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#21
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Walter Maxwell wrote:
Cecil, define 'sloshing.' Hi Walt, I'm having trouble with my news-server so I am posting from Google using procedures to which I am not accustomed. The classic wave reflection model indicates that forward power travels from the source to the load where it is incident upon the load. At a load mismatch, some of the forward power is rejected and travels back from the load toward the source as a reflected wave. For instance, in the following lossless example, we have 104.17 watts of forward wave and 4.17 watts of reflected wave on the 75 ohm line. This is all in line with "Reflections" and my unpublished article. 100W--50 ohm line--+--1/4WL 75 ohm line---112.5 ohm load As I infer/understand what Roy, and others, have said while objecting to the material in my unpublished article: The only real forward power wave is the one that is dissipated in the load. The reflected power wave doesn't travel from the load back toward the source and the extra 4.17 watts in the forward wave doesn't travel from the match point back toward the load. The energy associated with the reflected waves just "sloshes" around in the transmission line and doesn't move very far or very fast and certainly not in the form of EM wave components. So Roy's use of the word "slosh" in the context in which he used it, is all I can give you. Roy hasn't defined the word and neither has the IEEE. :-) -- 73, Cecil http://www.qsl.net/w5dxp |
#22
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Cecil, W5DXP, quoting others, wrote:
"The reflected power wave doesn`t travel from the load back toward the source and the extra 4.17 watts in the forward wave doesn`t travel from the match point back toward the load." Users of the Bird and Similar wattmeters know that is what they see. Source and load power is the forward power minus the reflercted power. Zo of the coax enforces its volt to amp ratio on both the incident and reflected waves. Reflected power is again reflected at the match point because the matched source sees no reflection. That`s the point of producing a match in the load. Best regards, Richard Harrison, KB5WZI |
#23
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Tom Donaly wrote: Tim Wescott wrote: Cecil Moore wrote: It has been said that the energy stored in the standing waves of a transmission line just "sloshes" around. We can demonstrate standing waves using a laser beam normal to a perfect mirror. There are points of maximum irradiance and points of minimum irradiance in the standing waves. So does the EM energy in the standing waves of light in free space "slosh" around like the energy in the standing waves in a transmission line? Yes -- there's energy actively bouncing around in that there beam; if you could reduce it down to one laser burst that was shorter than the distance between the laser and the mirror you'd (in theory at least) be able to see it. If so, where does the inductance and capacitance in free space come from to generate that 377 ohms of characteristic impedance? They don't. The behavior of EM radiation in free space is described by Maxwell's laws. The 377 ohms of characteristic impedance comes from the permittivity and permiability of free space but inductance and capacitance are only meaningful concepts if you have conductors in your model. If not, then why do the EM waves in a transmission line behave differently than the EM waves in free space? Because they're bounded by conductors. What different laws of physics do photonic waves in transmission lines obey than do photonic waves in free space? None. They obey Maxwell's laws. Of the E-field and H-fields rules for EM waves in free space, which of those rules are violated by EM waves in a transmission line? None. Is there one set of Maxwell's equations for free space and a separate set for transmission lines? No, just different boundary conditions to start. All this is covered in a good college E&M course. I wish I had an E&M book that I could recommend for self-study, but I don't. Mine is "Elements of Engineering Electromagnetics", but I took a course. I don't think I would have been able to just pick up the book and learn it from there. Did Maxwell ever mention the scientific concept of "sloshing"? Who knows? And was he talking about light waves or a wee dram of whiskey at the end of the day? As hard as it may be to believe for anyone who's gone through an E&M course the original form of Maxwell's equations were more difficult to comprehend than the way there're usually presented now -- the vector notation that is currently used either wasn't around then or wasn't in widespread use. ------------------------------------------- Tim Wescott Wescott Design Services http://www.wescottdesign.com Hi Tim, Cecil is just trying to crowd Roy into slapping leather (figuratively speaking). Cecil thinks he already knows the answer to all these questions, so there's no point in answering him. Indeed. It appears things have not changed much since last i checked here! hehe! I'd really like to hear Cecil's "answers" to these questions! He'll be at it for awhile, until he realizes rhetorical confrontation won't work. 73, Tom Donaly, KA6RUH Depends on what your goals are... Slick |
