# Design Optimization of Reconfigurable Liquid Crystal Patch Antenna
## 3 Results and Discussion
### Liquid Crystal Patch Antenna Performance Enhancement Technique  
Next, we propose an idea for maximizing the previously analyzed frequency change rate and securing the bandwidth of the patch antenna. First of all, when fabricating a patch antenna based on a liquid crystal, an alignment layer must be laid on the upper and lower surfaces of the liquid crystal to secure the dielectric anisotropy of the liquid crystal sufficiently. At this time, the alignment force of the alignment layer is rapidly weakened at a thickness of 100 \(\upmu\)m or more [8]. Therefore, it does not make much sense to use a liquid crystal layer with a thickness of 100 \(\upmu\)m or more when manufacturing a liquid crystal patch antenna.  
On the other hand, in terms of antenna radiation performance, since the frequency bandwidth increases as the thickness of the substrate increases, it is advantageous to make the thickness of the patch antenna substrate as thick as possible unless unwanted modes occur. Since a patch antenna operating in the several GHz bands can secure a reasonable bandwidth when using a substrate with a thickness of several hundred \(\upmu\)m, this paper proposes a method to increase the thickness of the antenna substrate while maintaining the alignment force. As shown in Figure 10a, this is a method to increase the thickness of the entire substrate by additionally inserting an alignment layer for each 100 \(\upmu\)m-thick liquid crystal layer. In this case, the thickness of the liquid crystal layer does not exceed 100 \(\upmu\)m, and the overall thickness of the antenna substrate can be increased. However, as previously analyzed, since the substrate used as the alignment layer is additionally inserted, the ratio of the thickness occupied by the liquid crystal layer among the thicknesses of the entire substrate is reduced.  
Figure 10b shows a graph comparing the frequency bandwidth and frequency variation of the antenna while increasing the alignment layer inserted in the middle one by one. The thickness of one liquid crystal layer is set to 100 \(\upmu\)m, and the thickness of the inserted alignment layer is also set to 100 \(\upmu\)m. For example, when one alignment layer is inserted, the liquid crystal layer becomes two, one above and one below, so the ratio of the liquid crystal cell gap to the total thickness is 2/3. When two alignment layers are inserted, the liquid crystal layers are divided into three, and the ratio of the liquid crystal cell gap to the total substrate thickness is 3/5. As expected, as the number of 100 \(\upmu\)m-thick liquid crystal layers inserted, the total thickness of the antenna substrate increased, increasing the frequency bandwidth, while the rate of frequency change decreased. In case of the thickness of the layers inserted for the LC alignment to be less than 100 \(\upmu\)m, it is possible to suppress a decrease in frequency change rate due to an increase in the active layer ratio (\(\sum\)\(d_{\mathrm{LC}}\)/(\(\sum\)\(d_{\mathrm{LC}}\) + \(d_{\mathrm{sub}}\)))). Therefore, when designing a liquid crystal patch antenna, it is  
Figure 10: (**a**) 3D view and detail of the multi-layered LC substrate. (**b**) Frequency change rate and \(-\)10 dB bandwidth according to the number of liquid crystal layers.  
expected to be able to design it by appropriately selecting the number of liquid crystal layers in consideration of the desired bandwidth and frequency change rate.