The antenna requirements of a cellular or cordless telephone handset are primarily that it should be compact and omnidirectional. For a handset operating within the frequency range of 800 MHz to 2 GHz the antenna is typically an extendible rod having a length approximately equivalent to a quarter wavelength when extended, or a helical wire having several turns. The antenna is usuallv mounted partlv within the handset unit and partly <BR> <BR> <BR> <BR> projecting from the end of the unit ad-lacent the earphone. A disadvantage with small antennas such as those designed for personal telephone use is that. in general. they have poor gain over the frequency band in which they are required to operate. It is also known that small resonant antennas generally have a narrower fractional bandwidth than their larger counterparts designed to operate at lower frequencies. Another disadvantage is that. the smaller they are. the greater is their tendency to generate intense near-field electromagnetic radiation, i. e. radiation which is perceived to represent a health hazard when such an antenna is used close to the head for transmission of signals. The measurement of this effect is conducted to produce a parameter usuallv referred to as the Specific Absorption Rate (SAR).

The latter disadvantage has been addressed to some extent in the Applicant's co-pending British Patent Application No. 2309592 which discloses a twisted loop antenna exhibiting a radiation pattern with an azimuth null when oriented with the axis of the radiating helices upright. With appropriate mounting of the antenna on the housing of a portable telephone, this null can be directed towards the user's head to reduce irradiation in that direction.

It is an object of the invention to provide a small antenna which combines improved bandwidth with good SAR performance.

According to a first aspect of this invention, an antenna for operation at frequencies in excess of 200 MHz comprises: an insulative core of a solid material having a relative dielectric constant greater than 5 the outer surface of the core defining a volume the major part of which is occupied by the solid material : a feeder structure comprising a length of transmission line; an electrically conductive structure including a plurality of radiating elements connected to the transmission line at a first position, and a link element separately connecting the radiating elements to the transmission line at a second position, spaced from the first position along the feeder structure, the electrically conductive structure being disposed on or adjacent the outer surface of the core; wherein the core and conductive structure are configured such that the antenna has at least two different modes of resonance which are coupled thereby to define together an operating frequency band for signals fed to or received from the transmission line, the different modes of resonance being associated with different respective radio frequency (, r. f.) current patterns in the conductive structure, each pattern including the said radiating elements.

The antenna may be configured such that (a) a first mode of resonance occurs at a first frequency within the said band and is associated with an r. f. current loop including the said antenna elements and beginning and ending at a location at which the antenna elements are connected to the transmission line at the first position, the link element acting as a high impedance blocking element at the first frequency, and such that (b) a second mode of resonance occurs at a second frequency within the said band and is associated with an r. f. current loop running from the location of the connection of the antenna elements to the transmission line at the first position, through the antenna elements and link element in series, to the connection with the feeder structure at the second position.

The antenna may further be configured so that the input reactance component of the load represented by the antenna is substantially zero within the operating frequency band only when the corresponding input resistance component is finite and substantially non-zero.

The corresponding Smith chart representation of the load impedance presented by the antenna within the operating band is typically in the form of a looped self-intersecting

locus.

In the preferred embodiment, there are two modes of resonance within a single operating band of the antenna, the first resonance being a balanced mode and the second mode of resonance being a single-ended mode. The antenna elements, the link element, in the form of a balun trap, and the transmission line all act as current-carrying elements in both modes of resonance. In this preferred embodiment, the core is cylindrical, having a central axis of symmetry, and the antenna elements are a plurality of axially co-extensive conductors extending between an end of the transmission line and the trap element. These antenna elements are the sole radiating elements, and the antenna has no other elements which act as significant radiating elements in either mode. Effectively, the antenna comprises a unitary structure with a unitary set of conductive elements which act together with both modes to yield two different structural modes of resonance.

It will be appreciated that such an antenna provides an improved operating bandwidth without using a large antenna structure or a plurality of separately fed antenna structures.

