AU767408B2 - Antennas for portable communications devices - Google Patents

Description

S&F Ref: 518424

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

Name and Address of Applicant Actual Inventor(s): Address for Service: Invention Title: Griffith University Kessels Road Nathan Queensland 4111 Australia David Victor Thiel, Jun Wei Lu and Steven Gregory O'Keefe Spruson Ferguson St Martins Tower 31 Market Street Sydney NSW 2000 Antennas for Portable Communications Devices ASSOCIATED PROVISIONAL APPLICATION DETAILS [33] Country [31] Applic. No(s) AU PQ1980 [32] Application Date 02 Aug 1999 The following statement is a full description of this invention, including the best method of performing it known to me/us:- 5815c ANTENNAS FOR PORTABLE COMMUNICATIONS DEVICES Field of the Invention The present invention relates to antennas for use in portable communications devices and, in one preferred form, to a physically small directional antenna assembly.

Description of the Prior Art The prior art in relation to antennas covers a broad spectrum. Antennas are used in a wide variety of applications both as transmitters and receivers of electromagnetic energy. One important consideration in many of these applications is the directivity of the antenna. It is generally desirable to maximise the directional properties of the antenna.

0• 0 This has been achieved in the prior art arrangements by techniques such as reflector screens, multiple antenna arrays, electronically steerable antennas and reflector elements.

Optimised antenna directivity is of particular concern in the area of mobile cellular communications. Improved directivity increases the range of mobile cellular telephones in relation to a cell site, and reduces the interference between adjacent cells. A reduction in power consumption, and hence less demand on the mobile telephone battery, also results from improved directivity of the antenna.

There are also presently concerns about the safety of mobile cellular telephones for users. Human tissue is a very good conductor of electricity, even at high frequencies, and it has been suggested that health problems may occur with prolonged use of such devices for reason of the antenna being very close to the user's skull resulting in very high strength electromagnetic fields concentrated about the antenna penetrating the skull and damaging brain tissue. The IEEE has published Technical Standard No. C95.3 in relation to recommended maximum exposure to electromagnetic radiation from antennas. A directional antenna can minimise the radiation directed towards the user, and from this point of view is most desirable.

A1 .13K 102(007 doc nixI Reduced exposure to mobile telephone radiation can also be achieved through the use of shielding devices. Such shields seek to protect the user by reducing the amount of radiation that is emitted towards the head of the user. However, there is a trade-off in that the absorbed energy is not used in transmission, thus reducing the overall efficiency of the mobile telephone. A further disadvantage of this method is that there is a certain amount of microwave energy that is diffracted around the edges of the shield. This diffracted energy reduces the effectiveness of the shield and therefore reduces the amount of protection that is given to the mobile telephone user.

B

The overall size of the antenna apparatus is another important consideration, particularly as electronic communications devices become ever more miniaturised. Large antenna apparatus are undesirable for reasons of portability, mechanical stability and appearance. Size is also an important consideration in achieving increased antenna directivity. In free space, the distance between radiating elements/reflectors is a .**substantial part of one free space wavelength of the radiation in air. This means that the antennas may be relatively large in more than one direction if directionality is required.

Reference also can be made to International Publication No. WO 94/28595 (equivalent to Australian Patent No. 679992) that discloses forms of physically small antennas.

It is an object of the present invention to substantially overcome or at least ameliorate one or more of the prior art deficiencies and/or preferably to provide a physically small directional antenna that provides protection to the user against electromagnetic radiation.

R:\LIBK02007.doc:mxl Summary of the Invention In accordance with one aspect of the present invention there is disclosed an antenna assembly comprising: a substantially planar structure of dielectric material without a ground plane; and an array of at least three antenna elements mounted on said structure, the array including an active element having a feed connection point, a first passive element being parallel with and spaced apart from the active element, and a second passive element being parallel with and spaced apart from the first active element in an opposed direction to said first passive element, the second passive element being connected with signal ground when in use.

The invention further discloses a portable communications device having an antenna assembly as described immediately above.

In one advantageous form, said antenna elements are substantially elongate.

Furthermore, said second passive element can further comprise a transverse portion to be substantially L-shaped, and of greater length than the active element to act as a reflector.

The L-shaped second passive element is arranged to at least partially surround the active element. The first passive element can be of equal or lesser length than the active element 20 to act as a director. The second passive element can further pass through said dielectric structure and extend over at least a portion of the opposed surface of the structure. The three antenna elements can be mounted on a common surface of the structure.

