2. Techniques for Measuring the Efficiency and Losses of Rectenna Elements 2. 2. 1 Measurement Equipment Raytheon Company, in part supported by contracts from Marshall Space Flight Center and Jet Propulsion Laboratory, has established a number of experimental techniques which were used to advantage in this technology development contract. One of these techniques is the measurement of individual rectenna element performance in an expanded waveguide fixture which tends to simulate its behavior and performance within the cell area that it occupies in the rectenna. This is a closed system from which there is no microwave leakage. The microwave input to the fixture can be calibrated to within 0. 5% of the power standard maintained at Raytheon and periodically checked at the National Bureau of Standards at Boulder, Colorado. A photograph of this set up is reproduced in Figure 2-1. The equipment is used to make accurate impedance measurements with the addition of a movable-probe VSWR indicator. Another piece of equipment which was effectively used in the measurements program to improve rectenna element design is the unbalanced version of the rectenna element and its associated coax-line test equipment. This piece of equipment is shown in Figures 2.-2 and 2-3. The technique is to use a ground plane to simulate one side of a balanced transmission line. This allows a coax line to be connected to the rectenna element so that measurements can be made over a very broad band of frequencies without the introduction of waveguide modes which would occur in the set up of Figure 2-1. This feature can be used to great advantage in the measurement of harmonic power at the input terminals of the low pass filter. Figure 2-2 shows a directional coupler placed before the VSWR probe for this purpose. Another useful aspect of the unbalanced set-up is that probes to the microwave current and voltage waveforms in the rectenna circuit of the rectenna element can be set into the ground plane. The ground plane test fixture can also be modified as shown in Figures 2-4 and 2-5 to accommodate the two thermistors of a thermistor bridge to measure the losses in the diode. This experimental technique is based upon the fact that the heat sink of the diode cannot detect the difference between heat which is generated by microwaves and heat which is generated by DC power. The thermistor bridge can therefore be accurately calibrated with DC power dissipated in the diode. The thermistor bridge technique for measuring diode dissipation was developed under this contract. A very important test equipment is the complete microwave power transmission system shown in Figure 1-13 in which DC to DC efficiency has been certified '10'. The major use of this equipment was to permit substitution of the two plane rectenna construction developed under this contract into the rectenna to determine its behavior in a full rectenna environment. 2. 2. 2 Calibration of Microwave Power Input In a rectenna element development program it is essential that the value of microwave power input to the rectenna element under test be accurately
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