The frequency responses associated with the respective modes couple together in the frequency domain so as to define the operating bandwidth.

By dimensioning the elements so that the two modes occur within a required band, e. g. the 1710 MHz to 1880 MHz DCS-1800 band for cellular telephones, or the 890 MHz to 965 Mhz European GSM band for cellular telephones, the whole of either of these bands can be accommodated with the bandwidth of the antenna, the two resonant modes coupling such that energy storage associated with one mode of resonance is shared with energy storage in the other mode of resonance, thereby forming a frequency response which is flat-topped or has a non-zero saddle between two resonant peaks. Typically, the modes of resonance are arranged to couple to achieve a combined gain characteristic for the antenna which maintains a response within the 3dB limits over a fractional bandwidth of at least 3% of the centre frequency of the operating band.

In the preferred antenna in accordance with the invention, the radiating elements comprise

a singe pair of elongate antenna elements interconnected at respective ends by the link element so as to form a path of conductive material around the core with the other ends of the antenna elements constituting a feed connection at a distal end of the transmission line. The antenna elements are co-extensive, each element extending between axially spaced-apart positions on the outer cylindrical surface of the core. The elements may be metallised tracks deposited or bonded onto the core and arranged such that at each of the spaced-apart positions the respective spaced-apart portions of the elements are substantially diametrically opposed. The spaced apart portions all lie substantially in a single plane containing the central axis of the core, and the portions at one of the spaced-apart positions are connected together by the link element to form the loop, the portions at the other of the spaced-apart positions being coupled to feed connections for the loop by cross elements extending generally radially on an end face of the core. The antenna elements are preferably of equal length and are helical, each executing a half-turn around the core between the spaced-apart portions. The feed connections may be connected to a coaxial feeder which forms the transmission line extending through the core on the axis. The other end face of the core is metallised, the resulting conductive layer forming part of the link element.

Where the transmission line emerges at this other end face of the core the coaxial outer (the screen) is electrically connected to the conductive layer, and the line forms a termination for the antenna. It is at this termination that the insertion loss and retlection coefficients can be measured to determine the bandwidth and load parameters referred to above.

The antenna is also extremely compact, for example, an antenna for operation in the DCS 1800 band of 1710 MHz to 1880 MHz typically has a cylindrical core with an axial length of 12.1mm and a diameter of lOmm, using a core material having a relative dielectric constant of about 36.5 or higher.

According to a second aspect of the invention, there is provided a handheld radio communication unit having a radio transceiver, an integral earphone for directing sound

energy from an inner face of the unit which, in use, is placed against the user's ear, and an antenna as described above coupled to the transceiver, wherein the antenna has a central axis and the said first and second positions on the transmission line are spaced apart along the axis, and wherein the antenna, over at least part of the operating frequency band, has a radiation pattern which, in a plane passing through the core normally to the axis, is substantially omnidirectional with the exception of a null. the antenna being mounted so that the axis of the antenna is generally parallel to the said inner face of unit and with the null directed generally perpendicularly to the inner face of the unit and, in use of the unit, towards the user's head.

With regard to orientation, in the case of the antenna core being in the form of a cylinder.

(which may be drum-or rod-shaped) with a pair of co-extensive antenna elements the ends of which lie in a plane containing the central axis of the core. the plane is preferably parallel to the inner face of the unit. Providing the antenna with a trap element or balun in the form of a metallised sleeve not only allows the antenna loop to be fed in a substantially balanced condition, but also reduces the effect of the comparatively small ground mass represented by the communication unit. In addition, it provides a useful surface area for secure mounting of the antenna, e. g. by soldering or clamping.

For reasons of physical and electrical stability, the material of the core is advantageously ceramic, e. g. a microwave ceramic material such as a zirconium-titanate-based material, magnesium calcium titanate, barium zirconium tantalate, and barium neodymium titanate, or a combination of these. The preferred relative dielectric constant (e,) is upwards of 10 or, indeed, upwards of 20,36. with a figure of 80 being attainable using a barium titanate material. Such materials have negligible dielectric loss to the extent that the Q of the antenna is governed more by the electrical resistance of the antenna elements than core loss.

