**4. Design of FSS-based reflectors**

In this section, we aim to design reflectors to be able to reflect the incident waves over the entire UWB band. A perfect electrical conductor (PEC) plane can be used as a reflector, but its performance cannot be guaranteed over a broadband frequency range as UWB band, especially over the higher frequencies. Frequency selective surfaces can be employed to alleviate this limitation as in [4]. As a result, a grounded FSS will serve as a broadband reflector. Also, this reflector needs to have a reflection coefficient that varies with frequency in a manner that stabilizes the gain of the UWB antennas over the entire operating band. Therefore, a grounded FSS with high resonance frequency is used to achieve these two features. Furthermore, a single-layer UWB stop-band FSS is designed and will be used as a UWB reflector, and its behavior will be compared with that of the other reflectors.

The dimensions of the unit cell control the width of the band over which the reflection phase varies between −90° and 90°, which can be called the in-phase band [10]. The choice of the used substrate is also an effective factor for enhancing the in-phase band. Therefore, we chose RT/duroid 5880, of a dielectric constant of 1.96, a dielectric loss tangent of 0.0004, and a thickness of hu =3 mm, to obtain a wide in-phase band. For the same purpose, parametric studies of both parameters W and g, around their initial values obtained from (2), were performed, from which the values that give a wide in-phase band centered at 7 GHz are selected. These

Ultra-Wideband FSS-Based Antennas http://dx.doi.org/10.5772/intechopen.79888 23

**Figure 5a** shows the reflection phase of the grounded unit cell with the selected parameters, computed using CST-MWS by considering the "unit cell" boundary conditions and Floquet port. From **Figure 5a**, we can see that broad in-phase band from 5.5 to 8.5 GHz, and AMC

The reflection magnitude of the unit cell without a ground plane varies, over the UWB band, from 0.3 at the lower frequency to 0.8 at the higher frequency, and it is totally reflective around 45 GHz, as shown in **Figure 5b**. As a result, the reflected amount by the ground plane is unequal, and it is proportionally decreasing with frequency. This unequal amount of transmission is the key feature to achieve a constant gain across the entire UWB, especially if the gain of the used radiator is higher at upper frequencies than it is at lower frequencies, which

In [9], the stability of the gain is obtained by allowing the transmission through no backed

The operation mechanism of the grounded FSS reflector treated in the last subsection can be realized differently, in a manner that leads to further improvement, by using the complemen-

multilayer FSS, while here it is achieved by only one partially reflective FSS.

values are W = 5 mm and g = 0.25 mm.

**Figure 4.** Structure of the FSS unit cell.

feature at 7 GHz are achieved as required.

is the case for most UWB planar antennas.

**4.2. Complementary reflector**

tary technique.

#### **4.1. Grounded FSS reflector**

An FSS with an array of conductors is fully reflective at resonance frequency where it acts as a metallic sheet, and it remains transparent at other frequencies. This feature of FSS with conductor array can be used to design partial reflector with a desirable reflection coefficient by choosing the location of its resonance frequency.

An FSS of conductor elements, at a high resonance frequency, is mostly transparent with a reflection magnitude that increases with frequency. After that, the transmitted waves can be reflected totally using a PEC ground plane. In this way, the grounded FSS will improve the gain of the used radiator across the entire UWB band with different amount of enhancement; hence, a constant gain can be achieved.

A grounded FSS can also be considered as a grounded dielectric slab loaded with periodic patches, which is similar to a high impedance surface with removed vertical vias. Although removing the vertical vias from HIS mushroom-like structures leads to the disappearing of the EBG, the surface waves can exist over the entire frequency band, and it has little effect on the in-phase reflection feature or AMC feature when the plane wave is normally incident, which is the main interest for our purpose.

Therefore, as a starting point, we used the initial parameters (2), suggested in [10] to analyze EBG ground planes, to design a grounded slab loaded with periodic square shape patches of width W and gap width g between the unit cells, as indicated in **Figure 4**.

$$W = 0.12\ \lambda o, \text{g = 0.02 } \lambda o, hu = 0.04\ \lambda o, \text{er = } 2.20\tag{2}$$

where λo is the wavelength at 7 GHz, which is around the center frequency of the UWB band, and hu is the thickness of the substrate of dielectric constant εr.

