Figure 7.202 Photograph of fabricated CBCSLA ([100], copyright C 2008 IEEE).

Figure 7.203 Topology of cavity-backed slot loop antenna; (a) original slot loop, (b) modified slot loop for size reduction, and (c) sectionalized slot loop for input impedance matching ([100], copyright C 2008 IEEE).

the geometry shows conceptually the proposed composite slot loop antenna, which is fed capacitively with a stub. To reduce the antenna dimensions, the edges of each slot are folded into a spiral-like shape that acts at the same time as inductive load on the edges. Figure 7.204 shows the antenna structure along with the antenna geometrical parameters. As can be seen in the figure, the coax feed is connected to the six Coplanar Waveguides (CPW), by which each of the slots is fed separately. At the edges of each CPW line, a corrugated stub is attached to control capacitance and thus improve impedance matching to the slot. Measured and simulated S11 of the CBCSLA with the cavity height hc = 12.7 mm is shown in Figure 7.205, and simulated radiation pattern is depicted in Figure 7.206.

7.2.5Applications of metamaterials

Real metamaterials (MM) are not available in nature; however, equivalent media may be constructed of either resonant particles (RP) MM [101a, b, 102] or transmission lines (TL) MM [103a, b]. The RP MM may consist of periodical arrays of sub-wavelength thin wires (Figure 6.18) [101] or split rings (Figure 6.19) [102]. They provide either negative

z

Theta

y

phi

x

Figure 7.206 Simulated radiation pattern of reduced-size CBCSLA ([100], copyright C 2008 IEEE).

MM has unfavorable properties; the typical ones are unsuitability for most microwave applications, narrow bandwidth, large loss, and necessity of volumetric formation. The TL MM, on the contrary, has advantages: applicability in microwave frequency regions, wide bandwidth, low loss, and suitability for implementing in not only planar, but also volumetric structures. They have been extensively studied, and various applications to electromagnetic devices and antennas have been introduced [104–106].

The MM has unique dispersive characteristics and phase delay of wave propagation in the media that are attributes of double negative constitutive parameters (ε < 0, μ < 0). These MM properties can be effectively used to develop novel antennas. In the CRLH MM, as the frequency ω becomes higher, the wave number β decreases, corresponding to increase in the wavelength λ, thus lowering the resonance frequency in the media. Hence, an appropriate design of the wave number β in an antenna system constituted of an LH medium to obtain desired frequency ω renders the antenna size reduction.

In a CRLH TL MM of length l, constituted of a finite number N multiple unit cells of length p, there would exist multiple resonance modes when length l is a multiple of half a wavelength λg (guided wavelength), that is, l = nλg/2, with n = 0, ± 1, ±2, . . . ± ∞. A CRLH TL can be treated as effectively homogeneous media when the electrical length of the unit cell is smaller than π /2, that is, p < λg/4. The dispersion diagram of the CRLH TL MM is shown in Figure 7.207. The CRLH structure can support negative resonance (n < 0) in the LH region, because of transfer of the phase origin from frequency zero to the transition frequency ω0, and also zeroth-order resonance (n = 0) at ω0 in addition to the ordinary positive resonance (n > 0) in the RH region [107] (Figure 7.207 and Figure 6.49). Multiple resonances may occur at frequencies of n π /N. Therefore, resonance at two or more frequencies in a medium can be attained, indicating possibility of a multiband antenna design. When n = 0, that is the zero-order mode, β = 0, and λ is infinite, but the group velocity vg is not zero, as the wavelength becomes infinite, and the amplitude and phase of the wave are the same anywhere in the media. This means that the resonance is not dependent on the dimensions of the resonator, but

Figure 7.207 Resonance spectrum of a CRLH structure ([116], copyright C 2008 IEEE).

solely on the values of lumped reactive components on the unit cells, and hence this property can be exploited to design an electrically small antenna.

In the DNG media, negative β (the wave number) suggests that direction of the power propagation in the media is opposite to that of the phase propagation. By introducing such condition as negative β in an antenna structure, a backward radiation can easily be produced, whereas an ordinary LW (Leaky Wave) antenna radiates only in the forward direction. A balanced-type CRLH TL [103a] can be used for a frequency scanned LW antenna, which is designed to produce backward radiation as well as forward by using the LH/RH property. By constituting a CRLH TL MM in 2D structure, a larger aperture is obtained and consequently a small antenna with enhanced gain can be designed. By dual feeding with phase delay circuits to the 2D CRLH TL MM, a circularly polarized small antenna can be realized.

Other than artificial materials, there are some materials which can exhibit negative permeability. For example, ferrite materials show negative-mu property near magnetic resonance, although the mu changes rapidly once reaching the positive peak value, turning to quickly descend to the negative lowest value through zero, and then gradually increasing to recover positive value. Then, over the frequency range in which the permeability takes negative values, the ferrite material can be used as a negative-mu (MNG) material. However, this frequency range is rather narrow, and that often checks the practical applications.

Meanwhile, as was described previously, real MNG materials have been realized with BaFe material and permalloy composite [108]. With the BaFe material, negative mu can be obtained in a lower-frequency region, 2 to 5 GHz, while the permalloy composite exhibits negative mu in higher-frequency regions, 9 to 18 GHz and beyond. Figure 7.208 provides variation of the permeability of the permalloy composite with respect to frequency. However, since these materials have some loss and a somewhat high permittivity, this problem has been targeted to be improved.