**1. Introduction**

The antenna is an important aspect of any wireless system. It ensures the transmission/reception of the signals and can be designed to comply with the systems' requirements, especially those with imposed regulations that must be respected such as UWB systems. Antennas are the components of the wireless communication system that are responsible for shaping and launching the emission as well as receiving the incoming radiation. As a result, the system's antennas control its coverage range and area. As the antenna directivity gets increased, its coverage area gets narrower, which may not be convenient for omnidirectional applications. Meanwhile, many real-world environments cause the distortion of the omnidirectional radiation and make it more exposed to interference with the surroundings. Hence, in these cases, the directional radiation becomes preferable, especially when it ensures a wiser use of the radiated power.

An FSS of metallic patches, like the one indicated in **Figure 1**(**a**), can effectively suppress surface waves because currents cannot travel across the gaps between the patches. Over the frequency range where it prohibits surface waves, it is partially reflective. It is only at very high frequencies, when the effective capacitors between neighboring plates behave as shorts, that surface waves can propagate. Hence, this FSS transmits low frequencies while reflecting

Ultra-Wideband FSS-Based Antennas http://dx.doi.org/10.5772/intechopen.79888 17

The complementary geometry of this capacitive FSS is the inductive one that consists of an array of square slots as shown in **Figure 1**(**b**). Since the inductive structure represents the complementary of the capacitive one, it has complementary transmission spectra. Thus, the

For the inductive surface, the waves that are short compared to the diameter of the holes will easily fit through the mesh, while longer waves will see the sheet as continuous metal. Therefore, the sheet of metal islands transmits long wavelengths while reflecting short wavelengths. At low frequencies, where it can prevent the propagation of surface waves, the capacitive sheet is not completely reflective. Conversely, while the inductive sheet is reflective at low frequencies, surface waves can propagate easily along the continuous metal wires.

If a ground plane is added to a capacitive FSS, the structure will become completely reflective, and it will own the favorable reflection phase properties of high-impedance surfaces, but the propagation of surface waves will still be permitted. It is only when both the ground plane and the vias are included that the important properties of high-impedance surfaces, namely, in-phase, 100% reflection, and suppression of surface current propagation, are obtained [1].

Free-standing doubly periodic arrays of metallic elements were studied for many years in the context of FSS and their behavior is well understood. The incident polarization is assumed to be suitable to excite the metallic elements, meaning that in the case of linear dipole elements,

latter transmits high frequencies while reflecting low frequencies.

**Figure 1.** Complementary surfaces. (a) Capacitive surface, (b) inductive surface.

**2.1. FSS in proximity to a ground plane**

high frequencies.

Appropriately designed UWB reflectors can bring directionality to existing UWB omnidirectional antennas, as well as shielding them from nearby metallic objects that would otherwise destroy their performance, providing them with the suitability for a variety of applications. It is obvious that planar metallic reflectors cannot provide these advantages over an ultra-wide bandwidth due to their out-of-phase reflection.

Developments in periodic structures have led to the development of planar surfaces that have, among other characteristics, the possibility to act as a perfect magnetic conductor (PMC) with in-phase reflection over a narrow bandwidth. The insertion of such surfaces enhances impedance matching, hence improving the efficiency of some antennas (e.g., printed planar antennas) when they have to be installed close to conducting surfaces, and creates a unidirectional radiation. These structures can be seen as a combination of frequency selective surfaces (FSSs) and conventional metallic reflectors. FSSs have helped to solve some crucial challenges in various fields and they have been proposed as UWB reflectors.

Furthermore, by proposing UWB FSSs, the applications of these structures can be extended to include UWB communication systems and radars or to enhance the performance of UWB components such as antennas, where UWB FSSs can be used as UWB reflectors to increase their gain and minimize their back radiation and create pattern diversity.

This chapter is a compendium of FSSs in antenna engineering that gives an introduction to how FSSs have been used in antenna fields and their potentials that can serve our purposes. Furthermore, it illustrates the principal physical concepts that can be used to explain the interaction between antennas and FSSs. Then, it itemizes the designs of the proposed FSS-based antennas using the concepts introduced in the first part.

### **2. Relations between FSSs and well-known structures**

Some well-known structures, such as high impedance surfaces (HIS), can be considered as evolved versions of FSSs, whereas these structures combine FSSs with metallic ground planes and metallic pins (vias). Hence, they can provide two important characteristics, namely, artificial magnetic conductor (AMC) and electromagnetic band gap (EBG) simultaneously.

An FSS of metallic patches, like the one indicated in **Figure 1**(**a**), can effectively suppress surface waves because currents cannot travel across the gaps between the patches. Over the frequency range where it prohibits surface waves, it is partially reflective. It is only at very high frequencies, when the effective capacitors between neighboring plates behave as shorts, that surface waves can propagate. Hence, this FSS transmits low frequencies while reflecting high frequencies.

