Yes, but same load, connected to same VNA, shows flat SWR when Zref is set to 50ohm! So I suspect that VNA software is somewhat faulty.
Do you mean that it shows a flat SWR of 4:1 across the HF range when you change the reference to 50 Ohms?
There's something very odd about the results. On your chart, the cursor position (15.738 MHz) shows parallel equivalent components of Rp=186 and Xp=-188. When you change Zref to 50 Ohms those values shouldn't change; the AIM would then show an SWR(50) very different than 4:1.
Be sure to leave the cursor at 15.738 MHz so that we can compare the load impedance values.
Steve G3TXQ
"In physical science the first essential step in the direction of learning any subject is to find principles of numerical reckoning and practicable methods for measuring some quality connected with it. I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely in your thoughts advanced to the state of Science, whatever the matter may be." (Lord Kelvin 1824-1907)
I've re-checked everything, also soldered to another cable/connector, to check whenever the fault was contact fault. No change. You can see below two graphs, same load, same range, same frequency. Just Zref is 50 ohms in one case and 200 ohms in another case.
The error is in thinking that a load impedance that produces a 4:1 SWR(50) necessarily produces a 1:1 SWR(200) - it doesn't!
Taking your load impedance at 27MHz as an example: Rp is about 160 Ohms and Xp about -100 Ohms; that translates to a series equivalent of 45-j72 Ohms.
A load of 45-j72 Ohms is an SWR of 4:1 referenced to 50 Ohms, but is an SWR of 5:1 referenced to 200 Ohms - just like the analyser plots show.
In other words, an impedance that shows an SWR of 4:1 referenced to 50 Ohms will likely not be 1:1 when referenced to 200 Ohms; a 12.5 Ohm resistor would be an obvious example!
Are you measuring the 200 Ohm load through a length of 50 Ohm coax. If so, that would create a "flat" 4:1 SWR when referenced to 50 Ohms but a variable SWR when referenced to 200 Ohms.
Originally Posted by 4L4AGU
