understanding that more efficiency could be obtained at lower power levels by doing three things: (1) reducing the junction capacitance of the diode, (2) reducing the drop across the Schottky barrier and (3) operating at a higher value of load resistance. Items (1) and (2) involve the design of the diode and item (2) represents a change in the basic metallurgy of the diode and was the basis for considerable effort under this contract. The implementation of a program to bring about these changes resulted in considerable improvement in efficiency at the lower microwave input levels. This is shown in Figure 3-1, in which the efficiency achieved from the new effort is compared with that obtained from a representative RXCV rectenna element. The newly achieved efficiencies are associated with a large range (four orders of magnitude) of power input level and are obtained from several diode and circuit configurations since it has been found impractical to cover the entire power range with a single diode and circuit configuration. A more detailed description of the data in Figure 3-1 is provided in Table 3-1 for cross referencing to Section 4.0 to better identify circuit arrangements and diodes and to indicate efficiencies better. One of these circuit and diode configurations (No. 5 in Figure 3-1) gives remarkably higher efficiencies at very low values of power input than does the standard RXCV element. On the other hand, the new efficiency of 80% obtained at the 50 milliwatt level is disappointing in that it did not reach either the contract objective or the efficiency predicted from previous studies. It is believed that there are two reasons for this reduced efficiency from that anticipated. The first one is that it was necessary to add an impedance transforming section between the low-pass input filters and the diode and its tank circuit to match the dipole antenna into the rectification circuit to minimize reflected power. There was undoubtedly some additional circuit loss in this section. A more important cause of the reduction in efficiency from that anticipated is the relatively high loss that occurs in a diode with a small junction area because of the high back contact resistance. The back contact resistance can be reduced but in doing so the effective spreading resistance is increased. These losses were not taken into consideration by our earlier analysis. It is believed that these losses are from 5% to 10% in absolute terms. It is believed that these losses can be reduced to 2% to 6%. 3.2.2 The Design and Construction of Circuits for more efficient operation at lower power levels There were three different circuits used to obtain data on more efficient operation at lower power levels. However, they were all based on the RXCV element type circuit - that is the half-wave rectifier circuit, with two low pass filter input stages.
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