Radio communications handset antenna arrangements

Abstract: This invention relates to internal antenna arrangements for radio communications handsets. Internal antenna size and shape represents a constraint on handset miniaturisation if good antenna efficiency and bandwidth characteristics are to be maintained. The use of acoustic enhancing volumes of free space about a handset's speaker unit also constrains further miniaturisation. The present invention provides an internal antenna arrangement which facilities further miniaturisation and which combines antenna volume with the acoustic enhancing volume.

Description: FIELD OF THE INVENTION The invention generally relates to radio communications handsets, and in particular to internal antenna arrangements. PRIOR ART Recent advances in mobile communications have been coupled with increasing demand for miniaturisation of mobile communications handsets. A significant limitation on such miniaturisation is the internal antenna size which cannot easily be reduced. Existing antennas used in radio communications handsets include extendible monopoles, microstrip patch antennas, inverted L and F antennas, and helix antennas. Half or quarter wavelength monopoles extend a significant length from the handset and have a number of disadvantages including the inconvenience of such a long protuberance which is easily broken and can be hazardous to users eyes for example. The microstrip patch , while having a low profile, small size and light weight, has low efficiency or a narrow bandwidth. The inverted L antenna requires a significant physical length (quarter wavelength) for efficient operation, this is generally not possible within a handset so that a shortened L is generally inefficient. This can be improved by using a tuning element in the form of a stub to the ground plane giving the antenna an inverted F configuration, however this still suffers from inefficiency and limited bandwidth in the physical size constraints applicable to a handset. The helix antenna, while conveniently short, still requires a significant cylindrical volume which may be extended outside the main body of the handset forming a short protuberance. While this facilitates to some extent miniaturisation of the main handset, the protuberance is inconvenient in practical use. The helix also suffers from a narrow bandwidth. Various meandering antenna arrangements are also known. US4021810 discloses a 3D array of meander structure conductors above a ground plane which is complex to produce and is susceptible to the vagaries of manufacturing tolerances. WO96/38882 discloses a printed meandering monopole antenna extending from a mobile handset. While the meandering monopole is shorter than a standard monopole, it still represents an inconvenient protuberance outside the handset. WO93/12559 discloses a planar metallic sheet inverted F antenna having dependant elements angled with respect to the planar structure. As such it is delicate and complicated to manufacture. In addition to the above mentioned antenna size and volume constraints on the miniaturisation of handsets, there is now an increasing need for a handset to be used in different communications systems such as mobile and cordless telephony or mobiles in different countries, which requires the handset to be operable over more than one frequency band. While a single antenna and a multiple band matching circuit may be employed, this can prove overly complex and costly so that in practice each handset may require a separate antenna for each frequency band together with sufficient spacing between adjacent antennas to minimise coupling effects there between. US-A-4 679 233 discloses a radio communications handset comprising a speaker having an adjacent volume of free space for acoustic enhancement and an antenna arrangement which incorporates said volume within the antenna arrangement, said antenna arrangement comprising a ground plane and a radiating element. The ground plane has a "u"-shaped form that encloses the free space volume. The radiating element is positioned above the ground plane remote from the free space volume location. OBJECT OF THE INVENTION It is an object of the present invention to facilitate handset miniaturisation by providing improved or alternative internal antenna arrangements for such handsets. SUMMARY OF THE INVENTION In accordance with a first aspect, there is provided a radio communications handset comprising: a speaker having an adjacent volume of free space V for acoustic enhancement; and an antenna arrangement which incorporates said volume within the antenna arrangement, said antenna arrangement comprising a ground plane and a radiating element, characterised in that the arrangement is such that the volume of free space adjacent the speaker is located between the ground plane and the radiating element. Preferably, the radiating element is spaced a non-uniform distance over its length from said ground plane. Preferably, the radiating element of the antenna arrangement comprises a meandering radiating element extending in a series of opposing bends from a radio-frequency feed point. Preferably also, said opposing bends in the radiating element are effected in more than one plane. Preferably further, the antenna arrangement further comprises a planar element connected to a free end of said radiating element and extending backwards of and substantially parallel with said radiating element Preferably further, the radiating element is a monopole antenna radiating element. Preferably further, the handset further comprises: an extendible external antenna; radio frequency transceiver means; and antenna switching means which is arranged to switch between said transceiver means and said external antenna or said antenna arrangement upon manual extension or retraction of said external antenna. Preferably further, said non-uniform spacing is such that the radiating element extends in an arc across the ground plane. BRIEF DESCRIPTION OF THE DRAWINGS In order that a greater understanding of the invention be obtained, embodiments of the invention will now be described with reference to the accompanying drawings, by way of example only and without intending