The standards provided with the Nano are obviously going to have a different reference plane than the attenuator. At least with the version of firmware I am using, I am not aware of it having a way to compensate for this.
The physical length from the SMA connector to the resistor on that test fixture I show is obviously MUCH longer than with the PCB. If I use the same value resistor on both and measure them using my homemade PCB cal standards, the one mounted on the PCB would be reasonable. The other I would expect to be off by a fair amount. If it were a perfect 50 ohm resistor and everything else was perfect, there would be no return loss and the length would not matter.
For fun, let's test a 1K ohm resistor. I'll stay with the 1MHz to 300MHz to avoid nanoproblems.
PCB1&2 are looking at the resistor mounted to the PCB. 1) looking at the Smith chart, there's not a lot of rotation and 2) we can see the resistance is fairly flat over the entire frequency range. If we were to put say a meter of coax on the nano, we would expect to see a lot of rotation, starting at the far right and working around the outer circumference. We would expect the same with the test jig as that's what we have done.
JIG1&2 are looking at the same 1K mounted in my test jig. No soldering. 1) notice that rather than a dot at 1K, we have this rotation. 2) if we calculate the resistance for this data, you can see even at 300MHz we are not even close.
There's about a 40mm difference between these two setups. Using my 166ps/inch rule of thumb (obviously this is GREATLY oversimplified) we would expect about 280ps of delay.
JIG3&4 notice that I have added a 340ps port extension. 3) notice that we now basically have our dot at 1K again and 4) the part appears to be somewhat flat.
Keep in mind these are very back of the napkin sort of measurements, but it should provide you with some idea how important the mechanics are. I suspect after building these attenuators, you're starting to see that things are much less critical at lower frequencies.
As I mentioned earlier, Mario is helping me sort out some home made standards. I made up a set that I thought would be good, but these were not even close. I machined up a new set of parts and knew Mario would not approve of them.
After the third attempt, I came up with something that appears reasonable. These standards will be used at much higher frequencies and again, are just for ballparkish measurements.