**4. Small size printed monopole array**

#### **4.1. Fractal monopole antenna array**

**Frequency(GHz) Simulated Peak Gain(dBi) / Radiation Efficiency**

0.96 1.5 (93.7%) 1.8 2.6 (91.9%) 1.9 2.5 (92.2%) 2.4 2.7 (77.3%) 3.5 2.8 (86.8%) 5.2 1.7 (67.9%)

72 Progress in Compact Antennas

**Table 2.** Simulated Peak gain and radiation efficiency of the proposed antenna at each frequency band [3]

**Figure 16.** Measured E-plane (X-Z Plane) and H-plane (X-Y plane) radiation patterns of the proposed multiband an‐

tenna at: (a) 960MHz; (b) 1800MHz; (c) 1900MHz; (d) 2.4GHz; (e) 3.5GHz; (f) 5.2GHz [3]

To increase the directivity of an antenna system, the use of antenna arrays is an effective solution if an additional antenna element can be added in the wireless device. A single feed antenna array has the advantages of easy fabrication and does not need any extra RF compo‐ nents such has a phase shifter. It is desirable to design such antenna in a planar structure as it can simplify the fabrication process and reduce the fabrication cost. One compact single feed multiband printed monopole antenna array using the 2nd iteration of the Minkowski fractal geometry designed for WLAN dual band application is presented in [22].

The 2nd iteration of the Minkowski fractal geometry (see Figure 1) was chosen for this design due to its compact size. Figure 17 shows the geometry of the proposed fractal monopole array. This antenna was fabricated on a Roger 4003 substrate of thickness 0.813 mm and relative permittivity 3.38. The substrate is 112 mm long and 65 mm wide, which is the size for a typical PDA terminal. The antenna is constituted by two equal 2nd iteration Minkowski fractal monopoles fed by a single microstrip line of 1.89 mm wide. The line width of the fractal geometries is 0.25 mm and they are connected to the feed line by another horizontal microstrip line of width 1.2 mm. The partial ground plane is printed on the back side of the substrate and the antenna is printed on the top side.

**Figure 17.** The single-feed fractal monopole array on a PDA size substrate proposed in [22]

As can be seen from Figure 17, on the top size of the partial ground plane, a rectangular stub is added. Without introducing the rectangular stub on the partial ground plane, it is found that the bandwidth of this antenna is not as good as expected: the bandwidth at the higher band (5 GHz) is quite narrow. Therefore, it is necessary to find a method to improve the bandwidth of the antenna at the higher band without affecting too much the resonant frequency at the lower band. Some common impedance matching methods such as quarterwavelength transformer line or microstrip taper line, besides their large size, they are not suitable for this application, since they can only be applied to single band antennas. After several attempts, it was found that by adding a stub on the top edge of the ground plane, the impedance match of the antenna can be improved with little influence on the original resonant frequencies.

Z0=50Ω


L1

Figure 18. Antenna with an L-Matching Network

easily tuned to the desired ones.

C1 Z(w)

Figure 19 shows the measured and simulated reflection coefficient of this design. It can be observed that further optimization is required, to further increase the operating bandwidth of the antenna: it has a 10 dB bandwidth from 2.32 to 2.49 GHz and from 5.1 to 5.88 GHz, which covers the required 2.4, 5.2 and 5.8 GHz bands for 802.11a/b/g applications. Comparing the measured and simulated results, some frequency shifts were observed, which might be caused by the fabrication accuracy or the uncertainty of the dielectric constant of the substrate. By adjusting the size of the fractal geometry, the resonant frequencies can be

http://dx.doi.org/10.5772/58815

75

Figure 19. The measured and simulated S11 of the antenna array on a PDA size substrate [22]

Low Cost Compact Multiband Printed Monopole Antennas and Arrays for Wireless Communications

The measured radiation patterns for this single-feed array show that at the lower band, the radiation pattern of this antenna array is similar to a normal printed monopole antenna, which has a isotropic radiation pattern at the H plane and two broadside radiation pattern at the E plane. In the upper band, the radiation patterns at both 5.2 and 5.8 GHz are more or less omnidirectional but there are some nulls in the E plane, which are due to the cancellation from the two radiation elements. The measurement results also indicate that the maximum gain of this printed monopole array can reach 2.3 dBi in the lower band and 5.6 dBi in the upper band. Compared to the case of a single radiation element, a minimum of 2 dB gain improvement has been achieved. Based on the simulation results, the radiation efficiency of this antenna array

array is 86% at 2.4 GHz, 82% at 5.2 GHz and 89% at 5.8 GHz.

is 86% at 2.4 GHz, 82% at 5.2 GHz and 89% at 5.8 GHz.

**4.2. Inverted-L antenna array for MIMO applications**

adopted in this study employs the 'neutralizing technique'.

**Figure 19.** The measured and simulated S11 of the antenna array on a PDA size substrate [22]

**4.2. Inverted-L antenna array for MIMO applications**

Multiple-Input-Multiple-Output (MIMO) techniques enable a wireless device to transmit or receive data with higher data rate. The recently announced IEEE 802.11n and Long Term Evolution (LTE) standard requires the wireless LAN devices and mobile devices to support MIMO. The use of antenna arrays can improve the diversity performance of the antenna, which in turn increases the channel capacity by reducing the fading, suppressing both the random frequency modulation and co-channel interference. The biggest challenge in designing compact antenna arrays is how to maintain a good isolation between antennas that are closely spaced. To have good space diversity, traditionally the space between each antenna elements is required to be approximately half of the wavelength. However, for most of the commercial wireless devices, it is impossible to follow this rule due to the size constraints. The objective of this work is to explore solutions to design compact antenna arrays. The methodology

