**2.1. Design of dual band-notched UWB antenna**

The compact CPW antenna used for studies is shown in Figure 2. It has a semi-circular radiator fed by a 50-Ω CPW. The antenna is designed on a Rogers substrate, RO4350B, with an area of 32 mm×35 mm, a relative dielectric constant of ε*<sup>r</sup>*=3.48, a thickness of 0.762 mm and a loss tangent of 0.0037. The width, *S*, of the central-strip conductor and the distance, *W*, between the feed line and ground plane are 3 and 0.3 mm, respectively, in order to have a characteristic impedance of 50 Ω. The ground plane is rectangular in shape plus a half ellipse with a minor-radius-to-major-radius ratio of 0.5 to reduce the beam tilt of the antenna [12]. Detail dimensions of the dual band-notched antenna are listed in Table 1.

**Figure 2.** (a) Layout and (b) side view of CPW antenna with two CPW resonators, and (c) layout of CPW resonator


(a) Notched band at 5.5GHz

238 Ultra Wideband – Current Status and Future Trends

notched band using meander lines [11].

reference antenna.

**Figure 1.** Basic design concept for band-notched UWB antenna

In general, the design procedure for a band-notched antenna can be described as follows. An UWB antenna without band-notched function is designed to have good impedance matching over the UWB, which is used as a reference antenna. Proposed resonant structures are added to the reference antenna to create notches at some specific frequencies. The dimensions of the resonant structures can be used to control the center frequencies and bandwidths of the notches. Different designs have been proposed to realize the bandnotched characteristic for UWB planar monopole antennas [5-18]. These include using parasitic elements [6], folded strips [7], split-ring resonators (SRRs) [8], quarter-wavelength tuning stubs [9], meander-ground structures [10, 11], resonated cells on the coplanarwaveguide (CPW) [12], fractal tuning stub [13], slots on the radiator [14-16] or ground [17], and slots or folded-striplines along the antenna feed line [18]. However, most of these designs targeted at creating a single-notched band and only one design targeted at a triple-

In this chapter, we study the applications of CPW resonators, λ/4-resonators and MLs to design single, dual, triple and quadruple band-notched characteristics for compact UWB monopole antennas. The studies are carried out using computer simulations and the simulated results are verified using the antenna measurement system, Satimo Starlab. The simulated and measured results on the return loss, radiation pattern, peak gain and efficiency agree well. The pulse responses and fidelities of the single, dual, triple and quadruple band-notched antennas are also measured and compared with those of the

The compact CPW antenna used for studies is shown in Figure 2. It has a semi-circular radiator fed by a 50-Ω CPW. The antenna is designed on a Rogers substrate, RO4350B, with

and a loss tangent of 0.0037. The width, *S*, of the central-strip conductor and the distance, *W*, between the feed line and ground plane are 3 and 0.3 mm, respectively, in order to have a characteristic impedance of 50 Ω. The ground plane is rectangular in shape plus a half

ε

*<sup>r</sup>*=3.48, a thickness of 0.762 mm

**2. Dual band-notched antenna design using CPW resonators** 

**2.1. Design of dual band-notched UWB antenna** 

an area of 32 mm×35 mm, a relative dielectric constant of

(b)Notched band at 3.5GHz

**Table 1.** Antenna dimensions for dual-band notch

**Figure 3.** Simulated return losses of reference UWB antenna for different values of (a) *R1* and (b) *R2*

The small area connecting the CPW and the semi-circular radiator is quite critical for impedance matching and so is smoothed by using two arc shapes with radii *R1* and *R2*. Figures 3(a) and 3(b) show the effects of different values of *R1* and *R2* on the return loss. It can be seen from Figure 3(a) that, when *R1* is small, say *R1* = 3 mm, the bandwidth (for return loss > 10 dB) of the reference antenna covers only few GHz, from around 2.5 to 6.3 GHz. Increasing *R1* improves the impendence matching in the lower frequency band, but degrades it in the higher frequency band. For *R2*, Figure 3(b) shows an opposite effect. That is, decreasing *R2* improves the impendence matching in the lower frequency band, but degrades it in the higher frequency band. Increasing *R2* improves the bandwidth towards the high frequency, but if *R2* is too large, say *R2* = 6 mm, the return loss at around 7 GHz reduces to less than 10 dB.

Creating Band-Notched Characteristics for Compact UWB Monopole Antennas 241

*a* and *Lda*

, but the return loss in the rest of

in the CPW

**2.3. Parametric study on dual band-notched CPW antenna** 

Simulation results on the return loss with different values of *Lc*

be seen that the resonant frequency varies with *Lc*

resonance.

In the notched antenna shown in Figure 2(a), two CPW resonators are placed in series, known as a dual-band CPW resonator, and used to create two different notched bands.

resonator using computer simulation are shown in Figures 5(a) and 5(b), respectively. It can

the UWB remains almost unchanged. This property provides the antenna designers a great freedom to set the frequency of the notched bands. It should be noted that a spurious response at about 7.9 GHz is observed in Figure 5, which is due to the 1st odd-harmonic

**Figure 5.** Simulated return loss of dual band-notched antenna with different values of (a) *Lca* and (b) *Lda*

To better understand the antenna operation, the surface-current distributions on the antenna in the passband and notched bands are studied using computer simulation and results are shown in Figure 6. In the passbands of 3 GHz and 11 GHz, Figures 6(a) and 6(d) show that majority of the current flows through the CPW into the radiator and then radiates to free space. However, at the notched frequencies of 3.5 GHz and 5.5 GHz, Figures 6(b) and 6(c) show that the energies are well confined in the upper and lower CPW resonators,

respectively, and do not get radiated, while the radiator remains cool.

