GB2350235A

GB2350235A - Reducing electromagnetic radiation in portable RF devices

Info

Publication number: GB2350235A
Application number: GB9911358A
Authority: GB
Inventors: Raymond Etienne Schemel
Original assignee: NEOTERICS Ltd
Current assignee: NEOTERICS Ltd
Priority date: 1999-05-18
Filing date: 1999-05-18
Publication date: 2000-11-22
Also published as: GB9911358D0

Abstract

Hand-held mobile telephones and other portable RF apparatus give rise to electromagnetic fields in the region of the head or body of the user. This invention enables the radiating element (the aerial, or antenna) to be moved away from the head, thereby appreciably reducing the local field in the region of the head or body. At the same time the efficiency of radiation can be improved because the aerial is free from blocking by the head or body and various other effects of the body are minimized. Where transmitter power control is in use any increased efficiency of radiation will reduce the local electromagnetic fields and can lead to a reduced battery consumption. Radiation is further reduced by connecting the aerial to the apparatus by a transmission line 6 and providing a counter poise (20, Figs 2, 2bis not shown) balun (30, Fig. 3 not shown) or magnetic sleeve (42, Fig 3 not shown). All these reduce unbalance currents which give rise to return currents which can pass through the head or external surfaces.

Description

2350235 APPARATUS AND METHOD FOR REDUCING ELECTROMAGNETIC RADIATION FROM

HAND HELD PORTABLE RF APPARATUS IN THE VICINITY OF THE HEAD OR BODY OF THE USER.

1. Background to the invention.

1. 1 This invention relates to an apparatus and method for ensuring that the aerial of a portable radio frequency (RF) apparatus, usually but not exclusively a hand-held mobile telephone, is placed as far as possible from the head or other nearby body parts of the user. By placing the aerial in such a position the electric, magnetic and electromagnetic fields caused by the aerial are reduced in the vicinity of the user, signal blockage caused by the head, either by reflection or absorption, is minimized, and the efficiency is increased, because: a), unwanted currents that would otherwise circulate in the body of the user are constrained to flow in a good conductor; and b), the variabilities caused by the head or body being at an indeterminate distance from the aerial (often referred to as hand capacity effects) are minimized.

2. Siummajy of prior art

2.1 Mobile telephones (or other similar hand-held devices) are typically used with the whole apparatus held against the head and ear of the user. In particular, a modern hand-held mobile telephone operates in a frequency band where the radiating and reception element (also called the antenna, but hereafter called the aerial), can be very small. In effect, the aerial, when the apparatus is transmitting, can be viewed as a concentrated source of radio frequency power which is in very close proximity to the head, the hand, and other nearby parts of the body.

2.2 The head and any other body parts at the frequencies of concern reflect, absorb, and propagate a portion of the energy in the electric, magnetic, or electromagnetic fields. At the same time they act as a return path for the aerial current back to the zero radio frequency potential point (in other words, the chassis or printed circuit board) of the handset.

2.3 Electric, magnetic and electromagnetic fields are all interrelated by Maxwell's equations via the velocity of energy propagation. In space this is, of course, the velocity of light, but in a medium such as the head it can be much lower. The electric and magnetic fields are only of importance very close to the aerial, the closeness usually being defined as a fraction of a wavelength in the medium where their strengths are greater than the electromagnetic field. Wavelength is defined as the propagation velocity divided by the frequency and at the frequencies of interest can be as low as a few centimetres.

2.4 If the medium has electrical losses, the various fields cause conduction currents to flow. Conduction currents flowing in the head or the body will give rise to heating effects because the body acts as a resistance and may also have physiological effects. Fortunately, conduction currents diminish rapidly with depth. At a distance from the surface called the skin depth, these currents are lle times that at the surface, where e=2.718. The skin depth is a function of frequency as well as the electrical constants of the body losses and cannot be predetermined, but is at most a few centimetres for the frequencies in use by portable transmitting devices.