In a further preferred form, the feed point of the active element is electrically 25 connected with a centre conductor of a coaxial feed line. The feed point connection can be at one end of the active element. The second passive element therefore is electrically c t t ;.connected to a signal ground conductor of the coaxial feed line.

oa [R:\LIBK]02007.doc:mnxl The antenna assembly can further comprise a ferrite body adapted to be received around the feed line in the vicinity of the dielectric structure.

Preferably, said dielectric structure has a relatively high dielectric constant, and can be fabricated from a printed circuit board substrate material such as fibreglass and is of a substantially rectangular configuration being preferably greater than 1.2 mm thick.

Preferably, said ferrite body is composed of low loss ferrite material of a relatively high magnetic permeability. It can be of a substantially cylindrical configuration with a coaxial signal cable.

In a preferred embodiment, the antenna assembly is mounted from the communications device in a manner such that the plane of the array is perpendicular to the user's head, with the second passive element being proximate thereto.

In a further embodiment, the antenna assembly is mounted from the communications device in a manner such that the antenna assembly can pivot about its o base.

S" 20 Embodiments of the invention provide an antenna that has less absorption by the user's head, increased signal strength due to improved directionality, less crosspolarisation and a minimal change in antenna impedance with the user's head position than those in the prior art. This then results in a reduction in power consumption of the electronic equipment to which the antenna is coupled (eg. a cellular telephone). There 25 further is an associated health benefit, since the electromagnetic energy absorbed by the user's head will be at a lower level than in the prior art.

One other specific advantage is that, because the antenna assembly can be directly substituted for prior art antennas in portable communications devices, the [R\LIBK]02007.doc:mxl foregoing benefits are gained without a need to replace the otherwise expensive device.

In one example, a physically smaller antenna having improved directivity can be substituted for an existing antenna in a cellular telephone. Thus the telephone casing can further be reduced in size to provide the user with greater portability.

A further specific advantage is that the antenna assembly is capable of being arranged so as to fold down alongside a telephone casing further reducing the overall size of the device and further providing greater portability.

Brief Description of the Drawings Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a front elevational view of a directional antenna assembly according to 15 the preferred embodiment; Fig. 2 is a rear elevational view of the directional antenna assembly shown in Fig. 1; Fig. 3 is a side elevational view of the directional antenna assembly shown in Figs. 1 and 2; Fig. 4 is a front elevation view of the directional antenna assembly shown in Fig.

1, but showing the directional antenna assembly mounted on a cellular mobile telephone which is in use; Fig. 5 shows a plot of gain with angle, showing directional properties of the antenna assembly of Figs. 1 to 4; [l :\LIB K]02007.doc:mxl

S

S.

Fig. 6 shows a plot of the impedance properties of the antenna assembly of Figs.

1 to 4; Fig. 7 is a side elevational view of the directional antenna assembly according to another embodiment; Fig. 8 is a front elevational view of the directional antenna assembly of Fig. 7; Fig. 9 is a reverse side elevational view of the directional antenna assembly of 10 Fig. 7; Fig. 10 is a front elevational view of the antenna assembly of Fig. 1 mounted on a mobile cellular telephone; Fig. 11 is a view similar to Fig. 10, but showing the antenna assembly pivoted down to be aligned with the side of the mobile cellular telephone; Fig. 12 shows plots of antenna impedance as a function of the reflector element's length; and

S.

S.

Fig. 13 shows plots of antenna impedance as a function of the reflector element's length.

Detailed Description of the Drawings The embodiments will be described with reference to mobile cellular telecommunications. It is to be appreciated, however, that the invention is cclqually applicable to radio communications in general, including electromagnetic geophysics, radar systems and the like.

I RA:\113K 102007.doc:nix I As illustrated in the drawings, the antenna assembly 1 of the first embodiment includes a substrate 3, three antenna elements 5-7 and a bead 9 which is associated with a coaxial feed line 11.

