In discussing the data that was taken to respond to these areas of interest it should be realized that the object was to develop this method to the point of usefulness in our measurements and not to refine it to its fullest capability. The sensitivity of the arrangement is best indicated by the measurements that were made in the calibration process and by the residual drift in the system. Figure 2’7 shows a typical calibration curve of output of the thermistor bridge as a function of DC power dissipated in the diode. The slope of the curve shown in Figure 2-7 is 0. 188 millivolts per milliwatt. On the other hand as Figure 2-8 indicates, the drift of the zero on the thermistor bridge shows that a variation of as much as 0.4 millivolts can occur within a time period of a minute. Thus it would appear that the resolution of the system is in the range of two milliwatts. Larger variations that occur over a time period of several minutes can be eliminated by rebalancing the thermistor bridge. Figure 2-9 indicates the typically fast response time of the measuring system to a step function of applied or removed power. The time constant is nine seconds, so that the steady state is reached in less than a minute. About one minute was allowed for taking a point of data if a high degree of accuracy was wanted. The ambient temperature of the system heat sink has an impact upon measurement accuracy and should be taken into consideration by taking the experimental data at the same ambient temperature at which the thermistor bridge is calibrated. The mass of the heat sink is about 2000 grams and its heat storage is about 160 calories or about 670 watt seconds. Hence, if there were no other means of dissipating the heat than absorbing it in the mass of the brass plate, about 20 minutes would be required for the plate to increase its temperature by one degree Centigrade. 2.2.4 A Check on Measurement Accuracies by Balancing Measurements of Input Microwave Power Against the Sum of the Measurements of DC Power Output, Diode Losses, and Circuit Losses A primary objective of the measurement portion of this technology development program has been the development of measurement techniques which will provide high confidence in the measured rectenna element’s overall efficiency and which will also provide an accurate measurement of the major losses in the system. This objective has been achieved by measuring the microwave power input as accurately as possible and comparing this with the sum of the DC power output, the diode losses, and the circuit losses. All of these measurements have some error associated with them. The DC power output measurement has a relatively small error associated with the measurement of voltage across a known resistance. The probable error of these measurements is estimated to be ±0. 20% of the absorbed microwave power.
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