332Design and practice of small antennas II

depending upon in which configuration the antenna is set. That is, a ratio Gϕ /Gθ can be higher or lower than unity. By taking advantage of this pattern performance, the antenna can achieve pattern diversity in terms of polarization. It can be also said that the antenna is applicable to diversity of not only polarization, but also power and phase by means of the configuration switching.

Diversity performance is evaluated both theoretically and experimentally. A measure of the diversity performance is the correlation coefficient ρ, which determines the quality of the communication channel in diversity and is defined as the correlation between incident wave envelopes of different polarizations [62]. It is a function of complex incident fields of two polarizations, XPD, a ratio of incident wave powers, and arrival angle distributions of incident waves. With assumption of uniform distribution in the angle of arrival (AoA) and XPD < –10 dB, the simulated ρ is less than 0.12, but with nonuniform distributions, ρ takes different values depending on the configuration, because incident waves have different envelopes depending on the AoA. Thus three envelope correlations, ρ12, ρ13, and ρ23, because of three available pattern configurations, must be taken into consideration. As for the non-uniform AoA distributions, a Laplacian distribution is assumed [59]. With assumption of the Laplacian distribution and XPD = 0 (indoor condition) and 6 dB (urban field condition), ρ is evaluated higher than that in case of uniform distributions, but is estimated to be less than 0.47, which is still a practical and useful value. Experiments verified these simulated results.

It can be said that the reconfigurable cubic antenna is well suited to apply pattern diversity and also power, phase, and polarization diversity.

8.2Design and practice of PCSA (Physically Constrained Small Antennas)

A PCSA is an antenna that is not necessarily ESA, but has portions sized similarly to ESA. Typical PCSAs would be low-profile antennas, having the height comparable with the size of ESA. They are microstrip antennas, PIFAs, and printed antennas.

8.2.1Low-profile structure

Low-profile structure can be comprised of an antenna element and a ground plane, which provides an image effect. When the ground plane is a PEC (Perfect Electric Conductor), the image effect is negative for an electric source placed parallel to the ground plane, whereas it is a positive effect for a magnetic source. The PEC ground plane can be replaced by a high impedance surface (HIS) or a surface that exhibits an electromagnetic bandgap (EBG) property that can produce a positive image of the antenna, reducing the antenna profile as well as improving the antenna performance. Use of EBG is quite beneficial not only for miniaturizing the dimensions, increasing gain, and bandwidth, but also for reducing the mutual coupling between two closely mounted antennas. Recently, much interest has focused on applications of DGS (Defected Ground Surface) associated with low-profile antennas and development of small antennas.

Figure 8.79 3D view of a mushroom-like HIS structure ([64], copyright C 2003 IEEE).

8.2.2Application of HIS (High Impedance Surface)

The concept of HIS was first introduced in 1999 [63], where a mushroom-like HIS surface was treated. The mushroom-like surface has periodic structure, consisting of a lattice of metal plates connected to a solid metal sheet by vertical conducting metal vias. Figure 8.79 shows a sketch of the typical mushroom-like structure [64]. The structure can be visualized as mushrooms protruding from the back-plane surface. Since the protrusions are small compared to the operating wavelength, their electromagnetic properties can be described by using lumped capacitors and inductors, behaving as a network of parallel resonant L-C circuits, which acts as a two-dimensional filter to block the current flow along the surface. The surface impedance is modeled as a parallel resonant L-C circuit, which exhibits high impedance over a predetermined tuned frequency band. The surface acts equivalently as a frequency selective surface (FSS). The HIS can be considered as a kind of two-dimensional photonic crystal that prevents the propagation of radio frequency surface currents within the bandgap. The surface does not support propagating surface waves and its image currents are not in phase reversal, but in-phase, allowing a radiating element to lie directly adjacent to the surface, while still radiating efficiently. Figure 8.80(a) shows an example of the transmission coefficient S21 of a surface with suppression bandwidth from 0.98 to 1.35 GHz and (b) gives the phase of the reflection coefficient of a surface with an operational bandwidth from 0.88 to 1.35 GHz [64].

The HIS has proven to be useful as an antenna ground plane on which the surface wave is suppressed, resulting in less radiation in the backward direction. The reflection phase is unusual, allowing an antenna to lie directly adjacent to the ground plane (HIS) without being shorted out, by which antenna gain is enhanced [63].

Design methodology for the mushroom-like HIS was described in [64].

An HIS can be constituted by surfaces other than the mushroom-like one. Use of periodically corrugated reflectors shown in Figure 8.81 is an example, where a planar hexagonal dipole is placed close to the surface with its axis parallel to the grooves, which in this case is referred to as a system of H-type corrugation surface [65]. When a dipole is placed with its axis perpendicular to the grooves, it is referred to as a system of E-type corrugation surface. By combining E-type and H-type corrugated reflectors, and by applying a UWB dipole with only 21-mm profile on the surface, a very wide