Full duplex RFID readers generate a continuous magnetic field which powers the tag to respond repeatedly. Tags communicate the 80 bit message at a rate of 30 times per second.
FDX tags can be made very thin due to their simple construction of a coil, ferrite rod and a chip. Very small tags have A short read range and so are primarily used for proximity or hand scanning.
FDX uses Amplitude Shift Keying (AM) where the 1's and 0's are represented by a increases and decreases in antenna voltage. The tag uses the internal coil to put and release an electrical load on the antenna signal to cause thee amplitude changes.
FDX antennas must be held rigid to prevent movement due to vibration. Moving wires cause small changes in tuning which affects the amplitude of the signal. This can interfere with the tag signal so FDX and FDX/HDX antennas are often installed in a rigid structures such as plastic pipe.