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I would amend Cecil's statement to:
'We hope antennas are linear systems.' A nonlinear antenna is a terrible beast indeed. It will mix every signal it receives with every other signal, creating a nasty mush of signals. On transmit it is not so bad, maybe generating harmonics and a wider signal than we'd like. We've all heard stories of a poor antenna connection causing problems, or a nearby raingutter joint causing TV inteference. These are nonlinear antennas. 73, Glenn AC7ZN |
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"Richard Clark" wrote: In the game of describing an antenna as a transmission line, the non-linearity is compellingly obvious. A non-linear system would generate harmonics so where are those harmonics? A 1mm wire strung 36 meters in outer space is certainly thin by engineering conventions, but it doesn't qualify as the current distribution misses the mark of Cosinality by 5 or 6% (the distribution of a poor fit demonstrates the non-linearity). Nobody said it was a perfectly ideal cosine curve. You seem to have a strange definition of non-linearity as anything that differs from the ideal. By that definition, everything is non-linear (including your definition). The definition of non-linearity being used here is "discontinuous". Exactly where does the current in an antenna become discontinuous? Heaven forbid the cosine curve exhibit the same accuracy as a resistor. Would you also assert that a 52 ohm resistor that is marked 50 ohms is exhibiting non-linearity? -- 73, Cecil, W5DXP |
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"Cecil Moore" wrote: The definition of non-linearity being used here is "discontinuous". Exactly where does the current in an antenna become discontinuous? For readers who don't understanding the meaning of "linear systems", here is a tutorial: http://doctord.dyndns.org:8000/cours...ar_Systems.htm -- 73, Cecil, W5DXP |
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On Thu, 18 May 2006 20:04:12 GMT, "Cecil Moore"
wrote: A non-linear system would generate harmonics so where are those harmonics? There's no need to repeat one post removed coverage so early. |
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Richard,
Wow. You spent a lot of time on this. Thanks. Let's abandon antennas and postulate a twin-lead vacuum-dielectric 100 ohm transmission line that is ideal in the respects we consider important (except loss). There may be (resistive) loss, but no coupling to outside sources or objects, no hysteresis, no electromagnetic radiation. The characteristic impedance is exactly 100 ohms real over our frequency of interest. The velocity factor, for convenience, is 0.5. We feed it on one end with a sinewave generator whose impedance is matched to the line. On the other end we terminate the line with a floating load of arbitrary impedance. The only requirement of the load is that it be perfectly linear and can be described completely as a real and imaginary impedance at any given frequency of interest. Would you consider this system linear? We are not talking antennas now...nothing is radiating. I'm not sure I'm going to go anywhere with this, and I'm not setting a trap. I'm just curious what conditions would have to be set before you would consider a transmission line system linear (you are welcome to add any conditions I might have forgotten). By the way, if you want to discuss any really subtle effects such a Stokes shifting, I define linearity as obeying the law of superposition within a reasonable dynamic range, say 140 dB, which is about 20 dB better than the input dynamic range of our best HF receivers. I understand you are in an argumentative mood with others in the group, but I am taking no sides (I do happen to like Cecil's motorbike...) and will try to keep things civil. If I go anywhere with this, I hope to explain it clearly enough and with enough supporting material that there will be no arguments. I am not a guru so don't expect anyone to believe anything on my word alone. So what say you Richard? Do we have linearity? 73, Glenn AC7ZN |
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Glenn, can you understand whatever it is poor, demented Richard is
waffling about? |
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On Thu, 18 May 2006 19:01:58 -0500, Tom Ring
wrote: I had not heard of the Stokes shift, nor the scattering you mentioned. I have some looking up and reading to do. Which, of course, Cecil does not, since it's not a Xerox moment. Hi Tom, It is pretty exotic, it only relates to radiation, reflection, refraction, heat, and conduction, topics that are alien to discussion here in more than TV Guide English it appears. Other difficult concepts include linearity, coherence, mixing, and gain. Stokes shift is the change in frequency due to the non-linear response of a media to excitation. Typically the excitation is a photon interacting with a phonon with radiation scattering following. Injecting an electron (current) can achieve the same end. The effect of power clamping in fiber optic transmission lines is due to SBS (Stimulated Brillouin Scattering) threshold. I've been working with this (Stokes and Anti-Stokes Shift) for some 20 years, and it fails easy access through a copier. The mention came only response to questions of linear response to what at first glance would be a rather pedestrian transmission line definition, but Glenn appears to have followed the clown instead of pursuing his own question - he warned me it may have been pointless. SBS and SRS (Stimulated Raman Scattering) would be suitable search engine terms (esp. SBS threshold), but I warn you, they lead to remarkably dense work where only one link in 20 will be accessible. 73's Richard Clark, KB7QHC |
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Richard,
Since we agree the transmission line setup is substantially linear, I'm not sure we have much to discuss. I do not agree with you on antenna linearity, but that is another subject for another rainy weekend, and I'm not the right person to be discussing that anyway. 73, Glenn |
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Hi. How's about you guys going to Dayton.
