3.4 The Reduction of Second and Third Harmonic Radiation with the use of Stub Lines Another area of activity involved taking steps to decrease the amount of second and third harmonic power radiated from the antenna. In this case the grounded plane fixture was used and the input impedance of the antenna was simulated by a matched 50 ohm line. This, of course, is an accurate simulation of the properties of the dipole antenna only at the fundamental frequency since the dipole antenna will apoear as a different impedance at the two harmonic frequencies. However, data on dipole antennas whose arms have a high ratio of thickness to length indicate a fairly good match (that is, not more than 75% of the power reflected) over a frequency range that includes the fourth harmonic. On this basis an attempt to reduce the harmonic content by placing cpen and short circuited stubs of appropriate length in parallel with the input terminals of the dipole antenna seemed worthwhile. There is a natural opportunity in the two-plane rectenna construction to short out the second harmonic at the dipole antenna terminals because the section of transmission line that extends to the next rectenna element in the two plane construction must look like an open circuit. It therefore looks like a short circuit for the second harmonic. Using the ground-plane construction it was found that this section of line could indeed be adjusted to greatly reduce the second harmonic content that got into the 50 ohm line. As shown in Table 3-3, experiment No. 5 indicated a 30 dB reduction in the second harmonic level. The experimental set up for this is shown in Figure 2-2. The connection to the 50 ohm line is made through a small hole in the geometric center of the ground plane. Figure 2-3 also shows a short stub which acts as an open circuit at a quarter wavelength at the third harmonic frequency. The data indicates a 15 dB improvement in harmonic level as shown in experiment 6 in Table 3-3. It is interesting to note that in comparison with other data in Table 3 -3 there was no appreciable change in efficiency caused by the reduction of the second and third harmonics. However, it was found in later work as reported upon in Section 5 that the adjustment for the reduction of the second harmonic in the foreplane of the two-plane rectenna structure by this means may be so critical as not to be practical. More investigative work needs to be done since the structures being worked with were not identical, and in the foreplane case the physical length of the X/4 section is shortened by capacitance loading. The method by which the harmonic content at the input to the rectenna element was measured is indicated in Figure 2“ 2. The relative levels of the forward directed fundamental power and backward directed harmonic power can be measured with the use of an HFA spectrum analyzer and a directional coupler which is designed to operate at the frequency of the harmonic power. No observation of harmonics greater than the third harmonic were obtained although the analyzer could have seen harmonics down by a factor of
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