A Review of Antenna Near-Field Measurement Technology

• Antenna Measurement Technology 

Since the advent of antenna engineering, antenna measurement has been one of the important topics that people have always paid attention to. The accuracy of the method is directly related to the practicality of the supporting system. With the continuous updating of communication equipment, the requirements for antennas are getting higher and higher. The conventional far-field measurement method of antennas
has many difficulties in implementation, and sometimes it is even powerless. Therefore, people are eager to calculate the radiation field by measuring the source field of the antenna. However, due to the less than ideal probe and too many approximations in the calculation formula, this method has not been put into practical use. In order to reduce the mutual influence between the probe and the antenna
under test in the 1950s to measure the amplitude and phase characteristics of the wavefront at a distance of several wavelengths from the antenna aperture. The experimental results were very exciting. This opened the prelude to the research of near-field measurement. The emergence of this technology solved many problems that antenna engineering urgently needed to solve but had not yet solved, thus making antenna measurement methods appear in front of the world with a new look. More than 40 years have passed, and near-field measurement technology has entered the stage of applied research from theoretical research, and has been extended from the frequency domain to the time domain. It can not only measure the radiation characteristics of antennas, but also diagnose the distribution of antenna apertures, providing a reliable and accurate design basis for design; at the same time, people use it to study the scattering characteristics of targets, that is, the research of stealth technology and anti-stealth technology, thus providing new research methods for the research of this technology, and then this research has entered the stage of exploring the target imaging technology using the near-field measurement method.

 

• Near Field Measurement Techniques

At a distance of 3 to 5 meters (λ is the working wavelength) away from the object to be measured, a probe with known electrical characteristics is used to scan and sample (sample according to the sampling theorem) the amplitude and phase data of the electromagnetic field on a plane or curved surface in the near area of ​​the object to be measured, and then the electrical characteristics of the far field of the object to be measured are calculated through strict mathematical transformation (Fast Fourier Transform, Fast Fourier Transform, abbreviated as FFT). This technology is called near-field measurement technology. If the object to be measured is a radiator (usually an antenna), it is called radiation near-field measurement (Radiation Near Field Measurement); when the object to be measured is a scatterer, it is called scattering near-field measurement (Near Field Seattering Measurement). For radiation near-field measurement, according to the different sampling surfaces, it can be divided into plane scanning technology, cylindrical scanning technology and spherical scanning technology. Similarly, for scattering near-field measurement, there are corresponding plane, cylindrical and spherical scattering near-field measurements.

• 
  Characteristics of near-field measurements

Since the 1970s, the near-field measurement method has been mainly used for antenna measurement (radiation problem measurement)
. It samples data in the near area of ​​the antenna under test (Antenna Under Test, abbreviated as AUT).
Compared with the conventional far-field measurement of antennas, this method has the following advantages: (1) The near-field measurement method is low-cost, and the accuracy of the calculated far-field radiation pattern is much higher
than that of direct far-field measurement . (2) It has a large amount of information. A three-dimensional radiation pattern with a large solid angle domain can be obtained by one measurement. (3) When measuring large antennas with this method, the limitation of far-field size is eliminated, and the difficulty of building a large test . (4) Near-field measurement can be carried out indoors, eliminating weather factors and can work around the clock. (5) The entire measurement process is automatically completed under computer control and has a high degree of confidentiality. In view of these unique advantages, the near-field measurement method of antennas has become a relatively reliable method for identifying antenna electrical indicators. Scholars in the antenna industry recognize that it can be used as a standard for modern antenna measurement. In recent years, near-field measurement has also been used to detect the surface accuracy of reflector antennas, infer the aperture field distribution of the antenna, and "diagnose" whether the array unit has failed. In particular, it has been used abroad to measure the radiation pattern of low or ultra-low sidelobe antennas.

The technical research of near-field measurement has developed from the 1950s to the present, and its research direction has roughly gone through four stages, as shown in Table 1.

1950-x1961│Experimental exploration stage without probe correction

1961-1965│Research stage of probe correction theory  

1965-1975│Experimental verification stage of probe correction theory

1975~Present │Technology Promotion Class