**Figure 4.** Structure of the FSS unit cell.

In [9], a four-layer FSS has been used to form a UWB reflector. The four layers are separated by dielectric layers of thickness of 1.58 mm, and a UWB microstrip slot antenna is placed at a distance of 19 mm above the FSS reflector. With this structure, an average peak gain of 9.3 dB was achieved with an oscillation of ±0.5 dBi, versus an average peak gain of 4 dBi and a variation of ±2 dB of the

In this section, we aim to design reflectors to be able to reflect the incident waves over the entire UWB band. A perfect electrical conductor (PEC) plane can be used as a reflector, but its performance cannot be guaranteed over a broadband frequency range as UWB band, especially over the higher frequencies. Frequency selective surfaces can be employed to alleviate this limitation as in [4]. As a result, a grounded FSS will serve as a broadband reflector. Also, this reflector needs to have a reflection coefficient that varies with frequency in a manner that stabilizes the gain of the UWB antennas over the entire operating band. Therefore, a grounded FSS with high resonance frequency is used to achieve these two features. Furthermore, a single-layer UWB stop-band FSS is designed and will be used as a UWB reflector, and its

An FSS with an array of conductors is fully reflective at resonance frequency where it acts as a metallic sheet, and it remains transparent at other frequencies. This feature of FSS with conductor array can be used to design partial reflector with a desirable reflection coefficient

An FSS of conductor elements, at a high resonance frequency, is mostly transparent with a reflection magnitude that increases with frequency. After that, the transmitted waves can be reflected totally using a PEC ground plane. In this way, the grounded FSS will improve the gain of the used radiator across the entire UWB band with different amount of enhancement;

A grounded FSS can also be considered as a grounded dielectric slab loaded with periodic patches, which is similar to a high impedance surface with removed vertical vias. Although removing the vertical vias from HIS mushroom-like structures leads to the disappearing of the EBG, the surface waves can exist over the entire frequency band, and it has little effect on the in-phase reflection feature or AMC feature when the plane wave is normally incident,

Therefore, as a starting point, we used the initial parameters (2), suggested in [10] to analyze EBG ground planes, to design a grounded slab loaded with periodic square shape patches of

*W* = 0.12 *o*, *g* = 0.02 *o*, *hu* = 0.04 *o*, *r* = 2.20 (2)

where λo is the wavelength at 7 GHz, which is around the center frequency of the UWB band,

width W and gap width g between the unit cells, as indicated in **Figure 4**.

and hu is the thickness of the substrate of dielectric constant εr.

UWB microstrip slot antenna without the reflector. **Figure 3** illustrates the structure in [9].

**4. Design of FSS-based reflectors**

22 UWB Technology and its Applications

**4.1. Grounded FSS reflector**

behavior will be compared with that of the other reflectors.

by choosing the location of its resonance frequency.

hence, a constant gain can be achieved.

which is the main interest for our purpose.

The dimensions of the unit cell control the width of the band over which the reflection phase varies between −90° and 90°, which can be called the in-phase band [10]. The choice of the used substrate is also an effective factor for enhancing the in-phase band. Therefore, we chose RT/duroid 5880, of a dielectric constant of 1.96, a dielectric loss tangent of 0.0004, and a thickness of hu =3 mm, to obtain a wide in-phase band. For the same purpose, parametric studies of both parameters W and g, around their initial values obtained from (2), were performed, from which the values that give a wide in-phase band centered at 7 GHz are selected. These values are W = 5 mm and g = 0.25 mm.

**Figure 5a** shows the reflection phase of the grounded unit cell with the selected parameters, computed using CST-MWS by considering the "unit cell" boundary conditions and Floquet port. From **Figure 5a**, we can see that broad in-phase band from 5.5 to 8.5 GHz, and AMC feature at 7 GHz are achieved as required.

The reflection magnitude of the unit cell without a ground plane varies, over the UWB band, from 0.3 at the lower frequency to 0.8 at the higher frequency, and it is totally reflective around 45 GHz, as shown in **Figure 5b**. As a result, the reflected amount by the ground plane is unequal, and it is proportionally decreasing with frequency. This unequal amount of transmission is the key feature to achieve a constant gain across the entire UWB, especially if the gain of the used radiator is higher at upper frequencies than it is at lower frequencies, which is the case for most UWB planar antennas.

In [9], the stability of the gain is obtained by allowing the transmission through no backed multilayer FSS, while here it is achieved by only one partially reflective FSS.

### **4.2. Complementary reflector**

The operation mechanism of the grounded FSS reflector treated in the last subsection can be realized differently, in a manner that leads to further improvement, by using the complementary technique.

While the ground plane of the grounded FSS reflects the partially transmitted waves through the square conductor unit cells, the complementary FSS operates by partially transmitting the incident waves and reflects most of them with different amount across the frequency band.

