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"Sloshing" EM Energy
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? If so, where does the inductance and capacitance in free space come from to generate that 377 ohms of characteristic impedance? If not, then why do the EM waves in a transmission line behave differently than the EM waves in free space? What different laws of physics do photonic waves in transmission lines obey than do photonic waves in free space? 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? Is there one set of Maxwell's equations for free space and a separate set for transmission lines? Did Maxwell ever mention the scientific concept of "sloshing"? -- 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 =---- |
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 |
On Fri, 17 Jun 2005 15:48:08 -0500, Cecil Moore
wrote: We can demonstrate standing waves using a laser beam normal to a perfect mirror. "We?" Let's see, a hypothetical argument, involving a hypothetical "We," performing a hypothetical analysis that contains 0 places of precision, yielding a hypothetical answer that will be hypothetically true and hypothetically false hypothetically "It was the best of times, it was the worst of times." |
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. I would recommend: Introduction to Electromagnetic Fields, by Paul and Nasar, 3rd edition. ISBN: 0070460833. Available from www.bn.com, used, from $59. The review of vector calculus in the first two chapters is excellent. The text covers plane waves incident on material boundaries (and the resultant standing waves). It also covers transmission lines. Regards, Frank |
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. He'll be at it for awhile, until he realizes rhetorical confrontation won't work. 73, Tom Donaly, KA6RUH |
"Richard Clark" wrote - "It was the best of times, it was the worst of times." ============================= Richard, now you're plagiarising Charles Dickens. ---- Reg. |
Tim Wescott wrote:
. . . 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. . . . A few months ago, I came upon a book that really looks like it might fill the bill: _Engineering Electromagnetics_ by Nathan Ida. The text is clear but doesn't skimp on math or theory. At the end of each section, there are numerous examples showing how the concept is applied in the solution of real problems -- something sorely missing in most other texts and, for that matter, in a lot of college courses. For example, after the "Inductance and Inductance" section in the "Magnetic Materials and Properties" chapter are the following fully worked and explained examples: Application: Self-inductance of a toroidal coil Application: Self-inductance of a long solenoid - Inductance per unit length Application: Inductance per unit length of coaxial cables Application: Mutual inductance between a wire and a toroidal core - core memory Mutual inductance between straight wire and loop Self- and mutual inductances in multiple coils It's sort of like a Shaum's Outlines and textbook combined, but in a way that you can see the transition from the theory to practice. It's also a good reference to use later on. And the answers to all the problems (but no details about how they were solved) are at the back of the book. I was lucky and found a used one at Powell's while browsing in their technical bookstore, but even new it's a bargain. Roy Lewallen, W7EL |
Did Maxwell ever mention the scientific concept of "sloshing"? No. The electron had not yet been discovered. |
Tim Wescott wrote:
All this is covered in a good college E&M course. Uhhhh Tim, those were rhetorical questions aimed at people who believe that the energy in EM waves can slosh around at sub-light speeds. Where the heck did that idea come from anyway? -- 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 =---- |
"Reg Edwards" wrote in message ... "Richard Clark" wrote - "It was the best of times, it was the worst of times." ============================= Richard, now you're plagiarising Charles Dickens. ---- Reg. Cecil, define 'sloshing.' Walt,W2DU |
Cecil Moore wrote:
Tim Wescott wrote: All this is covered in a good college E&M course. Uhhhh Tim, those were rhetorical questions aimed at people who believe that the energy in EM waves can slosh around at sub-light speeds. Where the heck did that idea come from anyway? If you're responding to something you should post things as a followup, even if it's a continuing argument -- saves us new folks to the list some confusion. Besides, energy in EM waves can and does travel (or "slosh around", if there's reflections) at sub-light speeds, specifically in coax, waveguides and just about any bulk material (including air) that has an index of refraction higher than 1. -- ------------------------------------------- Tim Wescott Wescott Design Services http://www.wescottdesign.com |
On Fri, 17 Jun 2005 22:37:05 +0000 (UTC), "Reg Edwards"
wrote: Richard, now you're plagiarising Charles Dickens. Ah Reg, "'t'is a far, far better thing that I do, than I have ever done" Straight from Ronald Colman, Old Son. (who the dickens is Charles?) 73's Richard Clark, KB7QHC |
A few months ago, I came upon a book that really looks like it might fill
the bill: _Engineering Electromagnetics_ by Nathan Ida. The text is clear but doesn't skimp on math or theory. At the end of each section, there are numerous examples showing how the concept is applied in the solution of real problems -- something sorely missing in most other texts and, for that matter, in a lot of college courses. For example, after the "Inductance and Inductance" section in the "Magnetic Materials and Properties" chapter are the following fully worked and explained examples: Application: Self-inductance of a toroidal coil Application: Self-inductance of a long solenoid - Inductance per unit length Application: Inductance per unit length of coaxial cables Application: Mutual inductance between a wire and a toroidal core - core memory Mutual inductance between straight wire and loop Self- and mutual inductances in multiple coils It's sort of like a Shaum's Outlines and textbook combined, but in a way that you can see the transition from the theory to practice. It's also a good reference to use later on. And the answers to all the problems (but no details about how they were solved) are at the back of the book. I was lucky and found a used one at Powell's while browsing in their technical bookstore, but even new it's a bargain. Roy Lewallen, W7EL Looks interesting. I just ordered it for $79 Canadian from www.amazon.ca Thanks, Frank |
Frank wrote:
Looks interesting. I just ordered it for $79 Canadian from www.amazon.ca Let us know if it says anything about "sloshing" EM wave energy including reflected light waves in free space. -- 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 =--- |
Did Maxwell ever mention the scientific concept of "sloshing"?