In the preferred antenna, the antenna elements are fed from a distal end, the core having a central passage housing a coaxial feeder structure extending from a proximal or mounting end of the core and opening out at the distal end where radial elements couple

the antenna elements on the cylindrical outer surface of the core respectively to the inner and outer conductors of the feeder structure. The link conductor may then be annular, and advantageously is constituted bv a cylindrical sleeve on the outer surface of the proximal part of the core.

According to a third aspect of the invention, there is provided a handheld radio communication unit having a radio transceiver and an antenna as described above, the antenna being coupled to the transceiver, wherein the transceiver has a transmitting band portion and a receiving band portion which are different but adjacent each other within the said operating frequency band of then antenna.

The invention will now be described by way of example with reference to the drawings in which:- Figure 1 is a perspective view of an antenna in accordance with the invention : Figure 2 is an equivalent circuit diagram representing the antenna in a balanced mode of resonance : Figure 3 is an equivalent circuit diagram representing the antenna in a single-ended mode of resonance; Figure 4A is a diagram showing the variation of antenna gain with frequency due to separate modes of resonance; Figure 4B is a diagram showing the variation of antenna gain with frequency of the antenna of Figure I with coupled modes of resonance; Figure 5 is a Smith chart representation of the antenna ; Figures 6A and 6B are graphs showing respectively the magnitude and phase of r. f.

currents in the radiating elements of the antenna with respect to frequency; Figure 7 is a perspective view of a telephone handset incorporating an antenna in accordance with the invention.

Referring to Figure 1, an antenna 10 in accordance with the invention has an antenna element structure with two longitudinally extending radiating antenna elements 10A, lOB formed as metallic conductor tracks on the cylindrical outer surface of a ceramic core 12.

In this context,"radiating element"means an element which, if the antenna is used for transmission, radiates energy to space. Such an element, when used for receiving signals, receives energy from space in a reciprocal way. The core 12 has an axial passage 14 with an inner metallic lining 16, and the passage houses an axial inner feeder conductor 18.

The inner conductor 18 and the lining in this case form a transmission line feeder structure for coupling a feed line to the antenna elements IOAq IOB at a feed position on the distal end face 12D of the core. The antenna element structure also includes corresponding radial antenna elements 10AR, 10BR formed as metallic tracks on the distal end face 12D connecting diametrically opposed ends lOAE, lOBE of the respective longitudinally extending elements 10A, lOB to the feeder structure. The other ends of the antenna elements are also diametrically opposed and are linked by an annular common virtual <BR> <BR> <BR> <BR> ground conductor 20 in the form of a plated sleeve surrounding a proximal end portion of the core 12. This sleeve 20 is in turn connected to the lining 16 of the axial passage 14 by plating on the proximal end face 12D of the core 12.

In this preferred embodiment, the conductive sleeve 20 covers a proximal portion of the antenna core 12, thereby surrounding the feeder structure 16,18, the material of the core 12 filling the whole of the space between the sleeve 12 and the metallic lining 16 of the axial passage 14. At any given transverse cross-section through the antenna, the antenna elements 10A, lOB are substantially diametrically opposed, and the proximal ends lOAF, IOBF of the antenna element branches are also substantially diametrically opposed where they meet the rim of the sleeve. The longitudinal extending elements are of equal length, each being in the form of a simple helix executing a half turn around the axis of the core

12.