**Figure 6.** Structure of the complementary unit cell in comparison with the original unit cell.

As the FSS with aperture unit cells has a high resonance frequency, its reflection magnitude decreases with frequency, which can be clearly seen in **Figure 7**. As a consequence, the amount of the transmission through the FSS will be smaller, at lower frequencies than it is at higher

It has been proved, in Section 3.2 that an FSS with UWB stop-band response can serve as a good UWB reflector when it can provide linearly decreased reflection phase over the UWB band. The proposed UWB FSS offers UWB stop-band response and linearly decreased reflection phase across the UWB band; hence, it owns the ability to serve as a UWB reflector. To design an FSS with UWB stop-band response, we follow the idea of merging two structures with the ability to resonate at adjacent frequencies while their dimensions and geometries can allow them to be integrated together. So the first step is to find such structures. Loop types resonate when their circumference is approximately a wavelength that makes them a good choice for our purpose especially that their geometries are flexible enough to be combined

The square loop resonates when its four sides' length takes a quarter of the wavelength as its

nates with a total length smaller than that of the circular ring. Combining these two structures leads to the construction of a dual-band response where the lower resonance frequency is dictated by the square loop, and the higher frequency is controlled by the circular ring. The pass-band between the two resonance frequencies is governed by the distance separating the two structures. As made known in the previous section, the dielectric substrate, over which the FSS is printed, plays a vital role in determining the FSS characteristics, especially its size at resonance. Therefore, the choice of the FSS dielectric substrate can be beneficial in miniaturizing unit cell size, as a high dielectric constant substrate can reduce the size of the unit cell

π. Hence, the square loop reso-

Ultra-Wideband FSS-Based Antennas http://dx.doi.org/10.5772/intechopen.79888 25

frequencies.

**4.3. UWB FSS reflector**

with each other or even with other types.

value and the circular ring resonates with a diameter of D <sup>=</sup> \_\_<sup>λ</sup>

**Figure 5.** Reflection characteristics of FSS. (a) Phase reflection of the grounded FSS with optimized dimensions, (b) reflection magnitude of the FSS without a ground plane.

The complementary structure of the previously used FSS is an array of square slots of width Wc and metallic gap width gc, where Wc and gc have the same values as those of W and g of the grounded FSS. The complementary FSS is printed on a similar substrate to that of the grounded FSS. **Figure 6** shows the structure of the complementary unit cell as in comparison with the grounded one.

This complementary structure will have interchanged S-parameters with the original one according to Babinet's principle. As a result, a reflector with similar behavior as the grounded FSS can be obtained.

**Figure 6.** Structure of the complementary unit cell in comparison with the original unit cell.

While the ground plane of the grounded FSS reflects the partially transmitted waves through the square conductor unit cells, the complementary FSS operates by partially transmitting the incident waves and reflects most of them with different amount across the frequency band.

As the FSS with aperture unit cells has a high resonance frequency, its reflection magnitude decreases with frequency, which can be clearly seen in **Figure 7**. As a consequence, the amount of the transmission through the FSS will be smaller, at lower frequencies than it is at higher frequencies.

#### **4.3. UWB FSS reflector**

The complementary structure of the previously used FSS is an array of square slots of width Wc and metallic gap width gc, where Wc and gc have the same values as those of W and g of the grounded FSS. The complementary FSS is printed on a similar substrate to that of the grounded FSS. **Figure 6** shows the structure of the complementary unit cell as in comparison with the grounded one.

**Figure 5.** Reflection characteristics of FSS. (a) Phase reflection of the grounded FSS with optimized dimensions, (b)

This complementary structure will have interchanged S-parameters with the original one according to Babinet's principle. As a result, a reflector with similar behavior as the grounded

FSS can be obtained.

24 UWB Technology and its Applications

reflection magnitude of the FSS without a ground plane.

It has been proved, in Section 3.2 that an FSS with UWB stop-band response can serve as a good UWB reflector when it can provide linearly decreased reflection phase over the UWB band. The proposed UWB FSS offers UWB stop-band response and linearly decreased reflection phase across the UWB band; hence, it owns the ability to serve as a UWB reflector. To design an FSS with UWB stop-band response, we follow the idea of merging two structures with the ability to resonate at adjacent frequencies while their dimensions and geometries can allow them to be integrated together. So the first step is to find such structures. Loop types resonate when their circumference is approximately a wavelength that makes them a good choice for our purpose especially that their geometries are flexible enough to be combined with each other or even with other types.

