Wireless Communication Device with Near Field Control
Transcript of 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.
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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,
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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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