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Question about "Another look at reflections" article.
On Mon, 24 May 2010 14:23:59 -0700 (PDT), lu6etj
wrote: Regarding that output impedance of a rig be quite near to 50+j0 I think that depends on the concrete design; such a condition is not necessary and in my opinion neither optimal. Hi Miguel, Problems arise from qualitative statements or questions instead of quantitative statements or questions. output impedance of a rig be quite near to 50+j0 is a quantitative fact (depending upon the qualitative "near"). A fact can be measured and compared by in dependant and objective means. in my opinion neither optimal is a qualitative judgment. A judgment is only as authoritative as is the authority making it - and even then it comes at a discount. We get many judgments here by folks with little authority (and such judgments are usually adorned with anti-authority messages). I have measured the characteristic Z of my two transistorized HF rigs by different methods that agree in their results. I have also measured the Z of active loads that employ the same circuitry found in power sources. The values of Z for my HF rigs wander from 35 Ohms to 75 Ohms. The variation is a function of 1. frequency; 2. power; 3. which of the two is being measured. What is notable, for those values that are furthest from 50 Ohms (the design goal), my rigs experience issues (instabilities, poor efficiency, distortion...). The variation by frequency is found at the margins (at the ends of the HF band). The variation by power is found in inabilities to maintain a constant power deliver in every band - which weakly correlates to Z offset from 50. Variation by power also defines Z. Low power operation is not going to give you a 50 Ohm source (the manufacturer designs Z for rated power). The variation by set is due to different capabilities: one set is capable of 150W, and the other 100W; thus the first is more robust. As these variations of Z are not remarkable, trying to turn them into quantitative efficiency results are speculative at best (common designs are not optimal by any stretch when you add in the complication of modes). 73's Richard Clark, KB7QHC |
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