The square loop resonates when its four sides' length takes a quarter of the wavelength as its value and the circular ring resonates with a diameter of D <sup>=</sup> \_\_<sup>λ</sup> π. Hence, the square loop resonates with a total length smaller than that of the circular ring. Combining these two structures leads to the construction of a dual-band response where the lower resonance frequency is dictated by the square loop, and the higher frequency is controlled by the circular ring. The pass-band between the two resonance frequencies is governed by the distance separating the two structures. As made known in the previous section, the dielectric substrate, over which the FSS is printed, plays a vital role in determining the FSS characteristics, especially its size at resonance. Therefore, the choice of the FSS dielectric substrate can be beneficial in miniaturizing unit cell size, as a high dielectric constant substrate can reduce the size of the unit cell

**Figure 7.** Reflection and transmission magnitude of the complementary unit cell.

by a factor of 1/ √ \_\_\_\_\_\_\_\_\_\_ (∈*<sup>r</sup>* <sup>+</sup> <sup>1</sup>)/2 compared with a unit cell printed on a foam substrate (with dielectric constant equals unity). Consequently, RT/duroid 6010.2LM with a dielectric constant of 10.2, a tangent loss of 0.0023, and a thickness of 0.625 mm has been chosen as the dielectric substrate over which the proposed FSSs are printed.

After choosing the geometries of the FSS unit cells that serve our purpose, we need to study these unit cells using a 3D simulator for a further investigation of their behavior and how they react to the parameter variations.

It is important to mention that all the graphs presented in the next subsections are for a normal incidence. Due to the symmetry of all these FSSs, the obtained responses under TE-polarization are similar to those obtained under TM-polarization. Hence, for the sake of brevity, only the responses obtained under TE-polarization is presented.

We have studied the effects of varying the parameters of the square and circular ring unit cells, more details can be found in [11, 12], from which the design guides can be derived. Now, we shall study the effect of combining them. Although they respond to the variation of their parameters in similar ways, the square loop resonates at smaller dimensions compared to the circular ring. Consequently, for a miniaturized unit cell, the circular ring should be integrated into the square loop as depicted in **Figure 8**.

To visualize the effect of the spacing between the two elements, only "Rout" is varied while all the other parameters are kept constant, and the transmission coefficient is calculated for each case. The results of this study are illustrated in **Figure 9**.

The results of the previous studies can help deliver a design guide of the combined UWB FSS

Ultra-Wideband FSS-Based Antennas http://dx.doi.org/10.5772/intechopen.79888 27

**Figure 9.** Parametric study of transmission coefficient for different values of Rout.

• The square loop resonates at smaller dimensions compared to the circular ring. As a result, the circular ring should be integrated into the square loop to obtain a miniaturized unit cell.

as follows:

**Figure 8.** Circular ring inserted within square loop unit cell.

As "Rout" increases, the resonance frequency of the circular ring decreases, approaching that of the square ring. Thus, the band-pass between the two resonance frequencies gets narrower while merging both structures results in a UWB band-stop response.

**Figure 8.** Circular ring inserted within square loop unit cell.

by a factor of 1/

√

26 UWB Technology and its Applications

react to the parameter variations.

\_\_\_\_\_\_\_\_\_\_

strate over which the proposed FSSs are printed.

**Figure 7.** Reflection and transmission magnitude of the complementary unit cell.

into the square loop as depicted in **Figure 8**.

case. The results of this study are illustrated in **Figure 9**.

while merging both structures results in a UWB band-stop response.

(∈*<sup>r</sup>* <sup>+</sup> <sup>1</sup>)/2 compared with a unit cell printed on a foam substrate (with dielectric

constant equals unity). Consequently, RT/duroid 6010.2LM with a dielectric constant of 10.2, a tangent loss of 0.0023, and a thickness of 0.625 mm has been chosen as the dielectric sub-

After choosing the geometries of the FSS unit cells that serve our purpose, we need to study these unit cells using a 3D simulator for a further investigation of their behavior and how they

It is important to mention that all the graphs presented in the next subsections are for a normal incidence. Due to the symmetry of all these FSSs, the obtained responses under TE-polarization are similar to those obtained under TM-polarization. Hence, for the sake of

We have studied the effects of varying the parameters of the square and circular ring unit cells, more details can be found in [11, 12], from which the design guides can be derived. Now, we shall study the effect of combining them. Although they respond to the variation of their parameters in similar ways, the square loop resonates at smaller dimensions compared to the circular ring. Consequently, for a miniaturized unit cell, the circular ring should be integrated

To visualize the effect of the spacing between the two elements, only "Rout" is varied while all the other parameters are kept constant, and the transmission coefficient is calculated for each

As "Rout" increases, the resonance frequency of the circular ring decreases, approaching that of the square ring. Thus, the band-pass between the two resonance frequencies gets narrower

brevity, only the responses obtained under TE-polarization is presented.

**Figure 9.** Parametric study of transmission coefficient for different values of Rout.

The results of the previous studies can help deliver a design guide of the combined UWB FSS as follows:

• The square loop resonates at smaller dimensions compared to the circular ring. As a result, the circular ring should be integrated into the square loop to obtain a miniaturized unit cell.