**Figure 6.** Distribution of surface current at (a) 3 GHz, (b) 3.5 GHz, (c) 5.5 GHz and (d) 11 GHz

*a* and *Lda*

#### **2.2. Design of simple CPW resonator**

λ*g*/2-open-ended CPW resonators and λ*<sup>g</sup>*/4-CPW resonators can be used to design bandstop filters [19, 20]. However, the sizes of these resonators are too large to be integrated onto the compact UWB antennas. Here we propose a new simple structure having a much smaller size, as shown in Figure 2(c), for the CPW resonator. The resonator is a simple rectangular slot with three open stubs from the opposite sides. With such a small structure, two CPW resonators with a separation of 2 mm can be etched on the feed line, as shown in Figure 2(a), to generate 2 band notches for the antenna. The lower CPW resonator is at a distance 8 mm from the lower edge of the ground plane. The resonance of the CPW resonator is determined by the length *Lc* and the small gap *Ld* as indicated in Figure 2(c). The narrowest microstrip line that we can make using the prototype-machine in our laboratory is 0.1 mm. So for convenience in our design process, we fix the stub width *W2* and stub spacing *W3* of the CPW resonator to 0.3 mm. The simulated return loss and insertion loss of a CPW resonator, with *W1* = *W2* = *W3* = *Ld* = 0.3 mm and *Lc* = 9 mm, are shown in Figure 4. It can be observed that the CPW resonator has a bandstop characteristic of about 27 dB at 3.5 GHz.

**Figure 4.** Simulated return loss and insertion loss of CPW resonator

#### **2.3. Parametric study on dual band-notched CPW antenna**

240 Ultra Wideband – Current Status and Future Trends

reduces to less than 10 dB.

λ

**2.2. Design of simple CPW resonator** 

*g*/2-open-ended CPW resonators and

The small area connecting the CPW and the semi-circular radiator is quite critical for impedance matching and so is smoothed by using two arc shapes with radii *R1* and *R2*. Figures 3(a) and 3(b) show the effects of different values of *R1* and *R2* on the return loss. It can be seen from Figure 3(a) that, when *R1* is small, say *R1* = 3 mm, the bandwidth (for return loss > 10 dB) of the reference antenna covers only few GHz, from around 2.5 to 6.3 GHz. Increasing *R1* improves the impendence matching in the lower frequency band, but degrades it in the higher frequency band. For *R2*, Figure 3(b) shows an opposite effect. That is, decreasing *R2* improves the impendence matching in the lower frequency band, but degrades it in the higher frequency band. Increasing *R2* improves the bandwidth towards the high frequency, but if *R2* is too large, say *R2* = 6 mm, the return loss at around 7 GHz

λ

**Figure 4.** Simulated return loss and insertion loss of CPW resonator

filters [19, 20]. However, the sizes of these resonators are too large to be integrated onto the compact UWB antennas. Here we propose a new simple structure having a much smaller size, as shown in Figure 2(c), for the CPW resonator. The resonator is a simple rectangular slot with three open stubs from the opposite sides. With such a small structure, two CPW resonators with a separation of 2 mm can be etched on the feed line, as shown in Figure 2(a), to generate 2 band notches for the antenna. The lower CPW resonator is at a distance 8 mm from the lower edge of the ground plane. The resonance of the CPW resonator is determined by the length *Lc* and the small gap *Ld* as indicated in Figure 2(c). The narrowest microstrip line that we can make using the prototype-machine in our laboratory is 0.1 mm. So for convenience in our design process, we fix the stub width *W2* and stub spacing *W3* of the CPW resonator to 0.3 mm. The simulated return loss and insertion loss of a CPW resonator, with *W1* = *W2* = *W3* = *Ld* = 0.3 mm and *Lc* = 9 mm, are shown in Figure 4. It can be observed that the CPW resonator has a bandstop characteristic of about 27 dB at 3.5 GHz.

*<sup>g</sup>*/4-CPW resonators can be used to design bandstop

In the notched antenna shown in Figure 2(a), two CPW resonators are placed in series, known as a dual-band CPW resonator, and used to create two different notched bands. Simulation results on the return loss with different values of *Lc a* and *Lda* in the CPW resonator using computer simulation are shown in Figures 5(a) and 5(b), respectively. It can be seen that the resonant frequency varies with *Lc a* and *Lda* , but the return loss in the rest of the UWB remains almost unchanged. This property provides the antenna designers a great freedom to set the frequency of the notched bands. It should be noted that a spurious response at about 7.9 GHz is observed in Figure 5, which is due to the 1st odd-harmonic resonance.

**Figure 5.** Simulated return loss of dual band-notched antenna with different values of (a) *Lca* and (b) *Lda*

To better understand the antenna operation, the surface-current distributions on the antenna in the passband and notched bands are studied using computer simulation and results are shown in Figure 6. In the passbands of 3 GHz and 11 GHz, Figures 6(a) and 6(d) show that majority of the current flows through the CPW into the radiator and then radiates to free space. However, at the notched frequencies of 3.5 GHz and 5.5 GHz, Figures 6(b) and 6(c) show that the energies are well confined in the upper and lower CPW resonators, respectively, and do not get radiated, while the radiator remains cool.

**Figure 6.** Distribution of surface current at (a) 3 GHz, (b) 3.5 GHz, (c) 5.5 GHz and (d) 11 GHz

#### **2.4. Results and discussions**

To validate the simulation results, the antenna is fabricated on a Roger substrate, RO4350B, as shown in Figure 7, and measured using the antenna measurement system, Satimo Starlab.

Creating Band-Notched Characteristics for Compact UWB Monopole Antennas 243

The simulated and measured results on the peak gain and radiation efficiency of the two antennas are shown in Figures 9(a) and 9(b), respectively. At the notch frequencies of 3.5 and 5.5 GHz, the measured gain is suppressed to -7.6 and -4.3 dBi, respectively, with the corresponding efficiency substantially reduced to 8.3% and 9.7%. Thus the dual-band resonator can work effectively to generate a dual band-notched characteristic for the UWB antenna. The spurious stopband at around 7.9 GHz causes a 10% drop in efficiency, as can be seen in Figure 9(b). Note that there are substantial discrepancies between the simulated and measured results in peak gain and efficiency, especially at low frequencies. This is mainly due to the small ground plane of the antenna, which results in leakage current flowing from the ground plane to the outer conductor of the feeding coaxial cable [21-23].

