**2. UWB antennas for wireless communications**

"UWB communication systems" include communications which serve a variety of purposes. In a more loose definition, ultrawideband communication system is a system that requires more than 500 MHz bandwidth. Consequently, applications can be found in several subsections of the electromagnetic spectrum, starting from the UHF band all the way up to mm wave and sub-mm wave frequencies. This section focuses on UWB antennas for communications within the FCC designated UWB frequency range, 3.1–10.6 GHz. When UWB communications started to concentrate increased researchers' attention, (First International Conference

**61**

**Figure 2.**

*antenna [22].*

**Figure 1.**

*CPW-fed (a) elliptical slot, (b) cactus and (c) compact-cactus UWB antenna.*

*Simulated and measured S11 for (a) CPW-fed slot antenna, (b) cactus antenna, and (c) compact-cactus* 

*Antennas for UWB Applications*

antenna designers.

*DOI: http://dx.doi.org/10.5772/intechopen.86985*

on UWB-ICUWB, in 2001, is considered a milestone), in the early 2000, the used antennas were rather large in size. Some of the earlier UWB antenna versions included self-complementary antennas which are frequency independent (spiral antenna) or wideband dipoles [22]. For the use of UWB technology for mobile handheld devices, the large antenna size was becoming a limiting factor. Consequently, size reduction became one of the primary objectives of the UWB

In the next years, self-complementary antennas and end-fire antennas were replaced with multiple resonator antennas. The design approach is to create several resonances on the S11 which are distributed along the 3.1–10.6 GHz frequency range. As a result, the reflection coefficient is "pushed" lower than −10 dB conventional matching threshold which is widely used as the "good matching" criterion. The existence of radiating segments of specific size causes the creation of the resonances in the reflection coefficient, and the size variation of the physical size of the resonators results in the corresponding resonance shift in frequency. **Figure 1** illustrates a series of multiple resonator antennas which progressively limit their overall (board) size. The antennas are CPW-fed, and the central (signal) conductor is linearly tapered to improve the matching. Details about the design and performance of the depicted antennas can be found in [2]. It has been shown that the transition from the conventional transmission line to the radiator and the ground size both affect the overall matching and consequently the radiation efficiency of the UWB antenna and should be considered by antenna designers as inherently integrated parts of the radiator itself. **Figure 1a** demonstrates an uneven U-shaped stub, radiating inside an elliptical slot. Under the observation of the limited effect of the slot on the matching, the slot was removed, and an additional linear segment was added on

## *Antennas for UWB Applications DOI: http://dx.doi.org/10.5772/intechopen.86985*

*UWB Technology - Circuits and Systems*

the Federal Communication Commission (FCC) designated UWB band, of

tured on demand using additive fabrication technology.

antennas used for sensing and radar applications.

**2. UWB antennas for wireless communications**

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 manufac-

"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

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

"UWB communication systems" include communications which serve a variety of purposes. In a more loose definition, ultrawideband communication system is a system that requires more than 500 MHz bandwidth. Consequently, applications can be found in several subsections of the electromagnetic spectrum, starting from the UHF band all the way up to mm wave and sub-mm wave frequencies. This section focuses on UWB antennas for communications within the FCC designated UWB frequency range, 3.1–10.6 GHz. When UWB communications started to concentrate increased researchers' attention, (First International Conference

**60**

use at home [21].

on UWB-ICUWB, in 2001, is considered a milestone), in the early 2000, the used antennas were rather large in size. Some of the earlier UWB antenna versions included self-complementary antennas which are frequency independent (spiral antenna) or wideband dipoles [22]. For the use of UWB technology for mobile handheld devices, the large antenna size was becoming a limiting factor. Consequently, size reduction became one of the primary objectives of the UWB antenna designers.

In the next years, self-complementary antennas and end-fire antennas were replaced with multiple resonator antennas. The design approach is to create several resonances on the S11 which are distributed along the 3.1–10.6 GHz frequency range. As a result, the reflection coefficient is "pushed" lower than −10 dB conventional matching threshold which is widely used as the "good matching" criterion. The existence of radiating segments of specific size causes the creation of the resonances in the reflection coefficient, and the size variation of the physical size of the resonators results in the corresponding resonance shift in frequency. **Figure 1** illustrates a series of multiple resonator antennas which progressively limit their overall (board) size. The antennas are CPW-fed, and the central (signal) conductor is linearly tapered to improve the matching. Details about the design and performance of the depicted antennas can be found in [2]. It has been shown that the transition from the conventional transmission line to the radiator and the ground size both affect the overall matching and consequently the radiation efficiency of the UWB antenna and should be considered by antenna designers as inherently integrated parts of the radiator itself. **Figure 1a** demonstrates an uneven U-shaped stub, radiating inside an elliptical slot. Under the observation of the limited effect of the slot on the matching, the slot was removed, and an additional linear segment was added on

**Figure 1.** *CPW-fed (a) elliptical slot, (b) cactus and (c) compact-cactus UWB antenna.*

**Figure 2.**

*Simulated and measured S11 for (a) CPW-fed slot antenna, (b) cactus antenna, and (c) compact-cactus antenna [22].*

**Figure 3.**

*Simulated and measured H-plane radiation patterns of (a) CPW-fed slot antenna, (b) cactus antenna, and (c) compact-cactus antenna at 5 GHz and (d) CPW-fed slot antenna, (e) cactus antenna, and (f) compactcactus antenna at 9 GHz [22].*

