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#21
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On Thu, 28 Aug 2003 12:18:44 -0500, W5DXP
wrote: My basic approach is to achieve a Z0-match and therefore forget about source impedance. Hi Cecil, This is a cavalier attitude if you can afford it. Otherwise, those who so desperately hammer out the last 0.1 dB antenna gain are going to fall to their knees in wrack when they discover that their rig's characteristic Z of, say, 70 Ohms meeting the discontinuity of their low pass filter's 50 Ohms turns that effort into heat behind the antenna jack. I have long since stopped being surprised by those who spin on like whirling dervishes over trivial matters in the face of 10 fold losses in front of them. This, of course, is even more trivial when they gush on about their premium equipment that behind the knobs "efficiently" transforms 20 - 25 Amperes of DC current into 100 Watts RF. Now, that puts perspective to the topic: smoke and reflection coefficient. 73's Richard Clark, KB7QHC |
#22
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#23
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Richard Clark wrote:
But, again, this discussion is generally reserved only for those interested in accuracy. :-) Like I say, my solution is to block any reflections from being incident upon the source. But I have a question. Since we are discussing coherent sine waves, it seems to me that any reflection from the source impedance will become indistinguishable from the generated wave. In fact, the present convention of generated power equals forward power minus reflected power is designed to overcome that very problem. -- 73, Cecil, W5DXP |
#24
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Richard Clark wrote:
W5DXP wrote: My basic approach is to achieve a Z0-match and therefore forget about source impedance. This is a cavalier attitude if you can afford it. It's all part of my "Work Smarter, Not Harder" nature. The elimination of reflected energy incident upon the source is extremely rewarding in multiple ways. -- 73, Cecil, W5DXP |
#25
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Tom, to save everybody a lot of trouble -
The greatest theoretical value of the magnitude of the reflection coefficient occurs when the angle of Zo is -45 degrees, and the terminating impedance is a pure inductive reactance of |Zo| ohms. Do you think I should have mentioned this when I began this and other threads by saying a reflection coefficient greater than unity can occur? The riot police can now return to barracks. ---- Reg, G4FGQ. ==================================== --- "Tom Bruhns" wrote "Reg Edwards" wrote By the way, you've told us only half the story. What's the value of the load impedance which maximises the reflection coefficient? ==================================== Hey, Reg, it's just a simple high-school (well, maybe first-year college) differential calculus problem. Just let Garvin work through it for us. Hey, good Dr., could you do that for us? Just write an expression for |Vr/Vf| = |(Zl-Zo)/(Zl+Zo)| in terms of Rl and Xl and find the partial derivatives with respect to those two variables, and set both equal to zero, while letting Ro=Xo. It's mostly just a bunch of bookkeeping. You should come up with values of Rl and Zl in terms of Ro, and you can check to be sure that's actually a maximum and not a minimum or saddle point. You should see a symmetry for Ro=-Xo, the more usual limiting case. (Of course, that's not quite right, as I'm sure the good Dr. and Reg both know. Since we're talking passive here, you need to insure that Rl stays positive, so you just may need to check along the boundary where Rl=0. And you should convince yourself that the most reactive possible line really does yield the largest possible |Vr/Vf|. So it becomes a task of finding the maximum value of a function f(Rl, Xl, Xo) with Ro fixed positive non-zero, under the constraints that Rl=0 and |Xo|=Ro.) |
#26
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On Thu, 28 Aug 2003 12:33:53 -0700, W5DXP
wrote: Richard Clark wrote: But, again, this discussion is generally reserved only for those interested in accuracy. :-) Like I say, my solution is to block any reflections from being incident upon the source. But I have a question. Since we are discussing coherent sine waves, it seems to me that any reflection from the source impedance will become indistinguishable from the generated wave. In fact, the present convention of generated power equals forward power minus reflected power is designed to overcome that very problem. Hi Cecil, So you DO want to perform this test? Your presumption of coherency is false unless you engineer the solution. I got there first and made sure that wasn't gonna happen. :-) Any random attempt has only a one in 360 chance of being correct within one degree of coherent. This is simple interference math after all. Most individuals would just notice a 10 degree error which would boost your chances to slightly less than 3% - not very good coherency wise. 73's Richard Clark, KB7QHC |
#27
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On Thu, 28 Aug 2003 12:39:25 -0700, W5DXP
wrote: Richard Clark wrote: W5DXP wrote: My basic approach is to achieve a Z0-match and therefore forget about source impedance. This is a cavalier attitude if you can afford it. It's all part of my "Work Smarter, Not Harder" nature. The elimination of reflected energy incident upon the source is extremely rewarding in multiple ways. Hi Cecil, If smarter were hotter, then you could toast bread at 10 feet. Casting back ten watts by burning 20 hardly qualifies for more. 73's Richard Clark, KB7QHC |
#28
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#29
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#30
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"George, W5YR" wrote in message ...
Finally, he clearly shows how terminating an actual physical line appropriately can result in a reflection coefficient as large as 2.41. This revelation DOES NOT imply that the reflected wave would bear more power than the incident wave. For a line to display this behavior, it must first of all have a high attenuation per wavelength. Due to this high attenuation, the power in the reflected wave is high for only a short distance from the termination. George, with all due respect, even if the SWR measurement was done right at a short or open, the highest rho you could get would be 1. If the power reflection coefficient is the square of the MAGNITUDE of the voltage reflection coefficient, how can you have a voltage RC greater than one without the power RC being also greater than one?? Slick |
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