On Fri, Feb 7, 2025 at 04:08 AM, Bob Ecclestone VK2ZRE wrote:
I may have missed something here, but it seems to me your trace labels are
reversed.
The "top - SMA barrel" trace has much larger SWR ripple and a predominant
capacitive reactance loop on the Smith Chart. I suggest this is actually the
SO239 barrel.
Th trace labels are correct. The "SMA barrel" plot was done by attaching a SMA to PL259 adapter to the NanoVNA. Then a SO239 barrel was added and the RG 213 attached to that. At the end was a 50 ohm non-inductive load. There will be more ripple in the SWR than using just a SMA to SO239 adapter. The reason is that SO239 barrels are not 50 ohms characteristic impedance. I have seen others report them as around 35 ohms.
Also, the markers are at different frequencies which make the upper trace SWR
at the marker "look" better.
The point was to show the larger ripple when there is more impedance mismatch. All plots used the same scale.
The lower trace has much lower SWR ripple and a much tighter reactance loop at
~50 Ohms on the Smith trace. More like an SMA barrel.
The lower SWR ripple trace was done using a SMA to SO239 adapter and then attaching the RG213. There is still an impedance discontinuity but it is not as bad as when an SO239 barrel is used.
Roger Need