Figure 8.82 The FSS unit cell ([66], copyright C 2010 IEEE).

impedance bandwidth was obtained, for VSWR < 2, from 2.75 to 8.35 GHz. The radiation pattern was stable and unidirectional, and gain over 6 dB within the frequency band of 2.75 to 6.0 GHz was obtained.

Another example is an FSS with a periodical array of cells, having a pattern that is a combination of a Jerusalem cross and a three-step fractal patch shown in Figure 8.82 [66]. The low-profile monopole antenna on the HIS ground plane with the substrate thickness of 0.07λ was able to obtain an enhanced gain of 7.73 dB from 5.35 dB.

8.2.3Applications of EBG (Electromagnetic Band Gap)

8.2.3.1Miniaturization

A probe-fed patch antenna backed by a mushroom-type EBG substrate is designed, and it demonstrates miniaturization that achieved 66.83% [67]. Miniaturization is achieved by employing the property of EBG structures, which supports slow wave propagation as its first propagating mode, having a longer effective wavelength than those in free space and dielectrics. The EBG structure, over which a patch antenna will be embedded, is designed to operate in its slow wave region in order to achieve a lower frequency operation of the patch antenna for the size reduction. Here the operating frequency of

2.4GHz is used, to which the size of a conventional patch antenna is designed to be

31.9mm in length and 40.9 mm in width. A proposed patch antenna embedded on the mushroom-type EBG surface is illustrated in Figure 8.83, in which a unit cell is shown by its top and side views along with dimensional parameters. The number of cells is designed to have slightly larger area than the patch, so a square surface with 4 × 4 cells to cover the patch is selected. In the initial design, the unit cell size a is chosen to be

5.5 mm, based on the wavelength of slow wave mode λslow = 34.5 mm. Other parameters are Dvia = 0.8, g = 0.25, h1 = 1.524, and h2 = 0.762. The dielectric constant of the substrate is 3.66. By doing optimization, starting from the initial values with λslow/2, the patch size, L (length) × W (width), respectively, is determined to be 18.3 × 17.4 mm. The reduction of the patch area from the area of a conventional patch is estimated to be 66.83% in its ratio, and 49.58% even when the area of the EBG surface is included.

Figure 8.85 Planer bow-tie antenna closely placed over the EBG ground plane ([71], copyrightC 2008 IEEE).

1.7GHz to 2.5 GHz, while measured and simulated bandwidths are 38% and 41%, respectively. Since the antenna is a balanced type, a balun is necessary for avoiding interference of unbalanced current produced on the feeding cable without a balun. By using a wideband balun, the antenna was shown to obtain wider bandwidth as compared with a case without a balun. The dipole is made of two metal strips with 1-mm width and 45.7-mm length, and is printed on the substrate with εr = 4.5, having the size of 94 × 94 mm. The EBG structure is composed with 6 × 8 cells, each cell (mushroom) having the size of 12.4 mm in width, and 6.0 mm in height, being arrayed with gaps of 0.4 mm. The bandwidth of 1.6–2.5 GHz covers frequency bands of several wireless systems such as DCS (Digital Communication Systems: 1.71–1.88 GHz), GSM (Global Systems for Mobile Communication: 1.85–1.99 GHz), PCS (Personal Communication Systems: 1.85–1.99 GHz), UMTS (Universal Mobile Telecommunication System: 1.92–2.17 GHz), and WLAN (Wireless Local Network: 2.4–2.485 GHz).

8.2.3.3Enhancement of bandwidth

With the close spacing of a thin dipole and its image element, currents of these elements are in phase, leading to decrease in the reactive energy surrounding the dipole.

Consequentially, the operating bandwidth increases. It was shown that the EBG ground plane requires a reflection phase in the range of 90◦± 45◦ for a low-profile straight wire

dipole antenna to exhibit a good return loss [70] and the design of the EBG ground plane follows to meet this requirement at a desired operating frequency. A dipole is designed to operate at the same frequency, with consideration to placing it closely over the EBG surface.

A dipole antenna introduced previously in [69] was designed by following the above design concept and it demonstrated a fairly wideband performance covering 1.7 GHz–

2.5GHz with center frequency of about 2.0 GHz.

A dipole antenna embedded over the EBG ground plane designed to exhibit about 1.4:1 impedance bandwidth is shown in [71]. In the same way, a bow-tie dipole antenna over the EBG ground plane is described to show much greater bandwidth. Figure 8.85 depicts the antenna placed over the EBG ground plane. The bow-tie dipole antenna has the thickness of 0.01λ, an overall length of 0.30λ, and an overall width of 0.26λ at 300 MHz. It is located 0.02λ over the EBG ground plane, which is composed of

Figure 8.86 A folded bow-tie dipole over the EBG surface ([71], copyright C 2008 IEEE).

Figure 8.87 A diamond dipole backed by the EBG ([72], copyright C 2007 IEEE).

8 × 8 cells (mushrooms), with each cell having the size of 0.12λ, via radius of 0.005λ, gap between cells of 0.02λ, and the substrate (εr = 2.2) thickness of 0.04λ at a frequency of 300 MHz. To obtain good matching for a wide bandwidth, the antenna is modified to a folded structure, with both edges of the bow-tie element folded by a narrow strip over the bow-tie element as shown in Figure 8.86. With this folded bow-tie structure, the –10 dB return loss bandwidth spanned a frequency range from 306 MHz through 419.5 MHz.

Other types of antennas than straight wire and bow-tie can be useful. A square-patch dipole, called a diamond dipole, shown in Figure 8.87 [72] is embedded on the EBG surface and achieved a wide return-loss bandwidth of 1.4:1 (33%) taking the radiation pattern into consideration. Also a sleeve dipole over the EBG is treated similarly and shown to have a bandwidth of 1.28:1 (26%) [72].

8.2.3.4Reduction of mutual coupling

Since the EBG structure has a feature of suppressing the surface wave propagation on the EBG surface because of its bandgap property, the mutual coupling between two