One thing that I did to make sure oranges were oranges and apples were apples.? I constructed a reactive termination for the analyzer.? Based on 10 MHz, I built up a series SMT structure of a 0603 50 ohm and a 0603 -j50 ohm cap to ground.? After all of the SOL calibration I put the reactive load on the analyzer and it better read 50-j50 ohm at 10.000 MHz.? What does this do?? It confirms that the calibration is linear from short to open.? Some of the analyzers have a problem here.? I sleep a lot better at night now.? I have junked a couple of analyzers that did not pass this test.
Mel, K6KBE
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On Friday, February 5, 2021, 02:54:29 PM PST, Jim Lux <jim@...> wrote:
On 2/5/21 2:34 PM, Bill AA6BD wrote:
I measured my 40M OCF Dipole with my NanoVNA-H4 and it shows that the minimum SWR of 1.68 is at 6.975 MHz.? I had the loan of an MFJ-259C, and it shows the minimum SWR of 1.6 is at 7.14 MHz.? I also have an Arduino based SWR analyzer designed by K6BEZ and it shows the minimum SWR of 1.7 is at 6.92 MHz.? The SWR figures align nicely, but the center frequency is not very close.? In the 40M band, this difference represents about half of the band.? I calibrated the NanoVNA, and scanned from 6.5 to 7.5 MHz so precision of the 101 steps should not be an issue.? Which device(s) do I believe?? How can I resolve this difference?
Your question would be "is it a frequency measurement problem" or "is it
a impedance measurement problem" -
The first one could be answered by measuring a piece of coax that is
shorted or open at the far end. It's pretty high Q, and would be
unaffected by "fixturing".
It's about a 2% frequency different you're observing, which is quite
large, considering all three are essentially referenced to a crystal
(timebase in the MFJ's counter, PLL ref in the Nano, whatever in the
K6BEZ, but almost certainly a crystal). And a 2% error in crystal
frequency is enormous.
So the next question is "what is the actual R+X being measured" (as
opposed to SWR). at resonance, do they give the same numbers?
And then, all 3 have varying degrees of sensitivity to RFI - you're
measuring an antenna that's outdoors.
It's an interesting problem, but I'd rule out frequency errors first
