Miniature Planar Antenna Design for Ultra-Wideband Systems Miniature Planar Antenna Design for Ultra-Wideband Systems

Demand for antennas that are compact and operate over an ultra‐wideband (UWB) fre‐ quency range is growing rapidly as UWB systems offer high resolution imaging capa‐ bility and high data rate transmission in the order of Gb/s that is required by the next generation of wireless communication systems. Hence, over the recent years the research and development of UWB antennas has been widely reported in literature. The main performance requirements sought from such antennas include: (1) low VSWR of <2; (2) operation over 7.6 GHz from 3 to 10.6 GHz; and (3) good overall radiation characteristics. Significant size reduction and low manufacturing cost are also important criteria in order to realize a cost‐effective and miniature system. Other desirable requirements include compatibility and ease of integration with RF electronics.


Introduction
Ever since the Federal Communications Commission (FCC) released a bandwidth of 7.5 GHz (from 3.1 to 10.6 GHz) for ultra-wideband (UWB) wireless communications, UWB technology has rapidly developed for high data rate wireless communications [1][2][3][4][5]. As is the case in conventional narrowband wireless communication systems, an antenna plays a very crucial role in UWB systems. However, there are greater challenges in designing a UWB antenna than a narrow band one. A suitable UWB antenna should be capable of operating over an ultra-wide bandwidth as defined by the FCC. At the same time, satisfactory radiation properties over the UWB frequency range are also necessary, including nondispersive nature in order to minimize distortion in the transmitted signal. The antenna needs to have a low voltage standing wave ratio (VSWR) (<2) over 3.1-10.6 GHz band and omnidirectional radiation characteristics [6]. Nowadays, in most applications significant size reduction of antennas is paramount in order to achieve minimization of communication systems. Other desirable features include being a planar structure that is cost-effective to fabricate in large volumes.
Design of a UWB antennas is challenging for systems operating at the lower part of the microwave spectrum. This is due to the fact that the wavelength is very large at these frequencies and the resulting physically large antennas are not desirable due to space limitations of modern systems. This problem is particularly severe in systems that operate at HF, VHF, and UHF bands. This is because in such applications having antennas with low visual signatures is of paramount importance. The current antennas of choice for these applications tend to be monopole whip antennas. These antennas, however, suffer from two major drawbacks. First, the relatively large heights of a conventional monopole whip antenna, when mounted on a vehicle, such an antenna significantly protrudes from the top surface of the vehicle drastically increasing the visual signature of the vehicle. The second issue with monopole whip antennas is their narrow bandwidths, which limits the types of waveforms that they can receive or transmit. Therefore, development of compact, low-profile, and ultra-wideband antennas is of particular interest in many communications systems that operate at HF, VHF, and UHF frequencies. To increase the bandwidth of monopole-type radiators, a number of different techniques have been examined. A variety of printed monopole antennas that provide UWB operation in a planar form factor are examined in the 3.1-10.6 GHz band [7][8][9]. However, at its lowest frequency of operation, a printed monopole tends to have relatively large dimensions.
Numerous techniques have been investigated and reported in recent years to reduce the size of microstrip antennas. These techniques are mainly based on loading a patch antenna with reactive components realized with suitably designed slots, shorting posts, and lumped elements. The effectiveness of these techniques is, however, limited in the reduction of the footprint of planar antennas which is required by modern wireless systems [10][11][12]. The size of patch antennas can also be reduced by fabricating the antenna on dielectric substrates with a high permittivity. This well-established technique, however, results in increased surface wave excitation over the antenna that degrades its impedance bandwidth, radiation efficiency, and its radiation characteristics [13].
Techniques mentioned above fail to meet the challenges for miniaturization of antennas. In this chapter, the exploitation of artificially engineered materials that are based on metamaterial transmission lines is shown to provide the solution for miniaturization of planar antennas [14][15][16][17][18][19][20][21][22]. In fact metamaterials, which are also accurately referred to as composite right-/ left-handed (CRLH) transmission lines, are a novel paradigm in electromagnetic engineering as such materials exhibit electrical characteristics, that is, negative permittivity and permeability, not possible with naturally occurring materials [11]. These properties are exploited in this chapter to design planar antennas with a small footprint using standard manufacturing photolithography techniques [11][12][13][14][15][16][17][18]. In particular, in this chapter, a unique CRLH-TL structure is employed in the design of an antenna using conventional microwave integrated circuit (MIC) manufacturing techniques. The unit cells of the CRLH-TL structure was realized by engraving L-and T-shaped slits on a rectangular conductor that is grounded through a high impedance microstrip stub that are spiraled. In the antenna the L-and T-shaped slits exhibit capacitive property, and the stub acts like an inductor. The antenna structure, which was created using these unit cells, was modeled and optimized for UWB performance while maintaining its radiation characteristics in terms of gain and radiation efficiency.

Composite right-/left-handed antenna design
A left-handed transmission line structure can be created from an arrangement consisting of a series capacitor and shunt inductor. This LC circuit configuration can be achieved for antenna applications by simply etching a dielectric slot in a metallic-radiating patch, where the patch is grounded using a high impedance microstrip line. The slot created in the patch acts like a series capacitance C L and the high impedance microstrip line acts like a shunt inductance L L . The inductive element in the structure can be coiled to reduce the overall footprint of the antenna structure [23,24]. This approach is used here to implement a compact antenna by cascading together a number of CRLH transmission line unit cells comprising left-handed LC unit cells.

