established. The efficiency of the rectenna element as given by the ratio of DC power output to microwave power input depends upon it. Moreover in this development dealing with refinements in efficiency, an efficiency improvement of 1% is important. It is therefore necessary that the measurement sensistivity be considerably less than 1%, and it is desirable that repeatability in the measurement of efficiency be within a small fraction of 1%. The microwave power standard that is used at Raytheon was calibrated at 2450 MHz at the National Bureau of Standards in Boulder, Colorado in the summer of 1975. The probable error in this calibration was ±0.34% It is estimated that another probable error of 0.5% is involved in the calibration of the test set-ups at the two to ten watt level where the use of a calibrated attenuator is necessary, and about 0.3% at the 100 milliwatt level. The 100 milliwatt level is the same level as the calibrated standard so it is not necessary to use calibrated attenuators at this power level. Calibrations were made for both the expanded waveguide test fixture and for the fixture with the RF Ground plane. The block diagram for the calibration of the incident microwave power upon the rectenna element test fixture with the use of the Raytheon microwave power standard calibrated by the Bureau of Standards is shown in Figure 2-6. The power standard is placed at the point where the test fixture attaches and the 43 2A power meter or other suitable power meter is calibrated against this standard. This arrangement eliminates the errors in the calibration of the directional coupler and in the power meter itself. A digital readout is used on the power meter to eliminate operator error in reading the power meter. Calibrations at the 90 milliwatt level and at the 4 watt level were made for each of the two test fixtures. The microwave power standard itself is calibrated at the 90 milliwatt level. It is therefore necessary to insert calibrated incident power at the 4 watt level. The block diagram shows this attenuation to be 15. 95 dB. Precaution is taken to eliminate errors in the calibration of incident microwave power, and errors in subsequent efficiency measurements on rectenna elements, caused by reflections of power at various interfaces in the system. The reading of power in the forward direction is affected by any reflection of reverse directed power from an impedance mismatch at the source. By means of the three port circulator and the coax matching section, this reverse directed power can be absorbed so well that the resulting forward directed power can be held to less than one percent of the reverse directed power. The result of this adjustment is that a change from a match condition to a full short at the input * The uncertainty of the accuracy of the microwave power standard will be a major factor in obtaining an accurate efficiency measurement on the rectenna element. For this reason, the present calibration of the Raytheon standard has been discussed with Mr. Paul Hudson, Program Chief, RF Power, Current and Voltage Section of the National Bureau of Standards, Boulder, Colorado. His statement to me was that if a perfectly matched termination is used on the standard, a 2-sigma confidence level corresponds to a ± 1% error. It follows that the probable error is ± 0. 34%.
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