2.5 A full exposition on electromagnetic fields may be found in any of the large number of texts on the subject. Suffice it to say here that from the foregoing it will be appreciated that any effects are primarily electromagnetic at distances of more than a few centimetres from a radiating source. Without loss of generality in what follows the term electromagnetic will therefore be used to describe all three fields.

2.6 The maximum intensity of electromagnetic radiation is controlled and currently an apparatus such as a hand-held mobile telephone on sale to the public meets prescribed limits for the power density. This limit has been set largely by the local heating effect but there is an awareness that other physiological effects may be present. For this and other reasons the present limits are under discussion.

2.7 Perhaps of more importance, the general public perceive that handheld mobile telephones in general may have unknown effects and have reacted by purchasing various add-on devices. As but one example, the mobile telephone itself can be kept in a pocket and a separate microphone and earpiece can be connected. By this means the hand-held mobile telephone is kept remote from the head. But now, energy is absorbed by the body, the path to and from the base station may be blocked and at the same time body proximity effects can have the effect of mismatching the aerial to the transmitter power amplifier stage. Other methods employ various screening techniques and even attempt to preferentially radiate electromagnetic energy away from the head, possibly increasing the path loss in a desired direction. With many of the systems now available the power level of the transmitter is in any case raised to compensate for the increased path loss, and so these methods can be self-defeating. Another simple method that has been observed is holding the handset as far as possible from the head. In this case the aural pathway rather than the radio frequency pathway may be compromised.

2.8 It is desirable, therefore, to provide an apparatus that removes the aerial and any other parts that radiate as far as possible from the head and body of the user. By this means electromagnetic field intensity is much reduced, signal absorption and blockage caused by the head or body, both during transmission and reception is also reduced, efficiency is improved because losses caused by aerial return currents in the body are reduced, and there is less variability in the match between the transmitter and the load created by the aerial. Not only this, in an apparatus where the transmitted power is controlled, the reduced path loss can result in a lowering of the local electromagnetic fields and extending the life of the battery as a bonus.

3. Summa!y of the invention In the first aspect of the current invention there is provided a method for reducing any undesired local electromagnetic field produced by an aerial radiating RF energy by mounting the aerial at one end (referred to as the far end) of a variable length extension from a handheld mobile telephone or other portable RF apparatus, the aerial being connected by means of a transmission line, which is preferably screened, to the transmitter.

3.1 In another aspect of the invention there is provided a method for reducing the electromagnetic field produced by unwanted RF currents flowing in a transmission line by mounting a further conductor or conductors (hereafter called the counterpoise) at the far end of a variable length extension from a hand-held mobile telephone or other portable RF apparatus, the counterpoise being connected to the far end of the transmission line in order to provide a return path for the aerial current.

3 3.2 In another aspect of the invention there is provided a method of reducing the electromagnetic field produced by unwanted RF currents (also called unbalanced currents) flowing in a transmission line by using a balance to unbalance transformer (a balun) at the far end of the extension to feed the aerial and the counterpoise.

3.3 In another aspect of the invention there is provided a method of minimizing unbalanced currents flowing in any conductor of the transmission line by the use of at least one of a sleeve of magnetic material within the extension, preferably located at a point of a current maximum and with induced losses, the transmission line passing through the sleeve.

3.4 In another aspect of the invention there is provided apparatus in the form of an extension from a hand-held mobile telephone or other portable RIF apparatus, the far end supporting an aerial, the extension being made of either insulating or conducting materials and of variable length.

3.5 In another aspect of the invention there is provided apparatus to reduce the flow of unbalanced currents in the conductors of a transmission line (which can include a screen), namely, a counterpoise placed at the far end of the line, the counterpoise being separate or integrated into the body of the extension.

3.6 In another aspect of the invention there is provided apparatus to reduce the flow of unbalanced currents within a transmission line, namely, a balun placed at the far end of the extension.

