J. Seyl L. Leopold Tracking & Communications Development Division IV-C-4 PHASING OF ANTENNA a. RATIONALE The antenna array must be coherently transponding and "central phasing" should be used on large, low mass arrays in order to transmit power from geosynchronous satellites to earth. An active retrodirective array is a self-phasing array which transmits a beam towards the source of an incident signal. All self-phasing transmitting arrays are based on the "phase conjugation" principle. Each subarray of the array is connected to a circuit which transforms the received phase wt - Br into transmitted phase, at + Br (w is the frequency of the incoming "pilot" signal, r is the distance from the pilot source to the subarray, and B = w/v where v is the phase velocity in the intervening medium). Since wt + Br is the phase of a wave traveling towards r = 0, the envelope of all the spherical waves radiated by each subarray is just the incident wave front reversed in direction. b. DEFINITION In order to efficiently transmit power from geosynchronous satellites using large phased array antenna systems, it is necessary to precisely control the phase of the radio frequency signal transmitted from each element in the array. To accomplish the required phasing precision, an array which is self-focusing is proposed. Such arrays are referred to as retrodirective in that the transmitted signal is directed toward a reference source or pilot signal transmitter at the rectenna. The pilot signal transmitter is located with the rectenna or ground receiving antenna and thus insures that the transmitted beam is properly phased for pointing in the desired direction. Figure IV-C-4-1 illustrates the basic concept of the retrodirective (adaptive phase control) array system. c. OPERATING CHARACTERISTICS Adaptive phase control is accomplished by the process of "phase conjugating" the received pilot signal. Each subarray of the transmitting antenna array is connected to a circuit which transforms the phase of the received pilot signal at that subarray (wt - Br), into a conjugated phase (wt + Br + 0d) for retransmission . w is the frequency in radians per second of the RF (radio frequency) signal, t is time, r is the distance from the pilot transmitter to the receiving subarray, and B is a constant dependent on the transmission medium, and Or is a constant phase shift resulting from the reference system phase control. It should be noted the eR is the same for each subarray in the array system. K Since the retransmitted signal from each subarray has a phase which is exactly equal and opposite to the phase shift that results as it propagates back toward the pilot source (rectenna), then all subarray signals will be received at the rectenna with the same phase eR and the envelope of the spherical wave fronts will be normal to the rectenna. Phase conjugating may be realized in several ways. A well- known technique employs heterodyning and is shown functionally in Figure IV-C-4-2. Other approaches include servoing phase shifting devices
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