FIELD OF THE INVENTION 
     This invention relates to an antenna for operation at frequencies in excess of 200 MHz, and to a radio communication system including the antenna. 
     BACKGROUND OF THE INVENTION 
     The applicant has disclosed a family of dielectrically-loaded antennas in a number of co-pending patent applications. Common features of the disclosed antennas include a solid cylindrical ceramic core of high relative dielectric constant, a coaxial feeder passing through the core on its axis to a termination at a distal end, a conductive balun sleeve plated on a proximal portion of the core to create an at least approximately balanced feeder termination at the distal end, and a plurality of elongate helical conductor elements plated on the cylindrical surface of the core and extending between, on the one hand, radial connections with the feeder termination on the distal end face, and, on the other hand, the rim of the sleeve. 
     In one of the co-pending applications, GB-A-2292638, there is disclosed a quadrifilar backfire antenna having four co-extensive helical elements formed as two pairs, the electrical length of the elements of one pair being different from the electrical lengths of the elements of the other pair. This structure has the effect of creating orthogonally phased currents at an operating frequency of, for example, 1575 MHz with the result that the antenna has a cardioid radiation pattern for circularly polarised signals such as those transmitted by the satellites in the GPS (global positional system) satellite constellation. 
     In GB-A-2309592, the antenna has a single pair of diametrically opposed helical elements forming a twisted loop yielding a radiation pattern which is ommnidirectional with the exception of a null centred on a null axis extending perpendicularly to the cylinder axis of the antenna. This antenna is particularly suitable for use in a portable telephone, and can be dimensioned to have loop resonances at frequencies respectively within the European GSM band (890 to 960 MHz) and the DCS band (1710 to 1880 MHz), for example. Other relevant bands include the American AMPS (842 to 894 MHz) and PCN (1850 to 1990 MHz) bands. 
     GB-A-2311675 discloses the use of an antenna having the same general structure as that disclosed in GB-A-2292638 in a dual service system such as a combined GPS and mobile telephone system, the antenna being used for GPS reception when resonant in a quadrifilar (circularly polarised) mode, and for telephone signals when resonant in a single-ended (linearly polarised) mode. 
     SUMMARY OF THE INVENTION 
     The applicants have found that, by manipulating the diameter of the conductive sleeve encircling the proximal portion of the core, it is possible to produce a resonance which is characterised by a standing wave around the sleeve rim (referred to herein as a “ring resonance”) and which occurs at one of the frequencies used in, for instance, mobile telephones or satellite positioning receivers. The ring resonance is effectively a resonance associated with a circular guide mode or ring mode. 
     According to a first aspect of the present invention, there is provided an antenna having an operating frequency in excess of 200 MHz, comprising a cylindrical insulative body having a central axis and formed of a solid material which has a relative dielectric constant greater than 5, the outer surface of the body defining a volume the major part of which is occupied by the solid material, a conductive sleeve on the cylindrical surface of the insulative body, a conductive layer on a surface of the body which extends transversely of the axis, the conductive sleeve and layer together forming an open-ended cavity substantially filled with the solid material, and a feeder structure associated with the cavity, wherein the said relative dielectric constant and the dimensions of the cavity are adapted such that the electrical length of its circumference at the open end is substantially equal to a whole number (1, 2, 3, . . . ) of guide wavelengths around the said circumference corresponding to the said operating frequency. 
     One of the difficulties associated with the known dielectrically loaded quadrifilar backfire antenna referred to above is that the bandwidth of the antenna for circularly polarised signals is relatively narrow. This means that manufacturing tolerances tend to be tight, and the antenna may need to be individually tuned to a required frequency. In an antenna in accordance with the present invention it is possible to arrange for the feeder structure to excite a rotary standing wave around the rim of the cavity at its open end, so as to produce an antenna which is resonant for circularly polarised waves and which has an associated cardioid radiation pattern suitable for receiving signals from satellites when used with its axis vertical. The applicants have found that the bandwidth associated with such a resonance is much wider than the bandwidth of the quadrifilar antenna. 
     It should be noted that the term “excite” is used in this context as a reference to not only use of the antenna for transmitting signals, but also use of the antenna for receiving signals, since the functional characteristics of the antenna such as its frequency response, radiation pattern, etc. obey the reciprocity rule with respect to corresponding transmitting and receiving characteristics. Similarly, references to elements or parts which “radiate” when used in the context of an antenna for receiving signals should be construed to include elements or parts which absorb energy from the surrounding space but which, by virtue of the reciprocity rule, would radiate if the antenna were to be used for transmission. 