The metallic sleeve 20 forms a cylinder connected to the lining 16 by the plating 22 of the proximal end face 12P of the core 12, the combination of the sleeve 20 and plating 22 forming a balun so that, at or near a first predetermined frequency. signals in the transmission line formed by the feeder structure 16.18 are converted between an unbalanced state at the proximal end of the antenna and a balanced state at an axial position approximately in the plane of the upper edge 20U of the sleeve 20. To achieve this effect, the axial length of the sleeve 20 is such that in the presence of an underlying core material of relatively high dielectric constant, the balun has an electrical length of about A/4 at the predetermined frequency. Since the core material of the antenna has a foreshortening effect, the annular space surroundin the inner conductor 18 is filled with an insulating dielectric material 17 having a relatively small dielectric constant, and the feeder structure distally of the sleeve 20 has a short electric length. As a result, signals at the distal end of the feeder structure 16,18 are at least approximately balanced and the antenna is considered to operate in a so-called balanced mode of resonance at the predetermined frequency.

A further effect of the sleeve 20 is that for signals in the region of the first predetermined frequency, the rim 20U of the sleeve 20 is effectively isolated from the ground represented by the outer conductor of the feeder structure. This means that currents circulating between the antenna elements I OA, I OB are confined to the rim 20U and the loop formed by the antenna elements are isolated. The sleeve thus acts as an isolating trap.

The antenna elements 1OA. IOB are connected respectively to the inner conductor 18 and outer lining 16 of the feeder structure by their respective radial elements. It will be seen, then, that the helical elements, the radial elements 10A, lOB and the sleeve 20 together form a conductive loop on the outer surface of the core 12, the loop being fed at the distal end of the core by a feeder structure which extends through the core from the proximal end and lies between the antenna elements I OA, 1 OB. The antenna consequently has an end-fed bifilar helical structure.

It will be noted that the four ends of the antenna elements lOAE. lOAF. I OBE. lOBF all lie in a common plane containing the axis 12A of the core 12. The common plane is indicated by the chain lines in Figure 1. The feed connection to the antenna element structure also lies in the common plane 24. The antenna element structure is so configured that in a balanced mode the resonance the integral of currents induced in the elemental segments of this structure by a wave incident on the antenna from a direction 28 normal to the plane 24 and having a planar wavefront, sums to zero at the feed position, i. e. where the feeder structure 16,18 is connected to the antenna element structure. In practice, the two elements 10A, lOB are equally disposed and weighted on either side of the plane, yielding vectorial symmetry about the plane. Each element 10A, I OB may be regarded as being made up of a plurality of increments, each one of which lies diametrically opposite a corresponding complementary increment of the other of the elements IOA, lOB at an equal distance from the central axis 12A.

It should be understood that as well as exhibiting a balanced mode of resonance an antenna as described above also generally exhibits a single-ended mode of resonance at a different frequency. By arranging for the respective frequencies of the two modes to be close to each other, the modes can be made to couple, thereby yielding an antenna which radiates over a wider bandwidth than the bandwidth corresponding to either of the individual resonances.

Equivalent circuit diagrams representing the antenna element structure and characteristic operation in the balanced and single-ended modes of resonance are shown in Figures 2 and 3 respectively.

As Figure 2 shows, when considering the operation of the antenna in the balanced mode of resonance the two antenna elements 10A, lOB may be represented by a transmission line section of an electrical length which is at least approximately ./2 (or (2n+1) v2) where k is the centre wavelength of the antenna operating in the balanced mode, and n = 0,1,2,3....... The sleeve 20 and the plated end face 22 of the core can be represented by a similar transmission line section due to the presence of the core material of a relatively

high relative dielectric constant within the sleeve 20 having a predetermined axial length. This has an electrical length between the elements IOA. lOB and the feeder structure of A/4. The inner metallic lining 16 of the axial passage and the inner feeder structure are likewise represented by transmission line sections with an electrical length of A/4 as a result of the foreshortening effect of the feeder dielectric material 17.