3.7 In another aspect of the invention there is provided apparatus to reduce the flow of unbalanced currents along any conductor of a transmission line, namely at least one of a magnetic sleeve located preferably at points of current maxima in the transmission line.

3.8 As will be evident from the above the invention will reduce the local undesirable fields because the aerial and any other radiating parts are no longer in the near vicinity of the head or body of a user.

3.9 The invention further improves the effectiveness of communication by placing the aerial in such a position that the head or body does not block the two way radio propagation path between the user and the distant point by reflection or absorption.

3.10 The invention further improves effectiveness of communication by reducing the losses caused by conduction currents flowing in the head or body of the user and by reducing any mismatch of the aerial loading caused by hand capacity effects.

3.11 The invention further lowers the nearby electromagnetic fields where there is transmitter power control because a reduced path loss ensures a lower radiated power. At the same time battery life may be extended.

4. Brief description of the drawings

4.1 The invention can be readily understood with reference to the accompanying drawings, in which; 4.2 FIG. 1 (a) shows a cross section of the extension and a stowable transmission line, one end (the far end) of which is terminated by an aerial. Preferably the transmission line is screened but internally it may be balanced (two internal conductors) or unbalanced (one internal conductor). For the purpose of description an unbalanced configuration with a screened outer conductor is shown. The extension is shown in the operating (extended) and

4 stowed positions. FIG. 1(b) shows the approximate electromagnetic field strength to be expected from a typical hand-held mobile radio aerial of the type illustrated in FIG. 1 (a) as an observer moves around in the plane of the paper.

4.3 FIG. 2 and FIG. 2 bis show two exemplary embodiments of apparatus for minimizing the flow of unbalanced currents by the use of a counterpoise mounted at the far end of the extension. FIG. 2 also shows an alternative method to that shown in FIG. 1 (a) for stowing the transmission line.

4.4 FIG. 3 shows two embodiments for minimizing the flow of unbalanced currents by the use of a balun mounted at the far end of the extension and by using a sleeve of magnetic material, the transmission line, including any screen, passing through the sleeve.

5. Detailed description of the particular embodiments

5.1 Referring to FIG. 1 (a), an aerial 2, which can be of arbitrary design, but which is shown here as a simple monopole for convenience, is placed at the far end of an extension 4 and is connected by a transmission line 6 to host apparatus 8. The transmission line is coupled to the host at its near end by an optional matching network 10. The extension has a variable length and can be made of a series of fitting concentric tubes of insulated or conducting material. Dependent upon an exact mechanical design supporting collars 12 may be necessary, and there may be a need for a sliding contact 14 to ensure a good connection to the extension where this is metallic and acts as the return path for the aerial currents. Preferably the length of the tubes is such that they (and the aerial) ran all fit into host apparatus 8.

5.2 The transmission line can be preformed to curl as shown, it then being stowed within the extension, or it can be folded or wound up by a so called "cable tidy". Yet another method is shown in FIG. 2. Here, the transmission line forms a convenient loop or wrist strap by which the host apparatus can be carried. The combination of the variable length of the extension and the means to take up any slack in the transmission line enables a user to set the extension anywhere between the stowed position, in which case the apparatus works in the conventional manner, and the fully extended position, whereby the principal advantages of this invention are obtained. Further, the extension may be performed manually or by mechanical means, e.g., a small electric motor.

5.3 When used for land hand-held mobile radio the aerial preferably has a polar diagram similar to that shown in FIG. 1(b), that is, the electromagnetic radiation is at a maximum broadside to the extension axis and at a minimum along the extension axis, represented here by the line "yy". Thus when extended away the head of the user maximum radiation is approximately ornnidirectional around the "yy" axis and the head itself can lie in a region of reduced field strength. A monopole as shown has such a polar diagram.