**Figure 9.** Simulated and measured (a) peak gains and (b) efficiencies of antennas

directions by the resonators and the average gain is about -10 dBi.

using the time-domain function (inverse FFT) of the PNA.

*2.4.2. Time-domain performance* 

The simulated and measured radiation patterns of the dual band-notched antenna at the frequencies of 3, 3.5, 5.5 and 11 GHz and in the vertical and horizontal cuts, i.e., in the x-z and x-y planes, respectively, are shown in Figure 10. It can be seen that the measured radiation patterns agree well with the simulated results. For UWB applications, an omnidirectional radiation pattern is normally preferred (i.e., in the x-y plane). The results of Figures 10(a) and 10(g) show that the radiation patterns in the passbands satisfy this requirement well. In the x-z plane, the radiation patterns in Figures 10(b) and 10(h) show two nulls occurring at the positive and negative z-directions, which is typical for monopole antennas. For the radiation patterns at the notch frequencies of 3.5 and 5.5 GHz, the gain is almost evenly suppressed in all

UWB radio systems typically employ pulse modulation where extremely narrow (short) bursts of RF energy are used to convey information [1]. Antennas with notches will introduce distortion to these bursts. To investigate this, the pulse response of the proposed antenna in the time domain is studied as follows [24]. Two antennas of the same type are placed side-by-side or face-to-face at a distance of 50 cm (to ensure in the far field region) inside the quiet zone of an anechoic chamber. The antennas are connected using coaxial cables to the two ports of the Agilent PNA N5230C. The transfer function (or S21) of the twoantenna setup is measured in the frequency domain. The time response is then obtained by

**Figure 7.** Photograph of CPW antennas without and with dual-band CPW resonator

#### *2.4.1. Frequency-domain performance*

The return loss, efficiency and peak gain across the UWB, and the radiation patterns in the passbands and notch frequencies of the reference antenna and dual band-notched antenna are all studied using computer simulation and measurement.

The return losses of the two antennas are shown in Figure 8. Across the UWB, excluding the notched bands, both the simulated and measured return losses of the reference antenna are larger than 10 dB which satisfies the UWB requirement. In the notched bands, the return loss of the notched antenna is substantially smaller than 10 dB. The measured results in Figure 8 show that the two notches at the frequencies of 3.5 and 5.5 GHz have the bandwidths of 585 and 758 MHz, respectively. A spurious response at about 7.9 GHz is observed in Figure 8, which is due to the 1st odd-harmonic of the resonant frequency at 3.5 GHz. The discrepancies between the simulated and measured results are due to the tolerances in prototype fabrication and measurements and also partly due to the SMA connector which is not included in our simulation.

**Figure 8.** Simulated and measured return losses of reference and dual band-notched antennas

The simulated and measured results on the peak gain and radiation efficiency of the two antennas are shown in Figures 9(a) and 9(b), respectively. At the notch frequencies of 3.5 and 5.5 GHz, the measured gain is suppressed to -7.6 and -4.3 dBi, respectively, with the corresponding efficiency substantially reduced to 8.3% and 9.7%. Thus the dual-band resonator can work effectively to generate a dual band-notched characteristic for the UWB antenna. The spurious stopband at around 7.9 GHz causes a 10% drop in efficiency, as can be seen in Figure 9(b). Note that there are substantial discrepancies between the simulated and measured results in peak gain and efficiency, especially at low frequencies. This is mainly due to the small ground plane of the antenna, which results in leakage current flowing from the ground plane to the outer conductor of the feeding coaxial cable [21-23].

**Figure 9.** Simulated and measured (a) peak gains and (b) efficiencies of antennas

The simulated and measured radiation patterns of the dual band-notched antenna at the frequencies of 3, 3.5, 5.5 and 11 GHz and in the vertical and horizontal cuts, i.e., in the x-z and x-y planes, respectively, are shown in Figure 10. It can be seen that the measured radiation patterns agree well with the simulated results. For UWB applications, an omnidirectional radiation pattern is normally preferred (i.e., in the x-y plane). The results of Figures 10(a) and 10(g) show that the radiation patterns in the passbands satisfy this requirement well. In the x-z plane, the radiation patterns in Figures 10(b) and 10(h) show two nulls occurring at the positive and negative z-directions, which is typical for monopole antennas. For the radiation patterns at the notch frequencies of 3.5 and 5.5 GHz, the gain is almost evenly suppressed in all directions by the resonators and the average gain is about -10 dBi.

#### *2.4.2. Time-domain performance*

242 Ultra Wideband – Current Status and Future Trends

*2.4.1. Frequency-domain performance* 

not included in our simulation.

To validate the simulation results, the antenna is fabricated on a Roger substrate, RO4350B, as shown in Figure 7, and measured using the antenna measurement system, Satimo Starlab.

The return loss, efficiency and peak gain across the UWB, and the radiation patterns in the passbands and notch frequencies of the reference antenna and dual band-notched antenna

The return losses of the two antennas are shown in Figure 8. Across the UWB, excluding the notched bands, both the simulated and measured return losses of the reference antenna are larger than 10 dB which satisfies the UWB requirement. In the notched bands, the return loss of the notched antenna is substantially smaller than 10 dB. The measured results in Figure 8 show that the two notches at the frequencies of 3.5 and 5.5 GHz have the bandwidths of 585 and 758 MHz, respectively. A spurious response at about 7.9 GHz is observed in Figure 8, which is due to the 1st odd-harmonic of the resonant frequency at 3.5 GHz. The discrepancies between the simulated and measured results are due to the tolerances in prototype fabrication and measurements and also partly due to the SMA connector which is

**Figure 8.** Simulated and measured return losses of reference and dual band-notched antennas

**Figure 7.** Photograph of CPW antennas without and with dual-band CPW resonator

are all studied using computer simulation and measurement.

