Quote:
Originally Posted by W7JZE
Hard to wrap my head around "... it will show 200 watts going up the transmission line..." even though the xmtr is only 100 watts. Intuitively that seems close to getting something for nothing. That old "energy can neither be created nor destroyed..." thing.
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Think about the tuner-feedline-antenna system. There's 100 watts going out (radiated by the antenna) and 100 watts coming in (from the transmitter). That's the same rate of energy-per-second (100 watts = 100 joules per second), so the same amount of energy is going out as coming in.
There's additional energy running back and forth. Where did it come from? When the key was first pressed and the transmitter started pouring 100 watts in, it took a while for the power radiated by the antenna to build up. At first, with 100 watts coming up, the antenna was radiating only 50 watts. Then, after the first bounce-bounce-round-trip, it was radiating 75 watts. As energy bounced back and forth, eventually the antenna reached a full 100 watts radiated. So, at first, the transmitter was pouring in 100 joules per second and the antenna was radiating less than 100 joules per second. The extra energy got stored and is running back and forth. Energy that's
traveling is
power, and that's how we get 200 watts going up and 100 watts coming down.
When you release the key, that gradual buildup is undone by a gradual slowdown, and the stored energy finally goes to zero.
Simple stuff, no?