3.2.3 Other Approaches to Efficient Operation at Lower Power Levels In looking at the design of circuits for more efficient operation at lower power levels a number of approaches were passed over. The first of these was the use of either a full wave rectifier circuit or a pseudo-full wave circuit. In view of the fact that the circuit losses have been demonstrated by experimental measurement and by computer simulation to be very low for the half-wave rectification circuit, it was felt that this approach would not be fruitful. Another approach was to feed more than one dipole into a diode. This would probably favorably impact the efficiency at the very low power input levels. Figure 3-1 would seem to indicate an improvement in efficiency from 80 to 82% by just doubling the input power from 50 milliwatts to 100 milliwatts, and an improvement from 45% to 56% by doubling the power from 1 milliwatt to 2 milliwatts. The latter power level is, of course, a factor of 50 below the lowest level at which the full scale SSPS rectenna is anticipated to be operated. It is observed that a rectenna array rapidly takes on the directivity characteristics of a conventional phased array when an attempt is made to operate many dipole elements into one rectifier element. There may be instances where this is desirable for experimental reasons but the desirability in the full- scale SSPS application is questionable. Since the experimental investigation of this approach requires an elaborate new test arrangement it was not felt to be cost-effective within the present contract. Another approach that was not investigated was the elimination of the input filter sections completely and the use of a X/4 impedance transformer between the terminals of the dipole and the rectification circuit. Under this proposed arrangement the 2nd harmonic and 3rd harmonic would be shorted out at the terminals of the dipole by stub lines. This approach would seem to place a burden upon the critical adjustment of the stub lines to prevent excessive harmonic radiation, but would, nevertheless be a worthwhile investigation to determine how efficient such an arrangement would be. 3.3 Initial Effort in Integration of Rectenna Element into a Two Plane Structure An initial investigation of the performance of a rectenna element when it was mounted in the waveguide test fixture at right angles to its normal position as shown in Figure 3-4 was carried out. It was found that there was no significant change in performance for the unshielded case as shown in Figure 3-4. However, there was a reduction in efficiency of about 1% when a short circuited quarter wavelength line was added to the structure to simulate a connection to the next rectenna element. The further integration effort is discussed in detail in Section 5.0.
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