#24
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Richard Harrison wrote:
Users of the Bird and Similar wattmeters know that is what they see. Source and load power is the forward power minus the reflercted power. But that's only what is printed on the meter scale. It doesn't make the "reflected power" real. For the (n+1)th time: the Bird so-called "wattmeter" does NOT sense forward and reflected power. It only senses RF voltage and current on the line. The meter scale calibration is a mathematical operation that depends on a lot of assumptions... most notably the assumption that "reflected power" has some physical reality. I am genuinely open-minded about that debate - which makes the all the more determined to be ruthless about bogus arguments on either side. And the most bogus argument of all is: "Users of the Bird and Similar wattmeters know that is what they see." -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
#25
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Ian White GM3SEK wrote:
But that's only what is printed on the meter scale. It doesn't make the "reflected power" real. I am genuinely open-minded about that debate - which makes the all the more determined to be ruthless about bogus arguments on either side. Ian, correct me if I'm wrong, but I infer that you are biased toward the "no reflected energy waves" side. That bias could be based on perceived knowledge. Seems to me, some people are begging the question. They assume there is no energy in reflected waves and call what the other side says, "gobbleygook", even when presented in scientific terms. I think we all understand the concepts behind net energy. What is in dispute is the next lower layer of dynamic energy movement in both directions at the same time during steady-state. So would you explain what happens to the reflections of a laser beam when aimed at a perfect mirror in free space. Offset the laser beam slightly from normal incidence to start with and observe the reflections with your naked eye. Then bring the beam to 90 degree incidence with the mirror. You can no longer see the reflections but now you can detect the superposition of the forward and reflected waves through interference "rings" or loops with an intensity maximum occuring every half wavelength and an intensity minimum occuring every half wavelength in between. Where is the EM wave energy just sloshing around and not traveling forward and rearward at the speed of light? -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
#26
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Cecil Moore wrote:
But that's only what is printed on the meter scale. It doesn't make the "reflected power" real. I am genuinely open-minded about that debate - which makes the all the more determined to be ruthless about bogus arguments on either side. Ian, correct me if I'm wrong, but I infer that you are biased toward the "no reflected energy waves" side. That bias could be based on perceived knowledge. I am trying very hard not to be biased about the actual problem - but I am very much against your methods of debate. Why should anyone consent to follow you into the realms of optics and laser physics, merely because that's where you want to go? Stand your ground right here on the log, Cecil, and talk strictly and exclusively about RF transmission lines. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
#27
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Cecil Moore wrote:
So would you explain what happens to the reflections of a laser beam when aimed at a perfect mirror in free space? Offset the laser beam slightly from normal incidence to start with and observe the reflections with your naked eye. Then bring the beam to 90 degree incidence with the mirror. You can no longer see the reflections but now you can detect the superposition of the forward and reflected waves through interference "rings" or loops with an intensity maximum occuring every half wavelength and an intensity minimum occuring every half wavelength in between. Incidentally, there is a very good discussion of standing waves in section 7.1.4 of "Optics", by Hecht, 4th Edition. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
#28
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Ian, GM3SEK wrote:
