In fact, a four - wire circuit can be established without the use of any wires at all, as in the case with a circuit established over microwave, satellite, or infrared transmission systems.
事实上，可以不使用任何线来建立四线电路，例如通过微波、卫星或红外传输系统。
Microwave: An electromagnetic wave that vibrates at 1GHz and above. Microwaves are the transmission frequencies used in communications satellites as well as in line-of-sight systems on earth.
微波：振动千兆赫兹或以上的电磁波。它是卫星通信及地面上的即时影像传送系统的播送频率。
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