not "receiving antenna") as the RF to DC conversion is conducted by rectifying the wave. We will then use "rectenna" when we consider a single element, and we will use "rectenna receiving array" when we consider the whole structure. Since the early beginning of these technology developments, severe requirements have been placed upon the collector-converter unit [Brown, 1977]: • large aperture and non directive, • minimal radio frequency interference, • high power handling capability, • high efficiency, • ability to passively radiate any heat resulting from inefficient operation, and of course: • high reliability, very long life, low cost, light weight,... As the rectenna concept has directly shown its ability to cope with almost all of these demands, the history of microwave collector-converter technology is mainly that of the development of the rectenna. Nevertheless, the radio frequency interference (RFI) requirement is probably the main problem for some rectenna designs. RFI, however in the form of harmonic power, is a special problem that confronts both the transmitter and the receiver. The harmonic level must be down to a very low level to meet non-interference requirements. This can be met by wave filters, but results in higher cost and reduced efficiency since these requirements are severe. A proper solution could be to have an allocation of frequencies for the harmonics that are generated in the system [Brown, 1977], The way the rectenna converts the RF signal to DC power is illustrated by Figure 7.29. An example of an early implementation is shown in Figure 7.30. Recent rectennas no longer look like this one as many technological improvements arose, but this picture gives the reader a concrete idea of the various components of the rectenna. The RF energy is collected by an antenna, rectified by a diode, and low-pass filtered in order to extract the DC component. A microwave low-pass filter can be found between the antenna and the diode, to attenuate the harmonic radiation, and to store energy for the rectification process. The rectenna global efficiency is given by the product of the power collection efficiency and the rectification or conversion efficiency. The collection efficiency characterizes the ability of the rectenna receiving array to capture the incident microwave power. This is measured by means of VSWR (Voltage Standing Wave Ratio) measurements of a probe in front of the array. (This gives access to the ratio of what is collected, and what is reflected away by the structure.) A high collection efficiency is then achieved by matching the surface impedance of the rectenna array to the impedance of the propagation media (the air or the vacuum). A proper design allows to reach collection efficiencies as high as 99% so that this factor is not limiting. The losses at various points of the conversion circuit make the conversion efficiency more critical. The filters losses are only few percent of the whole figure, so that it is difficult to try to reduce them. On the contrary, diode losses could vary between few percent up to few tens of percent. Because the diode rectifier is such a critical element, a search for diodes that would improve the efficiency and power handling capability of the rectenna has been a continuing process. These losses follow a curve that depends on the voltage drop in the forward direction of the diode. This voltage drop must be reduced as much as possible, in order to maximize the conversion efficiency. The gallium arsenide Schottky-barrier diodes have shown good performance for that purpose. The existence of this voltage drop across the diode has a very important consequence on the operating performances of the rectenna: the efficiency is directly related to the incident power following a non-linear function. A kind of threshold exists for the incident power, below which the rectification efficiency drops very quickly, from about 85% down to nothing. Usually, this problem is not frequently mentioned, as people look forward to high power transmission systems. Obviously, this assumption does not hold for the early low power demonstrations. Care must therefore be taken when designing these first systems
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