I'm sure a Tag Fight could be arranged for you all. Two Pin Falls, Two Submissions,or a KO to decide the winner. Maybe if you all meet face to face, you will come to some common agreement, or this thread wii continue ad nausium. Regards Mike. |
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Richard Clark wrote:
snip SBS and SRS (Stimulated Raman Scattering) would be suitable search engine terms (esp. SBS threshold), but I warn you, they lead to remarkably dense work where only one link in 20 will be accessible. 73's Richard Clark, KB7QHC Thanks. tom K0TAR |
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Richard Clark wrote:
Injecting an electron (current) can achieve the same end. How accurate are your electron position and velocity measurements? -- 73, Cecil http://www.qsl.net/w5dxp |
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Richard Clark wrote:
On Thu, 18 May 2006 19:01:58 -0500, Tom Ring wrote: I had not heard of the Stokes shift, nor the scattering you mentioned. I have some looking up and reading to do. Which, of course, Cecil does not, since it's not a Xerox moment. Hi Tom, It is pretty exotic, it only relates to radiation, reflection, refraction, heat, and conduction, topics that are alien to discussion here in more than TV Guide English it appears. Other difficult concepts include linearity, coherence, mixing, and gain. Stokes shift is the change in frequency due to the non-linear response of a media to excitation. Typically the excitation is a photon interacting with a phonon with radiation scattering following. Injecting an electron (current) can achieve the same end. The effect of power clamping in fiber optic transmission lines is due to SBS (Stimulated Brillouin Scattering) threshold. I've been working with this (Stokes and Anti-Stokes Shift) for some 20 years, and it fails easy access through a copier. The mention came only response to questions of linear response to what at first glance would be a rather pedestrian transmission line definition, but Glenn appears to have followed the clown instead of pursuing his own question - he warned me it may have been pointless. SBS and SRS (Stimulated Raman Scattering) would be suitable search engine terms (esp. SBS threshold), but I warn you, they lead to remarkably dense work where only one link in 20 will be accessible. 73's Richard Clark, KB7QHC Hi Richard, Stokes' law and the Raman effect can be found in physics texts dealing with quantum mechanics. Georg Joos, in his book _Theoretical Physics_ deals with such things. The reading is dense but the underlying concepts aren't too difficult. The difficulty might lie in understanding how they apply to this discussion. 73, Tom Donaly, KA6RUH |
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On Fri, 19 May 2006 15:08:11 GMT, "Tom Donaly"
wrote: Stokes' law and the Raman effect can be found in physics texts dealing with quantum mechanics. Georg Joos, in his book _Theoretical Physics_ deals with such things. The reading is dense but the underlying concepts aren't too difficult. The difficulty might lie in understanding how they apply to this discussion. Hi Tom, Certainly Joos would give some entry into the field, but finding work as accessible outside of a bookstore or library (in other words, through a search engine) makes for drinking out of a fire hose to quench a sip's worth of thirst. Insofar as HOW this applies, I've spoken to that and Tom shows interest. That alone goes beyond the typical churning that passes for discussion. The point is that these underlying concepts are fairly simple as you imply and they are certainly not remote from the usual topics of consideration here. What they lack is specifics that relate to our common applications, and there too I've offered discussion. However, few seem inspired to travel those paths and that fault can hardly be laid at my doorstep. 73's Richard Clark, KB7QHC |
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Richard,
Who, in your estimation, does qualify to discuss it? I would like to hear from someone who has actually measured the linearity of a simple antenna. I have not done this, so would only be one guy with an opinion. Plenty of us around. By 'simple antenna' I mean an antenna such as a wire dipole without traps, baluns or other things that could degrade linearity. Since the linearity of antenna systems in general is in question, the simplest setup that answers the question would be best. In the absence of measurement, can anyone comment on the modelling software? Does it assume and model a linear system? If so, do we know of any substantial nonlinear departures from the modelling software? Anyone? 73, Glenn AC7ZN |
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Richard Clark, KB7QHC wrote:
"Who. in your estimation, does qualify to discuss it?" If it`s about antennas, I nominate Kraus. If it`s about mathematics, many marhematicians qualify. In algebra, y = mx + b, (the point slope formula), is called linear because it is the graph of a straight line. In the discussion of transmission lines and antennas, you must admit that a uniform transmission line enforces a unique characteristic impedance (a resistance) on energy traveling in either direction. A resistance is linear because Ohm`s law prevails. In free-space, everyone agrees the characteristic impedance is about 377 ohms (a resistance). Kraus says on page 2 of his 1950 edition of "Antennas": "Let the transmission line now be connected to a dipole antenna as in Fig.1-2. The dipole acts as an antenna because it launches a free-space wave. However, it may also be regarded as a section of transmission line (see Sec. 1-2). In addition, it exhibits many of the characteristics of a resonator, since energy reflected from the ends of the dipole gives rise to a standing wave on the antenna. Thus a single device, in this case the dipole, exhibits simutaneously properties characteristic of an antenna, transmission line, and a resonator. Best regards, Richard Harrison, KB5WZI |