As seen in Figs 1 and 2, the substrate 3 is of a substantially rectangular configuration. The three elements 5-7 are printed on the front face 10 of the substrate 3 in a substantially parallel arrangement. The centre, (active) element 5 runs along the longitudinal axis of the substrate 3, extending from a point near the base 17 to substantially the centre point of the substrate 3. A grounded reflector (passive) element 7 and a director (passive) element 6 are equally spaced on either side of the centre element As seen in Fig 1, the director element 6 is of substantially the same length as the centre I°element 5 and is arranged on the left side 13 of the substrate 3. The reflector element 7 extends from a point near the base of the substrate 3, where it is electrically connected with the signal ground shield of the feed line 11, parallel to the base 17 to a point near the right side 15 of the substrate 3. The reflector element 7 then continues from this point, parallel to the right side 15, to a point near the top 19 of the substrate 3. This arrangement can be considered substantially L-shaped, such that the reflector partially surrounds the centre element As best seen in Fig 2, the reflector element 7 also continues onto the rear face of the substrate 3 by a via 23 passing therethrough. On the rear face 19, the director element 7 extends from a point near the top 19 of the substrate 3 to a point substantially half-way between the base 17 and the top 19 of the substrate 3. This arrangement maintains the electrical length of the director element 7 without increasing the overall physical length of the antenna assembly 1.

The bead 9 is of a substantially cylindrical configuration and is arranged at the base 17 of the substrate 3. The substrate 3 is mounted on one edge of the bead 9, as seen [RALI3K]02007.doc:mx in Fig 3, so that the bead 9 is arranged centrally relative to the base 17 of the substrate 3.

The substrate 3 is arranged substantially perpendicular with the top face 24 of the bead 9.

As best seen in Fig 3, the coaxial feed line 1 1 runs through the centre of the bead 9 and obtrudes from the top face 24 of the bead 9. The centre (signal) conductor 25 of the coaxial feed line 11 is electrically interconnected with the centre element 5. The outer conductor of the coaxial feed line 11 is electrically interconnected with the reflector element 7.

10 The substrate 3 is fabricated from a dielectric material, and is preferably at least 1.2 mm thick. In the preferred embodiment the material is a standard PCB material commonly called fibreglass FR4 which has a dielectric constant of 4-5 Eo. A conductor embedded in a dielectric material has an electrical length reduced by a factor proportional to the square root of the dielectric constant of the material. The effect of the dielectric material is to increase the effective length of the elements 5-7 and to increase the effective spacing between the elements, therefore allowing the antenna assembly 1 to be physically smaller than one constructed of wires in free space. For a conductor lying on the surface of an infinite dielectric halfspace with a relative dielectric constant Er, the effective dielectric constant, Seff, is given by the expression: Eeff (1 r)/ 2 The antenna elements 5-7 are configured on the dielectric substrate 3 in a manner commonly referred to as a Yagi arrangement, namely director(s) active element reflector, in the direction of an incoming wavefront. The Yagi arrangement is used in situations where optimised directionality of the transmitted and received antenna signals is required. Further improved directivity is achieved in the above described arrangement due to the effect of the dielectric substrate 3 in that a conductor located on the surface of or within a dielectric has an asymmetrical radiation pattern. Passive conductors of a dimension close to a resonant length and located within one wavelength of an active element act as reflectors, influence the radiation pattern of the antenna and decrease its [R:\LIBK]02007.doc:mxl resonant length. The centre element 5 excites the antenna structure. The director element 6 has been spaced so as to reinforce the field of the centre element 5, thus providing the antenna with a directional radiation (polar pattern) characteristic. The reflector element 7 is used to optimise the directivity of the antenna by reflecting the electric field of the centre element 5 back toward the director element 6. The above described arrangement may be regarded as an antenna structure which supports a travelling wave whose radiation characteristics are determined by the current distribution in each element of the antenna structure and the phase velocity of the travelling wave.

When used in a cellular mobile telecommunications application, typically at a frequency of 970 MHz, the antenna assembly 1 can have the following representative dimensions.

The substrate ofFR-4 material is 1.3 mm thick and 60 mm x 25 m in area. The antenna elements, formed from etched copper tracks, each are 2.0 mm in width; the centre active element is 38 mm in length, the director element 6 is 38 mm in length, and the reflector element 7 is 54 mm in length on the front face 10 and 34 mm in length on the rear face 20. The spacing between the three antenna elements 5,6,7 is 10 mm (centre to centre).

All of these distances in copper, scale linearly with frequency to aI first approximation. The size of the dielectric substrate 3 is chosen to accommodate the physical lengths of the copper antenna elements 5,6,7.

The position of the via 23 through the substrate 3 controls the lower centre frequency of the antenna. Thought of another way, the length of the grounded reflector element 7 affects the lower centre frequency. The relation is one of decreased length resulting in a higher centre frequency.

f R:\L13K]02007.doc:mnxl The bead 9 is fabricated from any convenient ferrite material and is effective to improve the Q of the antenna, and also reduces the effect of the user's hand on a handset 27 (to which the antenna assembly is attached) on the performance of the antenna.

