Lets Talk Transmission Lines（让我们谈谈输电线路）.pdf

Let’s Talk Transmission Lines Don’t neglect one of the most important parts of your station! By Edward J. Farmer, AA6ZM No one wants to discuss transmission lines. Many hams would rather chew aluminum foil. After all, transmission lines are just the cables that link your radios to your antennas. You hook them up and get on the air. Who cares? Boring! Well, the subject may not have much “sex appeal,” but if you want to get maximum performance from your station, give me your attention for a couple of minutes. These lines have a major influence on the results you’ll achieve on the air. A little extra knowledge will—literally—take you a long way! It’s a Question of Impedance Modern solid state radios expect a load impedance of 50 Ω, resistive. We want to connect them to an antenna, the feed-point impedance of which can be anything. It depends on the antenna design, plus how and where it’s installed. In general, the impedance is “complex,” meaning it has both resistive and reactive components. (Don’t worry—we’ll discuss reactance in a moment.) Antenna Impedance Even though the electrical properties are “distributed” throughout the antenna, it’s easier to think about a lumped impedance representing the load present at its feed point. Two resistors in series with an inductor and capacitor is a useful model (see Figure 1). Figure 1—The radio, antenna and transmission line. The object is to get as much power as possible from the radio into the antenna’s radiation resistance. One of the resistors is the desirable radiation resistance, which represents the useful work done by the antenna in converting your transmitter’s output into radiation that spans states or continents! The other is the loss resistance resulting from the resistance of the wire and connections. (Now you know why you need good solder joints and sound mechanical connections on your antenna.) Power consumed in the loss resistance does not increase the RF field—it merel