The chief sources of loss in a low power rectenna element are losses caused by the voltage drop across the Schottky barrier diode, and the I^r losses caused by the passage of the charging current to the diode capacitance through the series resistance of the diode. The efficiency loss caused bv the voltage drop across the Schottky barrier is closely given by V^/(V^ + Vl) where is the Schottky barrier voltage drop and V^c is the voltage across the load resistor. The voltage drop across the Schottky barrier is fixed so that the loss caused by the barrier can only be reduced by increasing the DC voltage. This can be done for a given microwave power input level by raising the resistance of the DC load. A fourfold increase in impedance will reduce the Schottky barrier losses by a factor of two, approximately. However, raising the voltage will increase the charging current losses into the diode so that decreasing the capacitance and therefore the contact area of the diode will be of some help. Raising the DC load resistance to decrease the Schottky barrier voltage presents a new problem in that the rectenna element will not be matched into space. The amount of power reflected is approximately: where Rl is the DC load resistance and Rlm is the load resistance that gives a match to the incoming power. In the RXCV rectenna element the input impedance to the dipole antenna is approximately 120 ohms as is the characteristic impedance of the low pass filters that are matched to it. The value of Rl that provides a good match into the 120 ohm impedance level is about 160 ohms. Therefore to match into a higher load resistance while still retaining the 120 ohm impedance of the low pass filters requires another matching section which is conveniently supplied by a quarter wavelength line of an impedance level which is the geometric mean of the two. In the first modification a quarter wavelength line at an impedance level of 240 ohms was used. This provided a reasonable match into a load resistance of 640 ohms. A photograph of the arrangement just described is shown in Figure 3-2. The data given as Items 3 and 4 for curves 3 and 4 in Figure 3- 1 were obtained with the use of the rectenna element shown in Figure 3-2. At values of incident power below 100 milliwatts there was an appreciable amount of power reflected with the circuit shown in Figure 3-2. To explore the impact of a higher impedance transformation upon operation the impedance of the X/4 section of line was raised by a factor of 1.25 to raise the matched impedance level for the DC load resistance by a factor of 1.56. This
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