A grating lobe occurs at each value of 0S, as given in Eq. 3, which is dependent upon the distance between phase control centers. The size of the subarray or power module determines the grating lobe location, and also, as will be shown later, the antenna mechanical pointing requirements. The grating lobes of interest, i.e., the ones incident upon the earth, occur at small values of 0^. In this spatial region, the periodic lobes are regularly spaced at angular intervals of X/Sx. In the discussion which follows, a simplified planer geometry is used which applies most accurately for equatorial rectenna locations. Asymmetries in the field pattern resulting from the curvature of the Earth and latitude of actual rectenna sites will be neglected. The amplitude dependence of the SPS grating lobes can be determined by first considering the signal Ea from a point source at location (Xa, Ya) in the satellite antenna at a height Zo above the ground as shown in Fig. 2. The far-field electric field intensity at a point on the ground (Xa, Y„, Zo) will lag the transmit phase by (w/c)R (where R is the path length, co is 2tt times 2450 MHz, and c is the velocity of light) and can be expressed as The transmission path length R to the field point on the ground is given by which can be approximated using the binomial expansion by
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