**6. UWB FSS-based antenna**

The plane wave analysis is the most common way to study and emphasize the performance of frequency selective surfaces where the incident waves are considered planar with angles of incidence around the normal on the FSS plane. Although this analysis method is useful for most of the applications that use FSSs as radomes and space filters, it is not that much useful when the FSSs are to be integrated into the near-field zone of an antenna.

However, several references used the plane wave approach to give a design guide for the FSS-based antennas [8, 14]. On the other hand, Ref. [15] showed that the interactions between antennas and FSSs could not be sufficiently addressed without a full-wave analysis of the actual antenna structure in the presence of the FSS.

Therefore, instead of using the FSS to enhance the bandwidth of narrowband radiators as in [15] or using FSS-based reflectors to improve the gain of predesigned UWB radiators as in [8, 9] and the previous section, we follow an approach that gathers the best of all, where the UWB radiator along with the FSS is designed together to achieve UWB, low profile, and high quasiconstant gain antenna. In the previous section, the study of effects of "h2" on the matching band of the antenna showed that as this parameter increases, the bandwidth of the antenna increases. However, one should pay attention to the fact that the parameter "h2" sets the profile of the antenna and hence, a minimum value of this parameter is needed to achieve a low profile.

The radiation patterns simulated and measured in E and H planes, which correspond to (YZ) and (XZ) as indicated in **Figures 18** and **19**, respectively, show the good effects of the reflectors on the radiation behavior of the antenna and confirm the gain enhancement, which is basi-

At higher frequencies, the radiation patterns start to be distorted with multiple side lobes due to the distortion and mean radiation tilt of the used radiator, which is a common behavior of UWB monopole antennas. However, the proposed reflectors have the ability to stabilize the peak gain despite the radiation distortion at higher frequencies, suggesting that using a more stable radiator can lead to constant and stable radiation. By taking into account their particularity and by following the proper design methodology, similar results can be obtained using

Each one of the proposed reflectors has a special operation mechanism. The first one uses the ground plane to reflect the incident waves transmitted through the FSS, where the main role of the latter is to stabilize the gain by controlling the transmission over UWB band. This reflector occupies both sides of the dielectric substrate and because of the ground plane, it is fully reflective at all frequencies even those outside the UWB band. This can be inconvenient for the nearby radiators as their radiation will be blocked even if they are not sources of interference. On the other hand, the complementary FSS, which covers only one side of the dielectric substrate, has a lower effect on the nearby radiators that operate outside the UWB band.

Meanwhile, the mounted radiator will not be completely isolated from surroundings.

the out of UWB band radiators because it is transparent outside UWB band.

when the FSSs are to be integrated into the near-field zone of an antenna.

actual antenna structure in the presence of the FSS.

The third reflector gathers the best characteristics of the two previously mentioned reflectors as its structure occupies a single side of the dielectric substrate and it is fully reflective over UWB band. Hence, it will isolate the radiator from surroundings without being an obstacle for

Due to the superiority that UWB FSS reflector shows, it will be used to design UWB FSS-based

The plane wave analysis is the most common way to study and emphasize the performance of frequency selective surfaces where the incident waves are considered planar with angles of incidence around the normal on the FSS plane. Although this analysis method is useful for most of the applications that use FSSs as radomes and space filters, it is not that much useful

However, several references used the plane wave approach to give a design guide for the FSS-based antennas [8, 14]. On the other hand, Ref. [15] showed that the interactions between antennas and FSSs could not be sufficiently addressed without a full-wave analysis of the

Therefore, instead of using the FSS to enhance the bandwidth of narrowband radiators as in [15] or using FSS-based reflectors to improve the gain of predesigned UWB radiators as in [8, 9]

cally because of the back radiation reduction.

36 UWB Technology and its Applications

other UWB monopole antennas as radiators.

antenna.

