| Summary: | In this paper, a low grating lobe array antenna with electrically large property is proposed based on TM<sub>50</sub> mode square patch antenna elements. By introducing transverse slots on the square radiating patch, four side lobes of the TM<sub>50</sub> mode element antenna can be reduced effectively. However, undesired grating lobes are created in the array pattern. That's because the antenna element distance is more than one wavelength on high modes. In order to solve this issue, a new method is proposed by making the grating lobes of the array factor pattern coincide with the null/(zero) ranges of element pattern. It can effectively suppress grating lobes caused by large element spacing. The method introduced here was supported by analytical 1×2 and 1×4 uniform liner arrays. Meanwhile, in order to further reduce transmission line loss and side lobes, substrate integration coaxial line structure and unequal amplitude feeding method are introduced. Finally, a 1×8 liner array is fabricated and measured for verification. When the element spacing is 1.4λ<sub>0</sub>, the grating lobes level at 22.3GHz is only -22dB.
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