5.4 When in operation a radio frequency current will flow in the aerial as shown by the upper part of graph 16 of FIG. 1 (a). The current will be substantially zero at the tip and will increase in amplitude towards the base. Any current flowing in the aerial must be balanced by an opposite current. Ideally this current flows back to the return side at the far end of the transmission line and all the radiation would take place from the aerial alone. In practice this does not occur and return currents flow in either the screen of the transmission line, the return conductor of same, or in the extension where this is metallic, as is shown by the lower part of graph 16. The distribution is approximately in the form of a decaying wave as the distance from the far end of the transmission line increases.

5.5 The return currents have no compensating current in the inner conductor of the transmission line and are said to be unbalanced. The return current can therefore give rise to residual electromagnetic radiation close to the host apparatus and hence in proximity to the head or body of the user. The problem of unbalanced currents is well covered in the literature and many different schemes are available to minimize them (see, for example, a reference work on aerials). Two such schemes which use counterpoises 20 and 22 are illustrated in Fig. 2 and FIG. 2 b"5 respectively. The two counterpoises shown represent but two embodiments out of many and have been chosen for illustration because each counterpoise may be stowed inside the host apparatus. For 20 a concentric cylinder, preferably and approximately one quarter of a wavelength long, is mounted as shown and connected to the screen of the transmission line via the metallic extension and contact 14. A slight modification of 20 is also shown in FIG. 3. An idealized current flow is shown in graph 24. Alternatively, the cylinder can be replaced by at least two and preferably more radial spokes 22, also shown in a plan view in FIG. 2b's, these again being preferably and approximately one quarter of a wavelength long. The spokes can fold before the aerial is placed in the stowed position but this is not shown in the diagram. The current distribution along the length of each spoke is the approximately the same as for 20, but the intensity in each is divided by the number of spokes. In both 20 and 22, the effect of the counterpoise is to reduce unbalanced currents in the transmission line.

5.6 The impedance of the combination of the aerial and the counterpoise is a function of the dimensions of the counterpoise; therefore changing these dimensions can assist in adjusting this impedance. Further, as will be readily appreciated, the impedance at the near end of a transmission line is a function of the load at the far end as well as its electrical length in wavelengths. Thus the transmission line can be preset in its length to assist in matching the load presented by it to the transmitter output stage.

5.7 In FIG. 2 and FIG. 2 bis counterpoises 20 or 22 ensure that the return currents from the aerial return only to the far end of the transmission line, as shown by graph 24, rather than in a distributed fashion along the length of the outer conductor as in graph 16 (FIG. 1 (a)). However, in practice there can still be residual unbalanced currents flowing in the transmission line, the screen, or a metallic extension. These can be caused by capacitive and inductive coupling from the counterpoise.

5.8 Yet a third embodiment for reducing currents in the outer conductor of the transmission line is shown in FIG. 3. A balun 30 is used to feed any aerial and counterpoise arrangement. Typically the balun is comprised of a small toroid and this can be conveniently placed as shown. The balun is mounted on an insulated collar 32, which can also serve for mounting suitable components (not shown) to assist in matching the aerial to the transmission line, and ultimately, to the transmitter power output stage. The aerial is also mounted on collar 32 by means of an insulated extension. The electrical connections for the balun are as shown in the inset while the main diagram shows a connection to an annular conducting ring 34 which makes contact with counterpoise 20 when the aerial 2 is extended. Optionally, a housing 36 can provide protection for the balun when exposed and can form a guide for the aerial. Such arrangements may be necessary for the type of balun in use and are only incorporated here as an aid to the understanding. The effect of the balun is to reduce substantially the flow of any unbalanced currents, that is, the sum of the currents at terminals a and b, and at terminals c and d, is nominally zero.

5.9 For the purpose of explanation FIG. 3 also includes yet a third type of counterpoise in which 20 of FIG 2 has been made an integral part of the extension 4. In this instance, the top part of the extension 4, labelled 40, is made of an insulating material such that the counterpoise 20 is isolated at radio frequencies, it only being electrically connected to the far end of the transmission line.

