**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 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

**Figure 5.**

*Simulated S11 microstrip-fed UWB (a) monopole with three pairs of resonators (b), stepped λ/4 open stubs (c), CLL resonators, and λ/2 parasitic linear segments (d).*

stub is connected and disconnected to the radiator, using a PIN diode switch. The J-shaped stub causes the notch, but when the diode is set to off state, the stub is disconnected, and the frequency notch is suppressed. In a similar design, the dynamically reconfigurable UWB antenna presented in **Figure 6b** uses a low-power field-effect transistor as switch (FET switch) to connect and disconnect the linear

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*Antennas for UWB Applications*

**Figure 6.**

**Figure 7.**

*(c) MEMS.*

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

reconfigurable notch band at 5.8 GHz.

**4. UWB antenna for chipless RFIDs**

segment inside the elliptical slot. The FET switch can be actuated without battery, using solely the ambient RF power which is collected from a 5.8 GHz rectenna. Finally, **Figure 6c** presents a CPW-fed elliptical slot which uses a U-shaped slot with a microelectromechanical system (MEMS) switch to implement the frequency notch reconfigurability. The MEMS switch is actuated without bias lines, while for the application of the differential voltage for the reconfigurable UWB antennas presented in **Figure 6a** and **b**, bias lines with RF choke inductors are necessary. The MEMS switch was fabricated in-house [25], while the PIN diode and the FET switch are off-the-shelf components. (**Figure 7**) presents the measured S11 of three different reconfigurable UWB antennas that show successful implementation of a

*Measured S11 of three different reconfigurable UWB antennas with (a) PIN diode, (b) FET switch, and* 

*Reconfigurable UWB antennas with (a) PIN diode, (b) FET switch, and (c) MEMS.*

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

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

*UWB Technology - Circuits and Systems*

**64**

**Figure 5.**

*CLL resonators, and λ/2 parasitic linear segments (d).*

stub is connected and disconnected to the radiator, using a PIN diode switch. The J-shaped stub causes the notch, but when the diode is set to off state, the stub is disconnected, and the frequency notch is suppressed. In a similar design, the dynamically reconfigurable UWB antenna presented in **Figure 6b** uses a low-power field-effect transistor as switch (FET switch) to connect and disconnect the linear

*Simulated S11 microstrip-fed UWB (a) monopole with three pairs of resonators (b), stepped λ/4 open stubs (c),* 

**Figure 6.** *Reconfigurable UWB antennas with (a) PIN diode, (b) FET switch, and (c) MEMS.*

**Figure 7.** *Measured S11 of three different reconfigurable UWB antennas with (a) PIN diode, (b) FET switch, and (c) MEMS.*

segment inside the elliptical slot. The FET switch can be actuated without battery, using solely the ambient RF power which is collected from a 5.8 GHz rectenna. Finally, **Figure 6c** presents a CPW-fed elliptical slot which uses a U-shaped slot with a microelectromechanical system (MEMS) switch to implement the frequency notch reconfigurability. The MEMS switch is actuated without bias lines, while for the application of the differential voltage for the reconfigurable UWB antennas presented in **Figure 6a** and **b**, bias lines with RF choke inductors are necessary. The MEMS switch was fabricated in-house [25], while the PIN diode and the FET switch are off-the-shelf components. (**Figure 7**) presents the measured S11 of three different reconfigurable UWB antennas that show successful implementation of a reconfigurable notch band at 5.8 GHz.
