Application Of Transmission Line Theory In Electromagnetic Interference Pretection Technology

Electronic and electrical equipments will generate electromagnetic field around them when they work. The electromagnetic field couples energy to other equipments in some forms, which causes other equipments to break down. At the same time, these equipments absorb energy from other equipments, which make themselves to break down. With more and more equipments operating in high frequency, high speed and high integration, interference between electronic equipments becomes more and more serious. So how to analyze the interference and propose pretection methods has been the hot topic. As we know, transmission line theory has been devopled fast and become a mature theory system since 1860s. If applying such perfect and easy-to-use theory to electromagnetic field, it will benefit analysis of electromagnetic compatibility problems. Transmission line theory has been an efficient method since Schelkunoff used it to analyze incident wave propagating in space and calculate shielding effectiveness of planar shield. This paper applys transmission line theory to more complex EMC problems and makes some deveplorment in interference analysis and shielding effectiveness calculation. The contribution of the paper is listed as follows:(1) Analyze the coupling between transmission lines using integral method and bring forward a general formula. The general formula can be used to analyze induced voltage in affected circuit. Parallel length of the two circuits can be unequal. The loss of circuit and mismatch of load are taken into account too. The integral method is mainly used in coupling analysis between power line and telecommunication line. But in this paper, it is developed for high frequency coupling analysis. The general formual is validated by other popular high frequency coupling analysis methods, such as Jarvis model. In addition, it is verified that results from Klewe theory and Jarvis model are specials cases of the general formulas. So the general formula is applicable in high frequency and low frequency coupling analysis.(2) Use nonuniform transmission line theory to analyze shielding effectiveness of multilayer plates. Uniform transmission line theory has been used in shielding effectiveness analysis widely, which should calculate re-reflection between plates. However, owing to discontinuity of nonuniform transmission line, it is suitable for modeling multilayer plates regardless of re-reflection between plates. Matrix presentation benefits calculation using programming. A general formula applicable to calculate shielding effectiveness of arbitrary multilayer plates is proposed. Special cases of double, triple and fourfold shield are investigated. The accuracy of this method has been validated by comparison to other analyticalmethods. This method is also applied to calculate shielding effectiveness when the plane wave is oblique incident.(3) An analytical formulation has been developed for the shielding effectiveness calculation of rectangular enclosure with an off-center aperture. It deals with more general case including arbitrary located aperture and high frequency incident wave by making use of high-order TEmn and TMmn mode transmission line model. It takes high order modes in enclosure and loss into account, which greatly removes limitation of frequency and enclosure size. Different with other literature, m and n value in TEmn and TMmn mode can be arbitrary other than n is equal to zero, which is more reasonable. Normal and oblique incident wave are analyzed in detail.