**2.4. Results and discussions** 

UWB radio systems typically employ pulse modulation where extremely narrow (short) bursts of RF energy are used to convey information [1]. Antennas with notches will introduce distortion to these bursts. To investigate this, the pulse response of the proposed antenna in the time domain is studied as follows [24]. Two antennas of the same type are placed side-by-side or face-to-face at a distance of 50 cm (to ensure in the far field region) inside the quiet zone of an anechoic chamber. The antennas are connected using coaxial cables to the two ports of the Agilent PNA N5230C. The transfer function (or S21) of the twoantenna setup is measured in the frequency domain. The time response is then obtained by using the time-domain function (inverse FFT) of the PNA.

Creating Band-Notched Characteristics for Compact UWB Monopole Antennas 245

−∞ = + (1)

λ**/4-**

= 3.5, a thickness

ε

is the time delay chosen to maximize the integral term. The

direction. Figure 11(b) shows that the received pulse for the dual band-notched antenna has

*F f t r t dt* max ( ) ( )

where *f t*( ) and *r t*( ) are the transmitted and received pulses, respectively, normalized to

calculated fidelities *F* given by (1), for using the reference and notched antennas, are shown in Table 2. It can be seen that the fidelities in the face-to-face and side-by-side arrangements for both antennas used are about the same, which should be the case for antennas with an omnidirectional radiation pattern. As expected, the reference antenna achieves the fidelity of more than 97%, very close to the transmit signal. Table 2 also shows that the antenna with a

*Reference Antenna Dual Band-Notched Antenna* 

<sup>∞</sup>

τ

To evaluate the quality of the received pulses, we define the fidelity as [22, 26]:

τ

late time ringing (or distortion) and lower power.

τ

dual band-notch still can achieve a fidelity of more than 95%.

*Face-to-Face 0.9825 0.9570 Side-by-Side 0.9744 0.9531* 

**3.1. Design of UWB monopole antenna** 

of 0.8 mm and a loss tangent of 0.003.

**Table 2.** Calculated Fidelity for Reference and Band-Notched Antennas

**3. Design of band-notched microstrip monopole antenna using** 

PCB with a dimension of 30 × 39.3 mm2, a relative dielectric constant of *<sup>r</sup>*

In our design of a single band-notched antenna for UWB applications, we propose to use a planar monopole antenna with microstrip feed to achieve a compact size for applications in wireless devices. The geometry of the design is shown in Figure 12 which consists of an elliptical radiator fed by a 50-Ω microstrip line, and a rectangular ground plane on the other side of the substrate. The antenna is designed on a polytetrafluoroethylene (PTFE) substrate

The parameters *r2* (*gap*) and *w2* of the antenna shown in Figure 12 are optimized for wideband operation using computer simulation and results are shown in Figure 13. The impedance bandwidth of a monopole can be increased by widening the radiator shape. In our case, Figure 13(a) shows that with *r2* = 3 mm (i.e., a thin vertical elliptical radiator), the antenna has two distinct narrow bands, resonating at about 2.8 and 7.4 GHz. The overall bandwidth is less than the UWB. When the width *r2* of the radiator increases, the bandwidth improves. However, with *r2* = 15 mm, the radiator is too wide which reduces the return loss at high frequencies. Figure 13(b) shows that the effects of the distance, *gap*, between the elliptical radiator and the upper edge of ground on the return loss. It can be seen that *gap* is

have unity energy, and

**resonator** 

**Figure 10.** Simulated and measured radiation patterns with dual-band CPW resonator. (a) 3 GHz in x-z plane; (b) 3 GHz in x-y plane; (c) 3.45 GHz in x-y plane; (d) 3.45 GHz in x-y plane; (e) 5.5 GHz in x-z plane; (f) 5.5 GHz in x-y plane; (g) 11 GHz in x-y plane; and (h) 11 GHz in x-y plane

To fully utilize the FCC's UWB, it would be better to select the transmitted pulse with spectrum as close as possible to the FCC's emission limit mask [25]. However, due to the limitation of the equipment, i.e. Agilent PNA N5230C, used in our laboratory, we only manage to generate pulses with a rectangular spectrum from 3.1 to 10.6 GHz as the transmitted pulses.

**Figure 11.** Measured pulse responses for (a) reference UWB antenna and (b) proposed dual bandnotched antennas

For comparison, the measured results on pulse responses of the reference antenna and dual band-notched antenna are plotted in Figure 11. It can be seen that, in both cases, the magnitudes of the received pulses are larger in the face-to-face arrangement than in the side-by-side arrangement. This is because, at higher frequencies, the planar structure of the monopole antennas causes the radiation patterns to become slightly directional at the face direction. Figure 11(b) shows that the received pulse for the dual band-notched antenna has late time ringing (or distortion) and lower power.

To evaluate the quality of the received pulses, we define the fidelity as [22, 26]:

$$F = \max\_{\pi} \int\_{-\infty}^{\infty} f(t) \, r(t + \pi) dt \tag{1}$$

where *f t*( ) and *r t*( ) are the transmitted and received pulses, respectively, normalized to have unity energy, and τ is the time delay chosen to maximize the integral term. The calculated fidelities *F* given by (1), for using the reference and notched antennas, are shown in Table 2. It can be seen that the fidelities in the face-to-face and side-by-side arrangements for both antennas used are about the same, which should be the case for antennas with an omnidirectional radiation pattern. As expected, the reference antenna achieves the fidelity of more than 97%, very close to the transmit signal. Table 2 also shows that the antenna with a dual band-notch still can achieve a fidelity of more than 95%.


