bolically concentrated waves are reflected to a collector strip in the back of the adjacent unit. 65,000 reflector/collector units are required for a single rectenna. Orientation and permanent positioning are important for a rectenna, but absolute flatness is not. As long as the reflector/collector units are relatively perpendicular to the axis of the beam, they will function as designed. This means that installations in desert areas can be made without undue regard for flatness, and installations at sea can take the motions of waves - even storms - without losing efficiency. For installation at sea, a rectenna is mounted on SWATH (Small Waterplane Area, Twin Hull) flotation supports. These place the supporting flotation body well below the water surface and the supported body well above, piercing the water plane only with support struts to minimize the effects of waves. Sea-based rectennas are anchored in place by cabling to the sea bottom. Dynamic tensioning of the anchoring cables enables an automatic positioning system to maintain orientation and position accurately with respect to the rectenna's assigned location. Power received is stored in an SMES and transmitted by cable as needed to the electrical grid. Depending on the location of the rectenna, electrolysis of water to hydrogen and oxygen may also be conducted on site. Energy in the form of hydrogen serves the needs of portable, mobile and remote applications, and large-scale hydrogen production is an important function of Project Phoenix's energy replacement system. Environmental concerns regarding microwave energy transmission take three forms: worry about biological effects of the radiation, concern that beams may drift away from the rectennas, and uncertainty about electromagnetic interference. For large scale applications such as suggested by Project Phoenix, concern for the addition of heat to the atmosphere may be added. The concerns are real and have been considered in several studies over recent years. Power transmitted to a rectenna is tightly restricted to a conical beam. While the power density at the center of the rectenna may reach 23 milliwatts per square centimeter, at the edge it will only be 1 milliwatt per square centimeter, and at a distance of 2.5 kilometers away from the Rectenna, it will only be 0.08 milliwatts per square centimeter. Safety standards in the United States and the western world allow up to 10 milliwatts per square centimeter. The major effect of exposure in the range above 10 milliwatts per square centimeter is believed to be heating. Birds flying through the beam would experience some elevation of body temperature. Passengers in an airplane should be more than adequately protected by the aircraft's metal skin and the short time of passage. Sea life below a sea-based rectenna are sheltered by the reflector/collector units. The beam from the SPS is triggered by a signal sent from the center of the rectenna. This signal provides the necessary phase control to produce a coherent beam. If the beam were to drift, its coherence would be lost and its energy would be dissipated to levels approximating normal communication signals on Earth.
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