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#17
August 29th 03, 04:07 AM
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W5DXP wrote:
wrote: And in a two-source DC circuit, you can indeed have net voltages derived from superposition of the two source voltages. Put them in series adding and you get a voltage maximum. Put them in series subtracting and you get a voltage minimum. But power can only be computed using the resultant voltage and current (or, if you prefer, NET voltage and current). What about that power plug on the wall? Too difficult? Too many sources, too many feedlines, too many loads. And the wavelength is really too long to illustrate RF transmission lines. I get a wavelength just over 3000 miles, which would stretch out straight from Texas to Alaska. Let me make it simpler then: the lamp cord from the wall to the table lamp; lamp off and lamp on. Most lamp cord looks like twin-lead; some have said it is approximately 100 Ohms but for the purposes of discussion feel free to assume any convenient Z0 for the cord. When we measure a voltage using an ideal voltmeter, how do we know if this voltage is composed of components ... For an RF transmission line, use a directional wattmeter to ascertain if reflected power exists. If your 50 ohm SWR meter is in a 50 ohm Z0 environment, it will indicate the presence or absence of reflected power. And if it is not an RF transmission line? If it is a DC circuit? Say your phone line when on hook? If it is a lamp cord? Do we really need directional wattmeters to understand what is happening? If you wish, the wall outlet provides an excellent opportunity for explanation by example. Nope, it doesn't. That wall outlet doesn't have one source, one feedline, and one load. If you had one 60 Hz generator hooked up to 3100 miles of lossless transmission line, you could certainly observe reflections over that one wavelength of wire. Please, replace wall outlet with lamp cord to table lamp. ....Keith |
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