The measured radiation patterns for this single-feed array show that at the lower band, the radiation pattern of this antenna array is similar to a normal printed monopole antenna, which has a isotropic radiation pattern at the H plane and two broadside radiation pattern at the E plane. In the upper band, the radiation patterns at both 5.2 and 5.8 GHz are more or less omnidirectional but there are some nulls in the E plane, which are due to the cancellation from the two radiation elements. The measurement results also indicate that the maximum gain of this printed monopole array can reach 2.3 dBi in the lower band and 5.6 dBi in the upper band. Compared to the case of a single radiation element, a minimum of 2 dB gain improvement has been achieved. Based on the simulation results, the radiation efficiency of this antenna

Multiple-Input-Multiple-Output (MIMO) techniques enable a wireless device to transmit or receive data with higher data rate. The recently announced IEEE 802.11n and Long Term Evolution (LTE) standard requires the wireless LAN devices and mobile devices to support MIMO. The use of antenna arrays can improve the diversity performance of the antenna, which in turn increases the channel capacity by reducing the fading, suppressing both the random frequency modulation and co-channel interference. The biggest challenge to design a compact antenna arrays is how to maintain a good isolation between antennas that are closely spaced. To have good space diversity, traditionally the space between each antenna elements is required to be approximately half of the wavelength. However, for most of the commercial wireless devices, it is impossible to follow this rule due to the size constraints. The objective of

The improvement of the impedance matching of the proposed fractal antenna with the addition of a stub on the partial ground plane can be explained by modeling the stub as an equivalent L-Matching Network, as shown in Figure 18. The value of the inductance (*L*1) and capacitance (*C*1) at each resonant frequency is determined by the size/shape of the stub and the thickness/ permittivity of the substrate. For the antenna presented in [22], due to the use of the fractal geometry, which has the advantage of self-affinity and exhibiting similar radiation character‐ istics at multiple resonant frequencies, the impedance matching was improved simultaneously at both resonant frequencies with the addition of an equivalent L-Network. This is one additional merit of employing fractals in monopole antenna design.

**Figure 18.** Antenna with an L-Matching Network

Figure 19 shows the measured and simulated reflection coefficient of this design. It can be observed that further optimization is required, to further increase the operating bandwidth of the antenna: it has a 10 dB bandwidth from 2.32 to 2.49 GHz and from 5.1 to 5.88 GHz, which covers the required 2.4, 5.2 and 5.8 GHz bands for 802.11a/b/g applications. Comparing the measured and simulated results, some frequency shifts were observed, which might be caused by the fabrication accuracy or the uncertainty of the dielectric constant of the substrate. By adjusting the size of the fractal geometry, the resonant frequencies can be easily tuned to the desired ones.

constant of the substrate. By adjusting the size of the fractal geometry, the resonant frequencies can be easily tuned to the desired ones. Low Cost Compact Multiband Printed Monopole Antennas and Arrays for Wireless Communications http://dx.doi.org/10.5772/58815 75

C1 Z(w)

Figure 19 shows the measured and simulated reflection coefficient of this design. It can be observed that further optimization is required, to further increase the operating bandwidth of the antenna: it has a 10 dB bandwidth from 2.32 to 2.49 GHz and from 5.1 to 5.88 GHz, which covers the required 2.4, 5.2 and 5.8 GHz bands for 802.11a/b/g applications. Comparing the measured and simulated results, some frequency shifts were observed, which might be caused by the fabrication accuracy or the uncertainty of the dielectric

Figure 19. The measured and simulated S11 of the antenna array on a PDA size substrate [22] **Figure 19.** The measured and simulated S11 of the antenna array on a PDA size substrate [22]

The measured radiation patterns for this single-feed array show that at the lower band, the radiation pattern of this antenna array is similar to a normal printed monopole antenna, which has a isotropic radiation pattern at the H plane and two broadside radiation pattern at the E plane. In the upper band, the radiation patterns at both 5.2 and 5.8 GHz are more or less omnidirectional but there are some nulls in the E plane, which are due to the cancellation from the two radiation elements. The measurement results also indicate that the maximum gain of this printed monopole array can reach 2.3 dBi in the lower band and 5.6 dBi in the upper band. Compared to the case of a single radiation element, a minimum of 2 dB gain improvement has been achieved. Based on the simulation results, the radiation efficiency of this antenna array is 86% at 2.4 GHz, 82% at 5.2 GHz and 89% at 5.8 GHz. The measured radiation patterns for this single-feed array show that at the lower band, the radiation pattern of this antenna array is similar to a normal printed monopole antenna, which has a isotropic radiation pattern at the H plane and two broadside radiation pattern at the E plane. In the upper band, the radiation patterns at both 5.2 and 5.8 GHz are more or less omnidirectional but there are some nulls in the E plane, which are due to the cancellation from the two radiation elements. The measurement results also indicate that the maximum gain of this printed monopole array can reach 2.3 dBi in the lower band and 5.6 dBi in the upper band. Compared to the case of a single radiation element, a minimum of 2 dB gain improvement has been achieved. Based on the simulation results, the radiation efficiency of this antenna array is 86% at 2.4 GHz, 82% at 5.2 GHz and 89% at 5.8 GHz.

