Thanks so much Jim, Dave, and Ron for your replies and all the information. I really appreciate the modeling and all the documents. I will need some time to review and understand all this information.
The coax is marked RG-58A/U on the outer jacket. It was purchased 40 years ago but is in perfect condition and has never been used outside. It is a smaller diameter than RG-8X or RG-59 and requires a thicker reducer when used with a PL-259 connector. I am actually using a UG-176 reducer but with some RG-8X outer jacket placed over the smaller coax.
The Smith Chart circles are centered on 50 ohms. I tried a 25 foot length of RG-8X and the Smith Chart circles were also centered on 50 ohms. However, with the longer feed line the frequency difference between the various peaks was about 15 MHz rather than 24 MHz.
I'll check the Smith Chart one more time with the RG-58A/U.
I attached an image of the adapter used for the calibration at the end of the coax cable. It may not be ideal, but it appears to work over a reasonable frequency range and it also allows use of the SMA calibration standards supplied with each NanoVNA.
73, Kent
AA6P