The complementary geometry of this capacitive FSS is the inductive one that consists of an array of square slots as shown in **Figure 1**(**b**). Since the inductive structure represents the complementary of the capacitive one, it has complementary transmission spectra. Thus, the latter transmits high frequencies while reflecting low frequencies.

For the inductive surface, the waves that are short compared to the diameter of the holes will easily fit through the mesh, while longer waves will see the sheet as continuous metal. Therefore, the sheet of metal islands transmits long wavelengths while reflecting short wavelengths. At low frequencies, where it can prevent the propagation of surface waves, the capacitive sheet is not completely reflective. Conversely, while the inductive sheet is reflective at low frequencies, surface waves can propagate easily along the continuous metal wires.

If a ground plane is added to a capacitive FSS, the structure will become completely reflective, and it will own the favorable reflection phase properties of high-impedance surfaces, but the propagation of surface waves will still be permitted. It is only when both the ground plane and the vias are included that the important properties of high-impedance surfaces, namely, in-phase, 100% reflection, and suppression of surface current propagation, are obtained [1].

#### **2.1. FSS in proximity to a ground plane**

**1. Introduction**

16 UWB Technology and its Applications

bandwidth due to their out-of-phase reflection.

various fields and they have been proposed as UWB reflectors.

antennas using the concepts introduced in the first part.

**2. Relations between FSSs and well-known structures**

their gain and minimize their back radiation and create pattern diversity.

The antenna is an important aspect of any wireless system. It ensures the transmission/reception of the signals and can be designed to comply with the systems' requirements, especially those with imposed regulations that must be respected such as UWB systems. Antennas are the components of the wireless communication system that are responsible for shaping and launching the emission as well as receiving the incoming radiation. As a result, the system's antennas control its coverage range and area. As the antenna directivity gets increased, its coverage area gets narrower, which may not be convenient for omnidirectional applications. Meanwhile, many real-world environments cause the distortion of the omnidirectional radiation and make it more exposed to interference with the surroundings. Hence, in these cases, the directional radiation becomes preferable, especially when it ensures a wiser use of the radiated power.

Appropriately designed UWB reflectors can bring directionality to existing UWB omnidirectional antennas, as well as shielding them from nearby metallic objects that would otherwise destroy their performance, providing them with the suitability for a variety of applications. It is obvious that planar metallic reflectors cannot provide these advantages over an ultra-wide

Developments in periodic structures have led to the development of planar surfaces that have, among other characteristics, the possibility to act as a perfect magnetic conductor (PMC) with in-phase reflection over a narrow bandwidth. The insertion of such surfaces enhances impedance matching, hence improving the efficiency of some antennas (e.g., printed planar antennas) when they have to be installed close to conducting surfaces, and creates a unidirectional radiation. These structures can be seen as a combination of frequency selective surfaces (FSSs) and conventional metallic reflectors. FSSs have helped to solve some crucial challenges in

Furthermore, by proposing UWB FSSs, the applications of these structures can be extended to include UWB communication systems and radars or to enhance the performance of UWB components such as antennas, where UWB FSSs can be used as UWB reflectors to increase

This chapter is a compendium of FSSs in antenna engineering that gives an introduction to how FSSs have been used in antenna fields and their potentials that can serve our purposes. Furthermore, it illustrates the principal physical concepts that can be used to explain the interaction between antennas and FSSs. Then, it itemizes the designs of the proposed FSS-based

Some well-known structures, such as high impedance surfaces (HIS), can be considered as evolved versions of FSSs, whereas these structures combine FSSs with metallic ground planes and metallic pins (vias). Hence, they can provide two important characteristics, namely, artificial magnetic conductor (AMC) and electromagnetic band gap (EBG) simultaneously.

Free-standing doubly periodic arrays of metallic elements were studied for many years in the context of FSS and their behavior is well understood. The incident polarization is assumed to be suitable to excite the metallic elements, meaning that in the case of linear dipole elements,

**Figure 1.** Complementary surfaces. (a) Capacitive surface, (b) inductive surface.

the electric field should have a component parallel to the direction of the dipoles. It is well known that at the resonant frequency of the array, the latter performs as a fully metalized screen and the incident waves are fully reflected with a phase reversal. Moreover, at resonance, the current is in phase with the incident field, i.e., the impedance seen by the incident wave is purely real, since the capacitive and inductive parts cancel each other. Also, a maximum current magnitude is excited on the elements.

**3. FSSs in antenna engineering**

**3.1. FSSs as reflectors and ground planes**

suitable only for a relatively limited frequency range.

frequency, away from the antenna's surface.

superstrates and as reflectors.

