Yes, employing a directional antenna can potentially be a solution to address a second adjacent channel waiver issue. Directional antennas are designed to focus the radiation pattern of the transmitted signal in specific directions while reducing it in others. By carefully configuring a directional antenna, it may be possible to minimize interference with neighboring stations on second adjacent channels, thus improving the chances of obtaining a waiver for such a setup. However, the feasibility and success of this approach would depend on various technical and regulatory factors, and it may require consultation with experts and relevant authorities.
I suggest considering the use of a directional antenna when your 100 dBu interference contour exhibits a slight overlap with the primary station's service contour. It's essential to note that the FCC has issued warnings regarding the use of directional antennas that might lead to a significant reduction in population coverage compared to a non-directional antenna at the same site. Implementing directional antennas entails additional engineering efforts both during the application process and before commencing broadcasting operations.