#### **4.2. Inverted-L antenna array for MIMO applications 4.2. Inverted-L antenna array for MIMO applications**

Z0=50Ω

L1

Figure 18. Antenna with an L-Matching Network

As can be seen from Figure 17, on the top size of the partial ground plane, a rectangular stub is added. Without introducing the rectangular stub on the partial ground plane, it is found that the bandwidth of this antenna is not as good as expected: the bandwidth at the higher band (5 GHz) is quite narrow. Therefore, it is necessary to find a method to improve the bandwidth of the antenna at the higher band without affecting too much the resonant frequency at the lower band. Some common impedance matching methods such as quarterwavelength transformer line or microstrip taper line, besides their large size, they are not suitable for this application, since they can only be applied to single band antennas. After several attempts, it was found that by adding a stub on the top edge of the ground plane, the impedance match of the antenna can be improved with little influence on the original resonant

The improvement of the impedance matching of the proposed fractal antenna with the addition of a stub on the partial ground plane can be explained by modeling the stub as an equivalent L-Matching Network, as shown in Figure 18. The value of the inductance (*L*1) and capacitance (*C*1) at each resonant frequency is determined by the size/shape of the stub and the thickness/ permittivity of the substrate. For the antenna presented in [22], due to the use of the fractal geometry, which has the advantage of self-affinity and exhibiting similar radiation character‐ istics at multiple resonant frequencies, the impedance matching was improved simultaneously at both resonant frequencies with the addition of an equivalent L-Network. This is one

Figure 19 shows the measured and simulated reflection coefficient of this design. It can be observed that further optimization is required, to further increase the operating bandwidth of the antenna: it has a 10 dB bandwidth from 2.32 to 2.49 GHz and from 5.1 to 5.88 GHz, which covers the required 2.4, 5.2 and 5.8 GHz bands for 802.11a/b/g applications. Comparing the measured and simulated results, some frequency shifts were observed, which might be caused by the fabrication accuracy or the uncertainty of the dielectric constant of the substrate. By adjusting the size of the fractal geometry, the resonant frequencies can be easily tuned to the

C1 Z(w)

additional merit of employing fractals in monopole antenna design.

L1

frequencies.

74 Progress in Compact Antennas

Z0=50Ω

desired ones.

**Figure 18.** Antenna with an L-Matching Network

Multiple-Input-Multiple-Output (MIMO) techniques enable a wireless device to transmit or receive data with higher data rate. The recently announced IEEE 802.11n and Long Term Evolution (LTE) standard requires the wireless LAN devices and mobile devices to support MIMO. The use of antenna arrays can improve the diversity performance of the antenna, which in turn increases the channel capacity by reducing the fading, suppressing both the random frequency modulation and co-channel interference. The biggest challenge to design a compact antenna arrays is how to maintain a good isolation between antennas that are closely spaced. To have good space diversity, traditionally the space between each antenna elements is required to be approximately half of the wavelength. However, for most of the commercial wireless devices, it is impossible to follow this rule due to the size constraints. The objective of Multiple-Input-Multiple-Output (MIMO) techniques enable a wireless device to transmit or receive data with higher data rate. The recently announced IEEE 802.11n and Long Term Evolution (LTE) standard requires the wireless LAN devices and mobile devices to support MIMO. The use of antenna arrays can improve the diversity performance of the antenna, which in turn increases the channel capacity by reducing the fading, suppressing both the random frequency modulation and co-channel interference. The biggest challenge in designing compact antenna arrays is how to maintain a good isolation between antennas that are closely spaced. To have good space diversity, traditionally the space between each antenna elements is required to be approximately half of the wavelength. However, for most of the commercial wireless devices, it is impossible to follow this rule due to the size constraints. The objective of this work is to explore solutions to design compact antenna arrays. The methodology adopted in this study employs the 'neutralizing technique'.

Designing a WLAN antenna for an USB dongle requires techniques for antenna miniaturiza‐ tion as the available volume left for the antenna is quite small compared to the wavelength at the required resonant frequency, which is quite challenging. As an example, in [23] a USB memory size antenna for 2.4 GHz Wireless LAN (WLAN) was achieved by using a folded trapezoidal antenna. In an USB dongle, the available volume for mounting the antennas is typically around 10×17×5 mm3 . With respect to the design of antenna arrays for USB dongles, it is a challenge task to improve the isolation between each antenna element, since the antennas have to be placed in close proximity. In [24], a dual band two antennas array was proposed. This antenna consists of an L-shape patch and a via trace connecting the via to the ground. To reach the expected performance, it needs precise fabrication and the experimental result shows that the isolation of this antenna array at 2.4 GHz is less than 9 dB. In [25], a MIMO antenna array for mobile WiMAX (3.5 GHz) was presented. This antenna has a 3D structure and the high isolation was achieved by using a common T-shaped ground plane. The disadvantages of this antenna array are that it is difficult to fabricate and the size of the ground plane can have a great effect on the radiation performance of the antenna due to the shorting structure. Regarding the design of compact planar antenna arrays for WLAN 5.8 GHz on a USB dongle, research has shown that there are few publications in this area, which is the main motivation behind this work. Recently, a new method named Neutralization Techniques has been proposed [26]. Using this method, the isolation of two Planar Inverted-F Antennas (PIFAs) can be improved through neutralizing the current of two antennas without the need of adding extra space for antenna design. So far, this method has only been applied in the design of PIFA antennas and there are few studies investigating the use of the neutralization technique. In this work, we further investigate this technique in the design of an Inverted-L antenna (ILA) array.

