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Richard Clark wrote:
Cecil would shrug off 59% worth of distortion to define it linear. Richard, seems you suffer from the same affliction as Howard Hughes, repeating the same psychotic nonsense over and over. -- 73, Cecil http://www.qsl.net/w5dxp |
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"Richard Harrison" wrote in message ... snip Antennas are in general distortion free. Yes, but "bad times" can make them non-linear. Consider the bolted joints that get corroded and semi-conductive over time, with rain and temperature changes to help. Nearby RF sources can generate distortion products in my antennas if I haven't kept up my maintenance. |
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wrote in message oups.com... ... By the way, generating new frequencies is not necessarily a violation of superposition (though it usually is). Consider a system undergoing a constant Doppler shift. 73, Glenn AC7ZN Glenn, boy! I can't read all these posts, so I was trying to see where you were going and asking by just skimming, primarily your posts. The above caught my eye. Doppler is not "generating new frequencies" as a non linearity does. A non-linear system will produce harmonics with one exitation frequency and produce the common mixing / IM with multiple exitation (superimposed) frequencies. I think trying to mix relativistic effects in with the stationary world is an unnecessary complication of a linearity discussion. If I have time I'll try to follow the thread to see what you're really after...but. If it takes mega watts to see some non linearity in an antenna, who cares? and more importantly how will you know whree it is occuring since things like the junction of two connectorc can produce enough IM to mask other, smaller sources. If you tried an experiment looking for IM / Mixing you might try to use a receiver becaue a receiver could be a very sensitive detector...but you'd have to have a pretty good receiver. Something like a kW LO and mixer to have a really good intercept. I'm not sure of the point here... Do antennas cause IM? Sounds like a deadend arena to me. 73, Steve, K9DCI |
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Thanks, Steve,
Good post. You do need to read a little mo by mentioning Doppler shifts, I was illustrating that a linear system can generate new frequencies without violating the law of superposition. No relativistic effects needed he a two-tone train whistle will not distort in the superposition sense under Doppler shift, but it can generate new (or different) frequencies. My point was that generation of new frequencies is not necessarily a valid test for nonlinearity. My position on antennas is the same as yours. Earlier posts were confusing as the term 'linearity' was being applied to antennas in two ways. One in the electrical superposition sense (by me) and the other in current distribution along the antenna element. Electrically, antennas are very linear (I believe) even when their current distribution along the element length is not. I stopped posting when I realized we were discussing two different things. 73, Glenn AC7ZN Steve N. wrote: wrote in message oups.com... ... By the way, generating new frequencies is not necessarily a violation of superposition (though it usually is). Consider a system undergoing a constant Doppler shift. 73, Glenn AC7ZN Glenn, boy! I can't read all these posts, so I was trying to see where you were going and asking by just skimming, primarily your posts. The above caught my eye. Doppler is not "generating new frequencies" as a non linearity does. A non-linear system will produce harmonics with one exitation frequency and produce the common mixing / IM with multiple exitation (superimposed) frequencies. I think trying to mix relativistic effects in with the stationary world is an unnecessary complication of a linearity discussion. If I have time I'll try to follow the thread to see what you're really after...but. If it takes mega watts to see some non linearity in an antenna, who cares? and more importantly how will you know whree it is occuring since things like the junction of two connectorc can produce enough IM to mask other, smaller sources. If you tried an experiment looking for IM / Mixing you might try to use a receiver becaue a receiver could be a very sensitive detector...but you'd have to have a pretty good receiver. Something like a kW LO and mixer to have a really good intercept. I'm not sure of the point here... Do antennas cause IM? Sounds like a deadend arena to me. 73, Steve, K9DCI |
