for the receiving rectenna. Technology requirements at 35 GHz have been assessed and several stages of development have been proposed. 2. Computer Analysis The theory and applications of free-space microwave power transmission have been studied extensively [1-3], The basic model of such a system is shown in Fig. 1. Input DC power is converted to microwave power, transmitted, collected, and then converted back to DC power. The overall efficiency of a free-space microwave power transmission system can be estimated from its subefficiencies [1]. The computer simulation utilizes the four subefficiencies shown in Fig. 1. These are: (1) t/dc: the DC to RF conversion efficiency of the microwave generator. (2) the efficiency of the transmitting antenna. (3) ?/ra: the collection or absorption efficiency of the rectenna. (4) t/rf: the RF to DC conversion efficiency of the rectanna. As shown in the figure, the overall system efficiency is simply the product of these four subefficiencies, or (1) The DC to RF and RF to DC conversion efficiencies depend on the availability of technology in microwave generators and rectennas. The efficiencies decrease as the operating frequencies increase. Simulations which utilize a sweep of frequency require user input of frequency-efficiency data pairs. Conversion efficiencies between the data points are calculated by linear interpolation. The efficiency of the transmitting antenna is assumed to be constant regardless of the operating frequency. The default value is 97%. The collection efficiency of the rectenna is based on a theoretical calculation. The collection efficiency is defined as the ratio of the power received to the total power transmitted. This efficiency can be optimized for a given system geometry by proper choice of a power gain taper at the transmitting antenna [2], The problem of determining an ‘optimum' taper has already been studied by Suddath [3]. One good ‘optimum' taper, 0(x), is of the form
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