The measuring device always becomes a part of the system, and changes it.
Different designs of SWR meters will not produce identical results. Theoretically one wants that to happen, but thats why plus or minus 10 percent. Who says they have anything that gives exact SWR measurements? What you are seeing, at best, is the SWR with that devicew a part of the system.
Then the old coax question. Is it 50 ohms, 52 ohms, 58 ohms. They are all sold/labeled as 50 ohm cable. Older cable almost always show additional loss due to migration of the center conductor, moisture, etc.
Then there is the power level question. Unless you have an almost perfect match, you will see one SWR at 20 watts, and a different reading at 1500 watts. Which is accurate? Both ! (plus of minus 10 percent). Additional power causes current to flow on the outside of the cable unless you have choked the dickens out of it.
We see arguments about what SWR "should be" if every thing were perfect : If we really knew the coax impedance, if the impedance of the load did not change with frequency and were completely resistive, if the impedance of the coax did not change with frequency, if the measuring device did not affect the impedance, etc.
Then we have the measurement variances of the imperfect world. With only 50 years of experience I have concluded that staring at swr measurements , if they are nominal, is about as useful as worrying about whether your microphone or speaker is perfectly matched. Maybe entertaining, but not all that useful.
Ken
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