10.3 Example (balanced antennas for mobile handsets)

conductor consists of a combination of polygons. In the IE3D simulator, the source program is not open to the public and only a brief description of the method of moments is given in the IE3D User’s Manual [8].

The FIDELITY simulator is the electromagnetic simulator based on the FDTD (FiniteDifference Time-Domain) method and its basic principle is to use a finite difference to represent the differential in Maxwell’s equations. The basis is to combine the electric and magnetic fields by employing Yee’s algorithm [9]. Primary features are: (a) modeling both planar and three-dimensional structures with isotropic or non-isotropic dielectric material, (b) three-dimensional electric field and magnetic field display with slicing capability, (c) automatic non-uniform meshing and meshing-independent geometry, and so on. In the FIDELITY simulator, the source program is also not open to the public and only a brief description of the FDTD method is given in the FIDELITY User’s Manual [10].

The HFSS simulator is the electromagnetic simulator based on the full-wave FEM (Finite Element Method). This simulator can analyze the characteristics of a small antenna automatically by solving Maxwell’s equations. Primary features are: (a) modeling three-dimensional structures with CAD and dynamic geometry rotation, (b) automatic adaptive mesh generation and refinement, (c) comprehensive materials database containing permittivity, permeability, electric and magnetic loss tangents for common substances, and so on.

Microwave Studio is based on the Finite Integration Technique (FIT) and it allows choosing the time-domain as well as the frequency-domain approach. The Finite Integration Technique first describes Maxwell’s equations on a grid space, maintaining properties such as energy conservation, and then forms the specific differential equations, such as the Poisson or wave equations. The Finite Integration Technique can be formulated on different kinds of grids, e.g., Cartesian or general non-orthogonal grids. In the time domain, which is our field of interest, the resulting discrete grid equations of the Finite Integration Technique are at least in “some” cases identical to the discrete equations derived with the classical Finite-Difference Time-Domain (FDTD) method. FDTD was introduced in the mid 1960s, and uses a coordinate-based staggered-grid system and the famous Yee cell. In contrast to the Finite Integration Technique, which is applied to the integral form of the field equations, FDTD (as a subset of the Finite Integration Method) is applied to the differential form of the governing Maxwell curl equations.

10.3Example (balanced antennas for mobile handsets)

This section describes an example of simulation for performance of a small antenna located in a complicated environment. A typical example is a built-in antenna installed in a small mobile terminal, where the antenna is surrounded by various materials, electronic components, and devices that seriously affect antenna performances, and furthermore influenced by an operator who holds the terminal unit. Mobile phone handsets and other wireless terminals in the recent decade have made remarkable progress, and their size

392Electromagnetic simulation

and weights have been dramatically reduced [11]. Accordingly, antennas used for such terminals must have followed their downsizing trends, for which some issues are taken up on the design of antenna systems.

Since antennas for mobile handsets cannot be designed in isolation from their host equipment, it is desirable to think of an antenna as a system. Moreover, the antenna performances are significantly influenced by the effects of the surrounding environments such as the human body which is considered as a big lossy medium existing near the antenna. The influence by the human body cannot be neglected as it disturbs the radiation, varies in the input impedance, and causes mismatching loss [11]. Accordingly, a system designer should consider some factors in designing antenna systems for mobile handsets as follows:

The antenna performances should be maintained or enhanced although the antenna was miniaturized.

The degradation of the antenna performances caused by a human body should be mitigated.

The effect on the human body from the antenna radiation should be reduced.

The antenna performances should be maintained under the multi-path radio environments.

The essential concept in designing antenna systems for handsets is that the antennas should be (1) small size and compact, (2) lightweight, (3) low-profile or built-in type, and have structure to (4) mitigate antenna performance degradation due to body effect, particularly, operator’s hand and head, and to (5) reduce the SAR value (Specific Absorption Rate) in the human head. The factors (4) and (5) are required for the latest handset design and need the new technology for realizing antenna systems.

In addition to the above issues, with new deployment of various wireless systems, there is an increasing demand for mobile terminals to enhance functions such as wideband or multiband operation, yet maintaining small dimensions. Thus, additional design considerations to meet such requirements have become an urgent issue.

The design concept so far applied to most built-in handset antennas is to use the GP (Ground Plane over which an antenna element is placed) as a part of the radiator. This is particularly true because antennas should have small size, and the assistance of the GP is needed in order to achieve enough gain and particularly wide-enough bandwidth. Regarding (1) to (3), e.g., small size and low-profile, antennas having magnetic current as the radiation source placed in parallel to the GP is not only to realize low-profile structure, but also to use its image of the antenna so that the twofold field-strength enhancement is obtained as a result of superposition of the field produced by the image, as shown in Figure 10.1 [12]. In order to realize (4) and (5), antennas having balanced terminals and being fed by a balanced line have been proposed. The effectiveness of using such antennas has been shown in some previous papers [13–16].