"I am trying not to be biased about the actual problem - " Open minds consider arguments. Wave velocity = frequency x wavelength Wave velocity in free space is 186,000 miles per second (300,000 km/sec). Velocity of disturbances on open wire transmission lines is almost equal to that in free space. At high radio frequencies, a wavelength may be measured in inches and centimeters. A transmission line containing a discontinuity produces a reflection from the change. As a short distance can produce a phase change of 360 degrees, the incident and reflected waves can combine to produce voltage variations along a short line length. These can be measured using simple instruments with confidence. Circuit theory works on transmission lines because the proximity of their conductors causes an effect on one conductor to be instantaneously imposed on the other. A transmission line cannot be analyzed as a simple series circuit because current in the wires is not everywhere the same. Volts and amps vary along the line depending upon construction, length. and load placed on the line. The line`s resistance, inductance, conductance, and capacitance are distributed and accumulate with its length. In a wave in either direction on the line, the volts and amps at any point on the line conform to Zo but also depend on summation of the incident and reflected waves at that point. Current in the linee is not independent of the voltage. A transmission line of any length terminated with an impedance equal to its Zo has an input impedance of Zo. Distribution of volts and amps on lines terminated with loads other than Zo has been demonstrated countless times and is well understood. Directional couplers are capable of separating forward and reverse directions on a line, and are also well known, understood and have proved useful. The Bird wattmeter uses a directional coupler, works as advertised, and may be inserted anywhere in a 50-ohm coax line. Best regards, Richard Harrison, KB5WZI |
#29
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Richard Harrison wrote:
Ian, GM3SEK wrote: "I am trying not to be biased about the actual problem - " Open minds consider arguments. Certainly. You begin with small steps that no-one will seriously dispute. Wave velocity = frequency x wavelength Wave velocity in free space is 186,000 miles per second (300,000 km/sec). Velocity of disturbances on open wire transmission lines is almost equal to that in free space. At high radio frequencies, a wavelength may be measured in inches and centimeters. A transmission line containing a discontinuity produces a reflection from the change. As a short distance can produce a phase change of 360 degrees, the incident and reflected waves can combine to produce voltage variations along a short line length. These can be measured using simple instruments with confidence. Circuit theory works on transmission lines because the proximity of their conductors causes an effect on one conductor to be instantaneously imposed on the other. A transmission line cannot be analyzed as a simple series circuit because current in the wires is not everywhere the same. Volts and amps vary along the line depending upon construction, length. and load placed on the line. The line`s resistance, inductance, conductance, and capacitance are distributed and accumulate with its length. In a wave in either direction on the line, the volts and amps at any point on the line conform to Zo but also depend on summation of the incident and reflected waves at that point. Current in the linee is not independent of the voltage. A transmission line of any length terminated with an impedance equal to its Zo has an input impedance of Zo. But here you pick up the pace. Instead of the detailed argument above, suddenly whole chapters flash by in a single sentence: Distribution of volts and amps on lines terminated with loads other than Zo has been demonstrated countless times and is well understood. Directional couplers are capable of separating forward and reverse directions on a line, and are also well known, understood and have proved useful. And it's all a calculated run-up to this huge flying leap: The Bird wattmeter uses a directional coupler, works as advertised, and may be inserted anywhere in a 50-ohm coax line. Sorry, Richard, but that isn't a constructed argument any more. It's just a declaration. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
#30
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Ian White GM3SEK wrote:
Richard Harrison wrote: The Bird wattmeter uses a directional coupler, works as advertised, and may be inserted anywhere in a 50-ohm coax line. Sorry, Richard, but that isn't a constructed argument any more. It's just a declaration. Do we need to go into the wave reflection model in detail? The Bird wattmeter accepts the wave reflection model as scientific fact as do most of my reference books. Given the wave reflection model and a 50 ohm environment, the Bird wattmeter reads forward and reflected power, i.e. the number of joules passing a point on the 50 ohm transmission line in each direction. The Bird wattmeter assumes: Forward V is in phase with forward I. Reflected V is in phase with reflected I. Vfor*Ifor = Pfor Vref*Iref = Pref Vfor/Ifor = 50 ohms, Vref/Iref = 50 ohms. Vfor^2/50 = forward power, Vref^2/50 = reflected power Ifor^2*50 = forward power, Iref^2*50 = reflected power All this is in any good textbook covering the wave reflection model and has been accepted as fact by RF engineers for the better part of a century. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
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