## 4 Discussion  
We have performed the first analysis of non-uniformity caused by insufficient [HTAB] and the effectiveness of HTAB as a surface-active agent with 5 CB LCs at different cell gaps, and measured the phase-change characteristics of the HTAB:5 CB mixture. Minimizing [HTAB] is essential for several reasons.  
The first is that the presence of dopants degrades the tunability of the LC. Previous studies for RF applications did not use LCs mixtures with dopants, and therefore did not consider the tunability of LC materials. Therefore, the first proposed concept of _WovR_ does not consider the tunability of LCs mixtures [26]. However, if a dopant is added to LC, _WovR_ on the tunable LCs region is reduced because the volume ratio of the tunable LC directors to the entire region is reduced. Therefore, if LCs are mixed with dopants, _WovR_ of the tunable LC region is reduced, so their tunability is reduced. However, this degradation can differ along materials and experimental conditions, so each case must be analyzed; we leave this task for future work, but it is essential for the exact design of the RF devices by simulation. Our results indicate that at a cell gap \(\leq\)20 \(\upmu\)m, LCs can be aligned using only 0.1 wt% HTAB, so the VA for RF devices design can be carried out using a small amount of HTAB.  
The second reason is the aggregation of the LC molecules' alignment [23]. Excessive [HTAB] in LCs mixtures can cause aggregation in the alignment of LCs and cause imperfect VA. This aggregation of alignment reduces the tunability of RF devices that use LCs, and reduces the stability of the initial homeotropic alignment, so [HTAB] should be minimized.  
The third consideration is the leakage current caused by the ionic dopant. An increase in [HTAB] causes an increase in the conductivity of the solution [40]. This high conductivity can degrade the operation of RF devices that use LCs that are driven by an applied electric field. To prevent this degradation, [HTAB] must be minimized in designs of RF devices. The specific effect of high [HTAB] on the degradation of the electrical characteristics of RF applications of LCs will be evaluated in future work.  
These experiments confirmed the limitation of the HTAB anchoring force when the cell gap is large. Strengthening the surfactant's anchoring force may be a solution to overcome this limitation. Some studies have sought methods to do this [36; 41; 42].  
Additionally, if substrate materials other than ITO are used to achieve homeotropic self-alignment in RF applications, the adhesion between the substrate material and surfactant must be considered. If different LCs than HTAB are used, the polar characteristics between the LCs and surfactants must be checked.  
The surface roughness of the substrate can also affect the arrangement of LCs. For a substrate that has a rough surface (e.g., PCB), coating it with dissolved HTAB as a VA alignment layer can be a solution, instead of using a mixture to avoid non-uniformity [21; 43; 44]. Furthermore, there is no prior study involving a morphological analysis of different temperature ranges or different concentrations when using HTAB for vertical alignment; we leave this task for future work.  
Until now, LC antennas have been developed using LCs only with positive dielectric anisotropy. If previously-suggested vertically-aligned LC molecules with a surfactant are widely used, negative LCs for LC antennas can be also developed for E-field driving. If we can use LCs with high dielectric negative anisotropy, we can reduce the manufacturing time with HTAB and improve the tuning range of LC-based RF devices at the same time.