No. The electron had not yet been discovered. ======================================= Electrons sloshing about in conductors, in the same general direction, always attempt to avoid each other. This unsociable characteristic results in a pressure which drives them to flow near to the surface of conductors in which they are sloshing. Hence skin and proximity effects. There is an opposite effect. When electrons slosh about in opposite general directions they form a great liking for each other. The result is a mechanical attractive force between a pair of parallel conductors carrying current in opposite directions. Also another proximity effect. It's all so simple. Can't imagine why you have sloshing problems. But no doubt Cecil will introduce reflections, standing waves on meters which don't measure them, and SHF scattering parameters. ;o) ---- Reg, G4FGQ |
Reg Edwards wrote:
But no doubt Cecil will introduce reflections, standing waves on meters which don't measure them, and SHF scattering parameters. ;o) How about I just introduce photons? EM waves are photonic energy whether they are traveling in free space or in a transmission line. How do the photons slosh around? The electrons that slosh around are the carriers of the EM wave and are not the EM wave. Who has published a scientific paper on photon sloshing? -- 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 =---- |
Reg Edwards wrote:
But no doubt Cecil will introduce reflections, standing waves on meters which don't measure them, and SHF scattering parameters. ;o) How about I just introduce photons? EM waves are photonic energy whether they are traveling in free space or in a transmission line. How do the photons slosh around? The electrons that slosh around are the carriers of the EM wave and are not the EM wave. Who has published a scientific paper on photon sloshing? -- 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 =---- |
Cecil, W5DXP wrote:
"If so, where does the inductance and capacitance in free space come from to generate that 377 ohms of characteristic impedance?" First, impedance is a voltage to current ratio as in Ohm`s law. It can be complex if reactance and resistance are involved, but it`s still a voltage to current ratio. Antennas radiate power which has the units of watts, and from the expanding radiation wavefront in free space, this amounts to watts per square meter. A wire one meter long placed for maximum excitation when swept by the passing wave will have a voltage induced across it equal to the wave`s signal strength in volts per meter. There are no volts or amps in the wave, only the ability to generate volts and amps in conductors. The 377 phms of characteristic impedance is the ratio of the electric field strength to the magnetic field strength in the wave. Its purpose is to get the units right. The ratio of energy in the electric and magnetic components of the wave is really one to one. It is really the same energy swapped back and forth between the electric field and magnetic field which physically are at a right angle and both are at right angles to the direction of travel. Kraus has done the math for us on page 170 of the 3rd edition of "Antennas". His answer is 376.7 ohms, a pure resistance. This is the far field in free space. Best regards, Richard Harrison, KB5WZI |
Richard Clark wrote:
On Fri, 17 Jun 2005 15:48:08 -0500, Cecil Moore wrote: We can demonstrate standing waves using a laser beam normal to a perfect mirror. "We?" Let's see, a hypothetical argument, involving a hypothetical "We," performing a hypothetical analysis that contains 0 places of precision, yielding a hypothetical answer that will be hypothetically true and hypothetically false hypothetically What if there were no hypothetical arguments? ;^) - Mike KB3EIA - |
Richard Harrison wrote:
Cecil, W5DXP wrote: "If so, where does the inductance and capacitance in free space come from to generate that 377 ohms of characteristic impedance?" First, impedance is a voltage to current ratio as in Ohm`s law. Thanks, Richard. The question was somewhat rhetorical and was aimed at the people who believe that EM wave energy "sloshes" around in a transmission line between the inductance and capacitance in the transmission line and that there is really no forward EM wave energy or momentum traveling at the speed of light and no reflected EM wave energy or momentum traveling at the speed of light. So I provided a mental example of a laser beam with reflections demonstrating standing waves in free space. Except for the wavelength, all field conditions are similiar to an RF transmission line with standing waves. So how does the light energy "slosh" around without the inductance and capacitance in free space? -- 73, Cecil http://www.qsl.net/w5dxp |
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 |
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 |
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 |
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 |
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 =---- |
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 |
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 =---- |