**Table 2.** Calculated Fidelity for Reference and Band-Notched Antennas

#### **3. Design of band-notched microstrip monopole antenna using** λ**/4 resonator**

#### **3.1. Design of UWB monopole antenna**

244 Ultra Wideband – Current Status and Future Trends

transmitted pulses.

notched antennas

**Figure 10.** Simulated and measured radiation patterns with dual-band CPW resonator. (a) 3 GHz in x-z plane; (b) 3 GHz in x-y plane; (c) 3.45 GHz in x-y plane; (d) 3.45 GHz in x-y plane; (e) 5.5 GHz in x-z

To fully utilize the FCC's UWB, it would be better to select the transmitted pulse with spectrum as close as possible to the FCC's emission limit mask [25]. However, due to the limitation of the equipment, i.e. Agilent PNA N5230C, used in our laboratory, we only manage to generate pulses with a rectangular spectrum from 3.1 to 10.6 GHz as the

**Figure 11.** Measured pulse responses for (a) reference UWB antenna and (b) proposed dual band-

For comparison, the measured results on pulse responses of the reference antenna and dual band-notched antenna are plotted in Figure 11. It can be seen that, in both cases, the magnitudes of the received pulses are larger in the face-to-face arrangement than in the side-by-side arrangement. This is because, at higher frequencies, the planar structure of the monopole antennas causes the radiation patterns to become slightly directional at the face

plane; (f) 5.5 GHz in x-y plane; (g) 11 GHz in x-y plane; and (h) 11 GHz in x-y plane

In our design of a single band-notched antenna for UWB applications, we propose to use a planar monopole antenna with microstrip feed to achieve a compact size for applications in wireless devices. The geometry of the design is shown in Figure 12 which consists of an elliptical radiator fed by a 50-Ω microstrip line, and a rectangular ground plane on the other side of the substrate. The antenna is designed on a polytetrafluoroethylene (PTFE) substrate PCB with a dimension of 30 × 39.3 mm2, a relative dielectric constant of *<sup>r</sup>* ε = 3.5, a thickness of 0.8 mm and a loss tangent of 0.003.

The parameters *r2* (*gap*) and *w2* of the antenna shown in Figure 12 are optimized for wideband operation using computer simulation and results are shown in Figure 13. The impedance bandwidth of a monopole can be increased by widening the radiator shape. In our case, Figure 13(a) shows that with *r2* = 3 mm (i.e., a thin vertical elliptical radiator), the antenna has two distinct narrow bands, resonating at about 2.8 and 7.4 GHz. The overall bandwidth is less than the UWB. When the width *r2* of the radiator increases, the bandwidth improves. However, with *r2* = 15 mm, the radiator is too wide which reduces the return loss at high frequencies. Figure 13(b) shows that the effects of the distance, *gap*, between the elliptical radiator and the upper edge of ground on the return loss. It can be seen that *gap* is quite sensitive to impedance matching (same as in the previous design of the CPW antenna). With smaller values of *gap*, the antenna has low return loss at high frequencies. With larger values of *gap*, it has low return loss at low frequencies. In our design, the optimized value for *gap* is 0.5 mm. Figure 13(c) shows the return loss for different values of *w2*, the width of the microstrip feed-line at the radiator end. Using the characteristic parameters of the substrate, the width *w1* needs to be 1.73 mm in order to achieve a 50-Ω characteristic impedance for the microstrip line [27]. It can be seen that, if *w1* = *w2* = 1.73 mm, the impedance bandwidth (for return loss > 10 dB) of the antenna cannot cover the whole UWB. Thus, the width of the upper 6 mm of the microstrip feed-line is tapered linearly to improve matching. With *w2* = 0.6 mm, the antenna has an impedance bandwidth (10-dB return loss) from around 2.5 GHz to over 12 GHz, which fully satisfies the FCC requirements for the UWB. This final design is used as a reference UWB antenna for comparison with our bandnotched antenna design. The dimensions are listed in Table 3.

Creating Band-Notched Characteristics for Compact UWB Monopole Antennas 247

/4-microstrip line with a shorting end) which has the

/4-resonators should be placed in the positions where

/4-resonators along these edges to create a

λ

/4-resonators

**Figure 13.** Simulated return loss of the planar monopole antenna with different values of (a) *r2* , (b) *gap*,

To generate a band-notched characteristic for the antenna, we propose to use a pair of

advantages of simple structure, easy design and fabrication, and low cost. To create an

majority of the current passes through before radiating into space. Figure 14 shows the simulated current distribution of the UWB antenna at 5.5 GHz. The current is mainly distributed at the edges of the microstrip line, the upper edges of the ground plane and the

are symmetrically placed at a distance *s4* from the center of the microstrip feed line and are

λ

and (c) *w2*

coupled-fed

**/4-resonator** 

λ

effective notch for the antenna, the

connected to the ground through a via.

/4-resonators (a

edges of the radiator. Thus, we should place the

λ

**Figure 14.** Current distribution of proposed UWB antenna at 5.5 GHz

λ

notch for the antenna. Figure 15 shows our proposed design, where the two

**3.2.** λ

**Figure 12.** Layout of UWB antenna: (a) top view and (b) side view


**Table 3.** Antenna dimensions (in mm)

**Figure 13.** Simulated return loss of the planar monopole antenna with different values of (a) *r2* , (b) *gap*, and (c) *w2*

#### **3.2.** λ**/4-resonator**

246 Ultra Wideband – Current Status and Future Trends

notched antenna design. The dimensions are listed in Table 3.