to be limited, in which: Figure 1(a) shows a preferred embodiment handset arrangement of the invention, and figure 1(b) shows a detail section of the handset's acoustic volume contained within the handset's antenna arrangement; Figures 2 (a) and (b) show in detail an preferred embodiment antenna arrangement of the invention in plan and elevation respectively; Figures 3 (a) and (b) show an alternative embodiment antenna arrangement in perspective and section respectively; Figures 4 (a) and (b) show a multi band embodiment of the antenna arrangement in plan and elevation respectively; Figure 5 shows insertion loss for a single band antenna; Figure 6 shows insertion loss for a dual band antenna; Figure 7 shows the azimuth radiation pattern for the single band antenna; Figure 8 shows the azimuth radiation pattern for the dual band antenna; and Figure 9 shows an external antenna switching arrangement. DETAILED DESCRIPTION Referring to figure 1(a), a handset 1 is there shown comprising a speaker unit 10 and an adjacent volume of free space V extending behind the speaker unit (as shown in detail 1(b)) for acoustic enhancement; an antenna arrangement 2 comprising a ground plane 4 and a radiating element 3 extending from a radio frequency feed point 8 on the handsets printed circuit board (PCB) 11. The radiating element 3 is curved with respect to the ground plane 4 and is arranged to fit around the peripheral edges of the acoustic enhancing volume of free space V, thereby incorporating the volume V within the antenna arrangement 2.. The ground plane may be formed on the handset's PCB 11, or a metallic plane may be formed perpendicular to the PCB 11 for example by a shielding case. The radiating element 3 of the antenna arrangement 2 is preferably a monopole type structure formed into a zig-zag pattern which consists of a series of opposing bends. The zigzag formation of the radiating element 3 maintains a small and convenient volume within the handset 1 while providing a self-resonant antenna 2 as described herein below. This particular antenna construction also provides good antenna efficiency and bandwidth characteristics. It should be noted that unlike conventional short antennas for handset applications, such as inverted F and folded monopole antennas, the radiating element 3 does not require tuning or matching stubs, nor grounding at any point along its length to achieve the desired resonant frequency from its compact dimensions. By contrast the radiating element is fed at one end while the other end is left free. This facilitates inclusion of handset elements such as speaker acoustic enhancing volumes between the radiating element 3 and the ground plane 4. The inclusion of the acoustics volume V between the radiating element 3 and the ground plane 4 reduces the combined internal antenna and acoustic volumes on further miniaturisation of handsets with this acoustic volume V. A preferred antenna arrangement is described in more detail with reference to figures 2 (a) and (b). The antenna arrangement 2 comprises a radiating element 3 and ground plane 4 connected to the handset's radio frequency transceiver circuitry 7 via a radio frequency feed point 8. The antenna 2 is shown in plan in figure 2a and in elevation in figure 2b. Referring to figure 2a, the radiating element 3 is a monopole structure which extends from the feed point 8 in a series of opposing bends which form a zigzag pattern of substantially parallel sections 6 separated by the bends 5. Referring to figure 2b, the radiating element 3 extends in a curve A with respect to the ground plane 4. Each bend 5 introduces an inductive element L bn into the antenna 2 which increases with sharpness (reduced radius r) of the bend 5. Capacitive elements C bn are introduced between adjacent sections 6 which are dependent on the respective parallel lengths I and distances d between adjacent sections. Further capacitive elements C gn are introduced between the radiating element 3 and the ground plane 4, each notional capacitance C gn being dependent on the distance between the ground plane 4 and radiating element 3 at that point. The combination of bends 5 and sections 6 can be thought of as a matching network composed of a variable inductor and capacitor in parallel, together with a shunt capacitor to ground. By varying the length I and separation distance d of the sections 6 the capacitance C b can be varied and by varying the bend 5 distance or radius r, the inductance L 6 can be varied. Similarly by varying the separation between the radiating element 3 and ground plane 4 and the radiating element radius R, the shunt capacitance C g can be varied. By varying these capacitive and inductive elements experimentally the antenna 2 can be made self-resonant at a desired frequency. The antenna 2 of the invention therefore does not require a matching network for tuning. The bandwidth of the antenna can be broadened by extending the total length of the radiating element 3. The capacitive elements C gn also influence the bandwidth of the tuned antenna 2. The centre frequency of the antenna 2 is influenced by the capacitive elements C gn and C bn and the inductive elements L bn . In practice these elements are varied experimentally to obtain the desired centre frequency and bandwidth of the antenna 2. The dimensions of the resulting antenna structure can then be mass produced as required. Preferably the radiating element 3 consists of a piece of plated wire bent into a series of bends to cause inductance and capacitance along its length. The whole radiating element 3 sits above the ground plane 4 of a PCB 11 in the handset 1, forming a variable impedance transmission line as the distance between the ground plane 4 and radiating element 3 varies. The series of bends 5 and sections 6 which form the radiating element 3 need not form a regular pattern as is shown in the preferred embodiment. The zig-zag pattern of the bends 5 and sections 6 is formed in a plane colinear with the direction of extension of the radiating element - denoted by curve A in figure 2b. While this plane is shown in figures 2a and 2b as perpendicular to the PCB 11 plane, the zig-zag pattern may be formed in any plane colinear