Solution
The flex antenna has to be placed on the terminal or the device you want to test.
Try to find the DUT’s antenna path and vary the position till the field strength detection reaches its maximum.
The LMA revision C helps you finding the best spot for the flex antenna on your PCD. The first LED (green) indicates low field strength. The second one (blue or Orange) indicates high field strength. You have to place your flex antenna in order to get the brightest LED.
For more precision, you could also connect le connector J3 or J4 of your LMA (depending if your testing PCD or PICC) to an Oscilloscope in order to visualize the field.