When operating at frequencies associated with the single-ended mode of resonance, the sleeve 20 and plated end face 22, the antenna elements 10A, IOB, and the feeder structure 16,18 may be represented by their respective electrical lengths It, 12 and 13, as shown in Figure 3. When operating in the single-ended mode the combined electrical path length (1) +l2 +h), i. e. the electrical length of the conductive path including the inner metalic lining 16 of the axial passage, the antenna element IOB, and the combination of the sleeve 20 and the plated end face of the core 22, is approximately equal to A (or n A. where n = 1,2,3....), whilst the electrical length li of the combination of the sleeve 20 and plated end face 22 of the core is approximately A/4 (or n A/4 where n = 1,2,3.....). Here. x is the centre wavelength associated with the single-ended mode of resonance.

Referring to Figure 2, when the antenna is operating at frequencies associated with the balanced mode of operation, the isolating effect of the sleeve 20 confines currents mainly to the rim 20U of the sleeve, this point representing a location of current maxima. By arranging for the balun length from the rim 20U of the sleeve to the feeder structure at the proximal end face 22 to be 90°, the balun acts as a current-to-voltage transformer so that the impedance ZL looking into the sleeve tends to infinity, as does the impedance at the distal end of the feeder structure, as indicated in Figure 2. Consequently, current flows from the inner conductor 18, through the first element lOA and around the rim 20U to the second element IOB, through the second element 20B to the outer conductor 16 of the feeder structure, the total path length around this loop being 360°. As such, at any given point in time, the amplitude of currents I,, I, in diametrically opposed portions of the antenna elements IOA, IOB are equal and opposite and. therefore, sum to zero at the distal end of the feeder structure i. e. where the feeder structure is connected to the antenna element structure. When considered in isolation (i. e. when not coupled with other modes),

the balanced mode of resonance produces an electric field in the form of a dipole which is oriented transversely to the central axis of symmetry 12A of the core 12.

The presence of different current patterns in the conductive structure due to the different modes of resonance, and in particular the differing current paths in the radiating elements, may be observed in practice by applying pairs of r. f. voltage probes to the radiating elements at spaced apart positions on the radiating elements, preferably adjacent current maxima such as a short distance above the rim 20U on the radiating elements IOA and IOB. To measure the r. f. current in 10A, for instance, probes may be applied at the junction lOAF of the element l0A with the rim 20U and a short distance along the element I OA from that point. The observed voltage drop between the probes is indicative of the current.

An illustration of the magnitudes and relative phases of the currents in the elements l0A and lOB appears in Figures 6A and 6B respectively. In Figure 6A. two network analyser traces are shown, one for the voltage drop on element 10A, and one for the voltage drop on element IOB. over a range of frequencies. Two peaks can be seen in each trace. These correspond to the resonant frequencies f, and f, associated with the balanced and single- ended modes respectively.

Referring to Figure 6B. it will be seen that at f, the two traces indicate a phase difference of 180-between the two currents, whereas at f, the currents are in phase, as would be expected from the polarity of the currents I, and I2 in the two modes as described above with reference to Figures 2 and 3.

Referring to Figure 3, at the frequency associated with the single-ended mode of resonance, the sleeve 20 no longer has a significant isolating effect and the impedance ZL looking into the sleeve tends to zero. Therefore the currents I,, L through the elements IOA. lOB sum together at the rim 20U of the sleeve 20 to produce a combined current I3 through the sleeve to the outer conductor of the feeder structure. The sleeve 20 may be connected to a ground plane, e. g. on a mobile communication unit, and so the summed

currents may flow through sleeve 20 to ground.

When considered in isolation, the single-ended mode of resonance produces an electric field in the form of a dipole. which is oriented generally parallel to the central axis of symmetry of the core.

In the general case, the single-ended mode is associated with different frequencies than is the balanced mode and produces its own resonant peak in the antenna gain characteristic. The individual peaks of resonance for this general case are shown in Figure 4A. Resonant peak 40 indicates a frequency response of the antenna associated with the single-ended mode of resonance, and resonant peak 42 indicates a frequencv response associated with the balanced mode of resonance.