Taking into consideration the entire design guide mentioned above, the geometry of the proposed UWB stop-band FSS unit cell and its final parameters are illustrated in **Figure 10** and **Table 1**, respectively. It is worth mentioning that the above design guide can be generalized to implement UWB FSSs with combined elements of different geometries.

**Figure 11** indicates the computed reflection and transmission coefficients of the proposed FSS. These graphs prove the ability of the proposed UWB FSS to act as a stop-band filter over a wide-band from 3 to 12 GHz, which includes the entire UWB band, with a reflection magnitude of 0 dB and a transmission magnitude less than -10 dB. At the main resonance frequency, around 7 GHz, the transmission magnitude becomes less than −75 dB. The computed

reflection phase of the FSS decreases linearly with frequency and has zero value around 7

The linearity of the reflection phase of the proposed FSS is satisfied across the whole band from 2 to 14 GHz. This special feature extends the recommended applications of our proposed FSS to include a variety of systems where a linearly decreasing phase is required.

For better evaluation of the proposed design concept, a UWB planar antenna is used as a radiator. The original design of this UWB planar antenna has been proposed in [13]. It is a CPW-fed circular disc antenna printed on a dielectric substrate as shown in **Figure 12**. In here, the employed substrate is RO4003C with a dielectric constant of 3.38, a dielectric tangent loss of 0.0027, and a thickness of 0.508. The dimensions of the CPW line are w<sup>f</sup> = 3 mm and sf = 0.28, where the former is the width of the main line, the latter is the gap between the main line of the CPW and the ground plane, and 's' is the slot gap between the circular patch and

GHz, where the transmission magnitude takes its lowest value.

**Figure 11.** Reflection and transmission coefficients of the proposed FSS.

**Lout Lin Rout Rin G** 8 mm 6 mm 3.1 mm 2.5 mm 0.25 mm

Ultra-Wideband FSS-Based Antennas http://dx.doi.org/10.5772/intechopen.79888 29

**Table 1.** Final dimensions of the proposed FSS unit cell.

**5. Reflectors with UWB radiator**

the ground plane.

**Figure 10.** Structure of the proposed unit cell.


**Table 1.** Final dimensions of the proposed FSS unit cell.

**Figure 11.** Reflection and transmission coefficients of the proposed FSS.

reflection phase of the FSS decreases linearly with frequency and has zero value around 7 GHz, where the transmission magnitude takes its lowest value.

The linearity of the reflection phase of the proposed FSS is satisfied across the whole band from 2 to 14 GHz. This special feature extends the recommended applications of our proposed FSS to include a variety of systems where a linearly decreasing phase is required.

#### **5. Reflectors with UWB radiator**

**Figure 10.** Structure of the proposed unit cell.

• The lower edge of the stop-band of an FSS, consisting of square loop unit cells, is governed by the external length of the square loop and the gap width between the unit cells; hence,

• After fixing the external length along with the gap width, for a desired lower edge of the stop-band, the width of the band as well as the resonance frequency can be adjusted

• The outer radius of the circular ring controls the lower edge of its operating band but, in

• The outer diameter of the circular ring should be equal or slightly superior to that of the

• The inner radius of the circular ring can be used to set the desired overall upper frequency.

Taking into consideration the entire design guide mentioned above, the geometry of the proposed UWB stop-band FSS unit cell and its final parameters are illustrated in **Figure 10** and **Table 1**, respectively. It is worth mentioning that the above design guide can be generalized

**Figure 11** indicates the computed reflection and transmission coefficients of the proposed FSS. These graphs prove the ability of the proposed UWB FSS to act as a stop-band filter over a wide-band from 3 to 12 GHz, which includes the entire UWB band, with a reflection magnitude of 0 dB and a transmission magnitude less than -10 dB. At the main resonance frequency, around 7 GHz, the transmission magnitude becomes less than −75 dB. The computed

they should be chosen so that the desired lower edge of the stop-band is obtained.

• The two element structures can be merged to obtain a UWB band-stop response.

here, it also controls the spacing between the combined elements.

to implement UWB FSSs with combined elements of different geometries.

internal length of the square loop for them to be merged.

through the variation of the internal length.

