Wireless Communication Device with Near Field Control

8/7/2019 Wireless Communication Device with Near Field Control

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A2·100

Fig. 1. Cellular phone SAR scan (a) without

Front housing floating shield and (b) with shield

Wireless Communication Device with Near Field Control

Lorenzo A. Ponce de Leon

Jan-Ove Mattsson*

Motorola iDEN, Plantation, FL, 33322 USA

E-mail: [email protected]

Introduction

Many wireless communication devices such as cellular phones suffer radiated

performance degradation when the devices are brought into close proximity to the

operator of the device. The mechanism for the degradation is the near field

deposition of energy from the antenna system into the operator's body. It would

be ideal for an antenna system on a wireless device to respond to the changing

radiation environment by automatically adjusting its near field energy distribution

to minimize deposition of energy if the communication device is brought in closeproximity to the operator's body. A switched floating element shield is proposed

that reduces near field energy deposition when active and becomes invisible to the

antenna system when deactivated. A sensing and control device controls the state

of the switching device.

Floating Element Shield

With standard wireless communication devices, radiated performance of an

antenna can be affected by the manner in which the devices are utilized and

radiated performance may impact the qualityof

service provided by the device.Typically users position the

communications device In

proximity to their head and this

may affect the antenna

performance A metallic floating

element (FE) shield (Fig. 1) or

reflector element placed in

between a hand-held transceiver

and a user's body has been shown

to improve the performance of the

devices and reduce near fieldenergy deposition as measured by

the SAR (Specific Absorption

Rate) [1]. In such a configuration

the side of the device that is

opposite to the user remains

unshielded to provide a radiating

surface for emitted energy.



However, the FE shield can degrade

the performance of the wirelesscommunication device when the

". 14 device is used in other modes. For

example in wireless communication

devices equipped with a speaker forhands-free operation the phone istypically used in a near free field

environment and a FE shield may

degrade performance of the

communication device.

Antenna (14) - - - - - - _ _ _ _

PCB Ground (16) - - _

---: __-1 - - - - - -"

~ - - - - - - - - - - - ~ - - - - -

Fig. 2. Switch enabled shield system

------E shield (18) - - - -

__ 2'0--

- - 16ousing (20, 21)The ideal FE shield system would be

designed to tum on the reflector when

the device is in proximity to theoperator and turn off the FE shield

when the device is an unloaded or ina near free field environment. Such asystem [2] is represented in Fig. 2. The FE shield 18 operates over a ground plane

16 and is normally connected to the ground plane. The FE shield is activated by

switches in the open position which break the FE shield's contact to the groundplane. This serves to change the near field energy distribution and radiation

pattern of the antenna 14 by directing radiated energy away from the FE shield

and thus away from any objects where the FE shield is between the antenna andthe object, including for example a user of the communication device.

The FE shield is a conductive layer or coating and is preferably metal. The FE

shield covers at least a portionof

the front areaof

the communication device.Further, the FE shield may extend to cover the sides of the communication deviceleaving the rear surface of the communication device uncovered.

1 - - - - - - ~ 2 . 2 5

20 1

o 10

25 +--------:: :: ; ; :00.- .: : : : :=----------+ 1.25

30 + - - - - - - - - - - - + - - - - - - = ~ - - + 1.5

35 + - - - - - - ~ ; : : : : _ _ _ _ _ _ _ + _ - - - - - + 1.75

45

50.-------------------,2.5

40 ~ - - - - - - - - _ _ _ _ _ r - - - +

Fig. 3. Shield spacing from PCB effect on

SAR and Efficiency over a flat phantom

The design of the FE shield is importantin order to achieve the potential benefits

simulations of this architecture. The

separation distance between the FEshield and the main structure of the

phone that carries the radiating currents(such as the PCB) should be maximizedto achieve minimal energy deposition

and maximum radiation efficiency (Fig.

3). Improvements of over 3dB have been

demonstrated in both simulation and in

measurements.

The FE shield can be a separate component installed within the front housingmember 20 or can be integrated with the front housing member (Fig. 4). That is,



Fig. 4. FE shield on phone

housing

the FE shield can be created by molding the fronthousing member out of conductive plastic, bysputtering, painting, or vacuum depositing a

conductive layer onto the inside surface or outsidesurface of the front housing member. Additionally, for

example, metal inserts, molded wires, molded wirescreen, conductive oxide or carbon layers or any

combination thereof can be molded into the fronthousing member to form the FE shield. The FE shield

can include a conductive lens or film to cover the

display area if the display is located on the front side

of the communication device. Also, the FE shield can

include conductive buttons if the keypad is located on

the front side of the phone. Further details regarding

FE shields that may be used with the communication device can be found in [2].

Proximity Sensing Architecture

For the FE shield to operate optimally a proximity sensing system is necessary

that determines when the communication device is in close proximity to an objectsuch as the body of a user. The proximity sensing circuit will cause the switching

devices to activate the FE shield when the communication device proximity is

detected. The FE shield is considered to be "active" when it is decoupled from theground plane.

Proximity sensing can be accomplished in many ways. For example, theproximity sensing circuit may include a photo-detector, an ultrasonic transducer, a

capacitance sensor, or a thermal sensor to determine that an object or that ahuman head is in close proximity to the communication device. Thus, software

executed by the sensing and control device periodically reads the sensor of the

proximity sensing circuit during a transmission mode of the communication

device to determine close proximity and to control the state of the switchingdevice accordingly. That is, a condition of the transmitter is sensed, and the

va.

o o + - - ~ - - + - - - - - i l - - - - - + - - - - - : l ~ ~ - - + - - - - - {

14

Fig. 5. SII effect of floating element (FE) and VSWR control system



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Wireless Communication Device with Near Field Control