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The present invention relates to an apparatus for mobile terminals reducing an user's
exposure to radiation emanating from components of the mobile terminal like e.g. an
antenna. The present invention relates in particular to radiation shielding in multiband
mobile terminals.
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User usually hold wireless communication devices in close contact to the ear while
calling. A users brain which is then directly exposed to the electromagnetic radiation
emitted from the antenna of the device hereby absorbs an amount of the emanated
radiation. The absorbed energy raises the average temperature of the brain. Legislators
have therefore set tolerance limits for a permissible radiation exposure intended to
guarantee that no respective temporary or permanent health risk will arise from the use
of mobile terminals.
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An awareness for electro-smog from mobile terminals representing a general health
hazard can be observed by a considerable part of the population. Mobile terminals are
of course not the only source for electro-smog; indeed many electronic devices like e.g.
computers, phones, TV sets, radar transmitters, and the like contribute to it.
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Irrespective of the public opinion, it is not clear at the moment if the electromagnetic
radiation emitted from a wireless communication device may or may not pose a
potential health hazard to users of mobile terminals. Reports about wireless
telecommunication devices affecting the human health are currently not leading to
secure conclusions. Only one scientific study found an indication for an increase in the
risk of a damage to DNA exposed to radiation of a frequency also used for mobile
communication (Lai H and Singh N.P, Acute Low-Intensity Microwave Exposure
Increases DNA Single-Strand Breaks in Rat Brain Cells Bioelectromagnetics,
1995,16:207-210). But it has to be admitted that the health risks for humans are still
unknown.
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To conclude that the use of mobile terminals will be harmless for human beings, only
because there is currently no prove of the reverse will probably be wrong. Therefore, a
manufacturer of wireless telecommunication devices takes care for his customers and
will take appropriate measures to protect users of his mobile terminals from
unnecessary radiation. From the point of view of efficient use of radiated energy, an
economical use of radiated power reduces the energy consumption of the mobile as
well. The operating time of a mobile terminal will be prolonged correspondingly.
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To this end, JP 8 288 895 proposes an antenna arrangement consisting of two bar-shaped
aerials extending from the body of a communication equipment, whereby the
signal supplied to the second aerial is phase shifted in respect to the signal supplied to
the first aerial by an amount adapted to suppress the emission of radio waves into the
direction of a user of the communication equipment. The use of two aerials contrasts
with the indispensable requirement for a compact design of modern telecommunication
devices. Unfortunately the proposed system is based on the geometry of the aerials
design and can therefore not be transferred to a system using patch antennas, which can
advantageously be integrated into the body of a compact mobile telecommunication
device.
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US 5 335 366 proposes a radiation shielding apparatus for a radio transmitting device,
whereby a radiation shield is disposed between a radiation component and a user to
prevent unwanted exposure of the user to radiation emanating from the radiation
component. The radiation shield can absorb, block and/or reflect electromagnetic wave
radiation. The shielding can be placed between the radiation emanating component and
the user or wrapped around that component. It is intended to absorb and/or block
and/or reflect microwave energy in a frequency range of approximately 800 to 900
MHz emitted into the direction of the cranium of a user.
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Radiation shields basically affect the near field of an electromagnetic radiation in the
immediate vicinity of radiation emanating components like an antenna, an antenna feed,
a RF-transceiver or the like. Changing to a different transmission band inevitably
modifies the near field condition around the antenna. The efficiency of a radiation
shield will consequently be quite different for one transmission frequency band than for
another. Using only one type of a radiation shielding apparatus will definitely not fit all
radiation conditions produced by a multiband wireless telecommunication device.
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It is therefore an object of the present invention to provide a radiation shield of high
shielding efficiency which is optimised to all frequency bands of a multiband wireless
telecommunication device.
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This object is achieved by a shielding arrangement as claimed in the independent
claims.
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In particular, the object is achieved by a method of shielding a user from
electromagnetic radiation emanated from a mobile telecommunication device with
operating components having electromagnetic radiation blocking characteristics, the
method comprising steps for providing switchable bonds at different positions of an
operating component with electromagnetic radiation blocking characteristics, and for
selectively switching the bonds depending on a selected operation frequency band to
different RF-ground potential terminals available on at least one further operating
component of the telecommunication device.
