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Richard Clark wrote:
Appeals of authority that are pegged to Cecil are like trying to tread water with a concrete life preserver. Your logic is blighted by a forced conclusion that has nothing to do with the obvious observation that antennas, as transmission lines, are quite evidently non-linear in their characteristic Z. This has been demonstrated and is historic from sources that even Terman's accepts. There exist transmission lines with a changing Z0 along their lengths. Those transmission lines are linear systems. -- 73, Cecil http://www.qsl.net/w5dxp |
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Richard Clark wrote:
"Demanding that "new frequencies" must exist AND then saying that they must be of such-and-such a magnitude to qualify is a hoot." Glad you got a kick out of that. It is not original. In analog microwave systems, often an baseband intermod monitor is used to alarm the operator that nonlinearity has arrived in his system. New frequencies have appeared and have reached a preset arbitrary amplitude sufficient to trigger an alarm. Nothing is perfect so there will always be some intermod. This requires setting a level of these intermod products which will trigger the alarm. This is a standard procedure. Best regards, Richard Harrison, KB5WZI |
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Richard Clark wrote:
"Choose your own numbers, or find an authority to quote a quantitave response." If you can`t detect it, it might as well not exist. If you do detect it, it`s up to you to correct it or not. How many antennas have troubled you with new frequencies? Best regards, Richard Harrison, KB5WZI |
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Richard Harrison wrote:
How many antennas have troubled you with new frequencies? Half a century ago, I installed a ceramic capacitor across the feedpoint of my dipole to change the resonant frequency. I'm sure it generated some new frequencies when it blew up and caught on fire. :-) -- 73, Cecil http://www.qsl.net/w5dxp |
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Roy Lewallen wrote:
I'm sure that somewhere in one of your texts you can find the definition of linear as applied to networks. Once you do, though, a little thought is required to discover that y = mx + b doesn't satisfy the criteria for network linearity. To be linear, a network has to satisfy superposition. This means that: If y1 is the response to excitation x1 and y2 is the response to excitation x2, then the response to x1 + x2 must be y1 + y2. Let's try that with your function. The response to x1 is: y(x1) = mx1 + b The response to x2 is: y(x2) = mx2 + b The sum of y(x1) and y(x2) is: y(x1) + y(x2) = m(x1 + x2) + 2b But response to x1 + x2 is: y(x1 + x2) = m(x1 + x2) + b These are not equal as they must be to satisfy superposition and therefore the requirements for linearity. Roy Lewallen, W7EL Richard Harrison wrote: Roy Lewallen, W7EL wrote: "But of course you realize that the function y = mx + b doesn`t meet the requirements of a linear function when applied to network theory." Works for me. Linear means the graph of the function is a straight line. f(x) = y = mx + b is called linear because its graph is a straight line. A straight line is the shortest distance between two points. In y = mx + b, m is a constant determining the slope of the line. x is is the independent variable. b is the offset or point along the x-axis where the line crosses. y then is a linear function of x because its slope is always mx, but displaced in the x-direction by a constant value, namely b. y is linear the same as IR is linear, or by substitution, E is linear in Ohm`s law where E=IR. For any value of I, voltage = IR and the graph of I versus E is a straight line with a slope equal to R. Resistance is a common factor in network theory. Best regards, Richard Harrison, KB5WZI Not that it means anything, but the linearity requirement is met when b = 0, which, of course, is a subset of the family of equations of the form y = mx + b. 73, Chuck NT3G |
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A good target number for antenna linearity would be one that does not
limit system dynamic range. Our best receivers have a dynamic range of around 120 dB as measured by the minimum discernable signal on the low end, and the point where two-tone third order distortion products are detectable on the high end. 140 dB seems reasonable for an antenna and would theoretically be measured the same way as receiver dynamic range, though setting up a noise-free environment, and coupling large distortion-free signals to a test antenna is a challenge, and is probably one reason we don't see these measurements. The other reason is that there is good evidence that a properly built antenna does not limit system dynamic range. That is, it is very linear in the superposition sense. 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 |
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Richard Clark wrote:
"Your explanation sounds like you are practicing psychaitry, not technoligy." I think quantification is valuable if the measured value is accurate and if the value makes a difference. Antennas are used with transmitters of megawatts of power. These have limitations by regulations on maximum noise and harmonic content. It depends on the jurisdiction, but maximum noise and distortion must be at least 50 dB below the fully modulated level in some locales. I`ve often used the H.P. noise and distortion analyzer to measure off the air to be sure we complied with the regulation. It never occurred to me that our antenna system had a part in noise and distortion production. I expected curvature in a tube`s characteristics or a failed component to cause a rise in noise and distortion. Not once do I recall our antenna system causing distortion anywhere except in the edges of pattern nulls.*This is normal. Receiving antennas on the other hand deliver a satisfactory signal having only microwatts of power. As one responder noted the dynamic range is enormous. This is not really an issue for concern among amateurs. Antennas are in general distortion free. Best regards, RIchard Harrison, KB5WZI |
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