Optimization of the Bandwidth of Electrically Small Planar Antennas

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Fig. 2 The 4-arm cylindrical folded helix antenna. © The MITRE Corporation. All rights reserved A number of 4-arm folded helices with different length to diameter ratios were studied. The ratio of the antenna Q to the lower bound (the Chu limit) of Equation 1 was calculated and compared against the Gustafsson limit for cylindrical shaped antennas [7]. This comparison is presented in Figure 3. Note that in calculating the Gustafsson limit presented in Figure 3, it was assumed that the antennas have a ka much less than 1, are purely vertically polarized and that the maximum achievable directivity is 1.5. In all cases, the Q of the antenna is above or at the Gustafsson limit. The objective of the present work is to perform a similar study for planar antennas as a function of their length-to-diameter ratio. We compare a number of different planar antennas, consider simple techniques to impedance match them and then compare their Q’s to the Gustafsson limit.
Fig. 3 Comparison of the cylindrical folded helix Q to the Gustafsson limit. Planar Antennas © The MITRE Corporation. All rights reserved In the previous section, we described two similar antenna designs that exhibit Q’s which approach and meet theoretical limits. In this section, we describe several planar antennas that are designed with the same objective, namely to develop antennas having a planar geometry which exhibit a matched impedance and Q’s that meet theoretical limits. While the Chulimit provides an absolute minimum for antenna Q as a function of occupied spherical volume, it provides no insight into what value of Q can be practically achieved using a planar geometry. Gustafsson provided data for the minimum achievable Q for planar antennas as a function of their length-to-diameter ratio [7]. It was assumed that the antennas are electrically small, exhibit a single resonance within their defined operating bandwidth and that they have a directivity equal to 1.5. A plot of the Gustafsson limit, expressed as the ratio of antenna Q to the Chu- Limit (Q/Qlb), is presented in Figure 4. From Figure 4, it is seen that the
Page 1 and 2: Optimization of the Bandwidth of El
Page 3 and 4: value of ka, where k is the free sp
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Page 13 and 14: Figure 8. Electrically small planar
Page 15 and 16: Figure 9. Radiation pattern of the
Page 19 and 20: Table 2. Performance comparison of
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Optimization of the Bandwidth of Electrically Small Planar Antennas

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