It is found in [27] that this antenna structure exhibits poor impedance matching at the desired frequency. The low input impedance of the ILA antenna is in fact one of its disadvantages [28]. The typical method employed to solve this problem for an ILA is to short the antenna element to the ground plane and change the feeding position, which in turn increases the input impedance of the antenna. Then the antenna becomes an Inverted-F antenna (IFA), whose input impedance is easier to be matched. However, shorting the antenna to the ground plane will increase the impact of the ground plane size to the radiation performance of the antenna. When connecting the USB dongle to a PC, for example, the equivalent size of the ground plane for the antenna is extended. In this scenario, the antenna may fail to operate at the desired frequency band. Moreover, the isolation between the antennas may also be influenced by shorting them to a common ground plane. Therefore, it is better to solve this limitation without resorting to short the antenna to the ground. Instead, the technique proposed in [27] improves the impedance matching of the antenna array by including one vertical stub in the middle of the neutralizing line, as depicted in Figure 21. From the aspect of the antenna array, where the isolation between the antennas is of concern, adding this stub has little influence on the isolation between the two antennas as the isolation is mainly controlled by the length, width and position of the horizontal neutralizing line. Meanwhile, for the single antenna itself, the equivalent antenna structure is one bent monopole with an L-shape stub, which operates as

L2

Antenna feeding

Low Cost Compact Multiband Printed Monopole Antennas and Arrays for Wireless Communications

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77

L1

**Figure 20.** The structure of a typical Inverted-L antenna [27]

Figure 22 shows the measured reflection coefficient and isolation of the proposed antenna array. The measurement results suggest that the proposed ILA array has a 10 dB return loss bandwidth from 5.7 to more than 6 GHz, which is more than the specification required for the WLAN 5.8 GHz frequency band of interest (5.725 to 5.875 GHz). This makes the performance of the proposed antenna more robust during product integration, such as immunity to

an impedance transformer.

In [27], a compact and low cost Inverted-L antenna array is proposed for the MIMO application. Figure 20 shows the structure of a classic ILA. The ILA can be viewed as a bent monopole antenna and the total length of the inverted-L, L1+L2, needs to be approximately one quarter of wavelength at the resonant frequency of interest. However, the challenge of this work is that the two antennas need to be closely located in a small area of an USB dongle.

Figure 21 presents the structure of the proposed ILAs. The proposed antenna is fabricated on 0.8 mm thick FR4 with relative permittivity of 4.4 and loss tangent of 0.02. The distance between the two feeding points is 0.15*λ*5.8GHz and the gap (d1) between these two antennas is only 0.02*λ*5.8GHz, where *λ*5.8GHz represents the free space wavelength at 5.8 GHz. This antenna array has two equal ILAs that are located within a small distance on the PCB board of the USB dongle. Based on the concept proposed in [26], a neutralizing line is added between the two antenna elements to increase the isolation. The length of the neutralizing line is critical in determining the frequency band where the isolation between the two antenna ports can be improved. Increasing the length of the neutralizing line can make the antenna array to have good isolation at the lower frequency band. According to [26], the location of the neutralizing line needs to be placed where the surface current is maximum (minimum E field) and the length of it needs to be approximately a quarter wavelength.

Low Cost Compact Multiband Printed Monopole Antennas and Arrays for Wireless Communications http://dx.doi.org/10.5772/58815 77

**Figure 20.** The structure of a typical Inverted-L antenna [27]

Designing a WLAN antenna for an USB dongle requires techniques for antenna miniaturiza‐ tion as the available volume left for the antenna is quite small compared to the wavelength at the required resonant frequency, which is quite challenging. As an example, in [23] a USB memory size antenna for 2.4 GHz Wireless LAN (WLAN) was achieved by using a folded trapezoidal antenna. In an USB dongle, the available volume for mounting the antennas is

it is a challenge task to improve the isolation between each antenna element, since the antennas have to be placed in close proximity. In [24], a dual band two antennas array was proposed. This antenna consists of an L-shape patch and a via trace connecting the via to the ground. To reach the expected performance, it needs precise fabrication and the experimental result shows that the isolation of this antenna array at 2.4 GHz is less than 9 dB. In [25], a MIMO antenna array for mobile WiMAX (3.5 GHz) was presented. This antenna has a 3D structure and the high isolation was achieved by using a common T-shaped ground plane. The disadvantages of this antenna array are that it is difficult to fabricate and the size of the ground plane can have a great effect on the radiation performance of the antenna due to the shorting structure. Regarding the design of compact planar antenna arrays for WLAN 5.8 GHz on a USB dongle, research has shown that there are few publications in this area, which is the main motivation behind this work. Recently, a new method named Neutralization Techniques has been proposed [26]. Using this method, the isolation of two Planar Inverted-F Antennas (PIFAs) can be improved through neutralizing the current of two antennas without the need of adding extra space for antenna design. So far, this method has only been applied in the design of PIFA antennas and there are few studies investigating the use of the neutralization technique. In this work, we further investigate this technique in the design of an Inverted-L antenna (ILA)

In [27], a compact and low cost Inverted-L antenna array is proposed for the MIMO application. Figure 20 shows the structure of a classic ILA. The ILA can be viewed as a bent monopole antenna and the total length of the inverted-L, L1+L2, needs to be approximately one quarter of wavelength at the resonant frequency of interest. However, the challenge of this work is

Figure 21 presents the structure of the proposed ILAs. The proposed antenna is fabricated on 0.8 mm thick FR4 with relative permittivity of 4.4 and loss tangent of 0.02. The distance between the two feeding points is 0.15*λ*5.8GHz and the gap (d1) between these two antennas is only 0.02*λ*5.8GHz, where *λ*5.8GHz represents the free space wavelength at 5.8 GHz. This antenna array has two equal ILAs that are located within a small distance on the PCB board of the USB dongle. Based on the concept proposed in [26], a neutralizing line is added between the two antenna elements to increase the isolation. The length of the neutralizing line is critical in determining the frequency band where the isolation between the two antenna ports can be improved. Increasing the length of the neutralizing line can make the antenna array to have good isolation at the lower frequency band. According to [26], the location of the neutralizing line needs to be placed where the surface current is maximum (minimum E field) and the length of it needs

that the two antennas need to be closely located in a small area of an USB dongle.

to be approximately a quarter wavelength.

