to the rectenna element test fixture will cause the measurement of incident power to vary by only 1%. Since the measurement of rectenna element efficiency takes place under reasonably well matched conditions, we can be well assured that the measurement of incident power is affected to less than 0. 1% by reflected power during any efficiency measurement of rectenna elements. Likewise, any errors in the measurement of reflected power from the expanded waveguide test fixture that might be caused by a mismatch in the coax to waveguide transmission unit is eliminated by terminating the WR 43 0 waveguide at the normal point of attachment of the expanded waveguide test fixture with a precision-matched waveguide termination load and then adjusting the waveguide matching section near the coax to waveguide transition so that a minimum of reverse power is indicated by the directional coupler. In the use of the microwave power standard for calibration purposes, any reflected power from the standard and from the wave guide-to-coax transition is matched out to a reflection of less than 0. 02% as measured by the slotted waveguide VSWR detector by adjustment of the matching section located on the outboard side of the slotted waveguide detector. Finally, in order to eliminate any measurement error caused by 2nd and 3rd harmonic output from the 10 watt TWT Amplifier, a low pass filter was inserted between the TWT and the input to the measurement system. 2. 2. 3 Measurement of Diode Losses The basis for being able to make accurate measurements of microwave losses in the diode is that the heat flow geometry for getting rid of the heat from dissipation losses in the semiconductor chip in the diode package is the same whether the heat comes from insertion of DC power or from microwave power. (It is assumed that all of the microwave losses and all of the DC losses taking place within the diode envelope originate in the semiconductor chip. ) If it is then possible to build a sensitive sensor to indicate the heat flow from the diode, one can calibrate this sensor by the injection of accurately measured DC power into the diode. The sensing arrangement for doing this is shown in Figures 2-4 and 2-5. The bridge consisting of two thermistors was used primarily because of the great sensitivity of the thermistors to a change in temperature. One thermistor was placed adjacent to the heat sink of the diode while the other thermistor was placed in the larger system heat sink across an impedance to the heat flow. A number of factors are of interest in this arrangement. One of these, of course, is the sensitivity of the system or how small an increment of heat dissipation it will resolve. Another is the system noise. The response of the system to a change in the heat input to the diode is of interest. Still another is the linearity of the system and its sensitivity to a change in the ambient temperature of the heat sink.
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