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The authors propose a sequential array composed of 4 inclined-slot antennas as a circularly polarized planar antenna worked on the broad-wall of an air-loaded waveguide with standing-wave excitation. The proposed antenna does not require any resistive termination, and works with high efficiency even in small size array. In addition, since the inter-element spacing for in-phase radiation can be chosen asλg/2, a broadside beam with low grating lobe is obtained. In this paper, several results simulated by using the software 'Microwave Studio ver.4.0' are presented, and their validity are also confirmed by experiments. This paper also presents a simple design procedure in which the degradation of axial ratio duo to the interactions from the adjacent element and the finite ground-plane are reduced by only adjusting the element spacing between the parallel slots. A 24-element planar array antenna employing the sequential slot array components is also presented. It shows that the proposed antenna structure is advantageous in terms of the grating lobe reduction that would be serious problem in a conventional slot array antenna cut on the broad-wall of an air-loaded waveguide.
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