So far the planar inverted F antenna (PIFA) has been used in many handsets, as it matches the needs (1), (2), and (3) mentioned above [17]. However, in practical use, gain degradation, which is serious sometimes, has been observed when an operator holds the handset. This is caused by the variation in the current distributions on the GP, which

Small loop

Ground plane

Magnetic current

Electrical image

Figure 10.1 Small loop antenna on a ground plane and magnetic current.

are produced by the excitation of the built-in PIFA element. These current distributions on the GP actually assist the antenna performance, contributing to improve gain and also bandwidth. In turn when the body effect exists, particularly when the operator’s hand grasps the handset, these currents vary and as a result the antenna performance degrades; sometimes serious gain reduction and frequency change are observed. It has been commonly observed in handsets where small antennas, not only PIFA but also other types of antennas like ceramic chips, are built in. Thus in order to avoid such antenna-performance degradation, reduction of the currents on the GP has been desired and antennas having balanced structure and being fed by a balanced line have been developed.

There has been a report that has shown another way to reduce the currents on the GP. It is the use of a half-wave monopole applied to a rectangular conducting box that simulates the handset body. The analysis of it has shown almost no current flow on the handset body with use of a half-wave monopole [18]. In practice; however, a 3/8 wavelength or a 5/8 wavelength monopole has been used for the handsets, as they have appropriate input impedance to match the load, and yet cause only a small current flow on the GP. The results of this analysis [8] have contributed to establishing the design concept for the antennas used in conventional PDC handsets. However, antennas designed with this concept do not satisfy the requirements (1) to (3) and (5).

Now it is rather natural that antennas for the latest handsets have been reconsidered and have progressed to be designed with advanced concepts. At the same time, the recent trends that antennas should be small in size, yet have enough performance and enhanced functions are consistently kept in mind and embraced in the design concept. In practice, in order to realize antenna systems which can satisfy the requirements (4) and (5) in addition to (1) to (3), new design concepts should be introduced.

For the purpose of realization of (4) and (5), various types of antennas having balanced structure have been taken into consideration [13–16]. Regarding small-sized and lowprofile antennas, use of antennas having a magnetic current source was introduced. One of these was a small loop antenna which was regarded as a magnetic dipole normal to the surface of the loop [12] and it was successfully applied practically to the box-type pager’s antenna [19]. A small loop antenna built into a pager was placed in the pager body in such a way that the loop surface was perpendicular to the human body instead of

394Electromagnetic simulation

to the GP so that the field produced by the image loop was superposed to obtain twofold field strength in front of the human body.

Pager systems have become disused in practice; however, the design concept employed for the pager antenna can still be applied to realize small antennas.

Another useful way to create a small antenna is the integration technique, by which an antenna is combined with other elements or devices so that the radiation mode is increased and the antenna performance is enhanced, even when the antenna size is made small. A folded loop antenna introduced here is a typical example of this type of antenna, which also has balanced structure.

The design concept for handset antennas thus favors a balanced structure in order to achieve small antennas and yet maintain performance that does not degrade in the vicinity of the human hand and head. In designing the latest handset antennas, other significant considerations are included; these are on the realization of very small antennas, feasibility of impedance matching for wide bandwidth, and so forth.

10.3.1Balanced-fed folded loop antenna

In the previous sections, it has been shown that antennas having balanced structures and being fed with a balanced line are very effective to mitigate the antenna performance degradation due to the body effect and also reduce the radiation toward the human head [13–16]. As Figure 10.2 shows, various types of antennas, such as rectangular loop and L-type loop, which have balanced terminals and are fed with a balanced line, have been introduced. The principle of using a balanced structure is to reduce the current flow on the GP excited by the antenna element placed on the GP.

In turn by applying this concept to the handset antenna design, a further serious issue has been raised. That is how to realize an antenna which has enough gain and bandwidth, even when the size has been made small and the assistance of the GP as a part of the radiator is removed. Problems are to design an antenna which has enough gain and bandwidth with a small-sized structure. The proper impedance matching is also a problem, because there is some difficulty in obtaining desired impedance by conventional antennas, as shown in Table 10.1. Use of a balun in order to feed antennas with balanced terminals provides a sort of complexity for impedance matching and usually makes the bandwidth narrow.

Then a folded loop antenna is taken into consideration as one of the candidates which solves these problems and satisfies the requirements in the design concepts. This antenna has an integrated structure, which is composed of a radiating element and a reactive element; these being constituted by using a two-wire transmission line, and folded at a quarter-wavelength to form a folded half-wave dipole equivalently. The equivalent folded dipole acts mainly as the radiator, and the two-wire line is used for adjusting the antenna impedance [20–22]. The antenna has a one-wavelength loop structure so that no unbalanced current may be produced on the feed line; that is to say, this antenna has a selfbalanced structure [21], which is useful to reduce the current flow on the GP. In addition, since this antenna can be built in a small volume by means of its folded structure, small size and low profile can be achieved. Furthermore, use of a two-wire transmission line can

Table 10.1 Input impedance of balanced-fed antennas (f0 = 186 GHz)

Input impedance [ ]

z Eθ

y

x

(a) Rectangular loop antenna (Type A)

z

y

Eφ

x

(b) Rectangular loop antenna (Type B) z

x

(c) L-type loop antenna

Figure 10.2 Balanced-fed antennas: (a) rectangular loop antenna (Type A), (b) rectangular loop antenna (Type B), and (c) L-type loop antenna.

make possible flexible adjustment of antenna impedance by changing parameters such as the length and the width of wires, and the distance between the two wires, and no balun is necessary. This antenna can be designed to have enough gain and bandwidth to be applied to handsets presently used in practice. In order to achieve wide bandwidth, the wire should be replaced by a ribbon-shaped element with wider width. A parasitic element placed along with the folded loop element is effective to achieve further wide bandwidth.