**4. UWB antenna for chipless RFIDs**

UWB monopoles are also used for the implementation of chipless UWB RFIDs [8]. Chipless tags are either backscattering-based or retransmission-based. Usually, UWB RFIDs are retransmission-based, and on-off keying (OOK) is performed by the presence or absence of a series of resonators which are coupled with the transmission line. Alternatively, the resonators may be perturbed and thus detuned by either short circuiting or open circuiting them. Backscattering occurs when a single antenna with high Q is used, while retransmission requires a second antenna which is preferably orthogonally polarized compared to the receiver, and it transmits a modulated signal after the unmodulated signal is received from the receiver antenna. The OOK modulation in chipless tags is performed in the frequency domain, and an

**Figure 8.**

*Operation principle and schematic of chipless UWB RFIDs.*

indicative signature of a retransmitted signal can be seen in **Figure 8**. The chipless UWB RFID schematic that is presented in **Figure 8** consists of a pair of UWB monopoles which are connected with a common feeding line. The feeding line is loaded with eight resonators forming an 8-bit word. Each resonator which has a slightly different size causes a frequency notch at a different frequency, and at the same time, it causes a phase discontinuity. With the use of a UWB interrogator, a wideband signal is sent, and the tag receives the signal, and it retransmits it back to the reader. The combination of resonators causes a unique electromagnetic signature, which identifies the tag. There is a variety of resonators that can be used, such as slits, CLLs, or slot ring resonators (SRRs). Batteryless, chipless, and entirely passive UWB RFIDs can be inkjet-printed on paper substrates and be massively and rapidly produced for disposable RFID tags, such as the baggage paper-based tags which are used to identify the checked-in luggage. A pair of microstrip-fed UWB monopole antennas is well-suited for the implementation of chipless UWB RFIDs.