**Figure 12.** Layout of UWB antenna: (a) top view and (b) side view

*r2* 9 *s3* 9 *t* 0.035 *s4* 1.0

**Table 3.** Antenna dimensions (in mm)

*Parameter Value Parameter Value Parameter Value gl* 15 *h* 0.762 *w1* 1.73 *gw* 30 *gap* 0.5 *w2* 0.6 *l* 39.3 *s1* 0 *Rvia* 0.3 *r1* 12 *s2* 0.5 *fl* 6

quite sensitive to impedance matching (same as in the previous design of the CPW antenna). With smaller values of *gap*, the antenna has low return loss at high frequencies. With larger values of *gap*, it has low return loss at low frequencies. In our design, the optimized value for *gap* is 0.5 mm. Figure 13(c) shows the return loss for different values of *w2*, the width of the microstrip feed-line at the radiator end. Using the characteristic parameters of the substrate, the width *w1* needs to be 1.73 mm in order to achieve a 50-Ω characteristic impedance for the microstrip line [27]. It can be seen that, if *w1* = *w2* = 1.73 mm, the impedance bandwidth (for return loss > 10 dB) of the antenna cannot cover the whole UWB. Thus, the width of the upper 6 mm of the microstrip feed-line is tapered linearly to improve matching. With *w2* = 0.6 mm, the antenna has an impedance bandwidth (10-dB return loss) from around 2.5 GHz to over 12 GHz, which fully satisfies the FCC requirements for the UWB. This final design is used as a reference UWB antenna for comparison with our band-

> To generate a band-notched characteristic for the antenna, we propose to use a pair of coupled-fed λ/4-resonators (a λ/4-microstrip line with a shorting end) which has the advantages of simple structure, easy design and fabrication, and low cost. To create an effective notch for the antenna, the λ/4-resonators should be placed in the positions where majority of the current passes through before radiating into space. Figure 14 shows the simulated current distribution of the UWB antenna at 5.5 GHz. The current is mainly distributed at the edges of the microstrip line, the upper edges of the ground plane and the edges of the radiator. Thus, we should place the λ/4-resonators along these edges to create a notch for the antenna. Figure 15 shows our proposed design, where the two λ/4-resonators are symmetrically placed at a distance *s4* from the center of the microstrip feed line and are connected to the ground through a via.

**Figure 14.** Current distribution of proposed UWB antenna at 5.5 GHz

**Figure 15.** Layout of single band-notched UWB antenna: (a) top view and (b) side view

The guided wavelength of the waves propagating along a substrate can be approximated by [27]:

$$
\lambda\_{\llcorner\_{\mathcal{S}}} = \lambda\_0 / \sqrt{\left(\varepsilon\_r \ast 1\right) / 2} \tag{2}
$$

Creating Band-Notched Characteristics for Compact UWB Monopole Antennas 249

λ

/4-resonators affect

λ/4-

**Figure 16.** Current flow around λ/4-resonators and feed line of single band-notched antenna at (a)

The width *s1* of the resonators, and the distances *s2* between the resonators and the upper edge of ground plane and *s3* between the feed line and resonators, determine the capacitance of the resonators. A parametric study of the single band-notched UWB antenna is carried

the characteristic of the band notch. Simulation results on the effects of the dimensions *s1*, *s2* and *s3* on the return loss of the antenna are shown in Figures 17. Figures 17(a) and 17(c) show that *s1* and *s3* determine the notch frequency. *s3* is more sensitive and so can be used for coarse adjustment of the notch frequency, while *s1* can be used for fine adjustment. Figure 17(b) shows that *s2* mainly affects the notch bandwidth. These plots also reveal that when the values of *s1*, *s2* and *s3* are changed, the return loss in the rest of the UWB band remains nearly unchanged. This property provides the designers with a great freedom to

**Figure 17.** Simulated return loss of single band-notched antenna with different values of (a) *s1* , (b) *s2*,

Figures 18(a) – 18(d) show the surface current distributions of the antenna at 3.5, 5.5, 7 and 12 GHz, respectively. At the passband frequencies, i.e., 3.5, 7 and 12 GHz, Figures 18(a), 18(c) and 18(d) show that majority of the current flows from the microstrip line to the radiator and so finally radiates into free space. However, at the notch frequency 5.5 GHz, Figure 18(b) shows that the current is confined much more around the areas near the

notch frequency 5.5 GHz and (b) passband frequency 3.5 GHz

λ

**/4-resonators** 

out using computer simulation to explore how the dimensions of the

select the notched-band frequency and bandwidth for the antenna.

**3.4. Parametric study on** 

and (c) *s3*

**3.5. Current distribution** 

with λ*0* being the free space wavelength and ε*<sup>r</sup>* the relative permittivity of the substrate. In our design, the PTFE substrate used has a relative dielectric constant of ε*<sup>r</sup>* = 3.5. To design a notch at 5.5 GHz which is the center frequency of the IEEE 802.11a WLAN band [28], applying (2) yields λ*g* ≈ 36.34 mm or λ*g* /4 ≈ 9.085 mm. While in our simulation studies, the required length of the λ/4-resonator for a notch at 5.5 GHz is *s*3 = 9 mm. Thus the difference between numerical calculation using (2) and simulation (or practical implementation) is only about 4.5%, which may be caused by the approximation of the expression for λ*<sup>g</sup>* in (2). The dimensions of the λ/4-resonators are listed in Table 3.

#### **3.3. Current flow on** λ**/4-resonators**

To better understand the working principle of the λ/4-resonators on the antenna, the currents flowing around the areas of the resonators and feed line at the notch frequency of 5.5 GHz and passband frequency of 3.5 GHz are simulated and shown in Figure 16. At 5.5 GHz, Figure 16(a) shows that the current is coupled from the feed line and the upper edge of the ground plane to the resonators and then flows to ground through the vias. This stops the energy on the feed line flowing into the elliptical radiator and radiating into free space. However, at the passband of 3.5 GHz, as shown in Figure 16(b), majority of the current flows to the radiator and radiates to free space.