As seen in Fig 4, in its normal operating position, the antenna assembly 1 is to be aligned generally perpendicular to the head of the user. In this position, the reflector element 7 is the closest element to the user with the centre element 5 and the director element 6 each positioned respectively further away from the user.

10 Figs 1 to 4 show an antenna assembly 1 that can be used in direct substitution for known antenna configurations, for example, in cellular mobile telephones. The assembly 1 can be mechanically arranged to fold down onto the top 29 of the mobile telephone handset 27.

The antenna assembly 1 described has a reduced physical size with respect to prior art arrangements. Size is an important design consideration in hand-held cellular

S..

telephones. A long single wire antenna (for example, an end feed dipole or a 3/4 wavelength dipole antenna) distributes the RF energy so that head absorption by the user a. is reduced. The antenna is also more efficient due to a larger effective aperture. The longer the antenna is, however, the less desirable it is from the point of view of portability and mechanical stability. The dielectric substrate 3 of the preferred embodiment has the effect of reducing the effective electrical length of the elements 5-7. This means that the mechanical dimensions of the antenna assembly 1 are smaller for any operational frequency than is conventionally the case; the electrical length and separation therefore are longer than the mechanical dimensions suggest. Therefore, the antenna assembly 1 as seen in Fig. 1, can achieve the same performance characteristics (ie. forward and backward gains, input impedance, bandwidth, front-to-back ratio, and magnitude of minor lobes) as the noted larger known types of antenna, but has the added advantage of being physically small.

IR I 111K])21)0)7 m\1 The directional properties of the antenna assembly 1 are shown in Fig. 5, having a front-to-back ratio of 10 dB, for a frequency of 960 MHz.

The impedance properties of the antenna assembly 1 are shown in Fig. 6 as SI 1 measurements relative to a 50 ohm cable. The S11 at the resident frequency is -35 dB, and the 10 dB bandwidth is 80 MHz. Fig. 6 illustrates a second resonance at 1.3 GHz.

This performance makes the antenna suitable also for use in a dual band mode, as will be presently discussed.

S* In a further embodiment, the antenna assembly 1 can be mechanically arranged to swivel about its base 17, as seen in Figs. 7 to 11.

Fig. 7 shows the coaxial feed 11 running substantially perpendicular to the substrate 3 in this embodiment. The ferrite bead 9 is substantially sandwiched between the substrate 3 and a handset chassis. As seen in Fig. 8, the reflector element 7 is arranged on the substrate 3 in substantially the same manner as in the previous embodiment. However, the centre (active) element 5 and the director element 6 are arranged on the rear face 20 of the substrate 3, as seen in Fig. 9. This arrangement minimises coupling of the radio frequency energy into the chassis of the handset 27.

In Fig. 10, the antenna assembly 1 shown in its extended position relative to the handset 27, such that the pivoting point located on the side 31 means that the antenna assembly 1 extends above the top 29 of the handset 27. In Fig. 11 the attachment point to the side 31 is such that the antenna assembly extends to be flush with the top 29 of the handset 27.

As discussed with reference to Fig. 6, the antenna assembly embodying the invention has a second resonance, making it suitable for operation as a dual frequency [R:\L13Kj02007.doc:nixI antenna. Dual frequency mobile communications will operate at frequencies in the range of 900 MHz and 1.8 GHz. Embodiments of the invention can be 'tuned' so as to be suitable for operation in both of the frequency ranges mentioned.

Fig. 12 shows a plot of antenna impedance as a function of the length of the 'ground line' (being the total length of the grounded reflector element 7 on the front and back faces), demonstrating how the lowest centre frequency can be shifted and still overlap with the GSM900 frequency bandwidth. Fig. 13 shows the variation in antenna impedance characteristics as a function of the length of the feed line the driven centre 10 element 5) on the strength of the upper resonance in the region of the DSCSI800 frequency bandwidth region. Accordingly, an appropriate choice of active element and :reflector element dimensions can result in an antenna that is able to service dual frequency mobile telecommunications systems.

As noted above, there are presently concerns about the effect of very high **strength electromagnetic fields associated with mobile cellular telephone antennas, on S° brain tissue. The overall improved directionality and efficiency of the antenna assemblies described means that the magnitude of radiation that is directed towards the head of the user of the mobile telephone is greatly reduced. In this connection the embodiments of the invention offers greater protection to users of mobile telephones than prior arrangements.

