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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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