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Abstract-A new configuration of Yagi antenna is proposed, which can improve the forward/backward ratio (f/b) significantly while maintaining a high gain. This configuration involves the addition of one radiating element to the original Yagi array. This additional element may arrange on parallel (side-by-side) or collinear to a radiating dipole of the original Yagi antenna. It is shown here that the technique is most effective for collinear configuration (exhibits smaller mutual effects) and that there then exists an optimum length and position for the added element. The amplitude of the excitation of the additional element determines the angular location of the back lobe reduction. To demonstrate the major benefits, comparisons are made among the proposed and conventional Yagi configurations. Numerical and measured results of our design show more than 20 dB front to back ratio at 2.4 GHz. Moreover, the proposed array represents a simple and valuable alternative to the stack Yagi antennas as the obtainable radiation characteristics are satisfactory in terms of both forward and backward gain.

The implementation of the feeding network in the arrays that composed of a large number of elements and using non-uniform excitations is a real challenging issue. The complexity of the feeding network can be greatly simplified by using uniformly excited arrays. However, such arrays suffer from sidelobe limitation. An optimised approach for sector sidelobe nulling in the equally spaced linear array pattern with uniform excitations except for two edge elements is presented. The genetic algorithm and particle swarm optimisation are independently used to find the optimal values of the amplitude and phase excitations for those edge elements.

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