• A microwave reference signal must be generated with its phase considered to be the 0 degree reference phase. • This signal must be split and distributed to each segment of the array. • The phase of the signal arriving at each segment must be independently controlled over a range large enough to fulfill the steering requirements, and with an accuracy compatible with the tracking accuracy requirements. • Each signal must be amplified to provide the overall power. The amplitude of each signal could also be controlled in order to maximize the flexibility in shaping the beam. The usual structure of a segment is illustrated in Figure 10.2.6. A local oscillator (LO) provides the reference signal. This signal is split and distributed to each segment. A digital phase shifter is then used to adjust the phase of the signal. As a reference, the METS experiment [Kaya, 1991] provides a 4-bit phase control, and a pointing accuracy better than one degree. Hence, it seems feasible to improve these specifications as this technology is already a few years old. After having adjusted the phase, the signal must be amplified up to a power of a few watts. This requires high efficiency and high power solid-state amplifiers. The phased array being developed by Matra Marconi uses solid-state amplifiers delivering the same order of power at 5.3 GHz, and with an efficiency of about 80%. To find an amplifier at 2.45 GHz should therefore not be a problem. The energy which is not transmitted is converted to heat, and raises the operating temperature. This problem must be addressed, and a key issue of the design would be to thermally connect all the power amplifiers to a sufficiently large radiating surface. Finally, each element of the array is fed with the controlled signal coming from the amplifier. These elements are simple antennae. Various kinds of antennae could be used: dipoles, crossed dipoles, helices, micro strip antennae etc. The latter has been chosen for its low thickness. Some special micro strip (MS) designs have very interesting properties. For instance, some circular MS antennae radiate the fundamental frequency very efficiently, while the harmonics are blocked. Further investigations should be done before the choice of antenna is made. Figure 10.2.6 Phased Array Functional Diagram Beam Control The beam emitted by the phased array could be controlled using retrodirective phase control technique. [Kaya, 1991; Chemoff, 1980] The rectenna emits a pilot signal to the power transmission antenna. By measuring the phase of the incoming signal on the different elements, it is possible to determine with very high accuracy the necessary phase shifts to steer the power beam on the target. This technology has already been validated on a small scale phased array. [Dickinson, 1978] Unfortunately, this requires a quite complex microwave circuitry, which would probably be too
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