Comparison of the antenna characteristics.
Next generation of wireless mobile systems calls for more compact and multiband antennas. This is because such systems need to be small and can operate over multiple wireless communication standards. The design and development of miniature antennas that function over a wideband are highly challenging. In this chapter, novel antenna designs are presented, which provide a solution to this deficiency. These antennas are based on composite right‐/left‐handed transmission line (CRLH‐TL) metamaterials. Unlike traditional right‐handed (RH) transmission materials, metamaterials based on left‐handed (LH) transmission lines have unique features of antiparallel group and phase velocities. Pure LH transmission lines cannot be implemented due to the existence of RH parasitic effects that occur naturally in practical LH transmission lines. In this chapter, novel CRLH transmission line structures are presented, which include right‐handed parasitic effects.
- compact antennas
- composite right‐/left‐handed transmission lines
- multiband antennas
- VHF/UHF antennas
The design and development of wideband antennas are highly challenging, especially for application in portable wireless communications systems . Due to the limited space assigned for the antenna in such systems, shrinking the size conventional antennas can lead to degradation in its performance and complicate mechanical assembly. An alternative solution is to employ metamaterial (MTM) technology in the design of antennas. MTM antennas have smaller dimensions because their size is independent of wavelength (
In this chapter, an innovative wideband antenna is designed, fabricated, and tested using a unique metamaterial transmission line unit cell structure. The MTM‐TL unit cell is based on distributed implementation of the series capacitor and shunt inductor realized with a slit (L‐ and F‐shaped) and spiral configurations, respectively. The radiating unit cells benefit from miniaturized size, planar structure, low profile, ease of fabrication, light weight, and low cost. In addition, the proposed MTM‐TL unit cell provides wideband operation with good radiation properties. The parametric study presented in the chapter shows that the number of unit cells and the slit dimensions can have a dramatic influence on the antenna's performance in terms of operational bandwidth and radiation characteristics. Two antennas are designed for RF applications with maximum size of 14 × 5 mm2.
2. Antenna design procedure
2.1. Equivalent circuit model
Equivalent circuit model of the CRLH‐TL unit cell, shown in Figure 1, consists of
The simplified circuit model and layouts of the proposed CRLH unit cell structures are shown in Figure 2a, where capacitance (
The CRLH‐TL unit cell topologies in Figure 2a has a propagation constant () given by:
Parameters , , and are a function of frequency and related to dispersion, sign function, impedance, and admittance of the antenna structure, respectively. The series and shunt resonance frequencies, respectively, are:
The phase and group velocities, respectively, are defined by:
The dispersion diagram of the proposed CRLH unit cells is shown in Figure 2b. Bandwidth of CRLH‐TL unit cells is defined between the high‐pass (left‐handed) cutoff frequency (and the low‐pass (right‐handed) cutoff frequency (. The cutoff frequencies and are given by:
The CRLH‐TL unit cells were designed and constructed on Rogers RT/duroid® RO4003 substrate with dielectric constant of , thicknesses of
2.2. Metamaterial antenna with L‐shaped slits
The CRLH‐TL unit cell consists of a rectangular radiation patch on which is engraved an L‐shaped slit, and the patch is inductively grounded with a high‐impedance spiral stub. The L‐shaped slit acts like a LH capacitance (
Besides the small dimensions, the bandwidth and good radiation properties are other main performance criteria in the antenna systems. With the proposed antenna design, the bandwidth can be increased by simply introducing more unit cells. Hence, there is a tradeoff between size of the antenna, bandwidth and radiation properties. Each cell occupies an area of 2.3 × 4.9 mm2. Dimensions of the L‐shaped slit antenna are 13.4 × 4.9 × 0.8 mm3 or 0.0089
For the L‐shaped antenna, the simulated bandwidth is 1625 MHz (195 MHz–1.82 GHz) using HFSS™, and the measured bandwidth is 1600 MHz (200 MHz–1.8 GHz) for VSWR ≤ 2, which corresponds to a fractional bandwidth of 160%. The reflection coefficient response of the antenna, shown in Figure 3, clearly indicates its resonates at four distinct frequencies of 600, 850, 1200, and 1550 MHz. The measured gain and efficiency of the antenna are 1.2 dBi and 34% at 600 MHz, 1.7 dBi and 45% at 850 MHz, 2.1 dBi and 62% at 1200 MHz, and 3.4 dBi and 88% at 1550 MHz. The measured 2D and simulated 3D radiation patterns of the antenna are shown in Figure 4 at the various resonance frequencies. The measured antenna gain and efficiency response are shown in Figure 4c.