16 5.10 FIG. 3 also shows another embodiment for the purposes of reducing unbalanced currents. One or more sleeves of magnetic material 42 are placed inside the extension, the transmission line passing through the sleeve, preferably at points where the unbalanced current is at a maximum. The effect of a magnetic sleeve is similar to the balun (indeed, the magnetic material with the transmission line passing through it may be regarded as a one kind of balance to unbalance transformer utilizing a single turn, as already described) but now the magnetic material can be chosen to have a high loss factor at the frequencies in use. As a result there can be further attenuation of unbalanced currents, the unbalance in this instance referring to the sum of all the currents that flow in conductors passing through the sleeve.

5.11 In summary, the various embodiments reduce electromagnetic radiation in the vicinity of the head or body of a user, at the same time allowing for an increased efficiency of radiation. The embodiments allow for several levels of reduction, namely: 1), by moving the aerial as far as possible from the head: 2), combining the aerial with a counterpoise: 3), by using a counterpoise in conjunction with a balun: 4), by incorporating a magnetic sleeve through which the transmission line, including its screen, passes; and 5), by enabling the aerial to be placed in such position that it radiates in a preferred direction, at the same time allowing the head to be in a region of reduced radiation. Further, in cases where there is transmitter power control, an increased efficiency of radiation will allow the total transmitted power to be reduced, with a consequent reduction in nearby electromagnetic radiation and a possible saving in battery consumption.

I

Claims

6. Claims

6.1 Claim 1. A method of reducing the intensity of electromagnetic radiation in the vicinity of the head or body of the user of a hand-held mobile telephone or other portable RIF apparatus, the method comprising the steps of::

a) placing the radiating element (the aerial) at the end of an extendable support such that its position can be varied between an upper limit to maximize the head to aerial distance and a minimum for stowage, the user having a choice of intermediate values, b) connecting the aerial to the host apparatus by means of a transmission line, preferably screened.

6.
2 Claim 2. The method of claim 1 whereby electromagnetic radiation is constrained to occur substantially from the aerial, and not from ancillary devices such as feeders, by the use of at least one of a counterpoise, a balance to unbalance transformer and one or more magnetic sleeves, the transmission line passing through the sleeve, and the said devices being placed remotely from the head or body of the user.

6.
3 Claim 3. The method of claim 1 whereby the performance of a hand-held mobile telephone or other portable RIF apparatus (be it transmit or receive or both) is improved by ensuring that the radio path between the local and the distant apparatus can be substantially free of blocking caused by the head or body of the user.

6.
4 Claim 4. The method of claim 1 and 2 whereby the performance of a hand-held mobile telephone or other portable RIF apparatus can be improved by constraining the return currents from the aerial to flow substantially in the transmission line rather than via the head and body of the user.

6.
5 Claim 5. Apparatus for reducing the intensity of electromagnetic radiation in the vicinity of the head or body of the user of a hand-held mobile telephone or other portable RIF apparatus, the apparatus consisting of:

a) an extendable support for an aerial, the length of which can be varied between an upper and lower limit set by design.

b) an aerial connected to one end of the extendable support, the other end being connected to the hand-held mobile telephone or other RIF apparatus via a transmission line.

6.6 Claim 6. Apparatus for constraining the radiation to be in the vicinity of the aerial of claim 5, the apparatus consisting of at least one of a counterpoise, a balun, and a magnetic sleeve.
6.8 Claim 8. Apparatus according to claim 5 and 6 whereby the performance of a hand-held mobile telephone or other portable RIF apparatus is improved by constraining the return currents from the aerial to flow in the transmission line rather than the head or body of the user.

6.7 Claim
7. Apparatus according to claim 5 whereby the aerial can be placed in such a position that the radio path between the user and the distant party is substantially free of signal blocking caused by the user.