Rectenna Characteristics The main constraints placed upon the rectenna are a light weight, low cost (which will probably be very low compared to the cost of the phased array) and good efficiency. As the distance between the transmitter and the receiver will be kept small, the power density would probably be high enough to provide a high rectenna conversion efficiency. Hence, no special care has to be taken, and a low cost, low profile and light weight thin-film technology rectenna can be used. Such rectennae have already reached conversion efficiencies over 85%. [Chang, 1991] A conversion efficiency of 70% may be assumed as a worst case. Thermal Control This section addresses the thermal control of the antenna system. A worst case scenario is assumed. In this scenario the antenna is in operational mode and fully faces the Sun at some point during its orbit. This is a conservative assumption since the orbit inclination is 51.6° (if the solar vector is not normal to the surface the solar flux is multiplied by cos0 where 0 is the incidence angle between the surface normal and the Sun). This implies that the antenna should not only radiate heat received from the Sun, but also the heat generated by the electronic equipment. In the chosen configuration the antenna does not receive albedo radiation nor Earth infrared radiation while in operation. The two main design criteria for the thermal control are: 1. low costs 2. no thermal interface to the Mir space station Passive thermal control is therefore a good choice. This implies the use of thermal control coatings. These are surfaces with special radiation properties that provide the desired thermal performance of the surface. The antenna will be side mounted on the Mir station. The antenna should be thermally decoupled from the Mir station as much as possible so as to limit heat exchange. This requirement arises from the fact that Mir only has a limited thermal control capacity. As a consequence the back of the antenna must be shielded and heat can only be radiated from the front of the antenna. For the thermal analysis several parameters need to be chosen, the absorptivity and emissivity of the antenna surface being the main ones. As a surface coating white epoxy (Al substrate) is considered. This coating has a small ratio of solar absorptivity (Os = 0.248) to infrared emissivity (ejr = 0.924) and a low equilibrium temperature. Degradation of the surface does not play a significant role since the mission duration is not more than one month. The power transmitted is taken to be 5000 W. A margin of 10% is assumed. The efficiency of the antenna system is 70%. Therefore 1650 W must be dissipated as heat. At thermal equilibrium the heat input to the antenna should equal the heat emitted. The heat input is due to the direct solar flux and the loss in the amplifiers (system efficiency = 70%). The general equation can be written as: [] Rewriting this equation for the antenna (per unit time per unit surface) yields: [] where o is the Stefan-Boltzman constant (5.67 x 10'8 Wnr2K4), Ta is the absolute antenna temperature in Kelvin, [] is the solar flux (1358 Wm-2) and Pdissipated ([]) is the power that needs to be dissipated. In the above equation it has been assumed for simplicity that the free space temperature is very small with respect to the antenna temperature. Solving for Ta an equilibrium temperature of approximately 75 °C is found. The operating temperature of the chosen antenna as well as the amplifiers mounted on the back of the antenna function well at high operating temperatures in the range up to 100°C. [Matra Marconi ,1992] Since structures are allowed to operate up to 65 °C or more, the equilibrium temperature of 75 °C is acceptable. Another important aspect to consider is the other temperature extreme, the cold case. The lowest temperature the antenna will experience is in eclipse when the antenna does not transmit power and does not receive any sunlight radiation (direct or indirect). The computation of this minimum
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