The radome is a structure that protects the antenna system from the external environment. It has good electromagnetic wave penetration characteristics in electrical performance, and can withstand the external harsh environment by mechanical properties. Outdoor antennas are usually placed in the open air and directly affected by storms, ice, snow, dust and solar radiation in nature, resulting in reduced antenna accuracy, shortened life and poor operational reliability.
The purpose of using the radome is to:
1 Protect the antenna system from wind, snow, sand, dust and solar radiation, which makes the antenna system work more stable and reliable, while reducing the wear, corrosion and aging of the antenna system and prolonging the service life.
2 Eliminate the wind load and wind torque, reduce the driving power of the rotating antenna, reduce the weight of the mechanical structure, reduce the inertia, and increase the natural frequency.
3 Relevant equipment and personnel can work inside the hood, not affected by the external environment, improve the efficiency of the equipment and improve the working conditions of the operators.
4 For high-speed aircraft, the radome can solve the problems caused by high temperature, aerodynamic load and other loads on the antenna.
However, the radome is an obstacle in front of the antenna, which absorbs and reflects the radiation wave of the antenna, changes the free space energy distribution of the antenna, and affects the electrical performance of the antenna to some extent.
The reason is:
1 The reflection of the radome wall and the diffraction of the uneven portion may cause the main lobe of the antenna to shift, which causes an aiming error;
2 The radome's absorption and reflection of high-frequency energy will cause transmission loss, which will affect the antenna gain (the system noise temperature will increase when receiving);
3 Antenna lobe distortion caused by the radome, the antenna main lobe width is changed, the zero depth is increased, and the side lobe level is increased;
Second, the radome increases the construction cost. The antennas on the aircraft are generally equipped with a radome, and the ground radome is not yet widely used.
Radome classification1 From the use of the two types of aviation and ground (including shipboard) type.
2 Electrically divided into a vertical incident radome and a large incident angle radome according to the incident angle of the antenna radiation wave. The angle between the radiation wave and the normal of the wall is the angle of incidence. A vertical incidence radome with an incident angle of less than 30°. The antenna scans anywhere in the hood, and the range of incident angles is relatively large (from 0 to 75 or more), which is called a large incident angle radome. The latter's electrical performance is much lower than the former.
3 According to the cross-sectional shape of the radome wall, the radome is divided into three types: uniform single-wall structure, sandwich structure and space skeleton structure.
4 According to the way the radome is formed, the ground radome is divided into a pneumatic cover and a rigid cover.
Structural designThe structure of the radome differs from other architectural structures in that electrical characteristics must be considered in terms of structural type, component size, wall thickness, material selection, and structural details.
1 Cover wall thickness: related to the operating wavelength. Electrically, in order to minimize reflections, it is necessary to design a uniform single wall thickness or sandwich core thickness at the operating wavelength. However, the wall thickness chosen must be able to withstand the expected maximum aerodynamic load and other loads without being damaged or causing large deformations. The specific choice of wall thickness should be based on the electrical and structural properties of the working wavelength, radome size and shape, environmental conditions, materials used, etc.
2 Material selection: The factors to be considered for the dielectric material used in the radome wall are: low dielectric constant and loss tangent at the operating frequency, and sufficient mechanical strength. In general, the inflatable radome is usually coated with a polyester fiber film of Heparon rubber or neoprene; the rigid radome is made of glass fiber reinforced plastic; and the sandwich in the sandwich structure is mostly made of a honeycomb core or foam. Aerospace radomes are typically made of fiberglass reinforced plastics, ceramics, glass-ceramics, and laminates.
3 The specific structure: the uneven part of the radome will cause the diffraction and reflection of high-frequency energy. Therefore, it is generally not suitable to set the rib on the radome wall where the high-frequency energy passes, because it may cause the casing radome to be generated. Local or overall instability, or large deformations, imposes many limitations on structural design and cover size. In order to facilitate manufacturing, installation and transportation, a large rigid radome must be made into a block type, and a flange is required at the ball joint, resulting in uneven wall.
Therefore, it is generally necessary to find a joint scheme with good comprehensive performance through electrical performance test and structural performance test at the time of design. In addition, the metal components or metal connectors used should minimize electrical occlusion.
Main structural formThe radome mainly has five structures: an aerial radome, a ground radome, an inflatable radome, a housing structure radome and a space skeleton radome.
Aviation radome
Generally it is a shell structure. Depending on the situation, a vertical incidence radome or a streamlined large angle of incidence radome can be used. To meet aerodynamic requirements, the radome is constructed in a streamlined configuration. However, when the antenna is scanned inside the radome, the incident angle changes greatly, making it difficult to obtain optimum electrical performance of the radome. If it is not streamlined, it is usually designed as a radome with an angle of incidence of less than 30°, which may be cylindrical, spherical or parabolic. Although the aerodynamic performance of such a vertical incident radome is poor, electrical performance is good.
Ground radome
Usually spherical (about three-quarters of the ball), can be divided into two types of inflatable cover and rigid cover. The rigid radome is divided into two types: a housing structure radome and a space skeleton radome.
Inflatable radome
The spherical film is fixed on the airtight platform by a pressure plate around the kerf, surrounded by a rope, or fixed by other means, and internally inflated. The utility model has the advantages that the cover wall is thin and uniform, the electrical performance is good, and the utility model is suitable for working in a wide frequency band; the cover body is soft and easy to fold, light in weight, small in size, convenient in transportation, storage and installation. The disadvantage is that it is necessary to continuously inflate the inside of the cover to maintain the shape of the cover and the necessary rigidity. If the inflator fails, the cover will collapse and damage the antenna. The United States has developed the largest inflatable hood, which has a diameter of 64.05 meters (210 feet) and is used as a radar antenna for the "Telestar".
Housing structure radome
The cover wall is typically formed as a curved housing and the structural loads are supported by the housing. Among them, the uniform single-walled shell structure is limited in size due to the working wavelength and size; the foamed plastic shell structure is low in tangent due to the dielectric constant and loss angle of the material, and electrically allows a thicker cover wall to meet Structural load requirements. The connection between the foam blocks can be glued to form a uniform overall shell, which has good electrical performance and is suitable for working at high frequency and wide frequency band; the sandwich shell structure is generally A-type sandwich structure. It consists of two equal-thickness symmetrical high-intensity skins and a low-density core. It has the advantage of high strength-to-weight ratio and stiffness-to-weight ratio, and is suitable for large ground radomes of a certain wavelength. However, the disadvantage is that the working frequency band is narrow, the manufacturing is complicated, and the cost is high. When the A-type interlayer is not satisfactory, a multilayer sandwich structure composed of odd-numbered layers may be employed. China's successful 44.4-meter-diameter laminated radome developed in 1972 uses an A-type sandwich housing structure.
Space skeleton radome
It consists of a metal (or medium) spherical mesh skeleton and a dielectric sheet (or film) attached thereto. The structure of the grid skeleton is mainly subjected to structural loads. The design principle of the grid skeleton is to make the electrical section of the rod section as small as possible while ensuring the mechanical properties. Metals have greater strength and stiffness than dielectric materials, so metal skeletons are often used. Its advantages are suitable for high frequency and wide frequency band operation, easy to manufacture and cheaper, suitable for large ground antennas. The world's largest metal space skeleton radome was built in 1964 and has a diameter of 45.75 meters (150 feet) for the US Haystack radar antenna.
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