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wrote:
By 'simple antenna' I mean an antenna such as a wire dipole without traps, baluns or other things that could degrade linearity. Seems the easiest measurement of nonlinearity would be the harmonics (if any) generated by the antenna that do not appear in the source signal. -- 73, Cecil http://www.qsl.net/w5dxp |
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"Richard Clark" wrote in message ... On 19 May 2006 03:26:29 -0700, wrote: I do not agree with you on antenna linearity, but that is another subject for another rainy weekend, and I'm not the right person to be discussing that anyway. Hi Glenn, By this very post you are discussing it. Who, in your estimation, does qualify to discuss it? Despite all outward appearances (and certainly the troll inspired name of the topic), this is NOT about one-upmanship competition. 73's Richard Clark, KB7QHC don't you call me a troll... i just set the stage with an aptly named thread to get things started for my own amusement... wait a minute, maybe that does make me a troll! but at least i was open about my motives, and boy am i enjoying it.... especially since its still raining! maybe you guys can keep going through this weekend??? |
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Cecil Moore wrote:
wrote: By 'simple antenna' I mean an antenna such as a wire dipole without traps, baluns or other things that could degrade linearity. Seems the easiest measurement of nonlinearity would be the harmonics (if any) generated by the antenna that do not appear in the source signal. Which wouldn't tell you a single thing about the current distribution along the length of the dipole. 73, Tom Donaly, KA6RUH |
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Richard Clark wrote:
On Fri, 19 May 2006 15:08:11 GMT, "Tom Donaly" wrote: Stokes' law and the Raman effect can be found in physics texts dealing with quantum mechanics. Georg Joos, in his book _Theoretical Physics_ deals with such things. The reading is dense but the underlying concepts aren't too difficult. The difficulty might lie in understanding how they apply to this discussion. Hi Tom, Certainly Joos would give some entry into the field, but finding work as accessible outside of a bookstore or library (in other words, through a search engine) makes for drinking out of a fire hose to quench a sip's worth of thirst. Insofar as HOW this applies, I've spoken to that and Tom shows interest. That alone goes beyond the typical churning that passes for discussion. The point is that these underlying concepts are fairly simple as you imply and they are certainly not remote from the usual topics of consideration here. What they lack is specifics that relate to our common applications, and there too I've offered discussion. However, few seem inspired to travel those paths and that fault can hardly be laid at my doorstep. 73's Richard Clark, KB7QHC Hi Richard, I didn't intend any criticism. People like Cecil, with home-grown theories, don't ever seem to want things considered in depth. That's understandable from a psychological standpoint, but it isn't any help to the rest of us when some of the things the theory ignores become significant. In the case of antennas, practically everything is significant. 73, Tom Donaly, KA6RUH |
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Reg,
Thank heavens there is someone here who knows the Queen's English. Glenn ======================================== The trouble with Richard is that he wraps everything up in Shakespearian prose, verse and poetry. The sonnets. Queen Elizabeth the First's language. ---- Reg. |
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On Fri, 19 May 2006 19:01:23 GMT, "Tom Donaly"
wrote: In the case of antennas, practically everything is significant. Les Mots Juste ;-) |
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Richard Clark wrote:
On Fri, 19 May 2006 15:08:11 GMT, "Tom Donaly" wrote: Stokes' law and the Raman effect can be found in physics texts dealing with quantum mechanics. Georg Joos, in his book _Theoretical Physics_ deals with such things. The reading is dense but the underlying concepts aren't too difficult. The difficulty might lie in understanding how they apply to this discussion. Hi Tom, Certainly Joos would give some entry into the field, but finding work as accessible outside of a bookstore or library (in other words, through a search engine) makes for drinking out of a fire hose to quench a sip's worth of thirst. Insofar as HOW this applies, I've spoken to that and Tom shows interest. That alone goes beyond the typical churning that passes for discussion. The point is that these underlying concepts are fairly simple as you imply and they are certainly not remote from the usual topics of consideration here. What they lack is specifics that relate to our common applications, and there too I've offered discussion. However, few seem inspired to travel those paths and that fault can hardly be laid at my doorstep. 73's Richard Clark, KB7QHC And a web search produced a couple very good links out of the first 20. One was a great discussion of problems in long optical fibers, and some relatively simple ways to work around some of them, or at least to mitigate them. tom K0TAR |
