Planar Antennas for Reliable Multiband RF Communications

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.


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 slitlines in the patch antenna [5][6][7][8][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][18][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 Ushaped 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 tanδ = 0.0022. The final second antenna occupies an area of 13.5 mm × 12.7 mm or 0.036λ 0 ×0 . 0 3 3 λ 0 , where the free space wavelength (λ 0 ) is 0.8 GHz. Numerical parametric analyses was used to (1) determine the location of the slits in the rectangular patch and (2) the optimum meandered-line feed structure. Verification of the antenna performance was done through measurement. It will be shown later the proposed antenna exhibits properties of low radiation loss, low cross-polarization, ease of manufacture and integration in RF transceivers, and no use via holes. These characteristics make it suitable for multiband applications, such as Ultra High Frequency (UHF) Radio Frequency Identification (RFID), Global Positioning System (GPS), Personal Communication Systems (PCS), Digital Communication Systems (DCS), Worldwide Interoperability for Microwave Access (WiMAX), Wireless Local Area Network (WLAN), WiFi, Bluetooth and other applications in the UHF, L, S, major part of C-bands.

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.

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 (C L ) and the meandered microstrip feed-line behaves as a shunt inductor (L L ). Other losses introduced by the antenna structure are represented by series right-handed resistance (R R ), shunt left-handed resistance (R L ), series conductance (G L ) and shunt conductance (G R ). Location and dimensions of the slit was optimized using 3D electromagnetic high frequency structural simulator (HFSS™) by ANSYS [21]. The electromagnetic fields and currents are concentrated in the vicinity of the patch by placing the slit at the center of patch. This prevents the fields from spreading on the ground plane of the antenna and thereby minimizes unwanted coupling. This technique contributes in the realization of a smaller patch antenna. The use of this antenna structure in an array would incur minimal mutual coupling which is important to de-correlate multipath channels in, for example, small cellular systems. The meandered microstrip feed-line ensures low radiation loss and low crosspolarization. It also helps to eliminate unwanted notch bands in the antenna'sr e s p o n s ed u et o impedance mismatch, thus providing bandwidth extension.
Length (L), width (W) and height (h) of the proposed antenna are 21.2 mm, 15 mm and 1.6 mm, respectively. The length (L p )a n dw i d t h( W p ) of radiation patch are 13.5 mm and 12.7 mm, respectively. The corresponding electrical size of the antenna and radiation patch is The antenna's simulated and measured reflection-coefficient (S 11 ) response is shown in Figure 2. The simulated impedance bandwidth of the antenna for S 11 < −10 dB is 2.58 GHz (2.05-4.63 GHz), and a fractional bandwidth of 77.2%. The antenna has a measured bandwidth of 2.3 GHz (2.2-4.5 GHz), and a fractional bandwidth of 68.7%. The simulated result shows the antenna to resonate at f r, sim = 3.6 GHz3.6 GHz, which is very close to the measured result at f r,measured = 3.5 GHz3.5 GHz. The discrepancy between the simulated and measured bandwidth is attributed to manufacturing tolerance and imperfect soldering of the SMA connector.  shows the measured co-polarization and cross-polarization radiation patterns in the E(yz)a n dH ( xz) planes at its resonance frequency of f r = 3.55 GHz3.55 GHz. The measured radiation pattern shows the antenna radiates omni-directionally in the E-plane and bidirectionally in the H-plane. The copolarization and cross-polarization patterns are similar to a typical monopole antenna.

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 Ushaped 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 L R and shunt capacitor C R . Series resonance is due to L R and C L , and shunt resonance due to C R and L L . At low frequency, C L and L L dominate and the transmission-line circuit exhibits lefthanded characteristics; at high frequency, L R and C R dominate and the transmission-line circuit exhibits right-handed characteristics. The propagation constant of the resulting structure is given by Refs. [22][23][24][25][26][27][28][29][30]: Where s ω ðÞ ¼ (3) and Z ω ðÞ ¼ j ωL R À 1=ωC L ðÞ (4)     Parameters β(ω), s(ω), Z(ω) and Y(ω) are a function of frequency and represent dispersion, sign function, impedance and admittance of the antenna structure, respectively. The series and shunt resonance frequencies are given by: 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 ω L to low-pass right-handed response with cut-off frequency ω R with no obvious stop-band. The cut-off frequencies ω L and ω R are given by: Individual slits embedded in the antenna resonate at specific frequencies, as shown in Figure 8.  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.

Parametric study
A parametric study is necessary to understand the effect of the slits and the meandered stripline feed on the characteristics of the second antenna. The results in Figure 12 shows that the length (L S1 ) and width (W S3 ) of H-shaped slit can substantially improve the impedance bandwidth of the antenna. Salient results are given in Table 2 where all other structural parameters were kept fixed.
The effect of inverted U-shaped slit's length (L S2 ) and width (W S4 ) on the antenna is shown in Figure 13. Figure 13 shows that by increasing L S2 and W S4 of the two inverted U-shaped slits the impedance bandwidth and impedance match of the antenna are improved significantly. The results of this analysis are given in Table 3.  Effect on the antenna impedance bandwidth as a function of H-slit length (L S1 ) and width (W S3 ). All other structural parameters given in Table 1 remain fixed.
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 L m , L m1 and L m2 , the width of W m has virtually no effect on the impedance    Table 3. Effect of length and width of the inverted U-shaped slits on the antenna bandwidth.

Metamaterials -Devices and Applications
bandwidth of the antenna, however it significantly improves the antenna's impedance match. Details of these results are provided in Table 4.
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 Cbands.