antenna illumination taper has been initially selected for maximizing power transfer to the rectenna while minimizing sidelobe levels. Consequently, the number of klystron tubes per subarray varies from 36 at the center to 4 at the edge of the antenna. Each subarray or each power module (that portion of a subarray fed by a single tube) has its own RF (radio frequency) receiver which processes the uplink pilot beam signal and inserts the conjugate of the uplink phase delay onto the input to the tube. The outputs from the subarrays or power modules combine to form a single coherent beam at the center of the ground rectenna where the pilot beam transmitter is located. The uplink pilot beam signal has a pseudo-noise code added to a square wave subcarrier operating at four times the code clock rate. The subcarrier biphase modulates the pilot carrier signal of 2450 MHz, thereby providing a suppressed carrier spectrum. This modulation allows sufficient RF filtering at the receivers in the antenna to isolate the pilot signal from the unwanted downlink power beam which also operates at 2450 MHz. The PN code is used to protect against intentional or unintentional jamming and to allow satellite discrimination between multiple uplink pilot beam signals. Within the antenna receivers, the PN code is despread and the carrier reconstructed, thereby allowing phase conjugation at the same frequency as the downlink power signal. The conjugated phase is routed to each power tube which has a phase control loop to compensate for phase delay variations between tubes and to maintain the output signal in phase with the conjugated pilot signal. The antenna also has a constant reference phase distribution scheme which delivers the same reference to each receiver. The reference distribution system is referred to as the master slave returnable timing system (MSRTS). This system has a four-level distribution tree with returnable timing to compensate for path-length variations within the distribution cables. The original SPS phase control configuration has individual phasing to the subarray level, implying that all power tubes within a particular subarray are adjusted to the same conjugated phase value. Phase control has since been extended down to the tube (or power module) level due to antenna mechanical alignment constraints imposed by the grating lobe characteristics and because of improvements in power transmission efficiency. 3. GRATING LOBE ANALYSIS FOR PHASE CONTROL TO SUBARRAY LEVEL Before proceeding to the detailed analysis of the magnitude and locations of the SPS grating lobes, a short discussion as to the reason for the occurrence of grating lobes is in order. Each subarray can be treated as a single-point phase source with an electromagnetic amplitude Eq, spaced meters apart. Consider a linear array of individual subarrays as shown in Fig. 1. The transmit signal will have a systematic change in phase with the angle of departure of the wavefront according to the expression where Sx = subarray or element spacing, 6 = transmission angle with respect to antenna boresight. The total output from a one-dimensional array is
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