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Since individual microstrip antenna radiats very small in θ=90 degree plane, the effect of grating lobe for microstrip array will be smaller than that of most other type antenna arrays. In many cases spacing close to λ shows small effect of grating lobe, but first look at your requirements.
then u know when designing an antenna array, what is feed line width? and how to calculate effective dielectric constant for right and left side impedence? i am using the book: "CAD Microstrip Antenna for wirless applications" and i am using the window application to use it.