WILEY ANTENNAS FOR PORTABLE DEVICES phần 3 ppt

44 Handset Antennas

of a calibrated standard-gain antenna – usually a horn or a standard dipole. Great care must

be taken in the design of cables and connections within the measurement system and careful

attention paid to mechanical stability and the protection of vulnerable components from

damage. The effects of temperature change on system calibration must be assessed; they

may be reduced by careful system design or by limiting the extent to which the ambient

temperature is able to vary.

2.6.5.5 Efficiency

Efficiency is measured by integrating the total power flux (or gain) measured over the whole

spherical surface containing the device under test. The details of computation vary according

to the distribution of the measurement points over the (virtual) measurement surface.

2.6.5.6 Specific Absorption Rate

SAR is measured by placing the handset under test next to a plastic phantom head filled

with a sugar-saline solution with similar dielectric properties to brain tissue. A probe is

moved inside the phantom and field levels are measured as its position is varied. Industrial￾grade robotic control is used in high-accuracy systems capable of absolute measurements for

certification purposes, while comparative tests can be made with less costly hand-operated

equipment.

2.6.5.7 Hearing Aid Compatibility

This is evaluated by measuring the axial and radial magnetic fields in the vicinity of the

user’s ear [21].

2.6.6 Design Optimization

This begins with the adjustment of the antenna and matching circuit to achieve a low input

VSWR over the working bands followed by measurement of the efficiency of the antenna

in place on the handset. Efficiency is usually optimized by investigating and modifying

the interactions between the antenna and other handset components and by reference to

simulations to assist with an understanding of the fields and loss processes which may be

occurring.

Optimization is not easily reducible to a simple procedural algorithm. It requires clear

understanding of the possible mechanisms at play, the development of insight into the

operation of the antenna and its interaction with the handset, experience, lots of patience and

a certain amount of luck.

2.7 Starting Points for Design and Optimization

The design of an antenna for a particular handset is constrained by the available dimensions.

These include any keep-out areas over components located under the antenna that might

need access – for example a test port connector, or a loudspeaker sufficiently thick to make

2.7 Starting Points for Design and Optimization 45

it unlikely that the antenna can extend over it. The design may begin with one of the familiar

canonic antenna models but the geometry will be modified to fit the available space.

2.7.1 External Antennas

The design of an external antenna is relatively straightforward. Extensive references to dual￾band external helical antennas are provided by Ying [15] and Haapala [26] (see Figure 2.19),

and design procedures for printed spirals are provided by Huang [27]. An alternative but

less common form of 3D branched monopole is proposed by Sun [28] (see Figure 2.20).

Figure 2.19 Non-uniform spiral antennas in cylindrical format [15] and concentric whip and spiral

[26].

Figure 2.20 Non-uniform spiral in flattened format [27] and 3D branched monopole [28].

46 Handset Antennas

Figure 2.21 A 2D branched monopole [29] and a hybrid dielectric-loaded/DRA [30].

2.7.1.1 Off-Groundplane Antennas

These often take the form of branched monopoles [29] (Figure 2.21), one or both of which

may be loaded with a dielectric pellet. The shape and dimensions of these are very variable

and they can be modified to suit the available space. An alternative format comprises a single

element which operates as a loaded monopole at one frequency and a dielectric resonator

antenna (DRA) at a higher frequency [30].

2.7.1.2 On-Groundplane Antennas

The almost universal format for on-groundplane antennas is some form of PIFA. These

exist in a wide variety of shapes and configurations [31]. The following paragraphs provide

general descriptions of a number of subclasses of PIFA. For all of them the basic relation￾ships between bandwidth, efficiency, antenna dimensions and chassis size apply. The more

complex forms have been created in an effort to increase the number of bands covered and

to squeeze the highest bandwidth and efficiency from a given geometry and environment.

In most cases the capacitive top of the antenna can be meandered, folded or convoluted to

reduce the maximum linear dimensions of the antenna – the exceptions to this being those

designs which themselves seek to optimize the geometry of the radiating element.

Simple Single-Band PIFAs

Because of requirements for multi-band operation, PIFAs are now most frequently used as

antennas for Bluetooth™, Zigbee and WLAN. To reduce their dimensions they are typically

dielectrically loaded, often with meandered conductors, and are produced in the form of

surface-mounted devices, using printed-circuit or LTCC techniques. Short-range protocols

demand less antenna efficiency than is needed for mobile phone applications – the low power

levels at which they operate allows some compensation for losses by increased power, so

more severe compression of dimensions is often accepted.

Multi-Band PIFAs

Early designs [16] have been elaborated to increase the available bandwidth as the number of

assigned mobile bands has increased. The simple two-pronged radiator is usually folded so

the low-band radiator either encloses the high-band radiator or lies next to it (Figure 2.22).

The choice between these configurations lies in the relative performance needed in the two

band groups – the radiator with the open circuit end on the outer edge generally performs

better, so Figure 2.22(a) has better high-band performance than Figure 2.22(b) which may be

preferable when the chassis is short or the height is restricted. It is often possible to reverse