Compact range anechoic chamber

Compact Anechoic Chamber

With increment of the aperture of the AUT and increasing of the antenna operating frequency, in order to meet the far-field condition, it requires the testing distance reaching several kilometers or even tens of kilometers. In addition, in order to avoid ground reflection, the installation height of the testing receiving/transmitting antenna becomes too high to be realized. This problem can be solved by using compact range technology. The compact range is formed by reflection plane principle, shortened the antenna range to test the distance. Its basic principle is: use one, two or even more reflection planes to transform the spherical wave radiated by feed source to plane wave at near distance (the typical value is 10~20m). Therefore, the compact range system can be deemed as a transformer to transform from spherical wave to plane wave within a near distance.

The compact range has several types, such as the reflective type, lens type, holographic compact range; the reflective type is as shown in Fig. 1.5; the reflective type compact range is further divided into single reflector, bicylindrical, front-fed Cassegrain and shaped reflector, and so on. The single reflector compact range uses a bias rotating paraboloid as the reflector, with the auxiliary antenna seat feed placed on the main focus, according to the geometric optical principle, the spherical waves radiated by feed source become plane waves through reflection of parabolic reflector. This area is just the position where the AUT must be placed. As the distance required by this area is greatly shorter than that required by the far-field, such antenna range is called compact range. In addition to the geometric optical principle, moment method is also the theoretical tool for analyzing and designing the compact range. But at high band, for the compact range with large electric dimension, the moment method, FDTD and finite element method are not practical. From the principle, diffraction geometry theory is suitable for analysis of compact range, but it is difficult to solve the diffraction problem of jagged edge reflective surface. At present, physical optics method may be the most practical and most effective method.

At present, the main applications of the compact range are as follows:

(1) Antenna measurement: it is mainly used for testing high performance antenna pattern, including amplitude pattern, phase pattern andcross-polarization pattern, antenna gain test, near zone side-lobe level and far zone side-lobe level test, which are extremely important to research, product development and performance test of the airborne radar, missile-borne radar and satellite antenna.

(2) Radome research: wave transmissivity test; pattern distortion, antenna Boresight error testing and research, etc.

(3) Radar target characteristic testing: radar cross section, i.e., analysis and testing of RCS, accurate RCS scaling; low and ultra-low RCS testing, etc.

(4) Millimeter-wave radiation and scattering research: millimeter-wave antenna testing and millimeter-wave band target RCS characteristic testing research.

(5)Targetrecognition technology research

Compared with other testing means and equipment, the compact range has the following advantages:

(1) Compact range transforming the spherical wave into plane wave with good performance within near distance can make the radiation and scattering measurement meet the far-field test condition, its background level may reach 1/1,000,000~1/10,000,000m²(-60~-70dBsm), which is 30~40dB less than the field background noise level, and makes the minimum RCS measurement ability and the test accuracy increased by 3~4 orders of magnitude. Thus the measurement accuracy is greatly increased.

(2) Compared with the near-field test method, compact range can make automation test at real-time without any mechanical scanning and digital conversion, therefore, it has the characteristics of high efficiency, high intensive cultivation, and high reliability.

(3) Compared with outdoor field test, compact range has the advantages of low background level, high test accuracy, less land occupation, low operation cost, and so on.

(4) It can work under all weather conditions, and has good confidentiality.

(5) Operating frequency of the compact range can be 1~100GHz, even higher. Large quiet zone, millimeter wave test ground requires the distance of tens kilometers to meet the far-field condition. To build such field of tens kilometers can be realized in no countries, only the compact range can achieve it.

(6) System is stable and reliable.

Main technical indexes of the compact range include the following aspects:

(1) Quiet zone dimension: reflect the size of the compact range plane wave test area, L×W×H, quiet zone of the compact range normally does not vary with the frequency variation;

(2) Operating frequency: reflect the service frequency range of the compact range, which is normally 2~100GHz;

(3) Quiet zone field amplitude variation: reflect the amplitude characteristics of the compact range plane waves, the smaller the amplitude variation the better, the amplitude variation of the compact range quiet zone is normally less than 1dB;

(4) Quiet zone field phase variation: reflect the phase characteristics of the compact range plane waves, the smaller the phase variation the better, the phase variation of the compact range quiet zone is normally less than 10°;

(5) Cross-polarization level: reflect the cross-polarization characteristics of the compact range plane waves, the smaller the cross-polarization variation the better, the cross-polarization variation of the compact range quiet zone is normally less than -27dB.

Although the basic principle of the compact range is simple, however, because of the following reasons (problems):

① The compact range realizes the plane wave on near distance, calculation of quiet zone field cannot be calculated as per the far-field calculation formula of the antenna, the calculation is complex and the calculation accuracy requirement is high;

② Size of the reflector is not infinite, edge diffraction must be considered;

③ Size of the reflector is very large, the reflector panel must be spliced, the influence of the splicing gaps must be considered;

④ How to realize the installation, mechanical structure, positioning, adjustment of such large sized compact range panels;

⑤ How to guarantee the panels have sufficient accuracy to meet the requirement of high frequency, etc.

A series of key problems of the compact range have to be tackled in research, these problems can be summarized as follows:

① Electrical design (including: near-field calculation, aperture design, layout design, side tooth design, feed design and error model, etc.);

② Accurate panel manufacturing (including: process scheme, raw materials, measurement, etc.);

③ Overall mechanical structure (including: structure scheme, dead weight deformation, temperature deformation, installation deformation, installation and adjustment mechanism, etc.);

④ Precise machinery positioning (including: positioning precision, surface fitting, field adjustment software, dynamic adjustment compensation, etc.);

⑤ Electric property detection (including: high precision scanning frame, microwave/millimeter wave amplitude and phase system, spatial spectrum analysis, etc.)