FSSs' valuable features emphasized through the analysis above have encouraged their use in antenna engineering to improve antenna performance and create further properties that would not be achievable otherwise. They have been used, to widen the operating band of backing reflectors and to enhance the performance of broadband reconfigurable antennas, as

Ultra-Wideband FSS-Based Antennas http://dx.doi.org/10.5772/intechopen.79888 19

Extending the bandwidth of backing reflectors is among the rich utilizations of FSSs. In [4], an FSS is sandwiched between a tightly coupled array and a metallic plane, providing an additional reflecting plane for a higher frequency band. In this way, the metallic ground plane will operate at lower frequencies and the FSS will cover higher frequencies, which leads to an extended bandwidth, while the location of the metallic plane without an FSS would be

Placement of the metallic plane at a quarter wavelength distance from the antenna allows obtaining a good matching with only modest degradation of the achievable gain, but the improvement of the front-to-back ratio will come at the expense of the antenna bandwidth. The targeted application in [4] forces the integration of two frequency bands: one corresponding to the typical radar X-band, 8.50–10.50 GHz, and the other corresponding to a Tactical Common Data Link (TCDL) system, 14.40–15.35 GHz. Therefore, the used FSS was designed to be reflective at the higher frequency range and to be practically transparent for the lower band where the metallic ground plane is in charge of the reflection. More importantly, the FSS should separate the two frequency bands. Therefore, a special FSS has been chosen to serve the design purposes. The chosen element exhibits a good performance against angular varia-

In [5], a novel FSS design aimed at enhancing the performance of a broadband reconfigurable antenna has been presented. Designing FSSs' subject to phase requirements was also elaborated, revealing that some compromise, in the response magnitude, should be made to achieve the desired phase requirements. The broadband requirements also presented the need for noncommensurate FSS designs, contrary to previous FSSs that were primarily designed on the basis of the reflection coefficient amplitude and were intended for radome applications rather than substrates. When traditional broadband antennas such as log-periodic are printed on substrates, their bandwidth characteristics are altered, and one approach to regain the broadband behavior of the antenna element is to employ frequency-dependent substrates or ground planes (GPs). From here comes the suggestion of using FSSs to create substrates on which broadband antennas can be printed without affecting their broadband behavior. This can be achieved by using multiple layer FSSs as part of the substrate in a similar manner to that used for designing broadband microwave filters. Each screen is resonant at a given frequency and is placed at a distance, of a quarter of the wavelength at the screen's resonance

tion and allows a packed lattice, with a further gain in angular independence.

For periodic arrays in proximity to a ground plane, some differences emerge. Due to the ground plane, incident waves are fully reflected at all frequencies. However, in this type of structures, careful investigation reveals that two distinct resonant phenomena occur for a normally incident wave. By assuming a free-standing array in proximity to an all-metal ground plane illuminated by a normally incident wave, the array resonance can be defined at the frequency where the currents excited on the array are in phase with the incident wave. At this frequency, the incident wave is reflected from the periodic array with a phase reverse, as in the case of the free-standing array resonance. However, it can be found that there also occurs a Fabry-Perot type of resonance at the cavity formed between the ground plane and the array. The Fabry-Perot resonance occurs at frequencies different from the array resonance. This strong cavity-type resonance excites maximum currents on the elements (which in general are out of phase with the incident wave), and the incident wave is reflected with a zero phase shift [2].

#### **2.2. AMC resonance cavity**

The presence of vias in a mushroom-type structure imposes an EBG at the same frequency range as the AMC property. In other words, the mushroom structure exhibits high surface impedance for both normally incident and surface waves at the same frequency band. Hence, at the same frequency, it reflects a normally incident plane wave with zero phase shift behaving, therefore, as an AMC and supports no surface waves behaving, therefore, as an EBG. It was demonstrated that the AMC operation is not directly related to the resonance of the FSS array. In fact, it is noticed that with varying array periodicity, the AMC band moves oppositely to the FSS resonance frequency. On the contrary, it is shown that the EBG frequency follows the trend of the FSS resonance [2].

#### **2.3. Complementary arrays**

It can be shown that the specular reflection coefficient for one array equals the transmission coefficient for its complementary array. This is a simple case of the general "Babinet's principle." Based on this observation, it is often expected that the investigation of one of the two cases is enough. However, this is in general not the case. First of all, the conducting screen must be a perfect conductor and "infinitely" thin, typically less than 1/1000 wavelength. If the screen is thick, the bandwidth of the dipole array will be larger while the bandwidth of the slot array will be smaller. Furthermore, if a thin layer of dielectric is added, the resonant frequency will be lowered somewhat for both the dipole and the slot arrays, but for a dielectric thickness of the order of λ/4 or more, the two cases behave vastly different [3].