. With respect to the design of antenna arrays for USB dongles,

typically around 10×17×5 mm3

76 Progress in Compact Antennas

array.

It is found in [27] that this antenna structure exhibits poor impedance matching at the desired frequency. The low input impedance of the ILA antenna is in fact one of its disadvantages [28]. The typical method employed to solve this problem for an ILA is to short the antenna element to the ground plane and change the feeding position, which in turn increases the input impedance of the antenna. Then the antenna becomes an Inverted-F antenna (IFA), whose input impedance is easier to be matched. However, shorting the antenna to the ground plane will increase the impact of the ground plane size to the radiation performance of the antenna. When connecting the USB dongle to a PC, for example, the equivalent size of the ground plane for the antenna is extended. In this scenario, the antenna may fail to operate at the desired frequency band. Moreover, the isolation between the antennas may also be influenced by shorting them to a common ground plane. Therefore, it is better to solve this limitation without resorting to short the antenna to the ground. Instead, the technique proposed in [27] improves the impedance matching of the antenna array by including one vertical stub in the middle of the neutralizing line, as depicted in Figure 21. From the aspect of the antenna array, where the isolation between the antennas is of concern, adding this stub has little influence on the isolation between the two antennas as the isolation is mainly controlled by the length, width and position of the horizontal neutralizing line. Meanwhile, for the single antenna itself, the equivalent antenna structure is one bent monopole with an L-shape stub, which operates as an impedance transformer.

Figure 22 shows the measured reflection coefficient and isolation of the proposed antenna array. The measurement results suggest that the proposed ILA array has a 10 dB return loss bandwidth from 5.7 to more than 6 GHz, which is more than the specification required for the WLAN 5.8 GHz frequency band of interest (5.725 to 5.875 GHz). This makes the performance of the proposed antenna more robust during product integration, such as immunity to

monopole with an L-shape stub, which operates as an impedance transformer.

operate at the desired frequency band. Moreover, the isolation between the antennas may also be influenced by shorting them to a common ground plane. Therefore, it is better to solve this limitation without resorting to short the antenna to the ground. Instead, the technique proposed in [27] improves the impedance matching of the antenna array by including one vertical stub in the middle of the neutralizing line, as depicted in Figure 21. From the aspect of antenna array, where the isolation between the antennas is of concern, adding this stub has little influence on the isolation between the two antennas as the isolation is mainly controlled by the length, width and position of the horizontal neutralizing line. Meanwhile, for the single antenna itself, the equivalent antenna structure is one bent

**5. Recent development**

It has been shown in this chapter that the usage of two strips can contribute to the design of a dual band printed monopole. Recent research in [29] demonstrates that instead of using only two strips, a triple-band monopole can be designed using three strips. The structure of the triple-band printed monopole in presented in Figure 23. As seen, the printed monopole has three strips, each of which corresponds to a resonant frequency. This implies that introducing multiple strips, a multiband printed monopole can be obtained. This is one of the advantages of the printed monopole compared to other types of antennas. However, the difficulty of this approach lies in how to match the monopole at different resonant frequencies and reduce the

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79

It is also known that either using the meandered lines or the introduction a chip inductor can contribute to the size reduction of the monopole. Recently, a broadband LTE/WWAN antenna was designed for the tablet PC application [30]. This monopole is designed by employing both the meandered lines and chip inductor, thus greatly reducing the size of the antenna, whilst reaching a multiple frequency band operation. Similarly, a small printed monopole for DVB application is achieved by introducing a varactor on the meander line monopole [31]. By adding the varactor on the antenna radiating element, not only the antenna size is reduced, but also the frequency reconfigurablility can be achieved. Therefore, it can be concluded that combining different antenna miniaturization techniques to design a small printed monopole is an effective approach. Figure 24 demonstrates one example of introducing a lumped element

influence from the mutual couplings between different strips.

**Figure 23.** The proposed triple band printed monopole by [29]

on a meander line structure.

Figure 21: The structure of the proposed ILAs array in [27]

**Figure 21.** The structure of the proposed ILAs array in [27]

proximity to other components and the product enclosure, thus providing some margin against proximity effects which can lead to some frequency shifts. It is also found that the isolation between the two antennas is always better than 10 dB from 5.5 to 6.0 GHz and within the desired WLAN operation band, an isolation of 12 dB or more is obtained. It is observed that there is some frequency differences (less than 100 MHz) between the measured reflection coefficients of the two ports of the antenna array. This is due to the fabrication accuracy and soldering of the feeding cable, which results in the asymmetrical response of the two antenna elements. The measured results indicate that the proposed antenna array exhibits a maximum gain around 2.5 dBi at 5.8 GHz. Figure 22 shows the measured reflection coefficient and isolation of the proposed antenna array. The measurement results suggest that the proposed ILA array has a 10 dB return loss bandwidth from 5.7 to more than 6 GHz, which is more than the specification required for the WLAN 5.8 GHz frequency band of interest (5.725 to 5.875 GHz). This makes the performance of the proposed antenna more robust during product integration, such as immunity to proximity to other components and the product enclosure, thus providing some margin against proximity effects which can lead to some frequency shifts. It is also found that the isolation between the two antennas is always better than 10 dB from 5.5 to 6.0 GHz and within the desired WLAN operation band, an isolation of 12 dB or more is obtained. It is observed that there is some frequency differences (less than 100 MHz) between the measured reflection coefficients of the two ports of the antenna array. This is due to the fabrication accuracy and soldering of the feeding cable, which results in the asymmetrical response of the two antenna elements. The measured results indicate that the

proposed antenna array exhibits a maximum gain around 2.5 dBi at 5.8 GHz.

