**1. Introduction**

There is a wide range of fundamentally different applications that use UWB antennas, and as a general approach, different customized antennas are needed depending on the desired radiation characteristics. The most common applications are data communication, localization and identification, and radar and sensing applications.

UWB communications have inherently very wide bandwidth in which, based on Shannon's theorem, these systems can support high data rates. Therefore, UWB transceivers are used for the transmission of high data rate, wireless communications, which are often used for PAN communications. One popular commercial application is the wireless USB (WUSB) which is designed to achieve 480 Mbit/s at distances up to 3 m and 110 Mbit/s at up to 10 m. PAN communications involve mobile handheld devices, in which case the used antennas should preferably have omnidirectional patterns with compact size and planar designs [1, 2]. Considering that UWB spectrum is shared with other technologies and standards such as the 3.6 GHz IEEE 802.11y wireless local area networks (WLAN) (3.6575–3.69 GHz), 4.9 GHz public safety WLAN (4.94–4.99 GHz), and 5 GHz IEEE 802.11a/h/j/n WLAN (5.15–5.35, 5.25–5.35, 5.47–5.725, 5.725–5.825 GHz), all operating within

the Federal Communication Commission (FCC) designated UWB band, of 3.1–10.6 GHz, the design of reconfigurable notch-band antennas has attracted a lot of attention [3–5] since they can potentially filter out the unwanted interferer.

UWB technology is used for positioning and location tracking. In the general principle, a UWB interrogator transmits a signal which is reflected by UWB tags which are identified, and depending on the number of interrogators and the utilized software, the position of the specific tag can be defined with relatively high accuracy. Currently there are companies such as UWINLOC which offer integrated solutions for smart and efficient asset management through real-time location systems (RTLS) by combining UWB technology with Internet of things (IoT) principles. Depending whether the UWB antenna is intended for the interrogator [6] or the UWB-RFID tag [7], the radiation characteristics and the size constraints may vary significantly. While the interrogator can combine multiple elements in arrays with beam-forming capabilities, the RFID tags need to be compact, lightweight, omnidirectional, and mostly, low-cost. In order to meet this last requirement, chipless UWB RFIDs [8] are used since they can be easily and massively manufactured on demand using additive fabrication technology.

"Radar devices" involve a wide variety of highly specialized applications for which UWB technology and UWB antennas are widely used even if many of the preferred UWB antennas radiate on different frequencies than the FCC designated 3.1–10.6 GHz band. Ground-penetrating radars (GPR) is one such application for which radars are used either to detect objects buried in the ground [9], to estimate soil characteristics (i.e., moisture) [10], or even to detect living beings trapped in ruins, after a physical disaster such as an earthquake or a hurricane. For this latter case, a more sophisticated radar—through-wall imaging radar—can also be used. Through-wall imaging systems are usually limited for use from law enforcement units, to monitor the position and movements of potentially dangerous targets [11], or even as airport security measure, in order to identify concealed weapons [12]. Microwave imaging [13] which is a more general category, under which the two aforementioned applications can be classified, includes the medical microwave imaging category which has attracted a lot of attention in the recent years. Medical microwave imaging is widely used for breast tumor detection [14, 15], and after several research efforts that investigated conformal UWB arrays [16, 17] and image reconstruction algorithms, several university spin-offs and other private companies proceeded with the implementation of commercial devices that were cleared for clinical studies with human subjects [18, 19]. These devices include both large-size devices facilitated at hospitals [20] and lightweight wearable devices for individual use at home [21].

This chapter presents an indicative list of UWB antennas which can be used for data communications, RFIDs used for identification and localization, and UWB antennas used for sensing and radar applications.