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wrote Electrically, antennas are very linear (I believe) even when their current distribution along the element length is not. I stopped posting when I realized we were discussing two different things. In a linear system, there only needs to be a straight line function between the input and output. The actual signals on the input wouldn't be very useful if only straight line functions were allowed on that input. The current distribution along an antenna element length only ever approximates a straight line. The only requirement for that current to obey the rules for a linear system is that it be a linear function of the source current and it is at every point. -- 73, Cecil http://www.qsl.net/w5dxp |
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Cecil Moore wrote: In a linear system, there only needs to be a straight line function between the input and output. The actual signals on the input wouldn't be very useful if only straight line functions were allowed on that input. The current distribution along an antenna element length only ever approximates a straight line. The only requirement for that current to obey the rules for a linear system is that it be a linear function of the source current and it is at every point. I mostly agree with your definition of linearity. Roy's point that an offset in the straight line violates superposition is an example of a straight line violating superposition. Also consider the throwing of two dice. If the dice act independently they can be considered a linear system with two outputs (the numbers that show on the dice) which obeys the law of superposition. If, however, the dice collide when thrown, they now influence each other, the system becomes nonlinear, and the law of superposition is violated. It's pretty hard for me to attach a straight line function to dice. Mixers are especially interesting beasts when viewed in the light of superposition. They are obviously highly nonlinear, yet we regularly speak of mixer linearity. Here is the trick: when the local oscillator is included in the input signal set, the mixer is highly nonlinear as the LO influences every signal that comes in drastically, in the sense of generating new frequencies. But it is convenient to think of the local oscillator as just an internal parameter of the mixer, and to not include it in the input signal set. Under this assumption all the RF input signals substantially obey the law of superposition (that is, they do not influence each other) and discussing mixer linearity has enough meaning that we can characterize it through standard tests. Note also that under this assumption the output frequencies do not match the input frequencies in general, yet the law of superposition holds over a wide dynamic range. Cecil's point that doubling antenna current will double the current at each point in the antenna (I'm rephrasing) has a dual in mixers: doubling any signal will double all the mixer's ouputs due to that signal alone, and not any other outputs. 73, Glenn Dixon AC7ZN |
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wrote:
Cecil's point that doubling antenna current will double the current at each point in the antenna (I'm rephrasing) has a dual in mixers: doubling any signal will double all the mixer's ouputs due to that signal alone, and not any other outputs. Yep, as long as system linearity is maintained. -- 73, Cecil http://www.qsl.net/w5dxp |
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wrote in message oups.com... Thanks, Steve, Good post. Thanks. You do need to read a little mo by mentioning Doppler shifts, I was illustrating that a linear system can generate new frequencies without violating the law of superposition. Hi Glenn, I got that. Perhaps my use of "relativitistic effects" was inappropriate. I still maintain that Doppler shift is not an example of (at least what I would call) "creating new frequencies" because an electronis system did not cause said shift. A system is linear or non-linear, but a system can't "make Doppler happen". It is a frequency change, yes, but an "electronic system" can't cause it. It occurs for reasons other than "system characteristics". Yes, a space craft can be considered part of a system in teh general sense, but not as I believe an "electronic system" should be thought of when discussing such things. Perhaps symmantics in your view, but not mine. I think it is a fundamental, but, perhaps can't explain adequately why. What a thread. usually when a thread gets this long it digresses far outside the original intent. ...Earlier posts were confusing as the term 'linearity' ...electrical superposition sense ... and ... current distribution along the antenna element. So you *were* questioning the linearity of antennas as we understand the term? 73, Steve 73, Glenn AC7ZN Steve N. wrote: wrote in message oups.com... ... By the way, generating new frequencies is not necessarily a violation of superposition (though it usually is). Consider a system undergoing a constant Doppler shift. 