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 |
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 |
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 =--- |
Richard Harrison wrote: Reflected power is again reflected at the match point because the matched source sees no reflection. I think you put your finger on it, Richard. That's exactly what inspired the 'sloshing energy' comment. 73, ac6xg |
Jim Kelley wrote:
Richard Harrison wrote: Reflected power is again reflected at the match point because the matched source sees no reflection. I think you put your finger on it, Richard. That's exactly what inspired the 'sloshing energy' comment. Except that the "sloshing energy" comment doesn't have the energy sloshing from the load to the match point and back at the speed of light. As I understand the concept of "sloshing energy" it is sloshing back and forth rather locally between the inductance and the capacitance. -- 73, Cecil http://www.qsl.net/w5dxp |
Tim Wescott wrote:
All this is covered in a good college E&M course. Do "good college E&M courses" cover the conservation of energy principle applied to canceled EM waves? That topic seems to be a black hole in the education of the average electrical engineer. -- 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 =---- |
Ian White, GM3SEK wrote:
"But here you pick up the pace. Instead of the detailed argument above, suddenly whole chapters rush by in a single sentence." Fair criticism. It reflects tiring of posting before its conclusion. The Bird wattmeter`s firectional coupler distinguishes between incident and reflected waves by their singular difference. Upon reflection of a wave, either the voltage or the current it generates is reversed in phase, but not both. Bird takes equal samples of voltage and current from the wave.. When there has been a reflection, the samples have opposite polarity and cancel. When there has been no reflection the samples from that direction of travel are in-phase and the sample total is double the contribution of either sample. To determine reverse power flow, the polarity of one of the samples is reversed. You don`t need to know how it works to use it and Bird never advertised how simple it is as far as I know. Best regards, Richard Harrison, KB5WZI |
Richard Harrison wrote:
Ian White, GM3SEK wrote: "But here you pick up the pace. Instead of the detailed argument above, suddenly whole chapters rush by in a single sentence." Fair criticism. It reflects tiring of posting before its conclusion. The Bird wattmeter`s firectional coupler distinguishes between incident and reflected waves by their singular difference. Upon reflection of a wave, either the voltage or the current it generates is reversed in phase, but not both. Bird takes equal samples of voltage and current from the wave.. When there has been a reflection, the samples have opposite polarity and cancel. When there has been no reflection the samples from that direction of travel are in-phase and the sample total is double the contribution of either sample. Bird assumes the wave reflection model is valid, i.e. Vsample proportional to Vtotal = vector sum of (Vfor+Vref) Isample proportional to Itotal = vector sum of (Ifor+Iref) Vfor in phase with Ifor, RMS Vfor/Ifor = 50 ohms, Vfor*Ifor=Pfor Vref 180 deg out of phase with Iref, RMS Vref/Iref = 50 ohms Vref*Iref=Pref These assumptions are valid for a 50 ohm feedline of reasonable length. These assumptions are obviously not valid if no feedline exists or if Z0 is not 50 ohms, which is, as I infer, Ian's objection. -- 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 =--- |
Richard Harrison wrote:
Ian White, GM3SEK wrote: "But here you pick up the pace. Instead of the detailed argument above, suddenly whole chapters rush by in a single sentence." Fair criticism. It reflects tiring of posting before its conclusion. The Bird wattmeter`s firectional coupler distinguishes between incident and reflected waves by their singular difference. Upon reflection of a wave, either the voltage or the current it generates is reversed in phase, but not both. Bird takes equal samples of voltage and current from the wave.. When there has been a reflection, the samples have opposite polarity and cancel. When there has been no reflection the samples from that direction of travel are in-phase and the sample total is double the contribution of either sample. Bird assumes the wave reflection model is valid, i.e. Vsample proportional to Vtotal = vector sum of (Vfor+Vref) Isample proportional to Itotal = vector sum of (Ifor+Iref) Vfor in phase with Ifor, RMS Vfor/Ifor = 50 ohms, Vfor*Ifor=Pfor Vref 180 deg out of phase with Iref, RMS Vref/Iref = 50 ohms Vref*Iref=Pref These assumptions are valid for a 50 ohm feedline of reasonable length. These assumptions are obviously not valid if no feedline exists or if Z0 is not 50 ohms, which is, as I infer, Ian's objection. -- 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 =--- |
Richard Harrison wrote:
