Small antenna with mushroom and lumped element

As mentioned, the main idea is proposing the compact mushroom structure to combine with the small antenna. In this section, we will use the concept of metamaterial technology to design the compact antenna. Figure 4.7 shows the properties of metamaterial antenna, it can also called CRLH antenna. The dispersion curve on the β >0side is the right-handed mode, while the dispersion curve on the β <0side is the left-handed mode. The electrical size of conventional antenna is restricted by its physical dimension, but if we can use the region ofβ <0, left-handed mode, the size of CRLH antenna can be reduced [16]. The method of realizing CRLH antenna is using series capacitance, and shunt inductance, like periodic structure or

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mushroom structure. So the antenna design is based on mushroom structure.

Figure 4.6 Dispersion diagram of right left handed region

Although, we can use more unit cell to get the lower operating frequency by Figure 4.6. Using left-handed modes with indexes more than −1can result in impedance matching issues and low-radiation-efficiency problems [16]. So how to design the exactly number of unit cell are the important skills in CRLH antenna. In this section, the two unit cell CRLH antenna is designed, shown in Figure4.7. This antenna is based on two mushroom structures. However, this mushroom structure has a low-series capacitance between two mushroom structures, calculated by Eq. (1.3). In order to increase the capacitance between two patches, the interdigital capacitor has been added instead of using lumped element [16].However, the concept of antenna design is based on mushroom structure, both two patched are not square, because of considering its matching network. For wireless technology, the antenna is designed at the 5.35GHz, which comfort to standard of 802.11 ac. The dimension of patches a and b are 4.2 5.35× mm, the length of interdigital capacitor L is 3.2mm, the gap between

Bandgap

Cutoff

Cutoff

RH region LH region

interdigital capacitor p is 0.65mm, the gap between interdigital capacitor and patch g is 0.2mm, the diameter of via place at the central of patches d is 0.4mm, the feed line is 50Ω, the substrate is RO4003, the dielectric constant of 3.55 and thickness h of 0.8mm, and the dimension of substrate s w× is 40 30× mm. The idea of using interdigital capacitor instead of lumped element is that we want to focus on inserting lumped element to produce the compact mushroom structure exciting the lower bandgap, not for application, and to avoid the confusion of lumped element effects.

The electric size of CRLH antenna is 0.28λ_o×0.095λ_o . Figure 4.8 shows the combination of CRLH antenna and mushroom structure in CST simulation software.

Dimension of mushroom structure is the same as Figure 2.8, and use 0.5pF capacitor to shift the bandgap at applicable place to match the resonant frequency of CRLH antenna at 5.35GHz, shown in Figure 3.2 (b)

Figure 3.2

c

Figure4.7 The top view of metamaterial antenna.

a

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Figure 4.8 Top view of CRLH antenna with mushroom structure and lumped element

Figure 4.9 shows the return loss of small antenna and small antenna with compact mushroom structure. The dotted line and solid line are original antenna and antenna with compact mushroom structure respectively. We can see there are three resonant frequencies at 5.35, 6.92 and 8.81 GHz in original antenna, but the bandgap of compact mushroom structure is only at 5GHz to 5.5GHz between first and second order mode, shown in Figure 3.2 (b). In this case, the only resonant frequency should be analyzed is 5.35 GHz. Figure 4.9 shows that the frequency with compact mushroom structure is shifted to 5.23GHz, but it still close to 5.35GHz and in the bandgap range.

s

w

Figure 4.9 Return loss of original antenna and antenna with mushroom structure and lumped elements

Then we will discuss the radiation efficiency after the return loss diagram is compared between antenna and antenna with compact mushroom structure. Figure 4.10 is the far-field pattern of original antenna, and we use the straightly method, linear scaling, to observe both efficiency. The radiation efficiency and total efficiency of CRLH antenna is 0.2916 and 0.2907 respectively. It seems this is not a good antenna, and the antenna gain is 1.176. From Figure 4.10, we know while we pursue to decrease the antenna size, but it also causing the reduction of the antenna’s performance. So we use the compact mushroom structure to solve this problem. Figure4.11 shows the performance of antenna after combining with compact mushroom structure. The radiation efficiency is improved from 0.2918 to 0.8064, the total efficiency is increased from 0.2907 to 0.5995, and the gain is raised to 3.648. The performance is improved after inserting compact mushroom structure.