 Figure 22: Measured reflection coefficient of the ILAs array proposed in [27] **Figure 22.** Measured reflection coefficient of the ILAs array proposed in [27]

## **5. Recent development**

proximity to other components and the product enclosure, thus providing some margin against proximity effects which can lead to some frequency shifts. It is also found that the isolation between the two antennas is always better than 10 dB from 5.5 to 6.0 GHz and within the desired WLAN operation band, an isolation of 12 dB or more is obtained. It is observed that there is some frequency differences (less than 100 MHz) between the measured reflection coefficients of the two ports of the antenna array. This is due to the fabrication accuracy and soldering of the feeding cable, which results in the asymmetrical response of the two antenna elements. The measured results indicate that the proposed antenna array exhibits a maximum

proposed antenna array exhibits a maximum gain around 2.5 dBi at 5.8 GHz.

monopole with an L-shape stub, which operates as an impedance transformer.

operate at the desired frequency band. Moreover, the isolation between the antennas may also be influenced by shorting them to a common ground plane. Therefore, it is better to solve this limitation without resorting to short the antenna to the ground. Instead, the technique proposed in [27] improves the impedance matching of the antenna array by including one vertical stub in the middle of the neutralizing line, as depicted in Figure 21. From the aspect of antenna array, where the isolation between the antennas is of concern, adding this stub has little influence on the isolation between the two antennas as the isolation is mainly controlled by the length, width and position of the horizontal neutralizing line. Meanwhile, for the single antenna itself, the equivalent antenna structure is one bent

Figure 21: The structure of the proposed ILAs array in [27]

 Figure 22 shows the measured reflection coefficient and isolation of the proposed antenna array. The measurement results suggest that the proposed ILA array has a 10 dB return loss bandwidth from 5.7 to more than 6 GHz, which is more than the specification required for the WLAN 5.8 GHz frequency band of interest (5.725 to 5.875 GHz). This makes the performance of the proposed antenna more robust during product integration, such as immunity to proximity to other components and the product enclosure, thus providing some margin against proximity effects which can lead to some frequency shifts. It is also found that the isolation between the two antennas is always better than 10 dB from 5.5 to 6.0 GHz and within the desired WLAN operation band, an isolation of 12 dB or more is obtained. It is observed that there is some frequency differences (less than 100 MHz) between the measured reflection coefficients of the two ports of the antenna array. This is due to the fabrication accuracy and soldering of the feeding cable, which results in the asymmetrical response of the two antenna elements. The measured results indicate that the

Figure 22: Measured reflection coefficient of the ILAs array proposed in [27]

gain around 2.5 dBi at 5.8 GHz.

78 Progress in Compact Antennas

**Figure 21.** The structure of the proposed ILAs array in [27]


S11| (dB)

**Figure 22.** Measured reflection coefficient of the ILAs array proposed in [27]

It has been shown in this chapter that the usage of two strips can contribute to the design of a dual band printed monopole. Recent research in [29] demonstrates that instead of using only two strips, a triple-band monopole can be designed using three strips. The structure of the triple-band printed monopole in presented in Figure 23. As seen, the printed monopole has three strips, each of which corresponds to a resonant frequency. This implies that introducing multiple strips, a multiband printed monopole can be obtained. This is one of the advantages of the printed monopole compared to other types of antennas. However, the difficulty of this approach lies in how to match the monopole at different resonant frequencies and reduce the influence from the mutual couplings between different strips.

**Figure 23.** The proposed triple band printed monopole by [29]

It is also known that either using the meandered lines or the introduction a chip inductor can contribute to the size reduction of the monopole. Recently, a broadband LTE/WWAN antenna was designed for the tablet PC application [30]. This monopole is designed by employing both the meandered lines and chip inductor, thus greatly reducing the size of the antenna, whilst reaching a multiple frequency band operation. Similarly, a small printed monopole for DVB application is achieved by introducing a varactor on the meander line monopole [31]. By adding the varactor on the antenna radiating element, not only the antenna size is reduced, but also the frequency reconfigurablility can be achieved. Therefore, it can be concluded that combining different antenna miniaturization techniques to design a small printed monopole is an effective approach. Figure 24 demonstrates one example of introducing a lumped element on a meander line structure.

Recently, a new approach based on the neutralizing technique, which has been introduced in this chapter, is proposed in [33]. Figure 26 shows the configuration of the neutralizing line. In this approach,a high isolation over a wideband frequency is reached by creating four current paths between the two antenna elements, which is achieved by attaching the neutralizing line to both the antenna elements and the feed line at its maximum current position. The meas‐ urement results provided in [33] show that this is an effective method to design a compact wideband printed monopole array for MIMO applications. This constitutes a further devel‐

Low Cost Compact Multiband Printed Monopole Antennas and Arrays for Wireless Communications

antenna feed line Connect to the

In this chapter, several techniques that can be employed to design compact and low cost printed monopole antennas and antenna arrays have been introduced. Either using some special geometry (e.g. fractals) or introducing lumped elements on the radiating elements, multiband monopole with reduced size can be implemented. The recent studies also show that combining both methods, additional size reduction can be achieved. However, the disadvant‐ age is that the antenna radiation performance will be influenced. In the field of compact printed monopole arrays, the use of the neutralizing technique has been proved to be an effective method that can be applied to the planar monopole design, which can result in a simple and low cost solution. To reach a wideband operation, the modified neutralizing line proposed in

antenna feed line

Connect to the antenna

http://dx.doi.org/10.5772/58815

81

opment for the conventional neutralizing technique proposed by [26].

