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Jeff Liebermann wrote:
I beg to differ somewhat. In order for the reflected power to contribute to the incident power, the reflected power would first be attenuated by the coax loss. It would then require a substantial mismatch at the transmitter, which is unlikely. However, assuming there is a mismatch at the source, some of the reflected power will be sent back to the load (antenna), after getting attenuated by the coax for a 2nd time. There may be some contribution, but it will very very very very small. Reflection of a single reflected wave is not the only mechanism that can redistribute energy back toward the load. Superposition of two waves at the source impedance (or at an impedance discontinuity in a feedline) accompanied by destructive interference can accomplish a similar feat. Non-reflective glass is a 1/4WL matching section of thin-film that accomplishes the same thing as a Z0-match. To the best of my knowledge, nobody is taking wave cancellation at the source impedance into account although it may be the major source of the redistribution of reflected energy back toward the antenna. It's explained on the following web pages: http://www.mellesgriot.com/products/optics/oc_2_1.htm "Clearly, if the wavelength of the incident light and the thickness of the film are such that a phase difference exists between reflections of p, then *reflected wavefronts* *interfere destructively*, and overall reflected intensity is a minimum. If the two reflections are of equal amplitude, then this amplitude (and hence intensity) minimum will be zero." (Referring to 1/4 wavelength thin films.) "In the absence of absorption or scatter, the principle of conservation of energy indicates all 'lost' reflected intensity will appear as *enhanced intensity in the transmitted beam*. The sum of the reflected and transmitted beam intensities is always equal to the incident intensity. This important fact has been confirmed experimentally." http://micro.magnet.fsu.edu/primer/j...ons/index.html "... when two waves of equal amplitude and wavelength that are 180-degrees ... out of phase with each other meet, they are not actually annihilated, ... All of the photon energy present in these waves must somehow be recovered or *redistributed* in a new direction, according to the law of energy conservation ... Instead, upon meeting, the photons are *redistributed* to regions that permit *constructive interference*, so the effect should be considered as a *redistribution* of light waves and photon energy rather than the spontaneous construction or destruction of light." Why does almost everyone seem to consider reflection the only way to redistribute reflected energy back toward the antenna? -- 73, Cecil http://www.w5dxp.com |
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