Metamaterial Antennas for Wireless Communications Transceivers

Limited space is given to antennas in modern portable wireless systems, which means that antennas need to be small in size and compact structures. However, shrinkage of con‐ ventional antennas leads to performance degradation and complex mechanical assembly. Therefore, the design of miniature antennas for application in wireless communication systems is highly challenging using traditional means. In this chapter, it is shown that metamaterial (MTM) technology offers a solution to synthesize antennas with a small footprint with the added advantage of low cost and excellent radiation characteristics.


Introduction
In this chapter, novel and compact planar antennas are presented including ultra-wideband (UWB) and hexa-band antennas. The UWB antenna presented in part 2 is based on metamaterial (MTM) transmission lines. In reality, the MTM antenna structure is more accurately described as a composite right-/left-handed transmission line (CRLH-TL) structure due to the resulting parasitic capacitance and inductance effects. The design of the UWB antenna is achieved by embedding E-shaped dielectric slits in the radiating patches. It is shown that the dimensions of such patches have a considerable influence on the radiation characteristics of the antenna. Parametric study is undertaken to demonstrate how the MTM unit-cell's parameters affect the antennas performance in terms of gain, radiation efficiency, and radiation patterns. A novel antenna structure referred to as 'hexa-band' coplanar waveguide (CPW)-fed antenna, which is presented in part 3, consists of asymmetric fork-shaped-radiating elements incorporating U-shaped radiators with a slit. Each of the branched radiators generates triple resonant frequencies within the L-, S-, C-and X-bands.

Ultra-wideband antenna
Geometry of an innovative ultra-wideband (UWB) antenna in Figure 1 shows that it is essentially a patch antenna with a rectangular slot. The antenna is terminated on the right-hand (RH) side with a grounded 50-Ω resistive load in order to reduce any impedance mismatch and thereby enhance the antenna's performance. Enclosed inside the slot are three radiating patches on which is etched an E-shaped dielectric slit. The middle patch of the antenna is excited through a coplanar waveguide (CPW) transmission line. A larger E-shaped slit is included in the antenna next to the 50-Ω load, which exhibits characteristics of the simplified composite right-/left-handed transmission line (SCRLH-TL) [1]. In the equivalent circuit of the antenna, the left-handed capacitance (C L ) results from the dielectric gap between the patches and the conductor next to the larger E-shaped slit. The impedance bandwidth of the antenna is affected by the smaller E-shaped slits and the dielectric gap represented by (C L ). In fact, the radiation properties of the antenna are affected by the dimensions of the three patches. Prototype of the antenna was fabricated on RT/duroid ® RO4003 dielectric substrate, with a thickness (h) of 0.8 mm, dielectric constant ( ε r ) of 3.38 and tanδ of 22 × 10 -4 . The antenna design was optimized using full-wave electromagnetic (EM) simulators, that is, of Agilent Advanced Design System (ADS), High-Frequency Structure Simulator (HFSS™) and CST Microwave Studio (CST-MWS). The simulation results were validated with the actual measurements.
The configuration of the E-shaped slit was determined through simulation analysis. As it will be shown later, the size of the three patches actually has an influence of the antenna gain and efficiency properties. The equivalent circuit of the E-shaped slit with the absence of lefthanded inductance ( L L ) and its microstrip structure is shown in Figure 2(a) and (b), respectively. Parameters of the simplified CRLH structure in Figure 2 are given by where W and l represent the width and length of the microstrip line, respectively, and h and ε r the thickness and relative permittivity of the substrate, respectively.
The E-shaped slit structure employed here is shown in Figure 2(b). Based on the resonance frequencies of this structure, the initial magnitude of C L , C R , and L R can be obtained from the dispersion relation of the SCRLH-TL. This is determined by applying the periodic boundary conditions related to the Bloch-Floquet theorem, given by The series impedance of the unit cell is represented by Z and its shunt admittance by Y. The series right-handed (RH) inductance is represented by L R , and its shunt RH capacitance by C R . By substituting Eqs. (4) and (5) into Eq. (3), the dispersion can be represented by Figure 3 shows the dispersion diagram of the antenna based on SCRLH-TL as determined by HFSS using Eq. (6).
Negative-order modes in the SCRLH-TL structure vanish with the absence of left-handed capacitance or inductance. Hence, the electrical size of the SCRLH-TL structure can be reduced by increasing the magnitude of C L , L R and C R . This can be achieved by adjusting the structure dimensions. An optimization technique has been adopted within ADS, HFSS™ and CST-MWS commercial tools, leading to the final size of the structure. The resulting antenna dimensions are annotated in Figure 1, and the corresponding values of the lumped elements are C L = 8.2 pF, L R = 5.8 nH and C R = 5.1 pF.
The antenna was fabricated using standard manufacturing techniques, and its performance  Anechoic chamber was used to accurately characterize the antenna's performance in terms of its gain and radiation efficiency. This involved applying RF power to the antenna and measuring the EM field radiated from it in the surrounding space. Radiation efficiency was calculated by taking the ratio of the radiated power to the input power of the antenna. Standard gain comparison technique was used to measure the antenna's gain. This involved using a pre-calibrated standard gain antenna to find the absolute gain of the antenna under test. Figure 4 shows how the vertical dielectric gap between small patches (L gap ), and the horizontal gap between small patches and the metallization next to the larger E-shaped slit (W gap ), affects the antenna's return-loss performance. It is evident from the simulation that both these dielectric gaps can have a significant effect on the impedance bandwidth of the antenna. The simulation results show that by reducing these gaps, the antenna's impedance bandwidth can be enhanced.  The antenna resonates at two distinct frequencies, as shown in Figure 5, which are measured at 4.75 and 7 GHz. Simulation results predict 4.15 and 6.8 GHz (ADS), 4.03 and 6.68 GHz (CST-MWS), and 4 and 6.6 GHz (HFSS™). The measured results agree well with the ADS prediction. The divergence in the results is attributed to manufacturing tolerances.
The distribution of the current density over the surface of the antenna at the two resonance frequencies of 4.75 and 7 GHz is shown in Figure 6. The current density distribution is symmetrical with reference to the feed line.
Excitation of the proposed antenna structure though the SubMiniature version A (SMA) connector's feed line can result in an imbalance in the current flow over the outer metallization  of the antenna, which can severely undermine the radiation characteristics of the antenna. To prevent this from happening, it was necessary to incorporate three smaller patches in the antenna slot. As is evident in Figure 1, only the middle patch is excited through the feed line, whereas the other two patches are used as parasitic radiators. This configuration facilitates the concentration of the EM fields in the proximity of the antenna structure instead of dispersing the field over its ground plane, which would otherwise contribute in unwanted coupling. Figure 7 shows that a larger number of patches can significantly improve the antenna's bandwidth and gain properties. The antenna was implemented on a RT/duroid ® RO4003 substrate with total dimensions of about 20 × 20 mm 2 . Within this size, the maximum number of inner patch arms was restricted to three. The resulting antenna gain and radiation efficiency at its operating frequency range are given in Table 1.
The radiation patterns of the antenna were simulated using ADS, CST-MWS and HFSS™. The simulated and measured radiation patterns are shown in Figure 8 at the two resonance frequencies.
The antenna radiates similarly to a dipole antenna with a large coverage angle at the two resonance frequencies. Figure 9 shows the simulated and measured radiation gains of the antenna. The measured radiation gain is greater than 1.2 dBi between 0.7 and 9 GHz, with a peak of 4.2 dBi at 4.8 GHz.      It has the second highest gain and its relatively small size is comparable to most previous works. These results clearly confirm that the antenna is great candidate for ultra-wideband communication systems.

