我google了near and far field, 还有magnet, here is the excerpt The near-field is remarkable for reproducing classical electromagnetic induction and electric charge effects on the EM field, which effects "die- out" with increasing distance from the antenna (with magnetic field strength proportional to the inverse-cube of the distance and electric field strength proportional to inverse-square of distance), far more rapidly than do the classical radiated EM far-field (E and B fields proportional simply to inverse-distance). Typically near-field effects are not important farther away than a few wavelengths of the antenna.
不用google, 就是简单的逻辑概念而已. google的结果告诉你 "magnetic field strength proportional to the inverse-cube of the distance and electric field strength proportional to inverse-square of distance", 你是选inverse-cube呢, 还是 inverse-square? 简单点说, 从源向外传播的电磁波有一定的能量. 如果是各向同性 的传播, 那么在距离源固定距离的球面上, 单位面积的能量是总能量 除以球面的面积. 当不存在介质, 或电磁波在介质中能量衰减可以忽略 不计时(我们现在的情况), 离源不同的距离, 总能量是一定的, 所以单位面积上(你的身体)的能量是与距离成反比的. 这个比去考虑磁场电场有多强要直接得多.
strength than farther
【在 j*****1 的大作中提到】 : 我google了near and far field, 还有magnet, here is the excerpt : The near-field is remarkable for reproducing classical electromagnetic : induction and electric charge effects on the EM field, which effects "die- : out" with increasing distance from the antenna (with magnetic field strength : proportional to the inverse-cube of the distance and electric field : strength proportional to inverse-square of distance), far more rapidly than : do the classical radiated EM far-field (E and B fields proportional simply : to inverse-distance). Typically near-field effects are not important farther : away than a few wavelengths of the antenna.