11. Directivity increases with the length of the antenna.12. A Yagi-Uda antenna's maximum directivity is 10.92 dBi. 13. The directivity should be as high as possible. 14. Waveguide directional couplers will have the best directivity . 15. The smaller the driver, the higher the frequency where this narrowing of directivity occurs. 16. Electrically small antennas, high directivity and tunable operational frequency are produced with negative magnetic permeability. 17. The word " directivity " is also sometimes used as a synonym for directive gain. 18. Design criteria are to achieve a substantially flat coupling together with high directivity over the desired band. 19. Also, a section about Directivity Control Waveguide was added and it seems it will be deleted. 20. This is because the directivity pattern of any wave source is the Fourier Transform of the source function.
