**1.3 Technologies**

**Microwave Transmitter:** The frequency range of choice for transmission is achieved by means of microwaves. Presently, an efficiency of 76% is possible using current technology for microwave power transmission. The waves must be focused so that all the energy transmitted by the source is incident on the wave collection device for increasing transmission efficiency. The high cost of transmitters and the relative low efficiency of current optical and infrared devices makes higher frequencies impractical. The most common transmitters for microwaves are klystron, traveling wave tube (TWT) and magnetron. The klystron has been the DC to microwave converter of choice however it is also somewhat expensive. The TWT is far too expensive and power restrictive making it impractical for the task of power transmission. The use of magnetrons because they are cheap and efficient is investigated by many researchers. The power transmission is more lenient to frequency fluctuations than the communication systems in magnetrons frequency output but is not as precisely controllable as the klystron or TWT. An array of magnetrons to be used as the transmitter can be one of the more common R & D investigation proposal. In range of 300 W to 1 kW magnetrons are already mass produced for

microwave ovens, it is one of the main advantages to using many smaller magnetrons as opposed to a few klystrons.

The generation of microwave power in the microwave power source and its output power is managed by electronic restrain circuits on the transmission side. To match the impedance between the transmitting antenna and the microwave source, a tuner is attached. Based on the direction of signal propagation by Directional Coupler, whose function is to divide the attenuated signals. The transmitting antenna emits the power uniformly through free space to the receiver antenna. An antenna receives the transmitted power and translates the microwave power to DC power on the receiving section. For setting the output impedance of a signal source equal to the rectifying circuit, both impedance matching circuit and the filter is provided. The Schottky barrier diodes which converts the received microwave power into DC power are connected in the rectifying circuit.

**Use of Microwave Power Transmission in Solar Power Satellites (SPS):** For transmitting power to earth stations, solar power generating satellites can be launched into space. Based on this idea, which was first proposed in 1968 based on experiments carried out in terrestrial laboratories. At high earth orbit in geosynchronous location, the SPS satellites are put in the orbits. This feature enables them to receive light almost whole year by up to 99% of the yearly time. A facility of a large rectenna array built on the Earth is for collecting the incoming microwaves. The satellite is required to be built with a retrodirective transmitter for maintaining a good lock on the rectenna. This helps in locking on to a pilot beam emanated from the ground station. Most of the research is done in the 2.4 GHz to 5.8 GHz range. Therefore, there are some spectrum regulatory issues to deal with their use. Also, the retro directive antenna system is unproven with present technology. The microwave beam could veer off target and can microwave some unsuspecting family. Therefore, this is the cause of the health concern [5].

**Magnetic Resonance:** In this technology, an oscillator is designed to generate the carrier signal for transmitting the power. Usually, oscillators are not intended to deliver the power, because a power amplifier is required to the oscillator for amplification of the oscillating signal. The output power to the transmission coil is transferred by the power amplifier. For receiving the transmitted power, a receiver coil is built. Since the power received at the receiver side is having an alternating current. Thus, a rectifier is needed for rectification of the AC voltage. An electric load is connected to the receiving coil to complete the circuit [6].

**WiTricity:** The new technology called WiTricity is based on using coupled resonant objects [1]. With the same resonant frequency, two resonant objects manage to exchange energy efficiently, while interacting weakly with extraneous off-resonant objects. The resonant nature of the witricity system guarantees the strong interaction between the sending unit and the receiving unit, while the interaction with the rest of the environment is weak. The design consists of two copper coils, each a selfresonant system. One of the coils, attached to the power source can be a solar power and is termed as the sending unit of the witricity. The irradiation of the environment with electromagnetic waves oscillates the space around it with a non-radiative magnetic field oscillating at MHz frequencies. The non-radiative field intervenes the power exchange with the receiving coil, which is built for the purpose of creating resonance with the field.
