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Jim Kelley wrote:
The only current flowing on an antenna is the current traveling from one end to the other. Since standing waves cannot exist without the underlying component traveling waves, to avoid conceptual blunders, one needs to deal directly with the component traveling waves. Your statement is based on a purely mathematical shortcut which exists only in the human brain, not in reality, and obscures the actual speed-of-light physics necessary for an EM wave to even exist. The current can be artificially parsed the way you are doing it but that parsing leads to the very misconception under which you are laboring. The same thing happened with w8ji's and w7el's "measurements" involving delays through loading coils. The actual component physics, as explained in any reasonably technical antenna book is: Total current = forward current + reflected current Itot = Ifor + Iref (phasor addition) Reference: "Antenna Theory", Balanis, 2nd edition Balanis, page 488: "The sinusoidal current distribution of long open-ended linear antennas is a standing wave constructed by two waves of equal amplitude and 180 degrees phase difference at the open end traveling in opposite directions along its length. ... The current and voltage distributions on open-ended wire antennas are similar to the standing wave patterns on open-ended transmission lines." Balanis, page 489: "Standing wave antennas, such as the dipole, can be analyzed as traveling wave antennas with waves propagating in opposite direstions (forward and backwards) and and represented by traveling wave currents, If and Ib in Figure 10.1a." In a standing wave antenna, e.g. a 1/2WL dipole, there exists a forward wave that gives up about 10% of its energy content to radiation. The remaining 90% of the wave encounters the open end of the antenna and is reflected. So, just as in the case of an open-circuit stub, we have a forward current component flowing in one direction and a reflected current component flowing in the other direction. Many of the mistakes and mis- conceptions about antennas are based on your false assertion above. I have the ARRL Antenna Book. :-) The ARRL Antenna Book doesn't even have "traveling wave antennas" in its index. It does state: "Unterminated long-wire antennas are often referred to as 'standing wave antennas'". Please reference a reasonably technical antenna book like "Antennas", by Kraus. "A sinusoidal current distribution (on a standing wave antenna) may be regarded as the standing wave produced by two uniform (unattenuated) traveling waves of equal amplitude moving in opposite directions along the antenna." I wonder if anyone is buying it? It doesn't matter if anyone is buying it. What matters is technical validity. Your first statement above is technical invalid. Given the free space description of standing waves of light given by Hecht in "Optics", your assertion above would lead one to believe that the photons comprising the standing wave of light must be at rest even though that's an impossibility (except in the human mind). Here's what a couple of references say about standing waves. "Electrical Communication", by Albert: "Such a plot of voltage is usually referred to as a *voltage standing wave* or as a *stationary wave*. Neither of these terms is particularly descriptive of the phenomenon. A plot of effective values of voltage, appearing as in Fig. 6(e), *is not a wave* in the usual sense. However, the term "standing wave" is in widespread use." "College Physics", by Bueche and Hecht: "These ... patterns are called *standing waves*, as compared to the propagating waves considered above. *They might better not be called waves at all*, since they do not transport energy and momentum." -- 73, Cecil, IEEE, OOTC, http://www.w5dxp.com "Government 'help' to business is just as disastrous as government persecution..." Ayn Rand |
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