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An approximate expression is derived for the phase velocity of wave propagation along an infinite Yagi structure and its dependence on the various parameters of the structure is discussed in detail. The structure is at first studied qualitatively by applying the transmission line analogy. The problem is next treated from the viewpoints of linear antenna and field theories. It is assumed that a traveling wave is propagating along the axial direction and it induces an axially symmetric and sinusoidal current distribution in each element The electric field at any point due to this current distribution is calculated by the Hertz vector method. After applying boundary condition to the electric field, an expression for the propagation constant is derived. The results are compared with existing experimental values. The agreement between theory and experiment is found to be within 5 per cent. The accuracy of the expression given is sufficient for practical purposes.