5. 5 Tests of the Separate Rectenna Elements in the Foreplane Structure with the use of the Expanded Waveguide Fixture Because the individual elements are isolated from each other in the foreplane structure it should be possible to check the elements individually. Therefore, extensive tests were carried out on the individual elements in the foreplane assembly using a modified expanded waveguide test fixture. The modifications consisted of cutting deep slots into the sides of the test fixture to accept the foreplane. After the foreplane is inserted into the test fixture, the lid is closed and clamped. Secure contact is made between the lid of the fixture and the back of the foreplane section with the use of machine screws, although good contact does not appear to be necessary for proper operation. Table 5-2 presents the results of the final tests that were made on each of the rectenna elements in the foreplane structure. A set of five GaAs-W Schottky barrier diodes fabricated as part of the diode development program was used. Each of the five diodes was also checked out in an RXCV element to ascertain if there was a substantial difference in efficiency between the RXCV element in the foreplane structure. A slightly lower efficiency (less than 1%) of these elements in the foreplane was noted. This loss was probably caused by the losses in the section of the transmission line that joined the rectenna section to the next within the foreplane construction. The average overall efficiency (the ratio of DC power output to incident microwave power) of the 5 elements in the foreplane was 87. 86%. The reflection loss was only 0. 26%. An incident power level of 4 watts was used for the test. Although it was possible to match the foreplane elements so that there was a very small reflection, in the cases of elements 4 and 5 the efficiency decreased substantially when the elements were matched out. This was not the case with the other three elements, and there was no obvious cause of the anomaly experienced with elements 4 and 5. 5. 6 Smith Chart Presentation of Reflection Data A Smith Chart presentation of microwave reflection as a function of incident microwave power level and the ohmic value of DC load resistance, made for a typical element in the foreplane structure is shown in Figure 5-11. This indicates, as had previous measurements of reflected power, that low values of reflected power (less than 1%) can be obtained over a relatively wide range of microwave power input if the proper load resistances are used. However, the reflection can be made to vanish only for a combination of load resistance and microwave power input. An adjustment of the load resistance primarily accommodates a mismatch of the resistance portion of the microwave load, but not the reactance part. The admittance is measured at the point where the half wave dipole input terminals connect to the input of the low pass filter. A temporary short at this point was used to find the corresponding reference point on the moving-probe slotted waveguide standing wave detector. The slotted waveguide standing wave detector is attached to the input of the expanded waveguide test fixture. Figure 5-11 utilizes only the central part of the Smith Chart in order to
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