Direction dependent modulation of an RFID tag

We introduce a simple radio frequency (RF) tag that can be configured to exhibit direction dependent modulation properties. The tag consists of a two-by-two array of pin diode loaded dipole elements that can be controlled by an external signal to modulate the RF scattering characteristics of the tag. The design of the tag allows for multilevel in-phase and quadrature modulation schemes to be implemented. It is also shown that the form of the modulation generated by the tag is direction dependent and that this property can be exploited to provide an additional degree of system security.