70 dB from the fundamental. It is therefore concluded that the fourth harmonic is down by at least this factor from the fundamental. The fourth harmonic was later observed in the rectifier tank circuit by means of a special probe which was coupled into the rectifier tank circuit in the ground plane test fixture. In connection with the measurement of harmonic power at the second and third harmonics, it is of interest to compare the level with that predicted by computer simulation. For both the experimental case and the simulated case the input to the rectenna element was terminated in a 50 ohm line. In the experimental case no stub lines were being used when the measurements were made. HARMONIC LEVEL WITH RESPECT TO FUNDAMENTAL * Sensitivity of equipment to 4th harmonic was -70 dB from fundamental. However, no 4th harmonic was observed. 3. 5 Reduction in radiated harmonic power by metallic shielding A considerable amount of harmonic power is radiated from the rectifier tank circuit directly into space unless some metallic shielding is placed around the tank circuit. Experiment No. 2 in Table 3 “3 shows that the addition of a metallic shield over the rectenna element resulted in an improvement in efficiency by nearly 1%; this despite the fact that there must be some skin losses in the shield. The second and third harmonic power being radiated from the unshielded structure was easily detected by monitoring equipment fifty feet away. The addition of the shield reduced any radiation to a level that the monitoring equipment could not detect. The radiation of harmonic power from the rectifier tank circuit should not be confused with radiation of harmonic power from the dipole antenna which must be transmitted through two or more low pass filter sections. 3.6 Improvement in the efficiency and in the consistency of efficiency measurements by refinements in the construction of the RXCV rec- tenna element. It was found that there was a considerable variation in the efficiency of some RXCV rectenna elements depending upon whether they were inserted into
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