**6. UWB FSS-based antenna**

Therefore, we set "h2" to be 10 mm, which is \_\_*<sup>λ</sup>* <sup>10</sup> at the lower frequency of UWB band, instead of 16 mm as in the previous section. Then, the new current distribution, over the structure of

**Figure 20.** Proposed structure of UWB FSS-based antenna. (a) UWB antenna above UWB reflector, (b) UWB FSS-based antenna, (c) UWB FSS-based antenna (front view).

the radiator, imposed by the integration of the FSS was studied and analyzed. The previous analysis of the effect of the reflector size on the matching band of the antenna revealed that the matching band of the antenna is mainly affected by the part of the FSS that is located directly under the source. Consequently, to reduce the time of simulation, we used an FSS consisting of only 8\*8 cells during the analysis process to achieve a wide operating band. Subsequently, the radiator was designed to have a wide matching band, and the reflector was designed for high and stable gain. Hence, the parameters of both structures were modified.

The structure of the proposed FSS-based antenna is illustrated in **Figure 20**. The feeding CPW line was tapered and its new parameters are defined in **Figure 20b**.

The modification of the feeding line is necessary to compensate some of the effects of the FSS on the matching band of the source antenna, especially that the tapered area has the most control on the matching band of the antenna due to the high current distribution around this area. This fact can be noted in **Figure 21** where the current distributions over the structure of the initial radiator, the initial radiator at distance 10 mm from FSS with 8\*8 cells, and the redesigned radiator with FSS of the same number of cells are shown at 4 GHz. The current distributions are studied at 4 GHz because an impedance mismatch appears at this frequency when the FSS is located at 10 mm from the initial radiator.

Tapering the CPW feeding line and changing the dimensions of the structure lead to the redistribution of the current in a similar manner to that of the antenna without FSS. Also, the current distribution over the FSS is weaker around the redesigned antenna compared with that over the FSS located at 10 mm from the initial antenna.

All the main parameters that control the matching band of the monopole antenna, such as r, w, s, the new feeding dimensions, as well as the dimensions of the FSS unit cells were optimized using CST-MWS.

After finding the parameters of the radiator that give the best performance, the number of cells was parametrically studied, as shown in **Figure 22**, to select the number of cells that gives a high gain with a minimum variation through the operating band. The final optimized dimensions of the proposed antenna are given in **Table 4**.

#### **6.1. Final results of FSS-based antenna**

**Figure 23** indicates the computed reflection coefficient of the proposed antenna, which shows that a reflection magnitude inferior to −10 dB is achieved along the band from 3.5 to 10.6 GHz. This matching band is obtained for overall profile thickness of 10 mm, which is around λ/10 at the lower operating frequency, which is wider than that achieved in previous section though the latter has a higher profile of 16 mm.

Different factors such as the linearly decreasing reflection phase of the proposed FSS and the small distance between the FSS and the radiator, which cannot be obtained using a flat metallic reflector, contribute in achieving the reached high quasi-constant gain. As a result, a low

**Lout Lin Rout Rin G L W R Wt Hs N h2** 8 6 3.1 2.5 0.25 52 55 17.5 2 3 12\*12 10

Ultra-Wideband FSS-Based Antennas http://dx.doi.org/10.5772/intechopen.79888 39

It should be mentioned that the proposed FSS reflectors through this chapter can be integrated with other UWB antennas to achieve further features such as reported in [16, 17], where a UWB dual-polarized antenna [18] was used as radiator to develop UWB FSS-based antennas

profile planar UWB antenna with enhanced quasi-constant gain is obtained.

with diversity operation.

**Figure 21.** Surface current distribution at 4 GHz.

**Figure 22.** Gain for different numbers of cells.

**Table 4.** Dimensions of the proposed FSS-based antenna (in mm).

Regarding the radiation behavior, the peak gain of the proposed antenna and that of previous section, across the frequency, are indicated in **Figure 24**. It is clear that the proposed antenna gain is higher, over higher frequencies of the UWB band than that of the antenna with FSS reflector of the previous section, and it is lower over lower frequencies of UWB band. This is due to the size of the FSS chosen that is smaller in the latter case. However, a quasi-constant gain with a maximum variation of 0.7 dBi across the UWB band is still provided.

