Dave - I had one other thought. Would differences in Velocity Factor explain the different results?
The velocity factor can vary quite a bit across different types of coax. I really noticed that when using the TLDetails program from AC6LA software. There is also a big difference between solid dielectric and foam.
A different velocity factor would change the electrical length of the transmission line. That would change the frequency differences between the various peaks and valleys.
I believe my cable is 14 feet 10 inches plus another 6 inches for the SMA to SO-239 adapter.
73, Kent
AA6P
