It is known that a mobile telephone, when transmitting, transmits microwave energy. When the telephone is held adjacent the ear of a user, that microwave energy is transmitted at a position which is very close to the brain of the user. It is thought that exposure to microwave energy may damage the brain of the user, and therefore it has been proposed that the amount of microwave energy being transmitted should be limited.

Various proposals have been put forward which involve the shielding of the antenna, or the application of a"load"to the antenna, but the prior proposals all suffer from disadvantages.

The present invention seeks to provide an improved communication device.

According to this invention there is provided a mobile communication device incorporating an antenna, the device including a transmitter and a receiver adapted to transmit and receive signals via the antenna, the mobile communication device incorporating a load or shield adapted to absorb or limit radiation transmitted by the antenna, the load or shield being movable relative to the antenna to adjust the strength of the signal actually transmitted by the antenna, there being circuitry to determine the signal strength of the signal received by the antenna, and to compare the signal strength with a threshold to generate a control signal, the control signal being supplied to a mechanism adapted to move the load or shield so that the received signal strength is substantially equal to the threshold.

The device may be in the form of a mobile telephone.

Preferably the load is in electromagnetic contact with the antenna.

Conveniently the load is in physical contact with the antenna.

Preferably the load or shield is mounted on a carriage, the carriage incorporating a threaded bore which engages a threaded shaft, a motor being provided to rotate the threaded shaft, the motor being connected to be controlled by the control signal In order that the invention may be more readily understood, and so that further features thereof may be appreciated, the invention will now be described, by way of example, with reference to the accompanying drawings in which: FIGURE 1 is a schematic view of a mobile telephone illustrating the antenna of the telephone in phantom, and FIGURE 2 is a partly schematic and partly block diagrammatic illustration of one embodiment of the invention.

Referring initially to Figure 1 of the accompanying drawings, a mobile telephone 1 is illustrated. The mobile telephone has a housing 2, and presents a display window 3 and a plurality of buttons in the form of a key-pad 4. The telephone has a microphone 5 and an ear-piece 6. Contained within the casing 2 of the mobile telephone is an antenna 7 which is typically an elongate element formed of a ferrite material or some other conducting material. The elongate element may be covered with a sleeve of insulating material, such as a plastics material.

Turning now to Figure 2, the present invention provides an arrangement in which a load is provided which is in electromagnetic contact and/or physical contact with the antenna, and in which a mechanism is provided to automatically physically adjust the position of the load relative to the antenna so that the signal transmitted by the antenna is maintained at a minimum practicable power. The load may thus be in direct physical electric contact with the antenna, or may be in electromagnetic contact by a capacitative and/or inductive coupling between the load and the antenna.

Referring to Figure 2 of the accompanying drawings, the antenna 7 is illustrated. The antenna 7 is connected to a receiver 8 adapted to receive the signal drawn in through the antenna.

One output from the receiver 8 is passed to a signal strength meter 9 which measures the strength of the received signal. The output of the signal strength meter 9 is fed to a processor 10 where the signal strength is compared with a pre-set threshold. The threshold may be adjustable if desired. The processor 10 forwards a signal to a controller 11, the signal being indicative of whether the monitored signal strength is above or below the predetermined threshold. In the described embodiment, the controller 11 controls the actuation of a motor 12. The motor 12 rotates a threaded shaft 13, and the arrangement is such that the threaded shaft 13 rotates in one sense if the signal strength is below the threshold, and in the opposite sense if the signal strength is above the threshold.

Mounted on the threaded shaft 13 is a carriage 14, the carriage 14 defining a threaded bore receiving the threaded shaft. It will be understood that the carriage will move in one direction, upwardly in the orientation shown, if the signal strength is above the predetermined threshold and downwardly, in the orientation shown, if the signal strength is below the predetermined threshold.

The carriage carries a load 15 which is associated with a resilient finger 16, the resilient finger 16 physically touching the exterior of the antenna 7. The load, as electro-magnetically connected to the antenna by the resilient finger 16 acts as a"power sink"and reduces the capacity of the antenna to transmit, and receive, signals.

The load or shield may be a connection to a"earth"or may be any other energy or power-absorbing load. The resilient finger 16 serves to establish an electromagnetic contact between the antenna and the load. In preferred embodiments, the finger 16 actually contacts the exterior of the antenna 7, especially if the antenna 7 incorporates a plastics material sleeve.

The antenna 7 is also connected to a transmitter 17.

In operation of the arrangement shown in Figure 2, when the antenna receives a signal from a cell transmitter, the strength of the signal received by the antenna will depend upon various parameters including the distance of the antenna from the cell transmitter. The signal is received by the receiver and the strength of the signal received at any instant is measured by the meter 9. The processor 10 processes the signal comparing the signal with a pre-set threshold.

The pre-set threshold is selected to be a signal strength at which satisfactory communication with the cell can be maintained. The controller then actuates the motor 12 so that the carriage 14 is moved to such a position that the load 15 makes contact with the antenna 7 at an appropriate point so that the received signal strength is equal to the threshold. With the load in this position relative to the antenna 7, when the transmitter 17 transmits a signal, the signal strength transmitted being just sufficient to establish communication with the cell in question. Thus the power transmitted by the antenna is, at all times, effectively minimised, thus minimising the microwave radiation that will be experienced by the head of the user of the mobile telephone.

Whilst the invention has been described with reference to one specific type of load, it is to be appreciated that other types of load or shield may be utilised. Any load or shield must, however, serve to absorb a specific proportion of the energy transmitted by the antenna, and the load or shield must be movable relative to the antenna to increase, or decrease, the proportion of energy absorbed by the load or shield. Thus, in a modified embodiment of the invention, the load or shield could comprise a tubular shield surrounding the antenna movable from a retracted position in which most of the antenna is exposed to an extended position in which most of the antenna is shielded. In such an embodiment the shield would not necessarily be in physical contact with the antenna.

It is also to be understood that whilst, in the described embodiment of the invention, a particular mechanical arrangement has been illustrated for effecting movement of the load, in the form of the rotatable threaded shaft 13 and the co-operating carriage 14, other mechanisms to effect movement of the load may be utilised. Many mechanisms may be contemplated which utilise a conventional motor as the driving force, but other arrangements may include solenoid actuated arrangements and even hydraulically powered arrangements.

Various types of antenna are utilised in connection with communication devices such as mobile telephones. The invention has been described above with reference to an antenna of elongate form. It has been proposed to use antenna which are substantially flat plate-like antennae. Of course, a load or shield may be provided for use with such a flat antenna. For example, a load in the form of an energy-absorbing plate may be provided located at a position immediately adjacent the antenna and being moveable from a retracted position in which the load does not cover any of the antenna, to a masking or shielding position in which the shield substantially covers the antenna. The plate may be moved, for example, by two screw-thread-and-carriage arrangements of the type shown in Figure 2.

Whilst reference has been made to a physical shield or load, it is to be appreciated that a shield could be utilised which is formed of an energy- absorbing fluid, such as water. The fluid could be pumped into and out of one or more chambers which lie adjacent, or which surround the antenna. When the chamber or chambers are filled with liquid a substantial shielding effect will be provided whereas, when the chambers are emptied of liquid, reduced shielding effect will be experienced.

In the present Specification"comprises"means"includes or consists of' and"comprising"means"including or consisting of'.