M1 M2

Figure 8.88 Microstrip antennas separated by the mushroom-type EBG ([68a], copyright C 2007 IEEE).

antennas can be suppressed by locating them over the EBG structure, if the antennas operate within the EBG bandgap frequency band [68a]. For an array of two antennas, for instance, the EBG substrate is placed between two antennas to allow arraying them with close separation as shown in Figure 8.88 [68a]. The EBG structure, in this example consisting of four columns of cells (patches), is inserted between two patches. The two patches have the same size of 7 × 4 mm and are placed at a distance of 38.8 mm (0.75λ at 5.8 GHz). The substrate thickness of the system is 2 mm, and εr is 10.2. Here, the cell (mushroom) size of the EBG structure is considered for three cases, 2, 3, and 4 mm, the gap between cells is 0.5 mm, and the via radius is 0.3 mm. Figure 8.89(a) provides the return loss S11 and (b) gives S21, corresponding to the mutual coupling. In each figure, comparisons between cases with and without the EBG structure are shown. All the antennas resonate around 5.8 GHz and show better than –10 dB matches, although the EBG substrate may somewhat affect the impedance match of the antennas. Without the EBG structure, the antennas show strong mutual coupling of –16.15 dB, whereas the mutual coupling level changes with the EBG structure’s presence. When the cell size is 2 mm, since the EBG bandgap is higher than the resonance frequency, the mutual coupling is not reduced, leaving still the high level of –15.85 dB. In case of 3-mm cell size, the resonance frequency 5.8 GHz falls inside the EBG bandgap so that the surface waves are suppressed and the mutual coupling is greatly reduced to the level of –25.03 dB. When the cell size is increased to 4 mm, the EBG bandgap becomes lower than the resonance frequency. Thus the mutual coupling is not improved, leaving the level as strong as –16.27 dB.

8.2.4Application of DGS (Defected Ground Surface)

The concept of DGS emerged from studies of PBG structure in electromagnetics, now being referred to as the EBG structure [73]. An EBG substrate is implemented on the ground plane surface to achieve the stopband property over a frequency range and thus a “defected” ground plane surface (DGS) is created. The DGS structure is constituted of modified EBG structures, having some compact geometries such as a unit cell as a single defect, or in periodic configuration with a small period number on the ground plane, which provides stopbands and slow-wave nature over a frequency range with a different manner of EBG. Hence, the DGS can be referred to as a unit cell

(e)(f)

Figure 8.90 Some examples of DGS geometries: (a) ring (b) meander line (c) triangle

(d) dumbbell (e) spiral H-shape and (f) U-shape.

Figure 8.91 Concentric ring (CR) DGS: (a) configuration no. 1 and (b) configuration no. 2 ([74], copyright C 2006 IEEE).

is similar to #1 but with a small circle defect at the center. The DGS circles are etched on the ground plane and are shown by dark color in the figure. A pair of dotted lines represents a microstrip line etched on the reverse side of the substrate. The substrate εr is 2.33 and the thickness is 1.575 mm. The dimensions of the DGS are: 2e = 0.1λg = 2 mm, h = 0.08λ, and s = 3e, where the operating frequency fg = 10 GHz. The width of the microstrip line w = 4.7 mm. Figure 8.92 shows measured and simulated transmission characteristics S21 of DGS #1, comparing measured S21 with and without DGS. It shows the stopband effect of DGS by S21 < –10 dB over the frequency range of 9 to 11 GHz. With the DGS #2, an almost identical result was obtained.

As a modification of these DGSs, a case where the DGS structure is backed by a secondary ground plane, a metal plate, is studied, and the performance measured. In

Figure 8.93 Multiband printed monopole antenna with a DGS ([75], copyright C 2008 IEEE).

Jsur f [A /m]

1. 6553e+003

3. 0618e+002

1. 5081e+002

7.4287e+001

3.6592e+001

1.8024e+001

8.8782e+000

4.3732e+000

2.1541e+000

1.0611e+000

5.2265e-001

2.5744e-001

1.2681e-001

6. 2463e-002

3. 0768e-002

1. 5155e02

7. 4651e03

Figure 8.94 Simulated surface current on the conductors of the multiband monopole antenna on the DGS at 2.7 GHz ([75], copyright C 2008 IEEE).

Employing the DGS structure by inserting an L-slot on the ground plane, two additional resonances around 3 GHz result, while retaining the original high-frequency resonances around 5.6 and 8 GHz. Change in the surface current distributions on the ground plane due to the L-shaped slot can be observed in Figure 8.94. The L-slot forces current on the left-side of the ground plane to wrap around the slot, while the current