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Figure 4.10 Far-field of original antenna

Figure4.11 Far-field of antenna with mushroom structure and lumped element

Table 2 Performace of CRLH antenna and CRLH antenna with compact mushrooom structure

Rad. efficiency Tot. efficiency Gain

Small antenna (5.35GHz) 29.18% 29.07% 1.176

Small antenna with compact mushroom structure (5.28 GHz)

84.07% 73.03% 5.937

In Table 2 Performace of CRLH antenna and CRLH antenna with compact mushrooom structure, we can easily see the improvement after the small antenna combining with compact mushroom structure. But, how can we know that this improvement is realized by compact mushroom structure. Does the original mushroom structure also can enhance antenna’s performance? To verify this doubt, we propose another experiment.

We use the same CRLH antenna, shown in Figure4.7 to combine with mushroom structure without lumped element, shown in Figure 4.12. Figure 2.8 shows the bandgap of original mushroom structure is from 8.2GHz to 9.5GHz. This bandgap does match to resonant frequency of CRLH antenna at the 5.35 GHz. The return loss diagram result is shown in Figure 4.13. The solid line is small antenna, and the dotted line is small antenna with original mushroom structure. There are still frequency shift after combining original mushroom structure, which resonant frequency is 5.21GHz. It’s similar to small antenna with compact mushroom structure. So we can assume that frequency shift is caused by mushroom structure, not lumped element. Figure 4.14 shows the performance of small antenna with mushroom structure. It also demonstrates by linear scaling. Radiation efficiency is 0.2735, total efficiency is 0.2729, and the gain is 1.031. The farfiled result is even worse than CRLH antenna without any EBG structure shown in Figure 4.14. This simulation result proves that the compact mushroom structure can exactly improve antenna’s performance rather than other

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mushroom structure, which electric size is large and not at the right bandgap.

Figure 4.12 CRLH antenna with original mushroom structure

Figure 4.13 Return loss of CRLH antenna and CRLH antenna with original mushroom structure

Figure 4.14 Far-field result of CRLH antenna with original mushroom structure

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Chapter 5 Measurement results

In chapter 4, the simulation result had been shown that the antenna performance can be improved by compact mushroom structure. So in this chapter, we had fabricated these antennas to observe that the contributions of combining compact mushroom structure

(a)

(b)

Figure 5.1(a) Top view of patch anteena and (b) patch antenna with compact mushroom structure

Figure 5.2 return loss of patch antenna and patch antenna with compact mushroom structure

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.

(a)

(b)

Figure 5.3(a) CRLH antenna and (b) CRLH antenna with compact mushroom structure

Figure 5.4 Return loss of CRLH antenna and CRLH antenna with compact mushroom structure

Figure 5.1 shows the top view of patch antenna and patch antenna with compact mushroom structure. The measurement results of return loss are shown in Figure 5.2, in which the solid and dotted lines represent the patch antenna and the patch antenna with compact mushroom structure respectively. Both operating frequency are almost the same at 3.9GHz, and it’s similar to Figure 4.4. Then the far-field results will be shown in table. 3. Figure 5.3 shows the view of both CRLH antenna and CRLH antenna with compact mushroom structure. The measurement results of return loss are shown in Figure 5.4. Although the operating frequency is shifted to lower frequency which is compared to Figure 4.9, it still have the same trends between simulation results and experiment results. The matching network is affected by compact mushroom structure.

In addition, we also measure the efficiency and directivity of both cases, and gain can be converted by efficiency and directivity, as shown in table 3. The efficiency of small antenna by measurement is -5.468dB, and gain is 0.377dB. It seems that the efficiency

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is almost as same as total efficiency in table1, but gain is worse. As for the CRLH antenna with the mushroom structure loaded with 0.5pF capacitor, the efficiency is -2.84dB, and the gain is 3.465. It is observed that the efficiency and the gain are improved by accompanying mushroom structure with lumped elements.