Figure 10.3.2 Calculations for SGD-1 Phased Array Antenna The operating frequency of the microwave transmission system is 35 GHz. This choice was driven by the need to reduce the size of the transmitting antenna. Phased array antennas already exist for S- and C-band applications and similar technology is expected to be developed for higher frequencies in the near future. For a 35 GHz array the inter-element distance will be roughly 5 mm, corresponding to one half of the wavelength. A 10x10 meter antenna will have approximately 10$ elements discrete elements. It is unlikely that this could be designed using hybrid solid state technology. Monolithic microwave integrated circuits (MMIC) is a promising technology that could make large scale integration of discrete circuits possible. The antenna is a phased array with a retrodirective pointing system. The antenna receives a pilot signal from the rectenna and the phase information of this signal is used to direct the beam in the desired direction. The pilot signal is shifted in frequency with respect to the carrier to provide sufficient isolation between the two. The frequency shift should be as small as possible so that the individual phasing of the elements compensates for any atmospheric perturbation of the wave front. The proposed phase control scheme for the METS rocket experiment is an example of a retrodirective system using a two tone pilot signal. [Kaya, 1991] Modular antenna design is required to be able to integrate the large number of elements necessary for the antenna. Each element would have phase conjugating circuitry for the retrodirective system and a local oscillator to drive the antenna elements. To reduce complexity, several elements could be connected in parallel. This will introduce beam focusing which will reduce the maximum angular area in which the main lobe can be steered. A suggested block diagram for the cell architecture is shown in Figure 10.3.3. The received pilot signal is a low power signal compared to the emitted power carrier. This implies that very high isolation is required between receiving and transmitting channels. The received pilot signal is processed to extract phase information for the carrier. A phase locking circuit drives a low power oscillator feeding a power amplifier which in turn is connected to the antenna element. Every cell of the antenna is thus a stand alone device only requiring DC power supply. This eliminates the need for RF-power distribution within the antenna structure.
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