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SWR variation with feedline length
On Jun 1, 1:50*pm, "Antonio Vernucci" wrote:
Along several decades of radio hamming on the HF bands, I noted that the measured SWR of all the antennas I have mounted (Yagis, dipoles) slightly varies when the feedline length is changed by several meters. For 100W of forward power, the reflected power could vary somewhat, e.g. from 2W to 5W or so, measured on a Bird wattmeter. This behavior would seeem to deny the theory, according to which *SWR is independent of feedline length (as long as the cable attenuation remains constant). Clearly the measured SWR change cannot be due to the change in the feedline attenuation as, at HF, adding or cutting a few meters of cable would yield a very small change in attenuation and hence a negligible impact on measured SWR. Reading here and there, the most common theory explaining such phenomenon is that, in presence of RF on the coaxial cable braid, the SWR meter reading is influenced by the feedline length. I am not too convinced of that explanation, also because I have invariably experienced the measured SWR variation phenomenon with all antenna I have had, and I never had hot braid problems. At that regard I got an idea that could explain the phenomenon, at least part of it. Reading coaxial cable data sheet, I noted that manufacturers typically give a small tolerance on cable impedance (2 to 3 ohm). Let us then assume that the feedline cable has a 53-ohm impedance, whilst the Bird wattmeter is 50 ohm sharp. If the 53-ohm cable is terminated on an e.g. 75-ohm (purely resistive) antenna, the real SWR on the line would be 75/53=1.41 independently of feedline length (if the attenuation variation with length is neglected). But the impedance seen by the wattmeter obviously varies with the feedline length, and it can be easily calculated that the seen impedance range results in an apparent SWR, on the 50-ohm wattneter, reading that varies from a maximum of 1.5 (when feedline length is an even multiple of half wavelenght) down to a minimum of 1.33 (when feedline length is an odd multiple of wavelenght quarters). For 100W of forward power, the reflected power varies from about 4W down to about 2W. Repeating the exercise with an e.g. 85-ohm load, the apparent SWR measured on the 50-ohm wattmeter would vary from 1.7 down to 1.51 (reflected power varying from 7W down to 4W). You can get easily convinced that such variation is only due to the assumed 3-ohm difference in cable impedance. With older cables having a nominal 52-ohm impedance, instead of 50, the situation could get even more evident. Any comment would be appreciated. 73 Tony I0JX There is a very good possibility that your analysis is correct. I see the same effect, and in fact, it's of particular concern to me right now, because I'm putting what effectively is an SWR meter into production, and it's important that we have a test setup that accurately measures the performance. I've been specifically concerned that the test setup, as currently configured, may have trouble because the connecting cables may not be close enough to 50 ohms. As others have said, IF there is a problem with RF on the outside of the line, any variation in observed SWR is most likely because the change in line length has changed the load the other end of the coax is seeing, NOT because the meter is directly responding to the "outside" RF. The meter measures transmission line current and transmission line voltage, and the line itself is all the reference it needs to do that. Direct response to RF on the outside of the line could result from poor construction of the meter, but I wouldn't expect that from a Bird. One other possibility that I haven't seen mentioned, too, is that the impedance of the line is not constant along its length. With line of good construction that hasn't been abused, the variation should be small. You can detect it by running a network analyzer sweep of just the line, across a broad frequency range. But a line with polyethylene dielectric (and especially one with foam polyethylene) that's gotten too hot--perhaps because of high power at high SWR--can have the center conductor go "off-center" and change the impedance. If the effect you are seeing is the result of a line that's not quite the same impedance that the meter is calibrated to (which itself may be noticably different from 50 ohms), you could plot the change in indicated SWR as a function of line length and see it vary in a smooth and predictable manner. Most likely, though, what you're seeing is the sum of several effects, and the variation in indicated SWR or reflected power may not be all that smooth. Very often when I expect to see a nice smooth spiral centered on one point on my network analyzer's Smith chart display, what I see is a spiral that follows along some arc, because of various imperfections. (Sometimes it's fun to try to figure out just what the imperfections are...) Cheers, Tom |
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