# Compact Design of Annular-Microstrip-Fed mmW Antenna Arrays
## 3 Improvement on Multi-Directional Design
### Bridged Annular-Microstrip-Fed Antenna Array  
This structure is shown in Figure 18. This design based on a bridged annular microstrip feeding line for the same four directional linear arrays with 1 \(\times\) 3 rectangular patches as above structure. It proposes another solution for acquiring the characteristics of more uniform multi-directional radiation. The difference from the above three structures is that this structure uses central feeding mode. In addition to the outer microstrip line, this structure also contains mutually perpendicular straight internal microstrip lines with the same quantity as the array branches. One end of each internal microstrip line is connected to the ring midpoint of the connection point between the adjacent array unit and the outer ring microstrip line and the other end is at the center of overall structure. The internal microstrip lines are designed to match the central feeding mode which means each internal microstrip line should also implement the input impedance of 200 \(\Omega\). The length of each internal microstrip line should also be the integral multiple of the half-wavelength of the central frequency. With the central feeding mode, the structure has a high symmetry.  
Figure 16: Simulation results of dual-annular microstrip-fed antenna array, (**a**) gain of this antenna array, (**b**) reflection coefficient of this antenna array.  
Figure 17: Radiation pattern of dual-annular microstrip-fed antenna array (**a**) \(x\)-\(z\) plane, (**b**) \(y\)-\(z\) plane, (**c**) \(x\)-\(y\) plane.  
In order to gain a better simulation result, we set all the patches, microstrip line, and the mental ground of bottom layer as copper with the thickness of 0.006 mm which are different from the above three structures. However, the structure and material of the dielectric substrate are the same.  
Because of the difference of the feeding mode, some structural parameters will have a relatively larger adjustment. The values of the parameters are shown in Table 4. The meanings of the parameters are shown in Figure 18.  
The gain and the reflection coefficient of bridged annular-microstrip-fed antenna array is shown in Figure 19.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Value (mm)** & **Parameter** & **Value (mm)** \\ \hline R\({}_{7}\) & 19.21 & \(W_{z}\) & 1.050 \\ \(R_{6}\) & 18.95 & \(L_{z}\) & 0.780 \\ \(d_{10}\) & 0.226 & \(L_{13}\) & 0.830 \\ \(d_{11}\) & 0.130 & \(L_{14}\) & 2.580 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Parameters of bridged annular-microstrip-fed antenna array.  
Figure 19: Simulation results of bridged annular-microstrip-fed antenna array, (**a**) gain of this antenna array, (**b**) reflection coefficient of this antenna array.  
Figure 18: Bridged annular-microstrip-fed antenna array, (**a**), top view, (**b**), side view.  
The maximum gain of bridged annular-microstrip-fed antenna array of microstrip antenna array is 10.7 dBi at 59.0 GHz and the minimum reflection coefficient of it is \(-\)17.1 dB at 59.1 GHz. Our designs reach a good impedance matching.  
We depict the \(x\)-\(z\) plane, \(y\)-\(z\) plane, and \(x\)-\(y\) plane of the radiation pattern of bridged annular-microstrip-fed antenna array in Figure 18. With relatively uniform multi-directional radiation \(H\)-plane patterns shown in Figures 17 and 20, the latter two designs are multi-directional radiation antennas and have wide radiation ranges which achieve our goal.