**Figure 4.**

*Simulated and measured E-plane radiation patterns of (a) CPW-fed slot antenna, (b) cactus antenna, and (c) compact-cactus antenna, at 5 GHz, and (d) CPW-fed slot antenna, (e) cactus antenna, and (f) compactcactus antenna, at 9 GHz [22].*

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*Antennas for UWB Applications*

*DOI: http://dx.doi.org/10.5772/intechopen.86985*

the U-shaped stub, converting it into a "cactus-shaped" radiator consisting of three linear segments (**Figure 1b**). The last iteration presents a variation of the cactusshaped radiator, the "compact cactus," with smaller RF-ground patches (**Figure 1c**) and further decreased overall size. **Figure 2** presents the reflection coefficient plots

where the resonances are distributed along the entire UWB frequency band. Although the elliptical slot does not affect the matching significantly, it has more profound impact on the resulted radiation patterns. The removal of the slot modifies the radiation patterns to rather omnidirectional patterns along the H-plane (**Figure 3**), while the E-plane (**Figure 4**) patterns are characterized by a null in the direction of the feeding line. Several other published research works demonstrated the attractive features and the effectiveness of the so-called fat monopole or the elliptical disc and its variations, as UWB radiator, and it became one of the most

widely used UWB antenna types, for mobile handheld devices.

state, it causes the cancelation of the notch at 8.8 GHz.

Depending on the geometry of the resonator, a variety of electronic switches can be implemented. **Figure 6** presents three implemented UWB antennas [24–25], with reconfigurable notch bands at the WLAN band (5.8 GHz) which use a single resonator instead of a pair of resonators and, consequently, only one switch to implement the notch reconfigurability feature. A microstrip-fed monopole with a J-shaped stub inside a rectangular slot is presented in **Figure 6a**, where the J-shaped

**3. Reconfigurable UWB antennas for wireless communications**

UWB systems share the same spectrum with several other narrowband wireless systems which use sub-bands inside the 3.1–10.6 GHz range. The FCC mask limits the UWB EIRP to −41.3 dBm/MHz, which means that UWB signals are rather weak, to degrade the performance of the narrowband systems, significantly. UWB signals are considered white noise for narrowband systems. On the other hand, UWB systems suffer from the strong interfering signals which are used from the narrowband systems. In order to reduce the received noise level and improve the associated SNR, UWB antennas are designed with notch bands, which effectively filter out the received signals at the frequencies used from a competing narrowband system. Ideally these notch bands should be reconfigurable, in other words to appear when an interfering signal is detected and to disappear when no such signal is detected. Generally, notch bands are caused from added resonators which are placed on the radiator, or the feeding line, or even the RF ground patches. **Figure 5** presents a microstrip-fed monopole with three pairs of added resonators which cause three notch bands on the reflection coefficient [23]. Each pair of resonators is designed to cause a notch band at a desired frequency. Specifically, the two stepped λ/4 open stubs cause the notch at the WiMAX (3.5 GHz) band; the two capacitively loaded loops (CLLs) on the ground patch cause the notch at the WLAN (5.8 GHz) band, and the pair of linear λ/2 segments, printed on the back side of the radiator, causes the notch at 8.87 GHz. The use of three different pairs of resonators allows the control of the notch bands independently. The effect of the resonators can be made reconfigurable if suitable switches are used at the right place. **Figure 5b** shows that if the λ/4 open stubs are disconnected (switch in off state), the frequency notch disappears (blue solid line). In **Figure 5c** the reflection coefficient of the UWB antenna is presented when the switch on the CLL is in either on or off state. With the switch in off state, the WLAN notch exists and filters out the undesired high-power signal, while when the switch is in on state, the effect of the CLL is canceled, and as a result the UWB antenna radiates efficiently at the WLAN band. Finally, when the switch separating the λ/2 linear segment into two unequal parts (**Figure 5d**) is set to off

#### *Antennas for UWB Applications DOI: http://dx.doi.org/10.5772/intechopen.86985*

*UWB Technology - Circuits and Systems*

**62**

**Figure 4.**

**Figure 3.**

*cactus antenna at 9 GHz [22].*

*cactus antenna, at 9 GHz [22].*

*Simulated and measured E-plane radiation patterns of (a) CPW-fed slot antenna, (b) cactus antenna, and (c) compact-cactus antenna, at 5 GHz, and (d) CPW-fed slot antenna, (e) cactus antenna, and (f) compact-*

*Simulated and measured H-plane radiation patterns of (a) CPW-fed slot antenna, (b) cactus antenna, and (c) compact-cactus antenna at 5 GHz and (d) CPW-fed slot antenna, (e) cactus antenna, and (f) compact-* the U-shaped stub, converting it into a "cactus-shaped" radiator consisting of three linear segments (**Figure 1b**). The last iteration presents a variation of the cactusshaped radiator, the "compact cactus," with smaller RF-ground patches (**Figure 1c**) and further decreased overall size. **Figure 2** presents the reflection coefficient plots where the resonances are distributed along the entire UWB frequency band.

Although the elliptical slot does not affect the matching significantly, it has more profound impact on the resulted radiation patterns. The removal of the slot modifies the radiation patterns to rather omnidirectional patterns along the H-plane (**Figure 3**), while the E-plane (**Figure 4**) patterns are characterized by a null in the direction of the feeding line. Several other published research works demonstrated the attractive features and the effectiveness of the so-called fat monopole or the elliptical disc and its variations, as UWB radiator, and it became one of the most widely used UWB antenna types, for mobile handheld devices.
