of sampling points. However, in the optimization process, it is assumed that the amplitude distribution of the aperture surface field is unchanged, and the side lobe level is mainly determined by the initial edge illumination. In fact, the change of the phase of the aperture surface will cause the change of the surface shape of the reflection surface, which will cause the amplitude distribution of the aperture surface to change. Although this change is not obvious, it will also affect the accuracy of the far-field calculation. In addition, the selection of some basis functions does not guarantee that the coverage area with a very complicated boundary shape has a good shaping effect.
2.3 The field phase optimization method of the aperture surface grid The field phase optimization method of the aperture surface grid [2, 5] is basically an improvement of the aperture field optimization method. In order to overcome the shortcomings of the phase optimization method of the aperture field, the aperture surface is divided into many small grids. Before optimization, the field distribution on each small grid is considered to be of equal amplitude and in phase. In this way, the phase distribution of the field on the aperture surface no longer uses trigonometric functions Expansion expression, but an independent value. The optimization idea is to optimize the phase distribution of the aperture surface field so that the far-field gain approaches the target value. The shape of the reflective surface is determined by the phase distribution of the aperture surface field; the amplitude distribution of the aperture surface field is determined by the shape of the reflection surface and the amplitude phase distribution of the feed, as the amplitude distribution in the next phase optimization. Since this method considers the effect of the change in the aperture field amplitude on the far field, compared with the aperture field optimization method, its accuracy is correspondingly improved. In reference [5], by optimizing each grid on the reflective surface in the direction of the parabolic focal axis The amount of deformation on the paper proposes the concept of the phase influence factor when the mesh is deformed, optimizes the phase, and at the same time attaches the deformation restriction condition, which improves the problem of the discontinuity of the reflective surface.
In short, the above three methods all use geometric optical analysis methods. Among them, the aperture field optimization method and the field phase optimization method of the aperture surface grid determine the shape of the reflection surface by optimizing the amplitude and phase of the aperture surface grid. . Each of these methods has a disadvantage, that is, when optimizing the phase of the aperture surface field, the surface of the reflection surface may be discontinuous, resulting in more difficult processing of the reflection surface. Therefore, in the optimization process, the problem of discontinuity must be solved.
2.4 Direct expansion method of the reflecting surface In order to obtain a continuous and smooth shaped reflecting surface, Y. Rahmat-Sami et al. Use special function expansion to represent the shape of the reflection surface, and regard the expansion coefficient as the optimized characteristic parameter of the antenna system. The reflective surface is shaped. The characteristics of this method are reflected in the selection of the expansion of the orthogonal global function, which can be selected as Zernike function expansion, trigonometric function expansion, Bessel function expansion, Fourier series and so on. The final reflection surface is smooth and continuous, the boundary is strictly defined, and has a first-order continuous derivative. Theoretical techniques such as geometric optics, physical optics, geometric diffraction, and physical diffraction can be used in the theoretical method, and the far-field characteristics of the antenna such as side-lobe level and cross-polarization can be accurately controlled.
2.5 Analysis and comparison of the above methods As far as antenna analysis and comprehensive methods are concerned, the geometric optical forming technology is relatively mature and highly accurate, but one of its main disadvantages is that the diffraction effect is not considered when forming the reflective surface. Neglected diffraction effects include not only the diffraction of the reflection surface and edge, the near-field effect of the feed and reflection surface, but also the interaction between the main reflection surface and the secondary reflection surface (dual reflection surface and multiple reflection in the design Face case). When the geometric diffraction analysis technique is used to calculate the far-field radiation pattern of a geometrically-optically shaped reflector antenna, some characteristic parameters (such as side lobes) will deviate greatly from the expected value because geometrical optics shaping requires that the relative wavelength of the antenna system is sufficiently large. To meet the approximate conditions of ray trajectory. The first three methods all use geometric optics, so we must consider this issue. For the design of small antenna systems, more accurate analysis and comprehensive procedures, such as physical optics, are required. Geometric optical shaping algorithms are often used to synthesize the oral field instead of directly synthesizing the far field. The relationship between the oral field and the far field can be determined by the geometric optical method. The shaped reflective surface derived by the geometric optical algorithm is a series of points It is indicated that these points may cause the surface of the reflecting surface to be discontinuous and irregular in perimeter. Therefore, before processing and manufacturing the shaped reflecting surface, these separate points must be interpolated and fitted.