28 UWB Technology and its Applications

For better evaluation of the proposed design concept, a UWB planar antenna is used as a radiator. The original design of this UWB planar antenna has been proposed in [13]. It is a CPW-fed circular disc antenna printed on a dielectric substrate as shown in **Figure 12**. In here, the employed substrate is RO4003C with a dielectric constant of 3.38, a dielectric tangent loss of 0.0027, and a thickness of 0.508. The dimensions of the CPW line are w<sup>f</sup> = 3 mm and sf = 0.28, where the former is the width of the main line, the latter is the gap between the main line of the CPW and the ground plane, and 's' is the slot gap between the circular patch and the ground plane.

and hence, a less amount of transmission through the FSS. Moreover, it can be noted that for stabilizing the gain, the in-phase reflection should not necessarily be at the center frequency of the UWB band and that its location depends on the scattered waves from the ends of the reflectors and the different factors that set the added phase through the distance between the

Ultra-Wideband FSS-Based Antennas http://dx.doi.org/10.5772/intechopen.79888 31

Regarding the UWB reflector, the same dimensions of its unit cells, found in **Table 1**, were kept and parametric studies were performed to choose the best values of its distance from the

First, the effect of the distance between the radiator and the UWB reflector on the operating band is studied through a parametric study of the reflection coefficient of the antenna for different values of "h2," as shown in **Figure 13a**. As expected, the FSS affects the matching band of the radiator as when "h2" increases, the bandwidth of the antenna increases. Furthermore, the influence of this parameter on the radiation behavior of the antenna is clearly illustrated in **Figure 13b** where the peak gain, across UWB band, is computed for different values of "h2." This points out that the gain changes differently over frequency, which can be explained by

As a summary, we can derive that the operating band, stability, and the value of the maximum gain of the antenna are highly dependent on how far the radiator is from the

The size of the FSS determines the overall size of the antenna. Therefore, a deep study of the effects of the installed FSS size on the provided performance was performed. The results of this study are shown in **Figure 14**. **Figure 14a** contains the reflection coefficient of the antenna for different numbers of cells (n2), which models the size of the FSS, revealing that the matching band of the antenna is mainly affected by the part of the FSS that is located directly under the radiator. In other words, when FSS dimensions exceed those of the radiator, the band-

radiator and the number of cells. The results of these studies are shown next.

the fact that the phase shift added by h2 is a function of frequency.

width of the antenna becomes independent of the FSS size.

**Figure 13.** Parametric studies of the parameter h2. (a) Reflection coefficient, (b) peak gain.

reflector.

radiator and the reflector, which is a function of frequency.

**Figure 12.** Structures of the used UWB antenna and the proposed structure.

The dimensions of the used radiator are illustrated in **Table 2**. The radiator is located at a distance "h" above the grounded FSS, complementary FSS, and UWB FSS, as indicated in **Figure 12**.

Many parameters affect the overall performance of the combined structures such as the distance between the antenna and the reflector and the number of the unit cells that constitute the latter.

The parameters of the grounded reflector include W, g and the number of cells n1 that is constituted of, along with the distance "h1" were optimized in terms of maximizing the operating band and minimizing the peak gain variation over frequency. Thus, the optimized values that give the best performance were selected, and they are given in **Table 2**. The dimensions of the complementary reflector are the same as those of grounded reflector.

The optimization of the grounded reflector parameters reveals that the best results are obtained when the length of the square patches is very small (W = 3 mm), which results in a higher resonance frequency compared with that associated with the initial value W = 5 mm


**Table 2.** Optimized values of the parameters of grounded and complementary reflectors along with the dimensions of the UWB radiator (in mm).

and hence, a less amount of transmission through the FSS. Moreover, it can be noted that for stabilizing the gain, the in-phase reflection should not necessarily be at the center frequency of the UWB band and that its location depends on the scattered waves from the ends of the reflectors and the different factors that set the added phase through the distance between the radiator and the reflector, which is a function of frequency.

Regarding the UWB reflector, the same dimensions of its unit cells, found in **Table 1**, were kept and parametric studies were performed to choose the best values of its distance from the radiator and the number of cells. The results of these studies are shown next.

First, the effect of the distance between the radiator and the UWB reflector on the operating band is studied through a parametric study of the reflection coefficient of the antenna for different values of "h2," as shown in **Figure 13a**. As expected, the FSS affects the matching band of the radiator as when "h2" increases, the bandwidth of the antenna increases. Furthermore, the influence of this parameter on the radiation behavior of the antenna is clearly illustrated in **Figure 13b** where the peak gain, across UWB band, is computed for different values of "h2." This points out that the gain changes differently over frequency, which can be explained by the fact that the phase shift added by h2 is a function of frequency.

As a summary, we can derive that the operating band, stability, and the value of the maximum gain of the antenna are highly dependent on how far the radiator is from the reflector.

**Figure 12.** Structures of the used UWB antenna and the proposed structure.

complementary reflector are the same as those of grounded reflector.

**Figure 12**.

30 UWB Technology and its Applications

the latter.

the UWB radiator (in mm).

