Figure 13 shows an RF to DC conversion efficiency measurement system. Experimental results are shown in Fig. 14, where resistance means DC load resistance. It is very difficult to measure the RF to DC conversion efficiency because the input RF power into each antenna can not be precisely estimated. Data in Fig. 14 are ones obtained by using the input RF power based on a few assumptions. CONCLUSION This paper proposed the use of a circular microstrip antenna (CMSA) instead of the conventional linear antennas as the receiving antenna of the rectenna in the SPS project. It was experimentally clarified that the wide frequency band characteristics of the CMSA behaved as well as expected. However, the RF-DC conversion efficiency of the rectenna was very low. Main causes of low efficiency are as follows: 1. Mismatch loss of the receiving antenna to the free space because of small number of antenna elements. 2. Resistive loss of diode because of low input power (1.47 W). [Diodes used here are ones for communication (for small power use).] 7ig. 13. RF-DC conversion efficiency measurement system.
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