In the last method, because the reflection surface is directly developed, a geometrical optics synthesis method is not required, but a physical optics analysis method is used, which does not need to meet the ray trajectory approximation conditions. It is also applicable to small antenna systems and is more accurate than the geometrical optics analysis method However, when considering the accuracy of some far-field parameters (such as side-lobe level, cross-polarization, etc.), the physical optics method is still not accurate enough, and the physical diffraction theory technology must be selected to consider the physical diffraction theory fringe field.
Therefore, in theory, the geometric optical method is simple and intuitive, and it is relatively mature; the physical optical method has high accuracy and a wide range of applications; the physical diffraction theoretical method can improve the accuracy of the far-field parameters.
3 Domestic research status Over the past decade or so abroad, shaped reflectors have become an extremely important technology for spaceborne antennas. Many spaceborne shaped reflectors have been successfully designed, manufactured and put into use. In the domestic research on shaped antennas, there are many researches on multi-beam shaped antennas, but there are more theories and simulations, and less practical applications. There are few studies on shaped reflector antennas. Among them, Xi'an Branch of China Institute of Space Technology has designed and processed a shaped reflector antenna for communications satellites covering the Chinese territory in the Ku band. In theory, the geometric optical method is used for analysis, and in practice, the imported program POS is used for design and processing; Beijing University of Posts and Telecommunications has compared and calculated the single-feed forming reflector and the array-feed reflector antenna covering the Chinese layout. The overall design department of the Beijing Space Vehicle adopts the global basis Zernike function to expand the reflection surface, uses the PO method, takes the expansion coefficients as optimization objects, and brings it into the far-field radiation integration of the reflector antenna, and uses the trust region method to minimize the nonlinearity The square problem is optimized to determine the reflection surface of the single feed single reflection antenna. Because the researched object is also the shape-forming problem of single-feedback and single-reflection antenna, its research results have certain reference [18].
The current research hotspot of shaped reflector antenna is the direct deployment method of reflector. The advantage of this method is that there are fewer parameters to be optimized. For example, the method of optimizing the phase using a grid means that a reflective surface usually requires tens of thousands of phase parameters to be optimized, and the reflective surface expansion method only needs a dozen to dozens of The coefficient values ​​of the basis functions are optimized, which will greatly speed up the calculation. In addition, after the optimization of this method is completed, the expression of the reflective surface will be determined accordingly. There is no need to fit the data to get the expression of the reflection surface, and the normal or tangential equation of any point on the reflection surface is also easy to determine, which brings convenience to the manufacture, processing and measurement of the antenna. The focus of the research is mainly on what basis functions or expressions are used to represent the reflective surface, what methods are used in the analysis of physical optics to speed up the calculation, and what optimization methods are used to optimize the basis functions Coefficient value. Among them, the expansion of the reflection surface uses Zernike basis function to expand (as in the grasp software) is more extensive. In addition, broadband optimization, isolation station optimization, etc. are often encountered in engineering practice. There is still little research in this area in China, which deserves further attention.

For this Blue SMD LED, we can package with single chip 3528 SMD LED or 2 chips 3528 SMD LED , and the power can be 0.1W 3528 Blue SMD LED, 0.06W 3528 blue SMD LED, 0.2W 3528 blue SMD LED, 0.5W 3528 blue SMD LED and so on.

It is often used in LED lighting, LED Lamps, LED backlight, LED panel lights, LED furniture, grow light LED, fill light LED, LED aperture and other lighting products.

3528 Blue SMD LED

3528 Blue SMD LED,SMD 3528 Blue LED,Blue 3528 Diode SMD LED,SMD LED 5050 Blue

Shenzhen Best LED Opto-electronic Co.,Ltd , https://www.bestsmd.com