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On Fri, 19 May 2006 14:24:07 -0500, Tom Ring
wrote: And a web search produced a couple very good links out of the first 20. One was a great discussion of problems in long optical fibers, and some relatively simple ways to work around some of them, or at least to mitigate them. Hi Tom, In my early days in this game (late 80s), I sought to turn lemons into quantum-aide. That is, I sought erbium doped fiber optics to amplify nanowatt fluorescence signals with 10 to 50 µS decay times. Not one of those off-the-shelf commodities, however; so I had to amplify in the conventional way with an PMT. For those interested, long haul communications fiber optics (transoceanic grade) met with the same requirements for amplifiers placed along the length to maintain S+N/N. Erbium doped fibers were projected as a solution. You could pulse UV into the fiber to charge it, and a IR data pulse would be amplified, continuously along its length. The IR data pulse would be boosted by the previous charge of energy. This is an example of forward Raman scattering and is called Raman Amplification (which at the time would have been about 30dB and 10% efficient). 73's Richard Clark, KB7QHC |
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Tom Donaly, KA6RUH wrote:
"Actually, it`s supposed to be impossible to represent the current distribution along a dipole using simple mathematical formulas because integral equations have to be solved that are impervious to any solution other than numerical approximation." How many places do you attach to pi? First, what is linearity? It is the absence of nonlinearity. Millman and Seely wrote on page 525 of the 1951 edition of "Electronics" (one of my old textbooks): "Because of this nonlinear characteristic of the dynamic curve over the operating range, the wave form of the output wave differs slightly from that of the grid-exciting-voltage waveshape. Distortion of this type is called "nonlinear" or "amplitude" distortion.." All of the antennas I`ve worked with had no noticeable amplitude distortion. They caused no harmonics or mixing products. On page 235 of Kraus` 1950 edition of "Antennas" he sets out to solve Hallen`s equation for current distribution. On page 239, Kraus writes: "It is generally assumed that the current distribution of an infinitesimally thin antenna is sinusoidal, and that the phase is constant over a 1/2-wavelength interval, changing abruptly by 180-degrees between intervals." You can take what Kraus says to the bank. Best regards, Richard Harrison, KB5WZI |
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Richard,
Maybe someone can help us here. Linearity is well-defined in electronics by the law of superposition, and is characterized by well-known measurements such as harmonic generation, compression point, and third-order intercept point. I'm assuming antennas must follow the same law of superposition while transmitting and receiving to be linear. It is not clear to me that a nonlinear or even unpredictable current distribution along a wire antenna produces signals that violate the law of superposition. Under a strange current distribution the antenna radiation pattern will certainly distort, but how does that violate the law of superposition? That is, how can a strong received signal influence a weak one on an antenna with nonlinear current distribution? Maybe, like so many other threads in this group, we are discussing orthogonal concepts. 73, Glenn AC7ZN |
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In article .com,
wrote: Maybe, like so many other threads in this group, we are discussing orthogonal concepts. I believe you're correct. As I see it, in the *general* sense, linearity refers to a relationship between two variables, where the relationship is one of OUT = IN * F + C where F and C are constants (plus a dimensional factor in many cases). In other words, it's a straight-line relationship (hence, the name) between two variables of the same or different dimension. The sort of "linearity" that people usually refer to in electronics, involves voltages and currents (vs. one another). A theoretically perfect resistor, capacitor, or inductor is linear, because (e.g.) the peak current through it has a strictly linear relationship to the peak voltage across it. A semiconductor junction is described as nonlinear, because the current through it is not related to the voltage across it in a strictly linear relationship. The sort of "linearity" which Cecil seems to be referring to (if I understand what he's written correctly) involves a completely different sort of relationship. It's not current-vs-voltage, or voltage-vs-current - it's current-vs-distance. If I recall correctly, an infinitesimally-short "monopole" has a current-vs-distance relationship which is close to linear. A half-wave monopole does not. Nonlinearities of this sort would have entirely different effects on an antenna system than nonlinearities of the voltage-vs-current sort. They're two different beasts entirely. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
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Richard Harrison wrote:
