A chipless RFID tag system having a transmitter sending an input signal and a tag substrate. An ID generation circuit on the tag relies on microstrip transmission line patterns which are pre-designed to generate a unique code. The ID generating circuit may be designed based upon the transmission line properties, including signal delay, and/or reflection, and/or phase change. The tag may be formed on a flexible substrate having at least one microstrip and the microstrip having a first portion with a first impedance and a second portion with a second impedance different from the first impedance. The tag may further include a microstrip antenna for communication with the transmitter and a receiver system. The tag may also include sensors for detection of desired substances of interest. The system may further include a receiver detecting at least two reflections from an interface of first and second impedances and identifying relative time domain positions of the reflections to one another.