**Figure 16.** Current flow around λ/4-resonators and feed line of single band-notched antenna at (a) notch frequency 5.5 GHz and (b) passband frequency 3.5 GHz

#### **3.4. Parametric study on** λ**/4-resonators**

248 Ultra Wideband – Current Status and Future Trends

by [27]:

with λ

applying (2) yields

required length of the

The dimensions of the

**3.3. Current flow on** 

**Figure 15.** Layout of single band-notched UWB antenna: (a) top view and (b) side view

our design, the PTFE substrate used has a relative dielectric constant of

λ*g* /4 ≈

*0* being the free space wavelength and

36.34 mm or

**/4-resonators** 

λ*g* ≈

λ

λ

λ

flows to the radiator and radiates to free space.

To better understand the working principle of the

The guided wavelength of the waves propagating along a substrate can be approximated

<sup>0</sup> λ λ / ( +1) / 2 *g r* ≈ ε

ε

notch at 5.5 GHz which is the center frequency of the IEEE 802.11a WLAN band [28],

between numerical calculation using (2) and simulation (or practical implementation) is

currents flowing around the areas of the resonators and feed line at the notch frequency of 5.5 GHz and passband frequency of 3.5 GHz are simulated and shown in Figure 16. At 5.5 GHz, Figure 16(a) shows that the current is coupled from the feed line and the upper edge of the ground plane to the resonators and then flows to ground through the vias. This stops the energy on the feed line flowing into the elliptical radiator and radiating into free space. However, at the passband of 3.5 GHz, as shown in Figure 16(b), majority of the current

only about 4.5%, which may be caused by the approximation of the expression for

/4-resonators are listed in Table 3.

(2)

ε

/4-resonators on the antenna, the

*<sup>r</sup>* = 3.5. To design a

λ*<sup>g</sup>* in (2).

*<sup>r</sup>* the relative permittivity of the substrate. In

9.085 mm. While in our simulation studies, the

/4-resonator for a notch at 5.5 GHz is *s*3 = 9 mm. Thus the difference

λ

The width *s1* of the resonators, and the distances *s2* between the resonators and the upper edge of ground plane and *s3* between the feed line and resonators, determine the capacitance of the resonators. A parametric study of the single band-notched UWB antenna is carried out using computer simulation to explore how the dimensions of the λ/4-resonators affect the characteristic of the band notch. Simulation results on the effects of the dimensions *s1*, *s2* and *s3* on the return loss of the antenna are shown in Figures 17. Figures 17(a) and 17(c) show that *s1* and *s3* determine the notch frequency. *s3* is more sensitive and so can be used for coarse adjustment of the notch frequency, while *s1* can be used for fine adjustment. Figure 17(b) shows that *s2* mainly affects the notch bandwidth. These plots also reveal that when the values of *s1*, *s2* and *s3* are changed, the return loss in the rest of the UWB band remains nearly unchanged. This property provides the designers with a great freedom to select the notched-band frequency and bandwidth for the antenna.

**Figure 17.** Simulated return loss of single band-notched antenna with different values of (a) *s1* , (b) *s2*, and (c) *s3*

#### **3.5. Current distribution**

Figures 18(a) – 18(d) show the surface current distributions of the antenna at 3.5, 5.5, 7 and 12 GHz, respectively. At the passband frequencies, i.e., 3.5, 7 and 12 GHz, Figures 18(a), 18(c) and 18(d) show that majority of the current flows from the microstrip line to the radiator and so finally radiates into free space. However, at the notch frequency 5.5 GHz, Figure 18(b) shows that the current is confined much more around the areas near the λ/4-

resonators than those in the main radiator of the antenna. As a result, the energy does not get radiated.

Creating Band-Notched Characteristics for Compact UWB Monopole Antennas 251

λ

/4-resonators with

21(h) show that there are two nulls in the z-direction, which is typical for monopole antennas. The radiation patterns in Figures 21(c) and 21(d) at the notch frequency of 5.5 GHz

indicate that the gain is almost evenly suppressed in all directions by the

**Figure 20.** Simulated and measured return losses of single band-notched antenna

**Figure 21.** Simulated and measured radiation patterns of single band-notched antenna. (a) 3 GHz in x-y plane, (b) 3 GHz in x-z plane, (c) 5.5 GHz in x-y plane, (d) 5.5 GHz in x-z plane, (e) 7 GHz in x-y plane,

The peak gain and efficiency of the antenna are shown in Figures 22(a) and 22(b), respectively. The measured average peak gain over the UWB, computed by excluding the notched band, is around 3.5 dBi. However, at the notched band, significant reductions in gain and radiation efficiency can be observed. The measured peak gain is suppressed from

(f) 7 GHz in x-z plane, (g) 12 GHz in x-y plane, and (h) 12 GHz in x-z plane

an average peak gain of about -10 dBi.

**Figure 18.** Surface current distribution of single band-notched antenna at (a) 3.5, (b) 5.5, (c) 7, and (d) 12 GHz

## **3.6. Results and discussions**

The design of the band-notched antenna is fabricated using a Rogers substrate, RO4350B, as shown in Figure 19. The return loss, peak gain and efficiency across the UWB band, and the radiation patterns at 3, 5.5, 7 and 12 GHz are simulated and measured using the Satimo Starlab measurement system. The pulse responses between a pair of the antennas placed face-to-face and side-by-side are investigated.

**Figure 19.** Photograph of single band-notched antenna

## *3.6.1. Frequency-domain performance*

The simulated and measured return losses of the single band-notched antenna are shown in Figure 20. It can be seen that, the antenna can operate from 2.57 GHz to over 12 GHz with return loss > 10 dB, except in the WLAN band from 5.18 to 6.23 GHz, where the measured return loss is substantially less than 10 dB.

