The 10-step taper that provides the range of distance change Dmin/Dmax = 0.6 for r0= 1.7 (collection efficiency^=0.85) is shown in fig.3. The additional phase distributions, which maintain the desired value of the parameters and the corresponding power density distributions at the rectenna plane are shown in figs. 4-5. The phase distribution is changing from 0 to tt at the antenna edge, when the antennas are drawing together (see fig.4). This phase inversion increases the beam angular size (see fig.5). Analyzing various tapers we have observed an interesting fact: the tapers that provide the maximum value of collection efficiency and the uniform tapers are not good choices for an adaptive system, because in these cases it is not possible to change the distance between antennae: Dmax = Dmin (see fig.6-7). We think that this deals with the extremal properties of these tapers. The uniform tapers provide a maximum of antenna peak directivity (maximum of the peak power density at the rectenna), so these tapers are a stationary point for the first part of the penalty function (16). The tapers with the maximum value of collection efficiency are a stationary point for the second part of the penalty function (16).
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