The equivalent circuit model for a lossless CRLH transmission line unit cell is shown in
and where β ( ω ) , s ( ω ) , Z ( ω ) , and Y ( ω ) represent the dispersion relation, sign function, impedance, and admittance of the antenna structure, respectively. Series and shunt resonance frequencies, respectively, are given by The unit cell's permittivity and permeability are given by The prototype antenna based on CRLH transmission line unit cells, shown in Figure 2, consists of L-and T-shaped slits etched on a rectangular radiation patch. The patch is shortcircuited to ground through high impedance microstrip lines that are coiled to reduce the antennas' size. Appropriate number of CRLH unit cells is cascaded together in the antenna that is terminated in a matched load to achieve the required bandwidth and radiation characteristics. The antenna was fabricated on glass-reinforced epoxy FR-4 substrate with a dielectric constant of 4.6, thickness of 0.8 mm, and loss tangent of 1 × 10 −3 . Standard manufacturing technique was used to realize the antenna.
The antenna design was first analyzed and optimized using ANSYS high frequency structure simulator (HFSS™). Two waveguide ports were defined to represent the input and output of the antenna structure, as shown in Figure 2(a), in order to evaluate its performance. The antenna was excited at port 1 through an SMA connector, and terminated to a matched 50 Ω load at port 2 using surface mount technology (SMD1206) of dimensions 3.5 × 1.8 mm 2 .  A more accurate model of the proposed CRLH transmission line unit cell that is employed in the antenna is shown in Figure 3. The model includes loss components in the unit cell structure that are represented by right-handed resistance (R R ) and conductance (G R ), and lefthanded resistance (R L ) and conductance (G L ). These parameters account for the radiation emitted by the antenna. Optimized magnitudes of these parameters in the unit cell were obtained from ANSYS HFSS™, that is, L L = 6 nH, C L = 2.4 pF, L R = 2 nH, C R = 1 pF, R L = 5 Ω, R R = 3 Ω, G L = 4.5 ℧, and G R = 2 ℧.
The actual antenna's dimensions are 22.6 × 5.8 × 0.8 mm 3 or 0.037 λ 0 × 0.009 λ 0 × 0.001 λ 0 , where λ 0 is free space wavelength at 0.5 GHz. The simulated and measured impedance bandwidth of the antenna are 11.1 GHz (0.35-11.45 GHz) and 10.8 GHz (0.5-11.3 GHz), respectively, for voltage standing wave ratio (VSWR) < 2. This corresponds to a fractional bandwidth of 188% for the simulation result, and 183% for the measured result. The reflection coefficients and the measured VSWR of the proposed miniaturized antenna are shown in Figure 4. The simulated and measured group delay response depicted in Figure 5 shows the group delay variation is under 0.25 ns for a large frequency range up to 16 GHz.
Results of the parametric study are shown in Figure 6. It is evident from these results that the antenna's gain and radiation efficiency can be improved by increasing the number of CRLH unit cells. The peak gain and radiation efficiency are obtained at around 8 GHz. Gain of greater than 4 dBi from 2.9-12.6 GHz is achieved for a unit cell of four. Over this frequency range, the radiation efficiency exceeds 50%. As the proposed antenna had to fit within an area of 23 × 6 mm 2 , the number of unit cells selected in the design was therefore four.
The simulated and measured gain and radiation efficiency of the antenna at various frequencies are given in Tables 1 and 2. The results in these tables show a better performance obtained from the antenna over higher frequencies than at the lower frequencies. The simulated threedimensional (3-D) and measured two-dimensional (2-D) radiation patterns of the compact CRLH antenna at various frequencies are shown in Figures 7 and 8, respectively. The results show the cross-polarization is comparable or higher than copolarization at certain frequencies, however, over a small angular range in both the E-and H-planes. This is observed in other CRLH antennas too [25][26][27][28][29][30][31] which present scope for improvement of such antennas. Cross-polarization can be reduced by loading the shorting pins, which has been demonstrated for patch antennas [32][33][34][35]. The shorting pins are located in the centerline of a square patch to strengthen the surface current density near the feeding point at edge. Because of symmetric arrangement of these two shorting pins, surface current density on the patch is maintained as the odd-symmetric property with respect to the H-plane, thus tremendously degrading the cross-polarization level. Such a solution is worthy of investigation in CRLH antennas.
The measured gain and radiation efficiency of the CRLH antenna is shown in Figure 9. The antenna has a peak gain of 6.5 dBi and radiation efficiency of 88% at around 8 GHz. The antenna has a gain that exceeds 4 dBi over 3.5-12.5 GHz, and radiation efficiency over this frequency range is greater than 53%.          To summarize, this chapter presented the design and measured results of a novel antenna that is based on CRLH transmission lines. The antenna is highly compact planar structure with dimensions of 22.6 × 5.8 × 0.8 mm 3 and possesses desirable characteristics of ultra-wideband performance (500 MHz-11.3 GHz) with gain and radiation efficiency of 6.5 dBi and 88%, respectively, at 8 GHz. The low cost antenna is simple to design and easy to fabricate using standard manufacturing techniques. The CRLH transmission line unit cell constituting the antenna is realized by etching L-and T-shaped dielectric slots inside a rectangular patch, which is grounded through a high impedance transmission line. By cascading together several unit cells, the desired bandwidth and radiation characteristics can be obtained. In addition, the antenna can be easily integrated with RF electronics.