The simulated and measured radiation patterns of the antenna at the frequencies of 3, 5.5, 7 and 12 GHz in the two principle planes, the x-y and x-z planes, are shown in Figure 21. At 3, 7 and 12 GHz, Figures 21(a), 21(e) and 21(g) show that the antenna has approximately omnidirectional radiation patterns in the x-y plane. In the x-z plane, Figures 21(b), 21(f) and 21(h) show that there are two nulls in the z-direction, which is typical for monopole antennas. The radiation patterns in Figures 21(c) and 21(d) at the notch frequency of 5.5 GHz indicate that the gain is almost evenly suppressed in all directions by the λ/4-resonators with an average peak gain of about -10 dBi.

**Figure 20.** Simulated and measured return losses of single band-notched antenna

250 Ultra Wideband – Current Status and Future Trends

**3.6. Results and discussions** 

face-to-face and side-by-side are investigated.

**Figure 19.** Photograph of single band-notched antenna

*3.6.1. Frequency-domain performance* 

return loss is substantially less than 10 dB.

get radiated.

GHz

resonators than those in the main radiator of the antenna. As a result, the energy does not

**Figure 18.** Surface current distribution of single band-notched antenna at (a) 3.5, (b) 5.5, (c) 7, and (d) 12

The design of the band-notched antenna is fabricated using a Rogers substrate, RO4350B, as shown in Figure 19. The return loss, peak gain and efficiency across the UWB band, and the radiation patterns at 3, 5.5, 7 and 12 GHz are simulated and measured using the Satimo Starlab measurement system. The pulse responses between a pair of the antennas placed

The simulated and measured return losses of the single band-notched antenna are shown in Figure 20. It can be seen that, the antenna can operate from 2.57 GHz to over 12 GHz with return loss > 10 dB, except in the WLAN band from 5.18 to 6.23 GHz, where the measured

The simulated and measured radiation patterns of the antenna at the frequencies of 3, 5.5, 7 and 12 GHz in the two principle planes, the x-y and x-z planes, are shown in Figure 21. At 3, 7 and 12 GHz, Figures 21(a), 21(e) and 21(g) show that the antenna has approximately omnidirectional radiation patterns in the x-y plane. In the x-z plane, Figures 21(b), 21(f) and

**Figure 21.** Simulated and measured radiation patterns of single band-notched antenna. (a) 3 GHz in x-y plane, (b) 3 GHz in x-z plane, (c) 5.5 GHz in x-y plane, (d) 5.5 GHz in x-z plane, (e) 7 GHz in x-y plane, (f) 7 GHz in x-z plane, (g) 12 GHz in x-y plane, and (h) 12 GHz in x-z plane

The peak gain and efficiency of the antenna are shown in Figures 22(a) and 22(b), respectively. The measured average peak gain over the UWB, computed by excluding the notched band, is around 3.5 dBi. However, at the notched band, significant reductions in gain and radiation efficiency can be observed. The measured peak gain is suppressed from about 2.5 dBi to -5.4 dBi and the radiation efficiency is reduced from about 80% to 15%. These results indicate that the λ/4-resonators effectively generate a single band-notched characteristic for the antenna.

Creating Band-Notched Characteristics for Compact UWB Monopole Antennas 253

λ

λ

λ/4-

/4-resonator into a ML, we

/4 long. Studies have shown that

/4 with a smaller size. With the compact

*Reference Antenna Single Band-Notched Antenna* 

*Face-to-Face 0.9825 0.9481 Side-by-Side 0.9744 0.9413* 

**Table 4.** Calculated Fidelity for Reference and Band-Notched Antennas

**4.1. Design multiple band-notched antennas using MLs** 

resonator used in section 3 is too large to be used. By folding the

**Figure 25.** MLs with (a) parallel-coupled fed and (b) direct-connected fed

of the ML, the total physical resonator length is no longer

we can make the electrical length of the MLs to be

along the ML.

**Figure 24.** Typical ML with 8 segments

**4. Multiple band-notched characteristics using meander lines (MLs)** 

Meander line (ML), also known as serpentine line, consisting of a number of transmission lines closely packed and jointed to each others, as shown in Figure 24, is an effective way for size reduction of a transmission line [29-36]. The idea behind meandering is to increase the electrical length per unit area of circuit board space when the signal is propagating

To design multiple band-notched characteristics for compact UWB antennas, the

can obtain a compact structure. Due to the mutual coupling between the adjacent segments

structures of MLs, we can easily place several pairs of MLs in different positions of the antenna to obtain a multiple band-notched characteristic. In our proposed design, two different types of feeding techniques, known here as parallel-coupled feeding (PCF) and direct-connected feeding (DCF), as shown in Figures 25(a) and 25(b), respectively, are employed. In the PCF ML, the signal is coupled from the transmission line to the ML, while in the DCF ML, the signal is fed directly to the ML. Details of the different band-notched designs for a compact UWB antenna using MLs are described in the following sections.

λ

**Figure 22.** Simulated and measured (a) peak gains and (b) efficiencies of proposed antenna

#### *3.6.2. Time-domain performance*

The measurement procedure for the time-domain performance is described in the previous section. For comparison, the pulse responses of the reference UWB antenna (without having the notched characteristic) are also measured and shown in Figure 23. It can be seen that, the pulse responses in the face-to-face arrangements have larger amplitudes than those in the side-by-side arrangements. This agrees with the radiation patterns shown in Figure 21 where radiations in the x-direction are slightly larger than those in the y-direction in most of the frequencies tested. The pulse responses for the reference antenna are only slightly larger than those for the notched antenna in the same arrangements. Late time ringing (or distortion) and lower power are observed in the received pulses for both antennas.

**Figure 23.** Measured pulse responses for (a) reference and (b) single-band notched antennas

The fidelities *F* of the time responses using (1) are computed and shown in Table 4. In both the face-to-face and side-by-side arrangements, the fidelities are about the same. As expected, the reference UWB antenna has the fidelity of more than 97%, very close to the source signal. The results in Table 4 show that the antenna with a single-band notch can achieve the fidelity of more than 94%.


**Table 4.** Calculated Fidelity for Reference and Band-Notched Antennas
