## 2 Antenna Design and Performance
### Fractal Absorber Design and Performance  
As shown in Figure 13a, the basic unit cell of the metamaterial absorber with a fractal geometry was designed to enhance the low isolation characteristics in the P-band. A metallic fractal structure and a metal ground plane are printed on opposite sides of a 3.2 mm thick FR4 substrate. This fractal-shaped patch has a Minkowski fractal geometry, and this structure has been widely studied as a metamaterial absorber [12; 13; 14; 15]. The advantage of this fractal-shaped absorber is that the resonance frequency can be easily tuned by only changing the fractal structure with a fixed substrate size [12; 13]. An array of the fractal-shaped patches is equivalent to a series \(RLC\) circuit. Here, \(R\) takes into account dielectric/ohmic  
Figure 12: Dual-polarization Tx/Rx half bow-tie dipole antenna: (**a**) geometry of antenna, (**b**) simulated S-parameters, and (**c**) simulated radiation patterns at 580 MHz when Port 1 is excited. losses, \(L\) represents the inductance of the cell due to the magnetic flux between the patch and the metal ground plane, and \(C\) represents the parasitic capacitance between the edges of adjacent patches. For a fixed substrate, the best absorption performance can be achieved by proper optimization of the \(C\) values [13]. Figure 13b shows the simulation setup used to compute the S-parameters and absorptivity of the designed basic unit cell. Perfect electric conductor and perfect magnetic conductor boundary conditions are assigned in the xy- and xz-planes, respectively. The wave port is assigned in the yz-plane. The absorptivity can be calculated as \(A(\omega)=1-\ |S_{21}(\omega)\ |^{\,2}-\ |S_{11}(\omega)\ |^{\,2}\), where \(|S_{11}(\omega)\ |\) and \(|S_{21}(\omega)\ |\) are magnitudes of the reflection and transmission coefficients, respectively [16]. However, the incident wave cannot propagate through the absorber due to the metal ground plane. Therefore, the magnitude of the transmission coefficient is zero, and the absorption can be expressed as \(A(\omega)=1-\ |S_{11}(\omega)\ |^{\,2}\). Figure 13c shows the simulated S-parameters and the absorptivity of the designed basic unit cell. Nearly perfect absorption performance around 727.66 MHz is calculated (absorptivity of 98.34%); however, the resonance frequency needs to be moved to the lower frequency side around the center frequency of the target band. The resonance frequency can be shifted to a lower band by changing the fractal-shaped patch. Figure 14a shows the structure of the optimized fractal geometry. The increased electrical length of the surface current leads to a lower resonance frequency without changing the substrate size. As shown in Figure 14b, the simulated absorption peak is formed around 602.65 MHz, with an absorptivity of 87.96%. To improve the isolation characteristics between the Tx and Rx antennas, the metamaterial absorber should  
Figure 13: Basic unit cell of fractal absorber: (**a**) geometry of basic unit cell, (**b**) simulation setup, and (**c**) simulated S-parameters and absorptivity. be positioned diagonally in the proposed antenna structure. Therefore, we investigated the absorptivity performances depending on the incident angle, as shown in Figure 14c. When the wave is incident at an angle of 45\({}^{\circ}\), a simulated absorptivity of 63.44% is achieved at 608.15 MHz. Figure 15a shows a panel structure of the proposed metamaterial absorber. The panel consists of three layers, each of which is an 8 \(\times\) 1 array of the optimized fractal-shaped patches. The proposed absorber consists of two back-to-back panels with a space of 3 mm between the panels. Figure 15b shows the structure in which the Tx/Rx antennas and the proposed absorber are combined. Figure 15c shows the simulated E-field distributions at 580 MHz for various absorber layers when Port 1 is excited. In the case without an absorber, the radiated wave propagates directly to the opposite antenna; this is the main reason for the low isolation characteristics.