73, Glenn AC7ZN Glenn, boy! I can't read all these posts, so I was trying to see where you were going and asking by just skimming, primarily your posts. The above caught my eye. Doppler is not "generating new frequencies" as a non linearity does. A non-linear system will produce harmonics with one exitation frequency and produce the common mixing / IM with multiple exitation (superimposed) frequencies. I think trying to mix relativistic effects in with the stationary world is an unnecessary complication of a linearity discussion. If I have time I'll try to follow the thread to see what you're really after...but. If it takes mega watts to see some non linearity in an antenna, who cares? and more importantly how will you know whree it is occuring since things like the junction of two connectorc can produce enough IM to mask other, smaller sources. If you tried an experiment looking for IM / Mixing you might try to use a receiver becaue a receiver could be a very sensitive detector...but you'd have to have a pretty good receiver. Something like a kW LO and mixer to have a really good intercept. I'm not sure of the point here... Do antennas cause IM? Sounds like a deadend arena to me. 73, Steve, K9DCI |
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I haven't been following this thread lately, but happened on this
particular posting... Seems to me you want to say "Linear TIME INVARIANT system" to get to not generating new frequencies. Certainly I can build a system that is linear but not time-invariant and generate new frequencies with that system. A simple one is a signal going into a potentiometer, coming out the wiper, in which the wiper is rotated continuously. It's linear, but not time invariant, and obviously any input will be amplitued modulated at the rate of the time variation. Is a double-balanced mixer with LO a linear system (for input signals in the intended amplitued range)? Increasing the input amplitude by 1dB causes the output amplitude to increase by 1dB, though the output is not at the same frequency as the input. If the response of the DBM/LO system to input x1 is y1, and to x2 is y2, then is (y1+y2) the response to input (x1+x2)? Is a DBM/LO system time-invariant: if I apply stimulus x1 at time t1 do I get the same response as if I apply it at time t2 (where the response is also shifted by t2-t1)? Perhaps this will be useful food for thought... Cheers, Tom Steve N. wrote: wrote in message oups.com... Thanks, Steve, Good post. Thanks. You do need to read a little mo by mentioning Doppler shifts, I was illustrating that a linear system can generate new frequencies without violating the law of superposition. Hi Glenn, I got that. Perhaps my use of "relativitistic effects" was inappropriate. I still maintain that Doppler shift is not an example of (at least what I would call) "creating new frequencies" because an electronis system did not cause said shift. A system is linear or non-linear, but a system can't "make Doppler happen". It is a frequency change, yes, but an "electronic system" can't cause it. It occurs for reasons other than "system characteristics". Yes, a space craft can be considered part of a system in teh general sense, but not as I believe an "electronic system" should be thought of when discussing such things. Perhaps symmantics in your view, but not mine. I think it is a fundamental, but, perhaps can't explain adequately why. What a thread. usually when a thread gets this long it digresses far outside the original intent. ...Earlier posts were confusing as the term 'linearity' ...electrical superposition sense ... and ... current distribution along the antenna element. So you *were* questioning the linearity of antennas as we understand the term? 73, Steve 73, Glenn AC7ZN Steve N. wrote: wrote in message oups.com... ... By the way, generating new frequencies is not necessarily a violation of superposition (though it usually is). Consider a system undergoing a constant Doppler shift. 73, Glenn AC7ZN Glenn, boy! I can't read all these posts, so I was trying to see where you were going and asking by just skimming, primarily your posts. The above caught my eye. Doppler is not "generating new frequencies" as a non linearity does. A non-linear system will produce harmonics with one exitation frequency and produce the common mixing / IM with multiple exitation (superimposed) frequencies. I think trying to mix relativistic effects in with the stationary world is an unnecessary complication of a linearity discussion. If I have time I'll try to follow the thread to see what you're really after...but. If it takes mega watts to see some non linearity in an antenna, who cares? and more importantly how will you know whree it is occuring since things like the junction of two connectorc can produce enough IM to mask other, smaller sources. If you tried an experiment looking for IM / Mixing you might try to use a receiver becaue a receiver could be a very sensitive detector...but you'd have to have a pretty good receiver. Something like a kW LO and mixer to have a really good intercept. I'm not sure of the point here... Do antennas cause IM? Sounds like a deadend arena to me. 73, Steve, K9DCI |
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