Structural parameters of the proposed antenna and meandered strip-line feed.
Abstract
Multiband functionality in antennas has become a fundamental requirement to equip wireless devices with multiple communication standards so that they can utilize the electromagnetic spectrum more efficiently and effectively. This is necessary to ensure global portability and enhance system capacity. To meet these requirements, microstrip technology is increasingly being used in communication systems because it offers considerable size reduction, cost-effectiveness as they can be easily manufactured in mass production, are durable and can conform to planar or cylindrical surfaces. Unfortunately, such antennas suffer from intrinsically narrow bandwidth. To overcome this deficiency, various techniques have been investigated in the past. In this chapter, a novel approach is presented to design antennas for applications that cover radio frequency identification (RFID) and WiMAX systems.
Keywords
- planar antenna
- meandered strip-line feed
- wideband antenna
- multiband antenna
1. Introduction
Innovative design concepts that have been facilitated by cutting-edge technology are contributing toward boosting wireless communications. Antennas are an essential component in such systems; however the large dimensions and narrow operating range of conventional antennas preclude them from application in the next generation of wireless communications systems. This is because future wireless systems impose strict requirements from antennas, such as large impedance bandwidth to support multiple systems, small physical size, low cost designs, high efficiency and reliability.
To utilize the electromagnetic spectrum more efficiently and effectively it has become necessary to equip portable wireless devices with multiple communication standards. This necessitates antennas to operate over a wideband [1, 2]. Nowadays microstrip integrated technology (MIT) is being increasingly used in the design of antennas for application in wireless communication systems because it offers considerable size reduction, cost effectiveness as it allows easy manufacture in mass production, durability and enables antennas to be configured for mounting on various irregular surfaces [3, 4]. Unfortunately, such antennas have intrinsically narrow bandwidth. To circumvent this deficiency, numerous techniques have been investigated recently, which include embedding slit-lines in the patch antenna [5–9]; employing unconventional feeding structures [10, 11]; insertion of parasitic elements in the vicinity of the patch antenna [12]; employing thick substrates and/or higher dielectric constant substrates [13, 14]; loading the antenna with an arrangement of electromagnetic band-gap (EBG) structures [15]; using meta-surfaces [16] and employing metamaterial (MTM) unit cells [17–19]. These techniques certainly improve the impedance bandwidth of antennas; however, it is not sufficient to support multiple wireless communications systems. Another interesting technique to enhance the bandwidth of patch antennas uses meandered strip-line feed which has the advantage of being less complex to implement in practice [20].
In this chapter, a planar microstrip antenna is proposed for multiband wireless communications systems. Embedded in the antenna’s radiation patch are an H-shaped slit and two inverted U-shaped capacitive slits. The antenna is fed through a meandered strip-line. The antennas described in this chapter are fabricated on RT/duroid® RO4003 substrate with permittivity of 3.38, thickness of 1.6 mm and tan
2. MTM antenna design
The design technique described in this chapter offers expansion of the impedance bandwidth of the antenna without compromising its size and salient characteristics. This is achieved by inserting dielectric slits in the antenna patch and exciting it through a meandered strip-line. The antenna design employs an H-shape and inverted U-shape slit.
To minimize design complexity and reduce manufacturing cost the proposed antenna structure avoids the use of via-holes.
