**4. Conclusions**

Bumps and tapering effects are efficient strategies for reducing the RCS of wind turbine towers. When separately applied on the towers, each of these approaches is capable of decreasing radar returns. In this research, we combined these two methods and created hybrid wind turbine towers. These hybrid towers out-perform towers with only tapering effects or only bumpy surfaces. Test results prove that these two RCS mitigation strategies are not in mutual conflict. Instead, being put together, they produce additive effects on RCS reduction.

The taper ratio of a tower significantly influences its RCS. The higher the taper ratio, the lower the RCS. However, for a pragmatic tower, the taper ratio should be within a limit. Otherwise, the costs to build the tower would be too high. Taper bump towers possess effective RCS reduction capabilities for various radar frequencies and taper ratios. The performance of taper bamboo towers may deteriorate if the radar frequency exceeds four GHz and the taper ratio is larger than 1.0/60. However, in other cases, it is the best tower shape to reduce RCS.

The efficiencies of the taper tower and taper bamboo tower are not affected by radar polarization. On the other hand, the taper bump tower produces different bi-static RCS if we alter the polarization method. When the radar waves are horizontally polarized, its bi-static RCS are lower. However, this phenomenon is not obvious in the major back scattering directions.

**Funding:** This research was partially supported by the Ministry of Science and Technology, Taiwan.

**Conflicts of Interest:** The author declares no conflict of interest.
