On uniform and linearly tapered long yagi antennas

Traveling-wave analysis of long Yagi antennas is reviewed briefly. The method of designing a Yagi antenna from this viewpoint is discussed and some experimental results are given in order to verify the analysis. A long Yagi antenna, when designed according to the Hansen and Woodyard condition, has a sidelobe ratio of 9.32 db in its radiation pattern, irrespective of the length of the antenna. It is shown that by varying the propagation constant linearly along the length of the antenna, the sidelobe ratio can be improved considerably without sacrificing much of the antenna gain. This linear variation of the propagation constant may be obtained by slowly tapering the element lengths and/or element spacings along the length of the antenna. An approximate theory is developed for the linearly tapered long Yagi antenna and it is verified by actual measurements. A comparison between the radiation patterns of the uniform and the tapered long Yagi antennas clearly shows the advantage of tapering.