The dimensions of the used radiator are illustrated in **Table 2**. The radiator is located at a distance "h" above the grounded FSS, complementary FSS, and UWB FSS, as indicated in

Many parameters affect the overall performance of the combined structures such as the distance between the antenna and the reflector and the number of the unit cells that constitute

The parameters of the grounded reflector include W, g and the number of cells n1 that is constituted of, along with the distance "h1" were optimized in terms of maximizing the operating band and minimizing the peak gain variation over frequency. Thus, the optimized values that give the best performance were selected, and they are given in **Table 2**. The dimensions of the

The optimization of the grounded reflector parameters reveals that the best results are obtained when the length of the square patches is very small (W = 3 mm), which results in a higher resonance frequency compared with that associated with the initial value W = 5 mm

**Table 2.** Optimized values of the parameters of grounded and complementary reflectors along with the dimensions of

**W g La = Wa R S h1 n1** 3 0.25 50 14.5 0.35 15.5 24

The size of the FSS determines the overall size of the antenna. Therefore, a deep study of the effects of the installed FSS size on the provided performance was performed. The results of this study are shown in **Figure 14**. **Figure 14a** contains the reflection coefficient of the antenna for different numbers of cells (n2), which models the size of the FSS, revealing that the matching band of the antenna is mainly affected by the part of the FSS that is located directly under the radiator. In other words, when FSS dimensions exceed those of the radiator, the bandwidth of the antenna becomes independent of the FSS size.

**Figure 13.** Parametric studies of the parameter h2. (a) Reflection coefficient, (b) peak gain.

**Figure 14.** Parametric studies of different values of number of cells n2 of UWB reflector. (a) Reflection coefficient, (b) peak gain.

The reflective behavior of the UWB FSS is computed by considering infinite FSS dimensions, which cannot be realized in practice where finite size structures are required. However, with a large number of cells, infinite dimensions can be approximated. Nonetheless, the size of the FSS affects mainly the radiation behavior of the antenna, which is shown in **Figure 14b** through a parametric study of the antenna peak gain, illustrating that as the number of cells increases, the gain also increases over the entire UWB band. A smaller reflector can be used but at the expense of the achieved gain.

Therefore, we chose the value of "h2" that gives a wide operating band and the number of cells that is associated with high gain and minimum gain variation across the achieved band. Finally, the parameters of the structure were optimized, using CST-MWS, to achieve UWB matching band with a quasi-constant gain. The final dimensions of the UWB FSS reflector are indicated in **Table 3**.

will show, in more general way, the ability of the proposed reflectors to enhance the gain over

Ultra-Wideband FSS-Based Antennas http://dx.doi.org/10.5772/intechopen.79888 33

**Figures 16–19** illustrate the final results of the simulation and measurement of the proposed designs. **Figure 16** shows the reflection coefficient of the UWB antenna mounted above the proposed reflectors, while **Figure 17** indicates the simulated peak gain over frequency, which is

**Figure 16.** Reflection coefficient of UWB antenna with and without reflectors.

the entire UWB band.

**Figure 15.** Fabricated prototypes.

#### **5.1. Numerical and experimental results**

The three proposed reflectors, with the optimized dimensions, along with the used radiator were fabricated and their photographs are shown in **Figure 15**.

In the three cases, the reflection coefficient of the UWB antenna and its peak gain and radiation patterns were computed numerically and measured.

It is worth mentioning that the peak gain is chosen for the evaluation rather than the gain at a specific direction because the radiation behavior of the used radiator is unstable, by nature, meaning that the direction of its mean radiation varies with frequency. Hence, the peak gain


**Table 3.** Final dimensions of the UWB FSS reflector used with the UWB radiator with single polarization (mm).

**Figure 15.** Fabricated prototypes.

**Figure 14.** Parametric studies of different values of number of cells n2 of UWB reflector. (a) Reflection coefficient,

The reflective behavior of the UWB FSS is computed by considering infinite FSS dimensions, which cannot be realized in practice where finite size structures are required. However, with a large number of cells, infinite dimensions can be approximated. Nonetheless, the size of the FSS affects mainly the radiation behavior of the antenna, which is shown in **Figure 14b** through a parametric study of the antenna peak gain, illustrating that as the number of cells increases, the gain also increases over the entire UWB band. A smaller reflector can be used

Therefore, we chose the value of "h2" that gives a wide operating band and the number of cells that is associated with high gain and minimum gain variation across the achieved band. Finally, the parameters of the structure were optimized, using CST-MWS, to achieve UWB matching band with a quasi-constant gain. The final dimensions of the UWB FSS reflector are

The three proposed reflectors, with the optimized dimensions, along with the used radiator

In the three cases, the reflection coefficient of the UWB antenna and its peak gain and radia-

It is worth mentioning that the peak gain is chosen for the evaluation rather than the gain at a specific direction because the radiation behavior of the used radiator is unstable, by nature, meaning that the direction of its mean radiation varies with frequency. Hence, the peak gain

**Lout Lin G n2 h2** 8 6 0.25 14 16

**Table 3.** Final dimensions of the UWB FSS reflector used with the UWB radiator with single polarization (mm).

(b) peak gain.