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The above object is further achieved by a mobile terminal with a radiation shielding
means for shielding a user from electromagnetic radiation emanated from the mobile
terminal having operating components with electromagnetic radiation blocking
characteristics, operating components providing RF-ground potential terminals,
switchable bonds at different positions of an operating component with electromagnetic
radiation blocking characteristic, and a control means for selectively switching the
bonds depending on a selected operation frequency band to different RF-ground
potential terminals available on at least one further operating component of the mobile
terminal.
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The present invention advantageously reduces the rate of energy that is absorbed by a
user of a corresponding mobile telecommunication device without affecting the
efficiency of the mobile terminals transceiving system. Furthermore, by utilising
operating components for a radiation shielding, no space consuming extra equipment
has to be incorporated into the already crammed device.
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Further advantageous features are claimed in the respective sub-claims.
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According to an advantageous development, at least two operating components with
electromagnetic radiation blocking characteristics are arranged to achieve a parasitic
coupling influencing the near field for further suppressing an electromagnetic radiation
directed to a user. The coupled components advantageously interact in the near field of
the antenna by modifying the power flux vector of the electromagnetic field emitted
from the antenna.
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The switching components of a selectively switchable bond are preferably formed by a
microelectromechanical system (MEMS) and/or a field effect transistor (FET) and/or a
PIN diode.
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A radiation shielding according to the present invention can be used in modern mobile
terminals for wireless telecommunication networks, in cordless phones and of course in
all radio installations, where a user comes in close contact to components emitting
electromagnetic radiation.
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In the following description, the present invention is explained in more detail with
respect to special embodiments and in relation to the enclosed drawings, in which
- Figure 1 shows a top view of a radiation shielding apparatus according to the present
invention,
- Figure 2 shows a side view of a radiation shielding apparatus according to the present
invention, and
- Figure 3 shows the main components of a mobile terminal according to the present
invention.
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Figure 1 shows an implementation of a radiation shielding means 10 according to the
present invention for a mobile terminal of a wireless telecommunication network. The
components utilised for putting the shielding into practice are quite ordinary operating
components which are present in most of today's mobile terminals. In detail, an ESD
(electrostatic discharge) frame 1 providing a protective measure against electrostatic
discharge is used in combination with a PCB 4 (printed circuit board) or PWB 4
(printed wiring board) on which the functional devices of the mobile terminal are
mounted. Bonds 3 made from electrically conductive material connect the ESD frame 1
with different RF-ground potential terminals on the PWB 4 from different positions.
Unless otherwise specified, the term 'ground potential' or 'ground' used in the
following will always refer to an RF-ground potential terminal of the wireless device
even when the RF-ground is not equal to a DC-ground potential. Switching elements 2
are placed between the bonds 3 and the grounds on the PCB/PWB 4 to control the
electrical connection of each bond separately.
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The principle sources for electromagnetic radiation in a mobile telecommunication
device generally are the antennae, the antenna feed lines, and the RF-transceiver
component. The exposure of a user to radiation emanated from a radio equipment is
characterised by the Specific Absorption Rate (SAR), which is a measure for the rate of
electromagnetic energy absorbed or dissipated in a mass of dielectric materials, such as
biological tissues. Usually, SAR is expressed in watt per kilogram (W/kg) or in
milliwatt per kilogram (mW/kg). With the biological absorption conditions of a user
given, the SAR can be used as a synonym for the rate of emanated radiation energy
absorbed in a users tissue.
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It is known, that the SAR-distribution in a users body is strongly correlated with the
distribution of the magnetic field (H) on the PCB/PWB 4 of the mobile
telecommunication device forming the source for the radiation exposure. The
distribution of H is quite essentially influenced by ground return currents from the
antennae and antenna feeds, respectively, to the various grounds on the PCB/PWB 4.
These ground return currents are distributed over the entire PCB/PWB 4. Modifying the
return current distribution on the PCB/PWB 4 will therefore modify the H-field
distribution of the emanated RF-radiation and thus the SAR in a users tissue. Modifying
the return current distribution on the PCB/PWB 4 therefore provides a potent means for
suppressing a users exposure to electromagnetic radiation emanated from a mobile
terminal.
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The current distribution on the PCB/PWB 4 is modified by grounding an electrically
conductive component 1 in different points, and thereby provides a blocking of
unwanted radiation 9 emitted into the direction of a user 8.
