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True power doesn’t travel

المؤلف:  Stan Gibilisco

المصدر:  Teach Yourself Electricity and Electronics

الجزء والصفحة:  308

2-5-2021

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True power doesn’t travel

An important semantical point should be brought up concerning true power, not only in ac circuits, but in any kind of circuit or device. A common and usually harmless misconception about true power is that it “travels.”
For example, if you connect a radio transmitter to a cable that runs outdoors to an antenna, you might say you’re “feeding power” through the cable to the antenna. Everybody says this, even engineers and technicians. What’s moving along the cable is imaginary power, not true power. True power always involves a change in form, such as from electrical current and voltage into radio waves.
Some true power is dissipated as heat in the transmitter amplifiers and in the feed line (Fig. 1). The useful dissipation of true power occurs when the imaginary power, in the form of high-frequency current and voltage, gets to the antenna, where it is changed into electromagnetic waves.

Fig. 1: True and imaginary power in a radio antenna system.