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measuring cable loss
Jim Lux wrote in news:f9i1i3$8v5$1
@nntp1.jpl.nasa.gov: BTW, directional wattmeters for the ham market are often not capable of reasonable accuracy on loads other than the nominal 50 ohm load. There are a range of tests that such an instrument should satisfy, but for hams, it is usually considered sufficient if the "reflected" reading is approximately zero on a 50 ohm load. I should think, though, that one could calibrate such a reflectometer/directional wattmeter. That is, you could test it with a suitable variety of source and load impedances and develop a fairly simple arithmetic correction that would be accurate. Yes Jim, some of the deficiencies of the instrument fall to things like an equal response from the separate forward and reverse couplers. Scale shape is an issue (especially where the sensitivity is continuously adjustable using a pot). Phase and amplitude response of the coupler over the frequency range is another issue not so readily calibrated out. A coupler that is long will underestimate rho, and some couplers insert more mismatch than they pretend to measure. In my experience, many of the instruments that are claimed to work up to 144MHz band might well indicate close to 1:1 on a dummy load, but they do not indicate rho=1 on a s/c or o/c. Whilst they may serve their purpose as a null indicator on a 50 ohm load, they are not suited to the loss measurement such as Jimmie is performing. The interesting question might be whether you could unambiguously take a particular fwd and rev reading and turn that into a true fwd and true rev, essentially solving for the mismatch. I don't think you can compensate for lack of f/b ratio in the coupler, for example because the coupled lines are too long. Down in the lab here at work we have a whole rack of precision misterminations (1.1:1, 1.2:1, 1.5:1, etc.) that some talented engineer built and calibrated some decades ago. They're built on the Maury bluedot N terminations. I have always though that a budget priced set of mismatches would be real handy, and have wondered why MFJ (or someone else for that matter) don't offer a set for checking / calibration of the MFJ259B etc. Owen |
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