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In the special embodiment of the present invention of Figure 1, the metallic structure of
an ESD frame 1 is used as the electrically conductive component. Different electrically
conductive components made e.g. from sufficiently doped semiconductor material,
metal oxides, conductive polymers or any other material showing a specific resistance
of about or smaller than 10-2 Ωcm can be used just as well. Besides just influencing the
return current distribution on a PCB/PWB, an electrically conductive component also
works as a block for electromagnetic radiation by absorbing and/or reflecting
electromagnetic radiation. The electrically conductive components are therefore also
denoted as radiation blocking components.
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The electrically conductive component 1 is connected to several switching devices 2
distributed over the PCB/PWB 4 by bonds. Anything providing a good electrical
contact like for instance spring contacts, pins, contact tongues, solder bumps, wire
bonds, a retaining jaw or the like can be used as a bond. When a switching device 2 is
turned in the conductive state, it connects the respective bond and thereby the radiation
blocking component 1 to a ground potential provided on the operating component of the
wireless device formed by PCB/PWB 4. In other words, the radiation blocking
component 1 is grounded by means of the switching devices 2.
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The resulting modification of the return current distribution depends on which RF-ground
or grounds are used for grounding the component 1. It is therefore possible to
control the distribution of the return currents on the PCB/PWB 4 with the switching
elements 2 only and control through this the emission of unwanted electromagnetic
radiation into the direction of a user. Or to put it another way, the switching
components 2 in combination with the bonds 3 and the radiation blocking component 1
form a SAR modulator.
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Figure 2 illustrates the arrangement for a radiation shield according to the present
invention in a side view. The transceiver circuit 5 with the antenna 6 is mounted on the
back side of the PCB/PWB 4. The main radiation 7 is emitted by the antenna 6 away
from the user 8 holding the mobile terminal to the side of his head. The unwanted
radiation 9 emanated from antenna 6 into the direction of user 8 is blocked off by a
metallic box enclosing the transceiver circuit 5 grounded to one of the ground potentials
available on the PCB/PWB 4. It also prevents radiation leaking from the transceiver
circuit itself.
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Besides the currently employed triple bands in mobile terminals with operating
frequency bands ranging around 900, 1800, and 1900 MHz, new UMTS operating
frequency bands ranging between 1900 and 2170 will be integrated in future devices.
The distribution of the return currents on the PCB/PWB 4 will be different for each of
the operating frequency bands used, so that more than one switchable bond formed by a
bond 3 and a switching device 2 have to be provided for selectively adapting the SAR
modulator to the respective return current distribution. For simplicity, the SAR
modulator in Figure 2 is shown with just one switchable bond active.
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More than one radiation blocking component 1 can be used for forming a SAR
modulator. Each of these can be directly connected to the PCB/PWB 4 by switchable
bonds, but alternatively some switchable bonds may be provided also or exclusively
between the components 1 themselves.
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The switching units 2 can be microelectromechanical systems (MEMS), field effect
transistors (FETs) or PIN diodes or the like. For each bond a different switching
element 2 may be used. PIN diodes are the most simple switching devices which can be
employed, when a DC or low frequency voltage can be used to through-switch the
device, and the switching DC potential will still provide a RF-ground potential. If this
is not feasible, MEMS and/or FETs are the choice.
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Apart from providing direct electrical connections between a second electrically
conductive component and a first one, and/or a further operating component 4 of a
mobile terminal, advantage can be taken of the parasitic coupling between a first and an
adjacent second radiation blocking component 1. This is shown for the Liquid Crystal
display window 11 having a metallisation, which is mounted in the front cabinet of the
mobile terminal directly on top of the metallic ESD frame 1. Both components are
electrically conductive and form, due to their close vicinity a parasitic coupling which
interferes with the unwanted radiation component 9 such, that the radiation emitted in a
users direction is considerably reduced.
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Referring now to Figure 3, a mobile terminal 20 according to the present invention is
shown. The radiation emanating components like e.g. the transceiver unit 5 and the
antenna 6 are integrated in the wireless telecommunication device. Operating
components 4 providing RF-ground potentials are connected to operating components 1
and/or 11 with radiation blocking characteristics by switchable bonds formed by bonds
3 and switching elements 2. The switching elements 2 are controlled by a control means
21, which controls the switching configuration of all switching elements according to
the respective near field conditions of each operation frequency band. The control
means 21 forms in combination with the switchable bonds (2 and 3) and the radiation
blocking component 1 a controllable SAR modulator.
