电工电子专业英语（第2版）Module A Unit 1-10-修改.ppt

Notes 法拉第（1791-1867），英国的化学家和物理学家，是当时最伟大的实验学家。电动机和发电机的运行都是基于法拉第电磁感应原理。为了纪念这位科学家，人们以他的名字“法拉”作为电容的单位。 Real Knowledge, like everything else of value, is not to be obtained easily, it must be worked for, studied for, thought for, and more all, must paid for. 真知，如同真宝，不能轻易获得，必须经过研究、学习和思考，最重要的是为之付出，才能有所收获。 Thomas Arnold Unit 8 Transmission Lines Student: I’m a little confused about last conversation about antennas. How does power get from the transmitter to the antenna? You didn’t make that very clear to me. Fig.8-1 Transformer and Transmission Lines Engineer:It’s done by what is called a transmission line（as shown in Fig.8-1）. Student: You mean an ordinary pair of wires? Engineer: I wish it were that simple! Unfortunately, if we use just a pair of wires, we will find that most of the power will be reflected back from the antenna. Student: Why is that so? Engineer: Because all antennas have a very important property called impedance. Student: What’s that? Engineer: It’s a bit complicated to explain in great detail. Let’s just say that impedance measures the opposition to current flow and is very similar to resistance. Student: But what does impedance have to do with transmission lines? Engineer: Clearly, we want to transfer as much power as we can from the transmitter to the antenna. Student: Naturally. Engineer: We do that by impedance matching. The transmission line also has a characteristic impedance just like the antenna. When the transmission line’s impedance is the same as the antenna’s, then virtually all the electromagnetic radiation. Student: So the entire system of the transmitter, transmission line, and antenna operates at its maximum efficiency. What happens if the impedance doesn’t match? Engineer: It depends upon the severity of the mismatch. With very bad mismatches most of the forward power is reflected back to the transmitter. Student: And when this power reaches the transmitter? Engineer: It is reflected back agai