Tom Donaly, KA6RUH wrote: "Actually, it`s supposed to be impossible to represent the current distribution along a dipole using simple mathematical formulas because integral equations have to be solved that are impervious to any solution other than numerical approximation." How many places do you attach to pi? First, what is linearity? It is the absence of nonlinearity. Millman and Seely wrote on page 525 of the 1951 edition of "Electronics" (one of my old textbooks): "Because of this nonlinear characteristic of the dynamic curve over the operating range, the wave form of the output wave differs slightly from that of the grid-exciting-voltage waveshape. Distortion of this type is called "nonlinear" or "amplitude" distortion.." All of the antennas I`ve worked with had no noticeable amplitude distortion. They caused no harmonics or mixing products. On page 235 of Kraus` 1950 edition of "Antennas" he sets out to solve Hallen`s equation for current distribution. On page 239, Kraus writes: "It is generally assumed that the current distribution of an infinitesimally thin antenna is sinusoidal, and that the phase is constant over a 1/2-wavelength interval, changing abruptly by 180-degrees between intervals." You can take what Kraus says to the bank. Best regards, Richard Harrison, KB5WZI It certainly is interesting how many supposedly knowledgeable people can't tell the difference between length and time. Millman and Seely were writing about cycles per _second_. Kraus was talking about distribution over _length_. Moreover, read Richard Clark's post on this subject. Brother! 73, Tom Donaly, KA6RUH |
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Richard Clark wrote:
In other words, the non-linearity shown by the lack of congruence to the Cosine curve is not a presumption of non-linearity by the modeler; it is merely reporting an analysis. You are being fooled by an illusion. Any deviation from single frequency sinusoidal signals would generate harmonics which we know doesn't happen. Your "non-linearity" is not really there. For instance, a decrease in VF may compress the waveform but that is not non-linearity. -- 73, Cecil http://www.qsl.net/w5dxp |
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Tom Donaly wrote:
What he believes is that since he can't detect any harmonics emanating from a sinusoidally fed dipole, the current along its length must be a sinusoid. The non-existence of harmonics is prima facie evidence that only single frequency sinusoids exist. A properly functioning antenna system is linear. Any perceived non-linearity is an illusion. -- 73, Cecil http://www.qsl.net/w5dxp |
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Tom Donaly wrote:
Cecil Moore wrote: Seems the easiest measurement of nonlinearity would be the harmonics (if any) generated by the antenna that do not appear in the source signal. Which wouldn't tell you a single thing about the current distribution along the length of the dipole. Yes it would. It would be proof that the current distribution along the length of the dipole is sinusoidal no matter what your illusionary perceptions are telling you. For standing wave antennas, if the source is a pure single frequency sine wave and if no harmonics are generated by the antenna system: 1. The forward wave is sinusoidal. 2. The reflected wave is sinusoidal and coherent with the forward wave. 3. Their superposition results in a sinusoidal standing wave with the same angular velocity. Any non-linearity would introduce harmonics. -- 73, Cecil http://www.qsl.net/w5dxp |
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Tom Donaly wrote:
People like Cecil, with home-grown theories, don't ever seem to want things considered in depth. That's understandable from a psychological standpoint, but it isn't any help to the rest of us when some of the things the theory ignores become significant. In the case of antennas, practically everything is significant. All of the theories I am quoting were developed long before I was born. Almost every technical explanation starts out with simple concepts and proceeds to more complex concepts. For the sake of teaching and understanding simple concepts, the secondary variables are often ignored for the time being. Thus, every textbook on transmission lines starts off with an explanation of lossless lines with perfectly resistive characteristic impedances even though such lines don't exist in reality. -- 73, Cecil http://www.qsl.net/w5dxp |
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Richard Clark wrote:
How much distortion has to exist before you hear it? As this directly relates to your quoted selection, are we to believe that distortion does not exist if you cannot perceive it? How about: Distortion can be measured. -- 73, Cecil http://www.qsl.net/w5dxp |
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Dave Platt wrote:
The sort of "linearity" which Cecil seems to be referring to (if I understand what he's written correctly) involves a completely different sort of relationship. It's not current-vs-voltage, or voltage-vs-current - it's current-vs-distance. Assuming thin constant diameter wires with a constant Z0 and VF. If the diameter of the wire changes, or Z0 changes, or VF changes, the 'K' term in the cos(KX) expression changes. A change in a constant does NOT produce non-linearity in a linear system. Just because a wave slows down in a medium with a low VF doesn't mean that the system has gone non-linear. -- 73, Cecil http://www.qsl.net/w5dxp |
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