Ian White, GM3SEK wrote: "But here you pick up the pace. Instead of the detailed argument above, suddenly whole chapters rush by in a single sentence." Fair criticism. It reflects tiring of posting before its conclusion. Sure. If you'd kept to the original very steady pace, you'd still be writing... which is not what we do in newsgroups. The Bird wattmeter`s firectional coupler distinguishes between incident and reflected waves by their singular difference. Upon reflection of a wave, either the voltage or the current it generates is reversed in phase, but not both. Yup. If I can fill this out a little... Bird takes equal samples of voltage and current from the wave.. This is done by the pickup loop, which is both inductively and capacitively coupled to the center line. The capacitive coupling gives the voltage sample, while the inductive coupling gives the current sample. The current sample runs through a resistor, which develops a voltage that is made exactly equal to the direct voltage sample. So now we have two RF voltages appearing in series. In the forward direction, the thing is built so that these voltages add in phase. When you rotate the slug by 180deg, the phase of the current sample reverses but the phase of the voltage sample does not, so now the two voltages subtract. If the instrument is terminated in its design impedance of 50 ohms, the voltages (should) cancel exactly, so the meter reading falls back to zero. There's a small capacitive tab on the pickup loop that allows the meter reading to be nulled exactly. When there has been a reflection, the samples have opposite polarity and cancel. When there has been no reflection the samples from that direction of travel are in-phase and the sample total is double the contribution of either sample. Er, yes, pretty much... To determine reverse power flow, the polarity of one of the samples is reversed. And here you've made that big leap again. Where did "power" come from? Nothing in what you or I have said above explains how come the meter can read "Watts". That's because it doesn't actually measure watts. It has been calibrated in watts under certain specific test conditions, using a different kind of wattmeter that actually does measure watts. You don`t need to know how it works to use it No, you don't. But if you choose to use it as "evidence" in a discussion about waves and reflections, then you do need to know how it works. and Bird never advertised how simple it is as far as I know. Possibly because it isn't actually as simple as it looks. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Ian White GM3SEK wrote:
Richard Harrison wrote: To determine reverse power flow, the polarity of one of the samples is reversed. And here you've made that big leap again. Where did "power" come from? Nothing in what you or I have said above explains how come the meter can read "Watts". Bird assumes the meter is being used in a 50 ohm environment. Bird assumes after the two sample voltages are superposed, that the calibration is accurate to within 5% of full scale. The calibration is done using 50 ohm matched lines. In a transmission line, the net power transfer is V*I*cos(theta). It can be proven mathematically that, for a transmission line with reflections, Pnet = V*I*cos(theta) = Vfor*Ifor - Vref*Iref The Bird sampling circuit allows one to read either (Vfor*Ifor) or (Vref*Iref) by turning the slug. Bird assumes Vfor/Ifor = Vref/Iref = 50 ohms. -- 73, Cecil http://www.qsl.net/w5dxp |
Cecil Moore wrote:
Ian White GM3SEK wrote: Richard Harrison wrote: To determine reverse power flow, the polarity of one of the samples is reversed. And here you've made that big leap again. Where did "power" come from? Nothing in what you or I have said above explains how come the meter can read "Watts". Bird assumes the meter is being used in a 50 ohm environment. Bird assumes after the two sample voltages are superposed, that the calibration is accurate to within 5% of full scale. The calibration is done using 50 ohm matched lines. In a transmission line, the net power transfer is V*I*cos(theta). It can be proven mathematically that, for a transmission line with reflections, Pnet = V*I*cos(theta) = Vfor*Ifor - Vref*Iref The Bird sampling circuit allows one to read either (Vfor*Ifor) or (Vref*Iref) by turning the slug. Bird assumes Vfor/Ifor = Vref/Iref = 50 ohms. -- 73, Cecil http://www.qsl.net/w5dxp The Bird sampling circuit certainly is a magical device if it can allow one to "read" a power directly. Energy and power are always calculated quantities. You don't have the math right, Cecil. Try again. 73, Tom Donaly, KA6RUH |
Tom Donaly wrote:
The Bird sampling circuit certainly is a magical device if it can allow one to "read" a power directly. It would be a magical device if it did that but it doesn't. The Bird wattmeter is simply an analog calculator. When the Bird is *calculating* power, it phasor adds/subtracts a sample voltage proportional to the total current to/from a sample volt proportional to the total voltage and comes up with a superposed voltage that is proportional to either forward power or reflected power depending upon slug position. -- 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 =---- |
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