**Conflict of interest**

*UWB Technology - Circuits and Systems*

MEMS switches can be used.

manufacturing technologies such as inkjet printing.

characteristics are required and different types of antennas are better suited. For wireless personal network devices, omnidirectional radiation patterns are preferred, and the compact size of the antenna with a low-profile planar design is the most useful, in most cases. There are several types of antennas that can be used; however, the monopole antenna is the most widely used design. Either CPW-fed or microstrip-fed UWB monopole is the most prominent solution. UWB monopoles have been used extensively for the implementation of reconfigurable UWB antennas which are capable of filtering out the interfering signals that share parts of the 3.1–10.6 GHz, FCC designated UWB spectrum. Frequency notch bands can be created with the addition of resonators which can be implemented on either the radiator or the feeding line or even the RF ground segments. Quarter-wavelength open stubs, half-wavelength linear segments, half-wavelength slots, and more complex in shape resonators, such as CLLs or SRRs, have been successfully used. For the reconfigurability attribution, RF switches are required, and depending on the resonator geometry and the biasing conditions, PIN diodes, FET switches, or

UWB antennas for positioning and tracking are used for both the interrogator and the RFID tags. Apparently different characteristics are needed for each case. For the interrogators high-gain, directive antennas with agile radiation patterns are preferred, while for the RFID tags, omnidirectional lightweight and low-cost antennas are needed. The RFIDs are often customized considering the electromagnetic characteristics of the items that they tag, and they can be either chipless or terminated with an IC load. The operation principle of chipless UWB RFIDs exploits the presence of a series of resonators which are coupled with the transmission line that connects the two antennas. One antenna receives the interrogator's signal, and the second one retransmits the modulated signal back to the reader. Chipless UWB RFIDs are entirely passive, and thus they can be easily fabricated using additive

For radar applications, and depending on the target characteristics and the size and cost constraints, a wide variety of UWB antennas are used. For relatively long-range monostatic radars, such as ground-penetrating radars or through-wall imaging devices, high-gain directive antennas are used. Vivaldi antennas and their variations are the most common UWB antennas used for monostatic radars. For multistatic radars, like the ones used for microwave imaging for breast tumor detection, the requirements are very different. Since the target is relatively small and the size of the object under detection can be smaller than 1 cm3

radiators form a concave array. Cylindrical and hemispherical configurations are used, and the desired UWB antenna elements must have unidirectional radiation patterns, and they must be shielded from cross-coupling, while their size should be small to allow a large number of elements in a relatively small volume. Medical microwave imaging systems using UWB technology keep developing, and there are currently several vendors which have developed products which are cleared for clinical trials. Cavity-backed radiators are used covering different sub-bands in the UWB spectrum or even lower-frequency bands. These UWB antennas have conformal surfaces and are combined to form conformal arrays. They are co-designed with the human tissue target, since the radiation characteristics of UWB antennas change significantly when the antennas radiate in close proximity

UWB antennas are integrated parts of devices which are used for a wide range of applications. For some applications, off-the-shelf UWB antennas can be successfully used or even antennas designed for similar scope, but for several other cases, UWB antennas must be customized and be co-designed with the surrounding environment since they can be easily mismatched and their assumed

, the

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with the human body.

All authors listed have contributed sufficiently to the project to be included as authors. The authors declare that there is no conflict of interest, in terms of financial or other regarding the publication of this book chapter.
