# Compact Design of Annular-Microstrip-Fed mmW Antenna Arrays
## 3 Improvement on Multi-Directional Design
### Dual Annular-Microstrip-Fed Antenna Array  
This structure is shown in Figure 15 as the third design. The main purpose of this design is obtaining the characteristic of more uniform multi-direction radiation. In the first two designs, we can find that the edge feeding mode will generally cause the current on the non-main feeding units which are not connected with the feeding point is too small, which leads to the problem of inhomogeneous circumferential radiation. Aiming at solving this disadvantage, our researchers try to optimize the surface current distribution of the arrays to be uniform. In this case, in order to attain the characteristic of multi-directional radiation, it is also important to maintain more directional linear arrays. Our thought is to achieve this goal under the same edge feeding mode. Initially, we attempt to use the simple cruciform microstrip lines to interconnect the four linear arrays. However, because the length of microstrip lines in this situation is hard to be designed precisely enough, the impedance matching cannot achieve a favorable consequence. So, compared with pitchfork-shaped microstrip antenna array, in this design, we employ dual (inner and outer contour) annular microstrip lines to interconnect the four directional linear arrays with \(1\times 3\) rectangular patches. The inner annular microstrip line will connect the other end of the four branches which effectively make the current distribution on the four branches tend to be uniform. The design method of the inner annular microstrip line is the same as it of the outer annular microstrip line which means the length of the inner annular microstrip line between every two branches should also be the integral multiple of the half-wavelength of the central frequency. What is more, in order to ensure normal current feeding of the inner annular microstrip line, the width of the inner annular microstrip line has to be the same as other adjacent microstrip lines.  
After analyzing and optimizing, we determine the values of the parameters which are shown in Table 3. The meanings of the parameters are shown in Figure 15.  
The gain and the reflection coefficient of dual-annular microstrip-fed antenna array of microstrip antenna array is shown in Figure 16.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Parameter** & \(\mathbf{d_{7}}\), \(\mathbf{d_{9}}\) & \(\mathbf{d_{8}}\) & \(\mathbf{W_{z}}\) & \(\mathbf{R_{4}}\) & \(\mathbf{L_{10}}\) & \(\mathbf{L_{11}}\) & \(\mathbf{L_{12}}\) & \(\mathbf{L_{z}}\) & \(\mathbf{R_{5}}\) \\ \hline
**value (mm)** & 0.130 & 0.130 & 1.050 & 16.180 & 0.580 & 0.830 & 2.580 & 0.780 & 1.590 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Parameters of dual-annular microstrip-fed antenna array.  
Figure 15: The dual-annular microstrip-fed antenna array, (**a**) top view, (**b**) side view.  
The results show that the maximum gain of dual-annular microstrip-fed antenna array of microstrip antenna array is 10.4 dBi at 59.0 GHz. In the frequency band of 59.0 GHz-61.0 GHz, this antenna can remain a relatively high gain. Besides, the minimum reflection coefficient of it is \(-\)21.58 dB at 60.9 GHz and the reflection coefficient near the frequency of 58.97 GHz can also attain below \(-\)10 dB.  
For further confirmation, we depict the \(x\)-\(z\) plane, \(y\)-\(z\) plane, and \(x\)-\(y\) plane of the radiation pattern of dual-annular microstrip-fed antenna array in Figure 17.