with curve A. For example figure 1 shows the radiating element 3 formed in a plane parallel with the PCB 11 plane. As a further alternative the radiating element zigzag pattern may be formed in more than one plane as is shown in figure 3 in which the pattern extends in two perpendicular planes - one parallel and one perpendicular to the PCB 11 plane. A further embodiment antenna 2 is shown in figures 4 (a) and (b) which comprises a dual band antenna 2 in which a plate or planar element 20 is connected to the free end of the radiating element 3 extending back from the connection and substantially parallel with the radiating element 3. The presence of the planar element 20 shifts the second harmonic of the fundamental resonant frequency of the antenna 2 along the frequency spectrum effectively introducing a further controllable frequency band. The planar element 20 shifts the second harmonic down the frequency spectrum depending on for example the planar elements length and distance from the radiating element 3. The dimensions of the planar element 20 and its physical relationship to the radiating element 3 are obtained experimentally for the desired frequency bands. Figure 4 shows the dual band antenna tuned to the 850 MHz and 1920 MHz frequency bands. In experimentation, the first preferred embodiment antenna arrangement has been shown to have an antenna efficiency of 75% at 850 MHz. For the second preferred dual band antenna arrangement of figure 4, the antenna efficiency at 850 MHz has been measured at 75%, and at the higher band of 1920 MHz an antenna efficiency of 91% has been achieved. This compares favourably with an antenna efficiency of 71% for a helix antenna at 920 MHz. Figure 5 shows the insertion loss of the single frequency antenna. It can be seen that adequate return loss (>10dB) is seen across the band, this can be improved by retuning. Placement of the intended speaker unit 10 inside the antenna 2 produced only a slight change in frequency which is readily retuned. Figures 6, 7 and 8 show respectively the insertion loss of the dual band antenna; the azimuth radiation pattern of the single band antenna; the azimuth radiation pattern of the dual band antenna at 850 MHz; and at 1920 MHz. Referring now to figures 1 and 9 and a further inventive aspect in which a switching arrangement is used to switch between the internal antenna 2 and an external antenna 13 such as a telescopically extendible monopole. This allows each antenna to be individually optimised without the detrimental influence of the other antenna being in circuit. The need for complex and expensive dual matching circuitry is therefore essentially eliminated. The use of the switching arrangement is not restricted to the particular antenna arrangement of the invention as described above, but could be used with any type of internal and external antenna. The switching arrangement is shown in more detail in figure 9 and makes use of the manual engagement or disengagement of the external antenna 13. As the external antenna 13 is pulled out a metallic contact 31 attached at its base engages a flat spring contact 32 which disconnects the internal antenna 2 from the transceiver output 33, and simultaneously connects the external antenna 13 to the transceiver output 33. The reverse occurs when the external antenna 13 is manually pushed back into the handset. The switching arrangement could also be modified to operate using external antennas which are folded out or which are physically connected to the handset when required. Various alternative switching arrangements are conceivable by a person skilled in the art, including electronic switching, capacitive coupling, and other mechanical switching means.

Claims: A radio communications handset (1) comprising: a speaker (10) having an adjacent volume of free space V for acoustic enhancement; and an antenna arrangement (2) which incorporates said volume within the antenna arrangement, said antenna arrangement comprising a ground plane (14) and a radiating element (3), characterised in that the arrangement is such that the volume of free space adjacent the speaker is located between the ground plane and the radiating element. A radio communications handset according to claim 1, characterised in that the radiating element (3) is spaced a non-uniform distance over its length from said ground plane (4). A radio communications handset according to claim 1 or claim 2, characterised in that the radiating element of the antenna arrangement (1) comprises a meandering radiating element (3) extending in a series of opposing bends (5) from a radio-frequency feed point (8). A radio communications handset according to claim 3, characterised in that said opposing bends (5) in the radiating element (3) are effected in more than one plane. A radio communications handset according to any preceding claim, characterised in that the antenna arrangement (2) further comprises a planar element (20) connected to the free end of said radiating element (3) and extending backwards of and substantially parallel with said radiating element. A radio communications handset according to any one of claims 3 to 5, characterised in that the radiating element (3) is a monopole antenna radiating element. A radio communications handset according to any preceding claim, characterised in that the handset further comprises: an extendible external antenna (13); radio frequency transceiver means (7); and antenna switching means (31,32) which is arranged to switch between said transceiver means (7) and said external antenna (13) or said antenna arrangement (2) upon manual extension or retraction of said external antenna. A radio communications handset according to any one of claims 2 to 7, characterised in that said non-uniform spacing is such that the radiating element extends in an arc across the ground plane.

Inventors: AMOS SONYA, DALBY ANTHONY, GWYNN PETER, LLEWELLYN IAN PAUL, ROBSON JULIUS GEORGE, SMITH MARTIN STEVENS, AMOS, SONYA, DALBY, ANTHONY, GWYNN, PETER, LLEWELLYN, IAN PAUL, ROBSON, JULIUS GEORGE, SMITH, MARTIN STEVENS
Applicants: NORTEL NETWORKS LTD, NORTEL NETWORKS LIMITED
Application year: 1998
Main classification: H01Q1
IPC codes: H01Q1, H04R1