Effect of the 50-Ω load on the antenna bandwidth
The return loss of the antenna with and without the 50-Ω load is shown in Figure 10. It is evident that the 50-Ω load enhances the impedance match so that the impedance bandwidth improves from 131.8% (without 50 Ω) up to 164.6% (with 50 Ω). Excellent match is discerned near the two resonance frequencies of the antenna, at 4.8 and 7 GHz, thus enabling effective radiation at the resonance frequencies. These results are summarized in Table 3.

Effect of the SMD load on the radiation performance
Antenna gain and efficiency performance, with and without the 50-Ω impedance load, are shown in Figure 11, and the results are tabulated in Table 4. These results show that a small improvement is achieved with loading the antenna.   Table 3. Antenna bandwidth with and without 50-Ω load.

Hexa-band antenna
Configuration of a novel hexa-band CPW-fed antenna, shown in Figure 12, comprises three asymmetric fork-shaped-radiating stubs with U-shaped-radiating elements. Embedded within the U-shaped elements is a dielectric slit. The tri-branched radiator generates three distinct resonant frequencies within the L-, S-, C-and X-bands. The U-shaped element also generates resonant frequencies at the lower band of the antenna. The asymmetrical forkshaped stubs improve the impedance-matching characteristics of the antenna and reduce its stopband. The antenna is fabricated on an FR4 substrate with relative permittivity ε r = 4.4 and thickness h = 1.6 mm and is fed through a coplanar waveguide transmission line with 50-Ω impedance. The substrate size is 35 × 26 mm 2 .  The antenna was excited through a 50-Ω coplanar waveguide transmission line. Figure 13 shows that it resonates at 1.3, 1.75, 3.35, 4.85, 6.5 and 7.6 GHz. There is good agreement between simulated and measured results. Any divergence between the simulated and measured results is ascribed to a number of factors, that is (1) the non-uniform current density distribution over the antenna structure, (ii) imperfect equivalent circuit models, (iii) undesirable EM coupling and (iv) manufacturing errors. This antenna is shown to function between 700 MHz and 11.35 GHz, and has a fractional bandwidth of 176.76% that includes several communication standards, in particular, GSM, DCS, PCS, Bluetooth, WLAN, WiMAX and WiFi along with the major parts of the C-and X-bands. The measured E-plane and H-plane radiation characteristics at 1.3, 1.75, 3.35, 4.85, 6.5 and 7.6 GHz are shown in Figure 14. The measured results show that the antenna radiates omnidirectionally at these frequencies.
In Figure 13, the impedance bandwidth of the lower passband at 1.      Coverage bands

Conclusion
To summarize, a compact antenna comprising three asymmetrical branched fork with Ushaped strips is shown to exhibit hexa-band characteristics. The antenna meets multi-communication standards including L-, S-, C-and X-bands for GSM, DCS, PCS, Bluetooth, WLAN, WiMAX and WiFi applications. Slits in the U-shaped strips are shown to excite additional resonant bands. The proposed structure reduces stopbands and improves the antenna's impedance-matching performance. The antenna exhibits good return loss, gain and radiation patterns, which makes it an excellent candidate for multiband and broadband communication applications.