[信息与通信]综合布线系统的演变及技术要点--耐克森华南总代_珠海诚信通提供.ppt

* * * * * * 2. Emission A current running in a cable induces a an electromagnetic field. If we follow the rule about induced electromagnetic fields, we can find out the direction of the field. A current running in the twisted pair induces an field in the first twist, but the current also induces a field in the second twist. These fields are exactly the same, except that they are opposite to each other. The sum of both electromagnetic fields will be 0. This is of course only when we have a perfect balanced cable. In reality, however, a perfect balanced cable does not exist. First of all there will be bendings which will untwist the cable. Then there is a second fact that any metal object in the near of my cable will form a capacitive coupling with my twisted pair and both currents will not anymore be exactly opposite. If that is the case, then there will be an electromagnetic field induced by the cable an my cable will start to emit. Again this problem is not so important at low frequencies but the higher the frequency becomes, the bigger the problem becomes. * 2. Emission A current running in a cable induces a an electromagnetic field. If we follow the rule about induced electromagnetic fields, we can find out the direction of the field. A current running in the twisted pair induces an field in the first twist, but the current also induces a field in the second twist. These fields are exactly the same, except that they are opposite to each other. The sum of both electromagnetic fields will be 0. This is of course only when we have a perfect balanced cable. In reality, however, a perfect balanced cable does not exist. First of all there will be bendings which will untwist the cable. Then there is a second fact that any metal object in the near of my cable will form a capacitive coupling with my twisted pair and both currents will not anymore be exactly opposite. If that is the case, then there will be an electromagnetic field induced by the cable an my cable