The idea was to use large orbiting stations to collect solar energy on geostationary orbit, to transform it into electricity and then into microwaves. Once the microwave energy is transmitted to the earth's surface, it can be reconverted into electricity and distributed in the conventional power networks. Microwave Tubes The microwave power production is currently performed by using tube technology. [Brown, 1981] Within a certain range of power and frequencies this technology is well confirmed and mature. At low frequencies (S-band) a power output of several MW can be obtained. But the power level decreases using higher frequencies. To transmit a given amount of power at a given frequency, two possible solutions can be adopted: single source or combined sources. The choice is made considering the most cost effective one. The single source solution requires a high power tube. The transmitting antenna can be a standard one, with feed horn and attitude control using thrusters. The second solution employs several low power tubes and this leads to different ways to combine those tubes. Usually in telecommunications and direct broadcasting satellites the output signals from the tube may be added in-situ through hybrids, with proper phasing, to feed a standard antenna. An other technic is used for radars and requires that each tube amplifier feeds one horn of a phased array antenna, which achieves a spatial summation of the signals at the desired location. All microwave tubes are based on the same principle of transferring part of the kinetic or potential energy of the free electrons in a vacuum to an RF field carried by a microwave structure. The primary energy of the electrons is supplied by high accelerating voltages generated by a power supply. Linear Beam Tubes: Klystrons Multicavity klystron amplifiers are longitudinal interaction tubes. They have four or five resonant cavities, separated from each other by narrow drift tubes. The input cavity is connected to the signal to be amplified and the output cavity to the useful load. The bandwidth is usually very small. At higher frequencies (10-30 GHz) the klystron can produce up to 100 kW and it is suitable to be used in small size satellite power stations. Crossed-field Tubes: Magnetrons A magnetron is a circular diode to which a magnetic field parallel to its axis is applied. Oscillations occur when the electrons are given specified values of angular velocity. It is a low cost, high efficiency (80%) tube. But its high power capability is limited to a small frequency range. For instance at 10 GHz, the power capability is few hundreds watts. Fast-wave Tubes: Gyrotrons Gyrotrons are new millimeter-wave vacuum tubes. They are presently under development and promise to generate extremely high power levels. Solid State Microwave Devices The use of klystrons for conversion of energy from de electrical power to microwave radio frequency energy leads serious concerns about the reliability of this devices. Solid state amplifier circuits, which use high efficiency, advanced technology (gallium arsenide transistors for dc/RF conversion) have been considered as a good alternative because of their high reliability. But in order to be competitive with klystrons, the solid state amplifier circuits and antenna combined efficiencies need to approach the same efficiency as the klystrons. For solid state devices two major problems have to be solved: the temperature of the amplifier base needs to be controlled to about 150 C for efficient and reliable operation and the working voltages must be low (about 40 volts). Rectenna The term "rectenna" is most often used to indicate the whole receiving part of a microwave power transmission system. From this point of view, the rectenna refers to the structure that intercepts the main lobe of the transmitting beam, in order to collect the major part of its power. This structure is an array of elements, where each of those is converting the locally incident RF power. The term "rectenna" originally referred to each of these elements. It stands for "rectifying antenna" (and
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