Connect to the

[31] provides a good solution.

**6. Conclusion**

**Figure 26.** The modified the neutralizing line proposed in [31]

Connect to the antenna

**Figure 24.** Demonstration of adding a lumped element on the meander line

To design a compact monopole array for the MIMO application, the key issue is to keep a high isolation between two or more radiating elements when they are closely spaced. The simula‐ tion results presented in [32] show that the orientation of the antenna elements can be critical in determining the isolation between the antennas. For example, with the spacing between two antennas elements of only one tenth of the wavelength, when the two monopoles are orthog‐ onally oriented as the one shown in Figure 25, about 10-dB improvement in isolation can be observed. This can be explained by the polarization diversity. However, this approach will not be practical if a single polarization for the receiving signal is required to be used.

Antenna 1 Antenna 2

**Figure 25.** Example of placing two antennas in orthogonal position

Recently, a new approach based on the neutralizing technique, which has been introduced in this chapter, is proposed in [33]. Figure 26 shows the configuration of the neutralizing line. In this approach,a high isolation over a wideband frequency is reached by creating four current paths between the two antenna elements, which is achieved by attaching the neutralizing line to both the antenna elements and the feed line at its maximum current position. The meas‐ urement results provided in [33] show that this is an effective method to design a compact wideband printed monopole array for MIMO applications. This constitutes a further devel‐ opment for the conventional neutralizing technique proposed by [26].

**Figure 26.** The modified the neutralizing line proposed in [31]

#### **6. Conclusion**

Lumped element (e.g. inductor, varactor)

**Figure 24.** Demonstration of adding a lumped element on the meander line

80 Progress in Compact Antennas

**Figure 25.** Example of placing two antennas in orthogonal position

To design a compact monopole array for the MIMO application, the key issue is to keep a high isolation between two or more radiating elements when they are closely spaced. The simula‐ tion results presented in [32] show that the orientation of the antenna elements can be critical in determining the isolation between the antennas. For example, with the spacing between two antennas elements of only one tenth of the wavelength, when the two monopoles are orthog‐ onally oriented as the one shown in Figure 25, about 10-dB improvement in isolation can be observed. This can be explained by the polarization diversity. However, this approach will not

Antenna 1 Antenna 2

be practical if a single polarization for the receiving signal is required to be used.

In this chapter, several techniques that can be employed to design compact and low cost printed monopole antennas and antenna arrays have been introduced. Either using some special geometry (e.g. fractals) or introducing lumped elements on the radiating elements, multiband monopole with reduced size can be implemented. The recent studies also show that combining both methods, additional size reduction can be achieved. However, the disadvant‐ age is that the antenna radiation performance will be influenced. In the field of compact printed monopole arrays, the use of the neutralizing technique has been proved to be an effective method that can be applied to the planar monopole design, which can result in a simple and low cost solution. To reach a wideband operation, the modified neutralizing line proposed in [31] provides a good solution.

#### **Author details**

Qi Luo1 , Jose Rocha Pereira2,3 and Henrique Salgado4,5

1 School of Engineering and Digital Arts, University of Kent, Canterbury, UK

[10] C. H. Chang, K. L. Wong, and J. S. Row, "Coupled-Fed Small-Size Pifa for Penta-Band Folder-Type Mobile Phone Application," *Microwave and Optical Technology Let‐*

Low Cost Compact Multiband Printed Monopole Antennas and Arrays for Wireless Communications

http://dx.doi.org/10.5772/58815

83

[11] C. I. Lin, and K. L. Wong, "Printed monopole slot antenna for penta-band operation in the folder-type mobile phone," *Microwave and Optical Technology Letters,* vol. 50,

[12] F. H. Chu, and K. L. Wong, "Simple Folded Monopole Slot Antenna for Penta-Band Clamshell Mobile Phone Application," *Ieee Transactions on Antennas and Propagation,*

[13] K. L. Wong, and C. H. Huang, "Printed loop antenna with a perpendicular feed for penta-band mobile phone application," *Ieee Transactions on Antennas and Propagation,*

[14] C. I. Lin, and K. L. Wong, "Internal meandered loop antenna for GSM/DCS/PCS mul‐ tiband operation in a mobile phone with the user's hand," *Microwave and Optical*

[15] Y. W. Chi, and K. L. Wong, "Half-wavelength loop strip capacitively fed by a printed monopole for penta-band mobile phone antenna," *Microwave and Optical Technology*

[16] P. Ciais, R. Staraj, G. Kossiavas *et al.*, "Compact internal multiband antenna for mo‐ bile phone and WLAN standards," *Electronics Letters,* vol. 40, no. 15, pp. 920-921, Jul

[17] C. I. Lin, and K. L. Wong, "Printed monopole slot antenna for internal multiband mobile phone antenna," *Ieee Transactions on Antennas and Propagation,* vol. 55, no. 12,

[18] H. Hsuan-Wei, L. Yi-Chieh, T. Kwong-Kau *et al.*, "Design of a Multiband Antenna for Mobile Handset Operations," *Antennas and Wireless Propagation Letters, IEEE,* vol. 8,

[19] A. C. K. Mak, C. R. Rowell, R. D. Murch *et al.*, "Reconfigurable Multiband Antenna Designs for Wireless Communication Devices," *Antennas and Propagation, IEEE*

[20] J. K. Ji, G. H. Kim, and W. M. Seong, "A Compact Multiband Antenna Based on DNG ZOR for Wireless Mobile System," *Ieee Antennas and Wireless Propagation Letters,* vol.

