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How much can the impedance of coax vary from its characteristic impedance?
On Aug 19, 9:28 am, wrote:
Hello, I bought some no name RG-59/U coax (RG-59/U-SP-95 Made in the USA). I went to cut and tune a 1/4WL matching section for 3.8Mhz. I cut the coax a little longer because I intended to tune it with my MFJ259B or.LP100 exactly to the design frequency. I have a 50 Ohm dummyload that actually is about 54 Ohms @ 3.8Mhz. Plugging the numbers into ON4UN's program for calculating impedance along a coax line: Given the coax is 75 Ohms and the load is 54 Ohms, the program reported that if the line was a 1/4WL long the impedance at the source end would be about 105+j0 Ohms. However my actual measurement with both the MFJ259B and LP100 showed an impedance of 74 + j0 Ohms. http://remote.wu2x.com:8888/lee/quar...-75-meters.jpg I plugged in a few numbers into ON4UN program and calculated it would take coax that had a characteristic impedance of 64 Ohms to see the transformation that I am seeing. Is there any error in my logic here? If this coax really is 64 Ohms, then I'd like to find something that really is closer to 75 Ohms so I can achieve the 2:1 ratio that I intended.I still have another 100 feet of it and can do more tests with the tools I have on hand. 73, Scott, WU2X My rule-of-thumb is that I shouldn't be surprised if the actual impedance of coax is anywhere within ten percent of the nominal value. A while back I went looking for precision 50 ohm coax to be used in a test system that among other things does a calibration of test instruments, and the impedance of the coax really does matter. Even cables that cost several hundred dollars for a one or two meter length (that's admittedly with connectors attached...) didn't guarantee impedance closer than a couple ohms out of 50. Add to that that I just went looking for typical loss specs for RG-59- type cable on the web and the first one I looked at lists it as 70 ohm line, not 75. Although the expected impedance you calculated is for 75 ohm line with no loss, adding a dB loss doesn't change things all that much, certainly not enough by itself to account for your reading. Seems like with your impedance analyzer, you could pretty easily find the impedance and the loss of your line, and use those in your formulas to see if the performance is what you expect. Have you measured the line with the far end shorted and with it open? Assuming accurate measurements, the line impedance will be the square root of the product of those two measured values. Then there's also the possibility that your impedance measuring device isn't all that accurate at some impedances.... Cheers, Tom |
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