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#21
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On Sat, 21 Aug 2004 18:07:16 GMT, "The other John Smith"
wrote: I tried sending you an email on this but you've spoofed your address so bad I guess you didn't get it. Pity. |
#22
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"Walter Maxwell" wrote in message ... On Sat, 21 Aug 2004 20:46:15 -0500, Tom Ring wrote: Tom Ring wrote: snip Vector to swerve to the right. If you hang the shorted end down, the reverse occurs, causing more current, or "conservative" current, and swings the Pointy Vector to the left. Sorry, should have read "causing more current, or "conservative" current to be canceled". tom K0TAR Verily, Walt, W2DU Snicker! You guys are a hoot! John, KD5YI |
#23
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"TOM" wrote in message ... John, negative resistance is indicated when the magnitude of the reflection coefficient is greater than one. The solution of the impedance equation for rho 1 yields negative values of resistance. This lends a lot of insight into possible causes: The calibration for open or short is slightly lossy. If a measured load has a little more reflection than the calibration loads, it would indicate as negative resistance. Numerical errors creep into the results. The Smith chart is extremely sensitive around the periphery. A few tenths of a dB. result in large movements at the edge of the chart. Thus, small calibration or computational errors can result in crossing over the rho=1 circle. And, as mentioned, detection of external RF fields increasing the output of the reflection detector past the calibration value can result in a measured value for rho 1. -- Tom, N5EG Yes, I see that now, Tom. Thanks. John, KD5YI |
#24
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"John Smith" kd5yiatmindspringdotcom wrote in message ... "TOM" wrote in message ... John, negative resistance is indicated when the magnitude of the reflection coefficient is greater than one. The solution of the impedance equation for rho 1 yields negative values of resistance. This lends a lot of insight into possible causes: The calibration for open or short is slightly lossy. If a measured load has a little more reflection than the calibration loads, it would indicate as negative resistance. Numerical errors creep into the results. The Smith chart is extremely sensitive around the periphery. A few tenths of a dB. result in large movements at the edge of the chart. Thus, small calibration or computational errors can result in crossing over the rho=1 circle. And, as mentioned, detection of external RF fields increasing the output of the reflection detector past the calibration value can result in a measured value for rho 1. -- Tom, N5EG Yes, I see that now, Tom. Thanks. John, KD5YI ======================================== Measured values of Rho greater than 1.0 are not necessarily due to measurement errors. They can be true. The true theoretical maximum value of Rho is 1 + Sqrt(2) which occurs on transmission lines only at extremely low frequencies and with a purely inductive terminating impedance. It arises due to resonance between line impedance and the termination and cannot be of any consequence at RF. Values of Rho greater than 1.0 are mathematically legitimate as may be demonstrated by exact computer programs when presented with exact data. In any case, the Smith Chart is inherently inexact at the lower frequencies in addition to the user's difficulties of reading from it. The reflection coefficient can quite easily lie in any of the four 90-degree quadrants. If the real component of the vector is negative it may be thought to correspond somewhere to an imaginary negative resistance. But when Rho is correctly used in line input and output impedance calculations it always gives the correct answers. ie., negative resistances cannot exist. Resistances are always lossy. ---- Reg, G4FGQ |
#25
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"Wes Stewart" wrote in message ... On Sat, 21 Aug 2004 18:07:16 GMT, "The other John Smith" wrote: I tried sending you an email on this but you've spoofed your address so bad I guess you didn't get it. Pity. Hi, Wes. Yes it is a pity that we have to go to so much inconvenience to keep spam to a tolerable level. Sorry about that. I tried to email you, too, but you may have not checked it by the time I post this. You can reach me through the ARRL reflector as kd5yi at arrl dot net. Thanks. John, KD5YI |
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