WILEY ANTENNAS FOR PORTABLE DEVICES phần 2 ppt

14 Handset Antennas

Table 2.2 SAR limits for the general public specified by various administrations.

Australia Europe USA Japan Taiwan China

Measurement

method

ASA

ARPANSA

(ICNIRP)

EN50360

ANSI

C95.1b:2004

TTC/MPTC

ARIB

Whole body 0.08 W/kg 0.08 W/kg 0.08 W/kg 0.04 W/kg 0.08 W/kg

Spatial peak 2 W/kg 2.0 W/kg 1.6 W/kg 2 W/kg 1.6 W/kg 1 W/kg

Averaged over 10 g cube 10 g cube 1 g cube 10 g cube 1 g cube 10 g

Averaged for 6 min 6 min 30 min 6 min 30 min

densities, dielectric constants, dielectric loss factors and complex shapes. This is a situation

which has to be simplified to provide handset designers with engineering guidelines with

which they can work, so for regulatory purposes a standard physical phantom head is

used in which the internal organs are represented by a homogeneous fluid with defined

electrical properties. With a handset positioned beside the phantom and with its transmitter

switched on, the fields are probed inside the phantom. They are translated into SAR values

and the pattern of energy deposition is mapped to determine the regions with the highest

SAR averaged over 1 g and 10 g samples. Simulations are often carried out using this

‘standard head’, but more realistic information is obtained using high-resolution computer

models based on anatomical data.

Extensive investigation of possible health effects of RF energy absorbed from mobile

phones has been carried out in many countries. Current results suggest that any effects are

very small, at least over the time period for which mobile handsets have been in widespread

use. Those interested should consult the websites of the major national occupational health

administrations and medical journals. The responsibility of the antenna designer is to

ensure that the user is exposed to the lowest values of SAR consistent with the transmission

of a radio signal with the power demanded by the network.

Hearing aid compatibility. Handsets operating with time-division multiplex protocols such

as GSM emit short pulses of radio energy. A hearing aid contains a small-signal audio

amplifier and if this is presented with a high-level pulsed radio signal the result of any

non-linearity in the amplifier will be the generation of an unpleasant buzzing sound. Some

administrations place networks under a responsibility to provide some proportion of their

handsets which are designed to minimize these interactions.

2.3 Electrically Small Antennas

The dimensions of handset antennas are very small compared with the operating wavelength,

particularly in the low bands. Not only is the antenna small, but the length of the handset

to which it is attached – typically between 80 and 100 mm – is also only a fraction of a

wavelength long. A typical handset antenna is less than 4 ml in volume (about one thousandth

of a cubic wavelength) and a 90 mm chassis is only 0.27 long at 915 MHz.

The operation of electrically small antennas is dictated by fundamental relationships which

relate their minimum Q-factor to the volume of the smallest sphere in which they can be

enclosed, often referred to as the Chu-Harrington limit [12, 13]. The Q relates stored energy

2.3 Electrically Small Antennas 15

and dissipated energy, and a small antenna intrinsically has a very reactive input impedance

with an associated very narrow bandwidth. We can compensate for the input reactance by

adding an opposite reactance, but the combination will have a higher Q and less bandwidth.

We can trade efficiency for bandwidth, but we want to achieve the highest possible efficiency

at the same time as enough bandwidth to cover the mobile bands – perhaps several bands.

Whatever ingenuity we apply, it is often impossible to obtain the combination of properties

we need from such a small device.

A simple small antenna is shown in Figure 2.1, where a short monopole is fed against a

groundplane. This antenna looks capacitive all the way from DC to the frequency at which

it is almost /4 long. The input impedance has the form Zin = R + jX, where R is small

and X is very large. The bandwidth will be limited by the Q of the device, where Q = X/R.

If the antenna is a very small fraction of a wavelength long, it is necessary to excite a

very large current in it to persuade it to radiate any significant power; put another way,

its radiation resistance is very small so it must carry a large current to radiate the required

power. Unfortunately the radiation resistance may be comparable with the loss resistance

in its conductors and the equivalent loss resistance of any insulating components needed to

support it. We are therefore confronted with a very small bandwidth and a problem with

efficiency – any current will create losses as well as radiation. The efficiency  will be

limited to a value given by Rr/Rl +Rrwhere Rl is the equivalent loss resistance and Rr

is the radiation resistance. To feed energy into the antenna we will need to match it to a

transmission line, and the matching circuit will contribute further losses.

Figure 2.1(a) shows a short vertical radiator over ground – for the moment we can regard

this as perfect ground. The current at the top of the radiator is zero and it rises linearly to some

maximum value at the bottom (it is approximately linear because although the distribution is

approximately sinusoidal, sin

≈

when

is small). We can improve matters by extending

a horizontal conductor from the top of the antenna (Figure 2.1(b)); this occupies no more

height but the current zero is now moved to the ends of the horizontal sections and a larger

and almost constant current flows in the vertical section. We have increased the radiation

resistance (Rr) and at the same time reduced the capacitive reactance Xc at the feedpoint, so

the Q of the antenna has fallen. Figure 2.1(c) shows an alternative configuration with similar

characteristics, known as an inverted-L antenna. In both cases the top conductor contributes

little radiation because of the proximity of its anti-phase image in the groundplane.

(a) Simple vertical radiator

(b) T antenna

(c) Inverted-L antenna

Figure 2.1 Short radiators over ground.

16 Handset Antennas

(a) Folded inverted-L

(b) Tapped inverted-L – an inverted-F

(c) Planar inverted-L antenna

(d) Planar inverted-L antenna with a

folded top

Figure 2.2 Derivatives of an inverted-L.

To further increase the value of Rr we can fold the antenna as in Figure 2.2(a), or tap it in

the manner shown in Figure 2.2(b) – an inverted-F antenna. This will be naturally resonant

when the total length of the upper limb is around /4, and by selecting the position of the

feedpoint the input impedance can be chosen to be close to 50 ohms.

We can replace the wire top of the inverted-L with a plate (Figure 2.2(c)) and slot the

plate to make the loading more compact (Figure 2.2(d)). Unfortunately we have still not

overcome the constraint created by the small volume of the antenna and we need another

trick to allow us to solve our problem. An important feature of all these configurations is

that they are unbalanced. If we conceive the ground as an infinite perfect conductor we can

envisage an image of the antenna in the groundplane and calculate the radiation pattern by

summing the contributions of the antenna and its image.

When we build one of these antennas on a handset, the groundplane is only around /4

long – about the same length as one half of a dipole. What we have created is a kind of

curiously asymmetrical dipole; one limb comprises the groundplane of the handset, while

the other limb is the F-structure we have fed against it. What properties might we expect of

this configuration?

Polarization. The polarization of the inverted-F antenna (Figure 2.2(c)) is vertical – orthog￾onal to the groundplane. We can envisage this from the direction in which we apply the

feed voltage, the current in the vertical radiating leg and the alignment of the E-field

between the top and the ground. By contrast, our asymmetric dipole is polarized in the

direction of its long axis, along which most of the radiating current flows.

Radiation patterns. The inverted-F antenna would have an omnidirectional pattern in the

plane of the ground, while the asymmetric dipole would be omnidirectional in the plane

bisecting the groundplane.

If we now examine the behavior of a typical handset we see that it really does have

these properties. The antenna has very little relationship to the prototypes from which we

derived it. The polarization is aligned with the long axis of the phone, and its radiation

pattern in the low bands looks very much like that of a half-wave dipole aligned with the

groundplane (see Figure 2.11 below).