# Multi-Band Array Antenna Sharing a Common Aperture with Heterogeneous Array Elements
## 2 Unit-Cell Design for the Multi-Band Array Antenna  
The geometries of the proposed multi-band array antenna are illustrated in Figure 1. The proposed multi-band array antenna is composed of one printed dipole antenna for S-band and nine E-shaped patch antennas for the X-band. They are perpendicularly mounted to each other. The radiating part of the antenna is made of 1 oz copper with a conductivity of 5.87 \(\times\) 10\({}^{7}\) S/m and is printed on dielectric substrates. The RF-35 substrate from Taconic (\(\epsilon_{\rm r}\) = 3.5, tan \(\delta\) = 0.0018, d\({}_{1}\) = 0.8 mm) is used for the S-band printed dipole antenna, and the TLY-5 substrate from Taconic (\(\epsilon_{\rm r}\) = 2.2, tan \(\delta\) = 0.0009, d\({}_{2}\) = 3.08 mm) is used for the X-band E-shaped antenna. To improve the bandwidth property, a coupled-fed structure is employed to the S-band antenna, and two parallel slots are used in the X-band antennas. The S-band antenna contains a folded arm structure for wide beamwidth and reduction of the antenna size. The length of the printed dipole's arm is represented by \(w_{1}\) and \(l_{1}\), and the dipole is fed from the bent strip line on the opposite surface of the substrate. The induced currents of the S-band printed dipole antenna flow in the \(z\)- and \(y\)-directions. In contrast, the induced currents of the X-band E-shaped antenna flow in the \(x\)-direction, perpendicular to the S-band antenna. Since these current directions are orthogonal to each other, the mutual coupling interference can effectively be diminished. Moreover, to decrease the mutual coupling interference among the X-band components, we installed cavities using multiple via surrounding the X-band components. The geometrical parameters are optimized using the FEKO electromagnetic (EM) simulation software tool [16], as listed in Table 1.  
To examine the design performance, the multi-band array antenna is implemented and measured in a full anechoic chamber. Figure 2a,b presents the side views of the fabricated printed dipole antenna for the S-band; the S-band element has a folded arm and a coupled-fed structure. Figure 2c illustrates the top view of the fabricated E-shaped antennas for X-band. Extended via cavity structures are employed around the E-shaped antennas in the X-band, as shown in Figure 2c. Figure 2d presents the assembled antenna unit-cell, where the different band layers are firmly combined utilizing a glue-type adhesive. The S-band and X-band antennas are sharing the ground. In addition, the cavity of the X-band antenna and the arms of the S-band antenna are electrically connected, as shown in Figure 2e. Since the unit-cell has one S-band printed dipole and nine X-band antennas, it has totally ten  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameters** & **Values (mm)** & **Parameters** & **Values (mm)** \\ \hline \(w_{0}\) & 37 & \(l_{0}\) & 29 \\ \(w_{1}\) & 16.75 & \(l_{1}\) & 14 \\ \(w_{2}\) & 5 & \(l_{2}\) & 9.5 \\ \(w_{3}\) & 6.75 & \(l_{3}\) & 10.5 \\ \(w_{4}\) & 16 & \(l_{4}\) & 15 \\ \(w_{5}\) & 8 & \(l_{5}\) &SMA connectors that are directly connected to the transmitting receiver modules (TRMs) for digital beamforming. Since MFR radar systems using digital beamforming need a shock-stable feeding network, the co-planar waveguide (CPW) feeding network interface with high-durability [17] is considered to be employed in future work. Figure 3 depicts the test setup of the fabricated antenna under test (AUT) with a Styrofoam zig in the full anechoic chamber. The active element patterns and reflections for all the 10 unit-cell elements are completely measured. Figure 4 represents the bore-sight gains and reflection co-efficients of the implemented proposed antenna. While the solid and dashed line illustrate the measured and simulated reflection co-efficient, the 'x' marks and dotted line denote the measured and simulated bore-sight gains, where the number of the measured points are 10 in S-band and 31 in X-band.