2.1. H-shaped slit patch antenna
The generic patch antenna configuration and its equivalent circuit model are shown in Figure 1. The slit essentially behave as series capacitor (
Length (
The measured gain and radiation efficiency of the antenna at spot frequencies of 2.2, 3.55 and 4.5 GHz are 0.65 dBi and 18.34%, 2.75 dBi and 47.15% and 1.90 dBi and 36.12%, respectively. The optimum measured gain and radiation efficiency of the antenna are 2.75 dBi and 45.15%, respectively, as shown in Figure 3, at
Figure 4 shows the measured co-polarization and cross-polarization radiation patterns in the E(
2.2. Patch antenna with inverted U-shape slits located on either of the H-shape slit
The above antenna was modified to improve its performance by inserting two inverted U-shaped slits on either side of the H-shape slit, as shown in Figures 5 and 6. The dimensions of the antenna structure are given in Table 1. The equivalent circuit model of the proposed symmetrical antenna structure inset in Figure 7 consists of the composite right/left-handed transmission-line (CRLH-TL), where parasitic series reactance is represented by inductor
21.2 | 15 | 12.7 | 13.5 | 3.6 | 12 | 8.5 | 2.4 | 2.4 | 0.6 |
0.6 | 1.2 | 1.2 | 3.5 | 4.5 | 0.9 | 0.9 | 0.3 | 0.3 | 1.2 |
4.5 | 3.1 | 3.1 | 2 | 5.1 | 2.3 | 1.6 | 0.8 | 1.3 | 0.5 |
With
Where
and
Parameters
The phase and group velocities are given by:
The antenna’s dispersion diagram in Figure 7 shows the bandwidth of structure changes from high-pass left-handed response with cut-off frequency
Individual slits embedded in the antenna resonate at specific frequencies, as shown in Figure 8. The resonance at 2.05 GHz is generated by the inverted U-slit on the left-hand side of the H-slit; the resonance at 3.7 GHz is generated by the H-slit and the resonance at 4.45 GHz results from the inverted U-slit on the right-hand side of the H-slit. Simulated and measured impedance bandwidth of the proposed antenna are 5.55 GHz (0.65–6.2 GHz) and 5.25 GHz (0.8–6.05 GHz), respectively; and the corresponding fractional bandwidths are 162.04 and 153.28%, respectively. These results confirm the antenna can operate over multiple wireless communications standards, in particular, UHF RFID, WLAN, WiMAX, WiFi, Bluetooth, GPS, PCS, and DCS [31, 32]. Electrical size of the antenna at 800 MHz is 0.056
The measured gain and radiation efficiency of the antenna in Figure 9 at 0.8, 2.05, 3.7, 4.45 and 6.05 GHz are 0.95 dBi and 25.8%, 3.85 dBi and 63.1%, 4.73 dBi and 75.9%, 5.35 dBi and 84.1% and 3.05 dBi and 50.2%, respectively. The optimum gain and radiation efficiency of the antenna are 5.35 dBi and 84.1% at 4.45 GHz. Figure 10 shows the antenna’s measured radiation pattern at spot frequencies.
The radiation field in the E-plane is omni-directional however this deteriorates at the first resonance frequency of 2.05 GHz. In the H-plane the antenna radiates bi-directionally across its operational bandwidth. This antenna provides the best cross-polarization compared to the above antennas. The current distribution over the antenna at its three resonance frequencies is shown in Figure 11.
3. Parametric study
A parametric study is necessary to understand the effect of the slits and the meandered strip-line feed on the characteristics of the second antenna. The results in Figure 12 shows that the length (
Length and width | Frequency bandwidth |
---|---|
1.75−4.63 GHz ≈90.28%, | |
1.2–5.47 GHz ≈128.03%, | |
0.65–6.2 GHz ≈162.04%, | |
The effect of inverted U-shaped slit’s length (
Length and width | Frequency bandwidth |
---|---|
2.2–4.8 GHz ≈74.28%, | |
1.46–5.37 GHz ≈114.49%, | |
0.65–6.2 GHz ≈162.04%, | |
The effect on the antenna’s performance by the meandered microstrip feed-line in Figure 14 shows that it greatly contributes toward improving its impedance match. In fact by increasing the length of
Notched-band | |||||
---|---|---|---|---|---|
1.5 | 0.3 | 0.4 | 0.15 | 0.15 | I: 2.55–2.95 GHz II: 3.85–4.15 GHz Impedance matching ≥ −20 dB |
3 | 0.6 | 0.8 | 0.15 | 0.15 | I: 2.6–2.8 GHz II: 3.95–4.05 GHz Impedance matching ≥ −30 dB |
4.5 | 0.9 | 1.2 | 0.3 | 0.3 | Eliminated Impedance matching ≥ −40 dB |
To summarize, a simple and effective technique has been demonstrated to extend the impedance bandwidth of patch antennas. This involves embedding three slits and exciting the antenna through a meandered strip-line. The radiating surface of the antenna is loaded with two inverted U-shape slits that are placed on either side of an H-shape slit. The antenna essentially behaves as a CRLH-TL structure. The antenna is shown to provide a fractional bandwidth of 223.27%, a maximum gain of 5.35 dBi and radiation efficiency of 84.12% at 4.45 GHz. Its radiation characteristics are similar to a monopole antenna. The proposed antenna should provide reliable wireless communication across UHF, L, S and C-bands.
Acknowledgments
The authors would like to give their special thanks to faculty of microelectronics for the financial supports.
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