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measuring cable loss
On Aug 14, 8:45 am, Jim Lux wrote:
I am rather fond of the coupled-line hybrid idea: it can be built in a way that everything stays ratiometric, so the coupling ratio is very nearly constant over temperature, and of course the directionality lets you observe things you can't just from monitoring voltage at a point. It's possible to build one with low coupling without too much trouble; -60dB coupling isn't out of the question, for sure. I'm imagining a design I could make reliably with simple machine tools that would work well for the OP's application: 100 watts at about 1GHz as I recall in the through line, and coupling on the order of -60dB to get to about -10dBm coupled power and have negligible effect on the through line. There's a free fields solver software package that will accurately predict the coupling, and with the right design and normal machine shop tolerances the coupling and impedance should be accurate to a fraction of a dB and better than a percent, respectively. Perhaps I can run some examples to see if I'm off-base on that, but that's what my mental calculations tell me at the moment. Actually, the exact coupling ratio probably isn't important in this application, because it could be "calibrated out". Stability would be a bigger concern, and it's certainly possible to design a coupler that is very temperature stable by choosing the right dimensions so that things change in the right ratios. Bingo. It's that ratiometric thing that is a big plus for stability. In a coupler made of all the same metal, or at least metals that have nearly equal coefficients of expansion, the ratios stay the same, and it's the dimensional ratios that establish the coupling and impedances, not the absolute size. Actually, the change in length does matter, but if you make the assembly a quarter wave long, the d(coupling)/d(length) is zero at that point anyway. In any event, I suppose the thermal coefficient of expansion of metals you'd be most likely to use is small enough that you'd be fine with a shorter coupler. There doesn't need to be anything terribly complex about the geometry of the whole thing, either. It's probably safe to say that changes in the dielectric constant of air due to air pressure and humidity aren't going to be significant in this case. ;-) Cheers, Tom |
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