The radiating properties of end-fire aerials

An end-fire aerial, such as a Yagi or a dielectric-rod aerial, can be regarded as a structure which supports the dipole type of surface wave. Radiation occurs at the end of the aerial, the radiation pattern being determined by the transverse field distribution of the surface wave. It is shown that this leads to a simple relation between the beam width of the pattern and the ratio of the wavelength of the surface wave to that in free space. Further, this approach shows that the beam width tends to a non-zero value as the aerial length increases, in contrast with previous theories which predict that it decreases to zero with increasing length. In applying the theory to practical aerials, allowance must be made for the radiation which also occurs from the end of the aerial at which the surface wave is excited. Theoretical and experimental patterns for several end-fire aerials are compared, and it is shown that good agreement can be obtained.