The foregoing describes only one embodiment of the present invention and modifications, obvious to those skilled in the art, can be made thereto without departing from the scope of the present invention. For example, the number of antenna elements is not restricted to three. There may be two or more passive elements acting as directors.

Other regular or irregular arrays of monopole or dipole elements, in close relation to a dielectric structure, are also contemplated.

[R:\LIBK 02007.doc:ix I

Claims (17)

1. An antenna assembly comprising: a substantially planar structure of dielectric material without a ground plane; and an array of at least three antenna elements mounted on said structure, the array including an active element having a feed connection point, a first passive element being parallel with and spaced apart from the active element, and a second passive element being parallel with and spaced apart from the first active element in an opposed direction to said first passive element, the second passive element being connected with signal ground when in use.

2. An antenna assembly as claimed in claim 1, wherein said antenna elements are substantially elongate.

3. An antenna assembly as claimed in claim 2, wherein said first passive element is of equal or lesser length than the active element, and the second passive element is of greater length than the active element. e*

4. An antenna assembly as claimed in claim 3, wherein said second passive 20 element includes a transverse portion to be substantially L-shaped and at least partially e e surround the active element. An antenna element as claimed in claim 4, wherein the second passive '.element passes through the dielectric structure to further extend over at least a portion of 25 the opposed surface of the planar structure. o 6. An antenna element as claimed in any one of the preceding claims, S•wherein said antenna elements are mounted on a common surface of said structure. [R:\LIBK]02007.doc:mx -14-

7. An antenna element as claimed in any one of the preceding claims, wherein said planar structure is formed by a sheet of dielectric material.

8. An antenna element as claimed in claim 7, wherein said sheet is of fibreglass material and at least 1.2 mm in thickness.

9. An antenna element as claimed in any one of the preceding claims wherein a feed point of the active element is electrically connected with the signal conductor of a coaxial feed line, and the second passive element is electrically connected with the signal ground conductor of the coaxial feed line when in use. An antenna element as claimed in claim 9, wherein, when in use, said connections are made at an end of said active element and said second passive element, respectively.

11. A portable telecommunications device comprising: a transceiver located within a hand-held casing; and .oan antenna assembly mounted from, or located within said casing and connected with the transceiver by a coaxial feed line, the assembly including: 20 a substantially planar structure of dielectric material without a ground *.99 plane; and an array of at least three antenna elements mounted on said structure, the array including an active element having a feed connection point electrically -connected with the signal conductor of the feed line, a first passive element being oooo 25 parallel with and spaced apart from the active element, and a second passive element being parallel with and spaced apart from the first active element in an ~opposed direction to said first passive element, the second passive element being S•electrically connected with the signal ground conductor of the feed line. [R:\LIBK]02007.doc:mnxl

12. A device as claimed in claim 11, wherein said antenna assembly is arranged so that, in use of the device, said second passive element is closer to the user's head than the active element.

13. A device as claimed in claim 12, wherein said antenna elements are substantially elongate.

14. A device as claimed in claim 13, wherein said first passive element is of equal or lesser length than the active element, and the second passive element is of greater length than the active element. A device as claimed in claim 14, wherein said second passive element includes a transverse portion to be substantially L-shaped and at least partially surround the active element.

16. A device as claimed in claim 15, wherein the second passive element passes through the dielectric structure to further extend over at least a portion of the opposed surface of the planar structure. sees S" 20 17. A device as claimed in any one of claims 11 to 16, wherein said antenna elements are mounted on a common surface of said surface.

18. A device as claimed in any one of claims 12 to 17, wherein said planar o structure is formed by a sheet of dielectric material.

19. A device as claimed in claim 18, wherein said sheet is of fibreglass m a o material and at least 1.2 mm in thickness. [R:\LIBK]02007.doc: mx -16- A device as claimed in any one of claims 11 to 19, wherein the connection to the signal conductor is made at one end of said active element.

21. A device as claimed in claim 20, further comprising a ferrite collar located proximate said feed point and through which said coaxial feed line passes.

22. A device as claimed in any one of claims 11 to 21 for use in cellular mobile telecommunications.

23. An antenna element substantially as herein described with reference to Figs. 1 to 3 and/or 7 to 9 of the accompanying drawings. DATED this twenty-seventh Day of August, 2003 Griffith University Patent Attorneys for the Applicant SPRUSON FERGUSON *fee oo 9 00 *e 99 9.* [R:\LIBK102007.doc: xi