**Figure 21.** Surface current distribution at 4 GHz.

the radiator, imposed by the integration of the FSS was studied and analyzed. The previous analysis of the effect of the reflector size on the matching band of the antenna revealed that the matching band of the antenna is mainly affected by the part of the FSS that is located directly under the source. Consequently, to reduce the time of simulation, we used an FSS consisting of only 8\*8 cells during the analysis process to achieve a wide operating band. Subsequently, the radiator was designed to have a wide matching band, and the reflector was designed for

The structure of the proposed FSS-based antenna is illustrated in **Figure 20**. The feeding CPW

The modification of the feeding line is necessary to compensate some of the effects of the FSS on the matching band of the source antenna, especially that the tapered area has the most control on the matching band of the antenna due to the high current distribution around this area. This fact can be noted in **Figure 21** where the current distributions over the structure of the initial radiator, the initial radiator at distance 10 mm from FSS with 8\*8 cells, and the redesigned radiator with FSS of the same number of cells are shown at 4 GHz. The current distributions are studied at 4 GHz because an impedance mismatch appears at this frequency

Tapering the CPW feeding line and changing the dimensions of the structure lead to the redistribution of the current in a similar manner to that of the antenna without FSS. Also, the current distribution over the FSS is weaker around the redesigned antenna compared with

All the main parameters that control the matching band of the monopole antenna, such as r, w, s, the new feeding dimensions, as well as the dimensions of the FSS unit cells were

After finding the parameters of the radiator that give the best performance, the number of cells was parametrically studied, as shown in **Figure 22**, to select the number of cells that gives a high gain with a minimum variation through the operating band. The final optimized

**Figure 23** indicates the computed reflection coefficient of the proposed antenna, which shows that a reflection magnitude inferior to −10 dB is achieved along the band from 3.5 to 10.6 GHz. This matching band is obtained for overall profile thickness of 10 mm, which is around λ/10 at the lower operating frequency, which is wider than that achieved in previous section though

Regarding the radiation behavior, the peak gain of the proposed antenna and that of previous section, across the frequency, are indicated in **Figure 24**. It is clear that the proposed antenna gain is higher, over higher frequencies of the UWB band than that of the antenna with FSS reflector of the previous section, and it is lower over lower frequencies of UWB band. This is due to the size of the FSS chosen that is smaller in the latter case. However, a quasi-constant

gain with a maximum variation of 0.7 dBi across the UWB band is still provided.

high and stable gain. Hence, the parameters of both structures were modified.

line was tapered and its new parameters are defined in **Figure 20b**.

when the FSS is located at 10 mm from the initial radiator.

that over the FSS located at 10 mm from the initial antenna.

dimensions of the proposed antenna are given in **Table 4**.

optimized using CST-MWS.

38 UWB Technology and its Applications

**6.1. Final results of FSS-based antenna**

the latter has a higher profile of 16 mm.

**Figure 22.** Gain for different numbers of cells.


**Table 4.** Dimensions of the proposed FSS-based antenna (in mm).

Different factors such as the linearly decreasing reflection phase of the proposed FSS and the small distance between the FSS and the radiator, which cannot be obtained using a flat metallic reflector, contribute in achieving the reached high quasi-constant gain. As a result, a low profile planar UWB antenna with enhanced quasi-constant gain is obtained.

It should be mentioned that the proposed FSS reflectors through this chapter can be integrated with other UWB antennas to achieve further features such as reported in [16, 17], where a UWB dual-polarized antenna [18] was used as radiator to develop UWB FSS-based antennas with diversity operation.

and improved gain is required. The effectiveness of the proposed FSS-based reflectors has been proved using CPW-fed circular disc monopole as a radiator. A peak gain of 8.5 dBi with a maximum variation of 0.5 dBi across the UWB band has been achieved with a maintained

