reflected powers are 2. 5% versus 0.7%. The difference of 1.8% in reflected power correlates with the 2% increase in aggregate power. However, the microwave input measurement accuracy and the validity of the reflected power percentage by the VSWR measurements make such good agreement appear to be coincidental. The type of VSWR measurements that were obtained are shown in Figure 5-18. The 25-ohm load definitely gives a higher VSWR at all probe positions. However, the variation in VSWR as a function of probe position is not characteristic of measurements that had been made previously in this structure without the foreplane structure, as shown in Figure 5-19 The fact that the 5 elements of the foreplane receive different amounts of microwave power but operate into a common load raises the question as to the effective load resistance each one sees. If the assumption is first made that the efficiency of the rectenna element, and therefore its power output is affected to only a minor degree by modest perturbation of its load resistance, it is simple to calculate these load resistances. Thus, out of a predicted foreplane power output of 18. 59 watts, 2. 94 watts comes from each of the two elements in Set 3, 4. 13 watts from each of the two elements in Set 1, and 4.46 watts from the central element. The voltage across the 35-ohm load resistance for 18.59 watts input is 25.51 volts. Since all the elements operate in parallel, the respective load resistances are 221, 157, and 145 ohms respectively. It is possible that the nonuniformity of reflection from these elements may contribute to the difference in VSWR patterns between Figures 5-18and 5.19. The reflected power that would be associated with these load resistances and these power levels for the individual elements tested in the expanded waveguide fixtures are given in Figure 5-11. According to this figure, 2.94 watts and 221 ohms gives 1% power reflection with an admittance component of 0, 8 and very little susceptance; 4. 13 watts and 157 ohms gives 1% reflection with an admittance component of 1. 1 and a susceptance component of . 16; 4.46 watts and 145 ohms gives a power reflection of 1.2% with an admittance component of 1. 2 and a susceptance component of about 0. 2.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==