If, now, the dimensions of the antenna 10 are arranged such that the individual frequency responses associated with the balanced and single-ended modes of operation are caused to move together and couple in the frequency domain so that energy is shared between them, the antenna has a significantly wider overall operating bandwidth. It should be appreciated that the operating bandwidth of the antenna 10 is not defined by the antenna simply exhibiting a plurality of separate. closely situated resonant frequency responses.

The two resonant modes couple to create an return loss characteristic the envelope of which comprises a coupled transition region associated with a certain antenna gain, the gain indicating the storage of energy in the antenna due to both modes of operation. This frequency domain coupling of the two modes of operation is clearly shown in Figure 4B.

A shallow'saddle'region 44 exists between resonant peaks corresponding to the respective modes.

It should be noted that the modes of resonance of the antenna are of differing types. In other words, the operating bandwidth is not defined by simply arranging for a plurality of resonant modes of the same type (e. g. a plurality of balanced mode resonances) to be situated at adjacent frequencies so as to be coupled. Here, the different modes, in this preferred case the balanced and single-ended modes of resonance, are characterised by the

different overall conducting paths through which the r. f. current flows over part of the antennas conductive structure. Each respective current pattern however, makes use of common radiating elements, here both of the helical elements 10A, IOB.

Preferably, the antenna dimensions are arranged such that the antenna exhibits a usable fractional bandwidth of at least 0.03 (3%), wherein the fractional bandwidth is defined as the ratio of the width of the operating band B to the centre frequency fc of the band, the return loss of the antenna within the band being at least 3dB less than the average return loss outside the band. The return loss is defined as 2010glo (Vr/Vj) where Vr and V, are the magnitudes of the reflected and incident r. f. voltages at a feed termination of the feeder structure.

It will be understood that for the gain of an antenna, there is an associated variation in return loss related to the energy reflected at the antenna termination or single port. As the return loss changes with frequency, there is an associated change in the reactance and resistance of the load represented by the antenna, and thus in the ability to provide an impedance match. The antenna 10 is matchable over a bandwidth, as is evident when the complex load impedance is plotted on a normalise reflection coefficient diagram (often referred to as a Smith chart). The Smith chart provides a diagrammatic representation of the range of impedances over which the antenna can be matche. as indicated by a polar impedance locus. The Smith chart representation of the preferred antenna appears in Figure 5, the resistive component tending to infinity at the right hand end of the chart.

The locus 50 for the present antenna, with coupled modes of resonance over a predetermined band of operation has an inner loop 50a encircling the centre of the chart (corresponding to a source impedance of 50 ohms), the locus self-intersecting once along its path, as indicated by the locus portion 50a in Figure 5. This intersecting region and the inner loop correspond to the return loss indicated by the shallow'saddle'region 44 of Figure 4B which occurs within the antenna's operating bandwidth. As such, the input reactance component (X) of the load represented by the antenna (the lines 48 being cirlces of constant reactance) only approaches zero within the operating frequency band when the

corresponding input resistance (as indicated by the circles 46) is neither zero nor infinity.

By considering the radiation pattern of the antenna solely for the balanced mode of resonance, the antenna element structure with half-turn helical elements IOA, lOB performs in a manner similar to a simple planar loop, having nulls in its radiation pattern in a direction transverse to the axis 12A and perpendicular to the plane 24. Since the operating band of the antenna is defined by a plurality of coupled resonances, the above radiation pattern and the generally toroidal radiation pattern (with nulls in a direction parallel to the axis 12A) associated with the single-ended mode procude an overall radiation pattern which is a hybrid of these two patterns. As such, the hybrid pattern depends on the dimensions and physical characteristics of the materials forming the antenna structure.

The reactance fields in the near-field region around the antenna, associated with the hybrid radiation pattern are able to achieve good SAR performance.

