A Study of the Effect of the Near Field and Coupling on the Normalized Site Attenuation

• Main Authors: Zhi-Hao Chen, 陳志豪
• Other Authors: Ken-Huang Lin
• Format: Others
• Language: zh-TW
• Published: 1999
• Online Access: http://ndltd.ncl.edu.tw/handle/65651737479068713950

碩士 === 國立中山大學 === 電機工程學系 === 87 === A standard testing environment, Open Area Test Site, is used to measure the EMI effect of an electronic product. In addition to the EUT and required equipment, an ideal site should be away from objects that can cause reflections such as buildings and fences. Only the measurement undertaken in a standard environment can the result be accepted by a Standard Authority. To establish the standards of the open area test sites, FCC (Federal Communications Commission) introduces the Normalized Site Attenuation (NSA) which is calculated from the ideal model. The standards have been recommended in ANSI C63.4. For an oats, its horizontal and vertical NSA are measured. If this measured NSA stays within 4 dB of the theoretical NSA for an ideal site, the measurement site will be considered acceptable for radiated electromagnetic field measurement by FCC. The theoretical NSA is developed and calculated from an ideal site attenuation model which is proposed by Smith, German,and Pate in 1982. The theoretical NSA can be expressed in terms of the antenna factors of the transmitting and receiving antennas, the frequency, and a field-strength term. The electric field strength is calculated from classical ground-wave propagation theory. The broadband antennas and tuned dipoles are used frequently in the EMI measurements. The advantages of broadband antennas include faster measurement time and more convenient setup. On the other hand, there are better and simpler theoretical models for tuned dipoles. The theoretical NSA is obtained using the calculation based on the point source. It uses only the part of the electric fields which fall off as . The assumption is appropriate only in far-field situations. In the practical measurements, this assumption may not be reasonable. So it is necessary to propose some correction factors in practical measurements. This paper emphasizes on the differences of the site attenuation variation (3 m~10 m) between the linear antennas and the point sources. At first we calculate the electric field of the half-wavelength dipole antenna to find the height at which maximal electric field can be received. The mutual impedance between antennas at this height is then determined. This mutual impedance is used to include the effects due to the mutual coupling. To simplify the calculation of the electric field from a half-wavelength dipole antenna, we assume a sinusoidal current distribution. Considering the half-wavelength dipole to be composed of many elemental dipoles, we can easily calculate the height at which maximal electric field can be received. For a horizontal transmitting dipole antenna (antenna height = 2m), our computations show a 0.66m deviation from Smith''s value at 30 MHz. For a vertical transmitting dipole antenna (antenna height = 2.75m), our computations show a 1.23m deviation from Smith''s value at 50 MHz. Such deviations have direct impact on the coupling effect. Both give practically the same result above 150 MHz. All these calculated results have been verified with the NEC-2 (Numerical Electromagnetics Code).