[21] K. L. Wong, and S. C. Chen, "Printed Single-Strip Monopole Using a Chip Inductor for Penta-Band WWAN Operation in the Mobile Phone," *Ieee Transactions on Anten‐*

*ters,* vol. 51, no. 1, pp. 18-23, Jan, 2009.

vol. 57, no. 11, pp. 3680-3684, Nov, 2009.

vol. 56, no. 7, pp. 2138-2141, Jul, 2008.

22, 2004.

pp. 3690-3697, Dec, 2007.

pp. 200-203, 2009.

8, pp. 920-923, 2009.

*Technology Letters,* vol. 49, no. 4, pp. 759-765, Apr, 2007.

*Letters,* vol. 50, no. 10, pp. 2549-2554, Oct, 2008.

*Transactions on,* vol. 55, no. 7, pp. 1919-1928, 2007.

*nas and Propagation,* vol. 58, no. 3, pp. 1011-1014, Mar, 2010.

no. 9, pp. 2237-2242, Sep, 2008.


4 Faculdade de Engenharia Universidade do Porto, Porto, Portugal

5 INESC TEC - Instituto de Engenharia de Sistemas e Computadores do Porto, Porto, Portugal

#### **References**


[10] C. H. Chang, K. L. Wong, and J. S. Row, "Coupled-Fed Small-Size Pifa for Penta-Band Folder-Type Mobile Phone Application," *Microwave and Optical Technology Let‐ ters,* vol. 51, no. 1, pp. 18-23, Jan, 2009.

**Author details**

82 Progress in Compact Antennas

2 University of Aveiro, Portugal

W.H. Freeman, 1983.

*for WLAN USB Dongle*, 2013.

10, pp. 2487-2491, Oct, 2008.

880-883, 2011.

3 Instituto de Telecomunicações, Aveiro, Portugal

, Jose Rocha Pereira2,3 and Henrique Salgado4,5

4 Faculdade de Engenharia Universidade do Porto, Porto, Portugal

*Physics,* vol. 19, no. 12, pp. 1163-1175, 1948.

1 School of Engineering and Digital Arts, University of Kent, Canterbury, UK

5 INESC TEC - Instituto de Engenharia de Sistemas e Computadores do Porto, Porto,

[1] L. J. Chu, "Physical Limitations of Omni‐Directional Antennas," *Journal of Applied*

[2] B. B. Mandelbrot, *The fractal geometry of nature*, Updated and augm. ed., New York:

[3] Q.Luo, "Design synthesis and miniaturization of multiband and reconfigurable mi‐ crostrip antenna for future wireless applications," University of Porto, 2014.

[4] Q. Luo, H. M. Salgado, and J. R. Pereira, "Fractal Monopole Antenna Design Using Minkowski Island Geometry," *2009 Ieee Antennas and Propagation Society International Symposium and Usnc/Ursi National Radio Science Meeting, Vols 1-6*, pp. 2639-2642, 2009.

[5] Q. Luo, J. R. Pereira, and H. M. Salgado, *Inverted-L Antenna (ILA) Design Using Fractal*

[6] N. Cohen, "Fractal antenna applications in wireless telecommunications." pp. 43-49.

[7] Q. Luo, J. R. Pereira, and H. M. Salgado, "Compact Printed Monopole Antenna With Chip Inductor for WLAN," *Ieee Antennas and Wireless Propagation Letters,* vol. 10, pp.

[8] A. D. Yaghjian, and S. R. Best, "Impedance, bandwidth, and Q of antennas," *Anten‐ nas and Propagation, IEEE Transactions on,* vol. 53, no. 4, pp. 1298-1324, 2005.

[9] K. L. Wong, and C. H. Huang, "Compact multiband PIFA with a coupling feed for internal mobile phone antenna," *Microwave and Optical Technology Letters,* vol. 50, no.

Qi Luo1

Portugal

**References**


[22] L. Qi, H. M. Salgado, and J. R. Pereira, "Printed fractal monopole antenna array for WLAN." pp. 1-4.

**Chapter 4**

**Miniature Antenna with Frequency Agility**

The need of both mobility and communication leads to the integration of antennas in miniature devices so far non-connected (particularly in medical areas). The dedicated volume for the antenna, including its ground plane, has to be kept at its acceptable minimum, involving low bandwidth. Moreover, due to their poor impedance bandwidth, small antennas tend to be very sensitive to the environment. Indeed, they are directly affected by their immediate surround‐ ings, which disturb their working band, their radiation and their performances [1]. To counter the low bandwidth of the antenna and to adapt it to variable conditions and surroundings, it

Thus, active components become highly suitable for the development of modern wireless communications. Indeed, they allow the miniaturization, shifting the antenna working frequency to be matched over a wide bandwidth by covering only the user channel and the adaptation of antennas to variable operating conditions and surroundings. It is in this framework that authors will propose in this chapter to detail the integration of active compo‐

The first part will address an overview of the most common used techniques for compact antennas to become active. In this goal, active antennas state-of-the-art will be presented:

**•** The first sub-section will present antennas integrating tunable components such as varactor diodes, MicroElectroMechanical systems (MEMS), Positive Intrinsic Negative (PIN) diode

**•** The second sub-section will focus on active antennas using tunable materials properties, i.e.

A second part will show relevant parameters for active antennas studies. It will exhibit both challenges and how to integrate active components in order to maximize the antenna per‐

> © 2014 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

L. Huitema and T. Monediere

http://dx.doi.org/10.5772/58838

can integrate active components.

and Field Effect Transistor (FET).

ferroelectric materials and liquid crystal.

**1. Introduction**

Additional information is available at the end of the chapter

nents in antennas to be more compact, smart and integrated.

