Start with an lf tune and a hf tune. If the voltage drops, the case is affecting things.
You could also use the Pilgrim approach and measure the range with some test cards and fobs, but this method is probably less sensitive.
And of course, a coil and the VNA can be used to measure the field strength, but you need to make sure that the antenna of the Proxmark3 and the coil don’t move relative to each other during the testing.
Looking forwards to the results of the tests!
Edit: i did some testing with an 8mm carbon fiber rod that I have:
On the LF side of things I’m getting 73.1V without the rod. When I place the rod on the antenna on the spot where I can get the biggest voltage drop I get around 72.5V. There is some voltage drop but it’s not that big.
This is where I got the lowest voltage
However, HF is a different story… With about 45.5V with nothing on the antenna. And only 28.4V at the point where things are impacted the most.
And here is the spot for HF
Sliding the carbon rod across the antenna even changes the brightness of the LED of one of my RDCs if I place it under the PM3.
I can’t wait to see the results with carbon filled filament.