The antenna has particular application at frequencies above 200 MHz. The radiation pattern is such that the antenna lends itself especially to use in a handheld communication unit such as a cellular or cordless telephone handset, as shown in Figure 7. With regard to the orientation of the antenna, it is mounted such that its central axis 12A (see Figure 7) and the plane 24 (see Figure 1) are parallel to the inner face 30I of the handset, (specifically the inner face 30I in the region of the earphone 32) and that the radial element lOAR which is connected to the inner feeder conductor 18 is oriented towards the edge 34 of the handset, i. e. with the helical element IOA connected to the inner feeder conductor 18 located on a portion of the core surface which is directed in the same direction as the inner face 30I of the handset. The axis 12A also runs longitudinally in the handset 30, as shown in Figure 6. Again, the relative orientations of the antenna, and the handset 30 are evident by comparing the axis system X, Y, Z as it is shown in Figure 1 with the representations of the axis system in Figure 7.

The preferred material for the core 12 of the antenna is a zirconium-titanate-based

material. This material has a relative dielectric constant of about 36 and is noted also for its dimensional and electrical stability with varying temperature. Dielectric loss is negligible. The core may be produced by extrusion, pressing or moulding.

The antenna elements 10A, IOB, lOAR, IOBR are metallic conductor tracks bonded to the outer cylindrical and distal end surfaces of the core 12, each track being of a width of at least four times its thickness over its operating length. In this embodiment, the antenna element tracks are approximately 30 thousandth of an inch in thickness, and the loop resistance of the antenna, comprising the combination of both radiation loss and dissipation loss is approximately 3.2Q. The tracks may be formed by initially plating the surfaces of the core 12 with a metallic layer and then selectively etching away the layer to expose the core according to a pattern applied in a photosensitive layer similar to that used for etching printed circuit boards. Alternatively, the metallic material may be applied by selective deposition or by printing techniques. In all cases, the formation of the tracks as an integral layer on the outside of a dimensionally stable core leads to an antenna having dimensionally stable antenna elements.

With a core material having a substantially higher relative dielectric constant than that of <BR> <BR> <BR> <BR> air, e. g. Or == 36, an antenna as described above for the DCS-1800 band in the region of 1710 MHz to 1880 MHz typically has a core diameter of about 10 mm and a core length in the axial direction (i. e. parallel to the central axis 12A) of about 12.1 mm. The aspect ratio of the axial length to the diameter of the core preferably lies within the range 1: 1 to 2: 1. For the DCS-1800 band, the length of the balun sleeve is typically in the region of 4.2 mm, the ratio of the axial extent of the sleeve to the combined axial extent of the sleeve and the antenna elements being in the range of from 0. 25: 1 to 0.5: 1. In the case of the antenna elements and the sleeve together spanning the full length of the core between its two end surfaces, then, the axial length of the sleeve is typically between 0.25L and O. 5L, where L is the axial length of the core. The width of the conductor tracks is in the region of 0.75 to 1.25 mm, with 0.9 mm as a typical figure.

The'saddle'region 44 of the operating bandwidth of the present embodiment represents an area of slightly reduced gain over a fraction of the bandwidth. As such, this region may

be convenientlv arranged to coincide with the guard band between transmit and receive channels in e. g. cellular telephone bands.

The small size of the antenna 10 renders it particularlv suitable in handheld devices such as a mobile telephone handset and other personal communication devices. The plated balun sleeve 20 and/or the plated layer 22 on the proximal end face 12P of the core 12 allow the antenna to be directly mounted on a printed circuit board or other ground structure in a particularly secure manner. Typically, if the antenna is to be end-mounted, the proximal end face 12P can be soldered to a ground plane on the upper face of a printed circuit board with the inner feed conductor 18 passing directly through a plated hole in the board for soldering to a conductor track on the lower surface. Alternativelv. sleeve 20 may be clamped or soldered to a printed circuit board ground plane extending parallel to the axis 12A. with the distal part of the antenna, bearing antenna elements 10A, lOB extending beyond an edge of the ground plane. It is possible to mount the antenna 10 either wholly within the handset unit, or partially projecting as shown in Figure 7. The antenna of the present embodiment may be connected with a matched cable of 15 Q characteristic impedance.