32 UWB Technology and its Applications

but at the expense of the achieved gain.

**5.1. Numerical and experimental results**

were fabricated and their photographs are shown in **Figure 15**.

tion patterns were computed numerically and measured.

indicated in **Table 3**.

will show, in more general way, the ability of the proposed reflectors to enhance the gain over the entire UWB band.

**Figures 16–19** illustrate the final results of the simulation and measurement of the proposed designs. **Figure 16** shows the reflection coefficient of the UWB antenna mounted above the proposed reflectors, while **Figure 17** indicates the simulated peak gain over frequency, which is

**Figure 16.** Reflection coefficient of UWB antenna with and without reflectors.

**Figure 17.** Peak gain of the UWB antenna with and without reflectors.

These results prove that the provident choice of the different dimensions gives antennas with operating band covering the FCC authorized band from 3.1 to 10.6 GHz and an average peak

Ultra-Wideband FSS-Based Antennas http://dx.doi.org/10.5772/intechopen.79888 35

The comparison between the reflection coefficient of the radiator installed at the chosen value of h2 (h2 = 16 mm) from the UWB FSS reflector and that of the radiator alone without reflector reveals that at this height the FSS affects the matching level of the antenna, especially over the lower part of the UWB band. Regarding the gain, **Figure 17** illustrates the maximum gain of UWB antenna with and without UWB FSS. It is evident that the antenna gain is enhanced across the UWB band as a result of using the UWB FSS. The amount of enhancement varies, across the band, from 6 dBi at 3 GHz to 3.5 dBi at 10 GHz, which led to a quasi-constant gain with a maximum variation in gain of 0.7 dBi. As a result, a planar UWB antenna with

It can also be noticed that the antenna above the three reflectors shows a similar behavior, except small differences, over the UWB band. The simulation and measurement results show

gain of 8.5 dBi with a maximum variation of 0.6 dBi for the first two reflectors.

**Figure 19.** Radiation patterns in H-plane (XZ) at different frequencies.

enhanced quasi-constant gain is obtained.

an acceptable agreement.

**Figure 18.** Radiation patterns in E-plane (YZ) at different frequencies.

extracted from the 3D radiation patterns calculated at each frequency. The direction of the maximum radiation can be shown clearer through the 2D radiation patterns, which will be shown later on. These figures also contain the corresponding results of the UWB antenna without reflectors to allow a clear visualization of the achieved improvement owing to the reflectors.

**Figure 19.** Radiation patterns in H-plane (XZ) at different frequencies.

extracted from the 3D radiation patterns calculated at each frequency. The direction of the maximum radiation can be shown clearer through the 2D radiation patterns, which will be shown later on. These figures also contain the corresponding results of the UWB antenna without reflectors to allow a clear visualization of the achieved improvement owing to the reflectors.

**Figure 17.** Peak gain of the UWB antenna with and without reflectors.

34 UWB Technology and its Applications

**Figure 18.** Radiation patterns in E-plane (YZ) at different frequencies.

These results prove that the provident choice of the different dimensions gives antennas with operating band covering the FCC authorized band from 3.1 to 10.6 GHz and an average peak gain of 8.5 dBi with a maximum variation of 0.6 dBi for the first two reflectors.

The comparison between the reflection coefficient of the radiator installed at the chosen value of h2 (h2 = 16 mm) from the UWB FSS reflector and that of the radiator alone without reflector reveals that at this height the FSS affects the matching level of the antenna, especially over the lower part of the UWB band. Regarding the gain, **Figure 17** illustrates the maximum gain of UWB antenna with and without UWB FSS. It is evident that the antenna gain is enhanced across the UWB band as a result of using the UWB FSS. The amount of enhancement varies, across the band, from 6 dBi at 3 GHz to 3.5 dBi at 10 GHz, which led to a quasi-constant gain with a maximum variation in gain of 0.7 dBi. As a result, a planar UWB antenna with enhanced quasi-constant gain is obtained.

It can also be noticed that the antenna above the three reflectors shows a similar behavior, except small differences, over the UWB band. The simulation and measurement results show an acceptable agreement.

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 basically because of the back radiation reduction.

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.

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

<sup>10</sup> at the lower frequency of UWB band, instead

Ultra-Wideband FSS-Based Antennas http://dx.doi.org/10.5772/intechopen.79888 37

Therefore, we set "h2" to be 10 mm, which is \_\_*<sup>λ</sup>*

antenna, (c) UWB FSS-based antenna (front view).

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 other UWB monopole antennas as radiators.

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

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