The proposed reflectors can also be of great importance for applications where the operation environment of the antennas can impact their behavior. The proposed reflectors can be used as shields to prevent the distortion of antenna behaviors. These reflectors, specially the UWB FSS reflector, can be used separately for UWB applications; therefore, a design guide of these types of structures has been proposed, which can be generalized to serve designing further

For further improvement of the UWB FSS-based antenna design, a new methodology of design was followed, which consists of designing the UWB FSS reflector and the UWB radia-

which corresponds to λ/10 at the lower frequency of operation, was obtained. This antenna can operate over the entire UWB band with unidirectional radiation characteristics and a peak

[1] Capolino F. Theory and Phenomena of Metamaterials. Boca Raton: CRC Press, Taylor

[2] Goussetis G, Feresidis AP, Vardaxoglou JC. Tailoring the AMC and EBG characteristics of periodic metallic arrays printed on ground dielectric substrate. IEEE Transactions on

[3] Munk BA. Frequency Selective Surfaces: Theory and Design. New York: John Wiley &

[4] Pasian M, Monni S, Neto A, Ettorre M, Gerini G. Frequency selective surfaces for extended bandwidth backing reflector functions. IEEE Transactions on Antennas and

[5] Erdemli YE, Sertel K, Gilbert RA, Wright DE, Volakis JL. Frequency-selective surfaces to enhance performance of broad-band reconfigurable arrays. IEEE Transactions on

) and low profile of 10 mm,

Ultra-Wideband FSS-Based Antennas http://dx.doi.org/10.5772/intechopen.79888 41

tor together. As a result, an antenna with a small size (10\*10 cm<sup>2</sup>

wide bandwidth.

UWB FSSs.

**Author details**

**References**

gain varying from 8.5 to 10.5 dBi.

Rabia Yahya\*, Akira Nakamura and Makoto Itami

Tokyo University of Science, Tokyo, Japan

and Francis Group; 2009

Propagation. 2010;**58**(1):43-50

Sons Inc; 2000

\*Address all correspondence to: rabiamintsidi@yahoo.fr

Antennas and Propagation. 2006;**54**(1):82-89

Antennas and Propagation. 2002;**50**(12):1716-1724

**Figure 23.** Reflection coefficient of the proposed antenna and that proposed in the previous section (UWB antenna with UWB FSS).

**Figure 24.** Peak gain of the proposed antenna and the one proposed in the previous section (UWB antenna with UWB FSS).

## **7. Conclusion**

The elaborate examples of the different utilizations of the FSSs in antenna engineering prove without any doubt the reliability and flexibility of these structures and their ability to enrich antenna research field, which can lead to further innovations. Meanwhile, the reliability and flexibility of FSSs make them very sensitive. Hence, their design should be performed carefully to attain the desired purposes.

In this chapter, we presented a proposed technique to gradually increase the gain of UWB planar antennas over frequency by using FSSs with low-profile subwavelength unit cells, thus eliminating the restriction of the UWB planar antennas to be used only for one to multiuser applications and extending their potential applications to include the ones where constant and improved gain is required. The effectiveness of the proposed FSS-based reflectors has been proved using CPW-fed circular disc monopole as a radiator. A peak gain of 8.5 dBi with a maximum variation of 0.5 dBi across the UWB band has been achieved with a maintained wide bandwidth.

The proposed reflectors can also be of great importance for applications where the operation environment of the antennas can impact their behavior. The proposed reflectors can be used as shields to prevent the distortion of antenna behaviors. These reflectors, specially the UWB FSS reflector, can be used separately for UWB applications; therefore, a design guide of these types of structures has been proposed, which can be generalized to serve designing further UWB FSSs.

For further improvement of the UWB FSS-based antenna design, a new methodology of design was followed, which consists of designing the UWB FSS reflector and the UWB radiator together. As a result, an antenna with a small size (10\*10 cm<sup>2</sup> ) and low profile of 10 mm, which corresponds to λ/10 at the lower frequency of operation, was obtained. This antenna can operate over the entire UWB band with unidirectional radiation characteristics and a peak gain varying from 8.5 to 10.5 dBi.
