Yes, R cannot be < 0, but my interpretation is simply that the -2 is error in some small number like, say 1 or 2 ohms. I will admit that many of you experts talk about how you attach your DUT very carefully, solder it, sometimes have to throw it away when done testing, and all this sounds bizarre to a non-EE who is simply playing with crystal radio. My nanoVNA attaches to my DUT via 2 alligator clips which are on little connectors (BNC maybe?) which screw into the VNA. I recognize that I am accepting error, even though I do my calibration at the same alligator clips, but except for R, I am convinced that the error I am accepting is negligible when my interest is 0.5 to 25 MHz, picking up AM signals. (I welcome any insights if you think I am wrong.)
Andy
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On Oct 12, 2021, at 10:05 AM, Jim Lux <jim@...> wrote:
On 10/12/21 6:05 AM, DougVL wrote:
On Mon, Oct 11, 2021 at 09:21 AM, Andrew Kurtz wrote:
R is quite low and may vary between -2 and 5 ohms.
That "-2" suggests to me that you're not actually reading 'Resistance' there. Resistance should be positive, but Reactance can be in negative ohms.
Depending on how the conversion from reflection or transmission coefficient (gamma) to R+jX is done, a coefficient with magnitude >1 will result in negative R.
If you're measuring low impedances (so |gamma| is close to 1) then noise or roundoff errors in a measurement can wind up with gamma>1