The simulated and measured radiation patterns of the antenna at various frequencies in the two principle planes, the
2.3. Metamaterial antenna with F‐shaped slits
In this section, the main goal is to design and implement an antenna with better performance than the above L‐shaped antenna. This was achieved by modifying the L‐shaped antenna by changing the slits to F‐shaped. The equivalent circuit model of the unit cell is shown in Figures 1 and 2. In this case, the optimized antenna employed five CRLH‐TL unit cells, as shown in Figure 5a. Each unit cell occupied an area of 2.06 × 4.40 mm2 (0.00075
The antenna was fabricated on RT/duroid® RO4003 substrate with thickness of 1.6 mm. The reflection coefficient response of the antenna in Figure 5b shows that the antenna has a measured impedance bandwidth of 1.99 GHz (110 MHz–2.10 GHz) for VSWR ≤ 2, which corresponds to a fractional bandwidth of 180.1%. The antenna resonates at 450, 725, 1150, 1670, and 1900 MHz.
The measured gain and efficiency of the F‐shaped antenna, shown in Figure 5c, are 1.0 dBi and 31% at 450 MHz, 1.8 dBi and 47% at 725 MHz, 2.5 dBi and 70% at 1150 MHz, 3.8 dBi and 89% at 1670 MHz, and 4.5 dBi and 95% at 1900 MHz.
The simulated and measured radiation patterns of the antenna in the
3. Analysis on antenna design parameters
To achieve the desired antenna performance, the number of unit cells (
(UC: unit cells)
|Dimensions (ES: electrical size,|
PHS: physical size)
|Fractional bandwidth||Gain (Max)||Eff. (Max)|
| b‐shaped antenna with 4×UC||ES: 0.047|
GHz, PHS: 14.2 × 6.32 × 0.8 mm3
|104.76% (1–3.2 GHz)||2.3 dBi||62%|
| b‐shaped antenna with 6×UC||ES: 0.051|
GHz, PHS: 19.2 × 6.32 × 0.8 mm3
|123.8% (0.8–3.4 GHz)||2.8 dBi||70%|
antenna with 8×UC
at 7.5 GHz, PHS: 22.6 × 7 × 0.8 mm3
|84.23% (7.25–17.8 GHz)||2.3 dBi||48%|
antenna with 7×UC
at 7.7 GHz, PHS: 21.7 × 7 × 1.6 mm3
|87.16% (7.8–19.85 GHz)||3.4 dBi||68.1%|
antenna with 6×UC
at 7.5 GHz, PHS: 18 × 7 × 0.8 mm3
|74.4% (7.5–16.8 GHz)||2.1 dBi||44.3%|
antenna with 5×UC
at 7.7 GHz, PHS: 16.7 × 7 × 1.6 mm3
|82.88% (7.7–18.6 GHz)||3.1 dBi||58.6%|
at 0.67 MHz, PHS: 60 × 16 × 1 mm3
|116.7% (0.67–2.55 GHz)||4.7 dBi||62.9%|
GHz,PHS: 18 × 18 × 1.6 mm3
|26.5% (1.8–2.35 GHz)||3.7 dBi||20%|
0.8 MHz, PHS: 60 × 5 × 5 mm3
|103.03% (0.8–2.5 GHz)||0.4 dBi||53.6%|
GHz, PHS: 18.2 × 18.2 × 6.5 mm3
|66.66% (1–2 GHz)||0.6 dBi||26%|
|Proposed F‐shaped slit antenna 5×UC||ES: 0.0053|
at 0.11 MHz, PHS: 14.5 × 4.4 × 1.6 mm3
|180.1% (0.11–2.1 GHz)||4.5 dBi||95%|
Figure 8 indicates that the larger the vertical length of the slit, the larger the bandwidth (for S11 < -10 dB). Increase in slit length from 1.3 to 2.3 mm increases the bandwidth by 36.5%. Figure 9 shows that the increase in the width from 0.2 to 0.3 mm increases the bandwidth by 9.3%. The gain and the radiation efficiency of the antenna are greatly affected by the number of unit cells. The peak gain and radiation efficiency increase substantially with the number of unit cells as shown in Figures 10 and 11. The measured peak gain and peak radiation efficiency are 4.5 dBi and 94.8% for five unit cells at 1890 MHz. Figure 12 shows that the spiral width, their separation and number of turns also affect the antenna's gain and radiation efficiency.
Features of the proposed two antennas are compared with other similar antennas in Table 1.
The authors would like to give special thanks to faculty of Microelectronics for financial support.
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