     One way of exciting circular standing waves in the sleeve is to employ elongate helical or spiral elements on the surface of the insulative body. In effect, the helical elements impart a tangential component of excitation at the sleeve or sleeve rim so that they may be regarded as tangential excitation or feed means. With appropriate choice of dielectric constant and dimensioning of the sleeve and the helical or spiral elements, the antenna can be made to operate as a dual-mode antenna, with a circular polarisation mode associated with the ring resonance, i.e. a standing wave around the rim of the cavity, and a linear mode associated with the loop resonance referred to above in connection with the twisted loop configuration. 
     Preferably, at the frequency of the ring mode resonance, the helical elements each have an electrical length equal to nλ g /4 wherein n is a whole number (1, 2, 3, . . . ) and λ g  is the guide wavelength along the elements at the frequency of the ring resonance. 
     In this connection, it will be appreciated by those skilled in the art that “guide wavelength” means the distance represented by a complete wave cycle at the frequency in question along the path used for measurement, i.e. the path along which the wave is guided. In the present case, the measurement path is the respective helical element or the sleeve rim, and the guide wavelength is less than the corresponding wavelength in space by a factor which is governed by the dielectric constant of the core material and by the geometry of the antenna structure. It is to be understood that, with the dielectric constant of the core material being substantially greater than that of free space, the guide wavelength λ g  around the rim of the sleeve or along the helical elements is much less than the wavelength in free space, but generally not the same in each case. In the case of the rim, the current path is very strongly affected by the dielectric material because the associated fields are largely within the material, whereas the current paths of the helical elements are less strongly affected, being at the boundary between dielectric material and air. 
     It is possible, then, to produce a multiple-mode antenna suitable particularly, but not exclusively, for circularly polarised signals without using the narrow band quadrifilar structure referred to above. Consequently, a preferred use of the antenna is for portable or mobile equipment such as multiple-band portable or mobile telephones, particularly cellular telephones, or, more particularly, portable or mobile telephones for the Globalstar and Iridium satellite telephone systems, as well as portable telephones or other units having a GPS or GLONASS positioning function, these satellite services being services which employ circularly polarised signals. 
     According to a second aspect of the invention, there is provided a radio signal receiving and/or transmitting system comprising a radio frequency front end stage constructed to operate at a first signal receiving or transmitting frequency and, coupled to the front end stage, an antenna which comprises: a cylindrical insulative body having a central axis and formed of a solid material with a dielectric constant greater than 5, the outer surface of the body defining a volume the major part of which is occupied by the solid material, a conductive layer on a surface of the body which extends transversely of the axis, the conductive sleeve and layer together forming an open-ended cavity substantially filled with the solid material, and a feeder structure associated with the cavity, wherein the said relative dielectric constant and the dimensions of the cavity are adapted such that the electrical length of the rim of the cavity at its open ends is substantially equal to a whole number (1, 2, 3, . . . ) of guide wavelengths corresponding to the first signal frequency. 
     The invention also includes, according to a third aspect, a dielectrically-loaded cavity-backed antenna for circularly polarised waves at a required operating frequency in excess of 200 MHz, comprising a cavity with a conductive cylindrical side wall and a conductive bottom wall joined to the side wall, the side wall having a rim defining a cavity opening opposite the bottom wall, a dielectric core substantially filling the cavity and formed of a solid material having a relative dielectric constant greater than 5, and a rotational feed system, characterised in that the said dielectric constant and the dimensions of the cavity are such that the circumference of the rim is substantially equal to a whole number (1, 2, 3, . . . ) of guide wavelengths at the required operating frequency, and wherein the feed system is adapted to excite a waveguide resonance at the rim of the cavity at the required operating frequency, which resonance is characterised by at least one voltage dipole oriented diametrically across the cavity opening and spinning about the central axis of the cavity thereby to produce a circular polarisation radiation pattern which is directed axially outwardly from the opening of the cavity and has a null in the opposite axial direction. 
     Further preferred features of the antenna and system are set out in the dependent claims appearing at the end of this specification. 
     The invention will be described below by way of example with reference to the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a perspective view of a portable telephone including an antenna in accordance with the invention; 
     FIG. 2 is a perspective view of the antenna appearing in FIG. 1; 
     FIG. 3 is a diagram illustrating the horizontal polarisation radiation pattern produced when the antenna is resonant in a loop mode; 
     FIGS. 4A and 4B are diagrams illustrating a ring mode resonance in the sleeve forming part of the antenna of FIG. 2; 
     FIG. 5 is a diagram illustrating the circular polarisation radiation pattern produced when the antenna is resonant in the ring mode; 
     FIG. 6 is a block diagram of the telephone in FIG. 1; 
     FIG. 7 is a diagram showing a coupler for the telephone shown in FIGS. 1 and 6; 
     FIG. 8 is a perspective view of a second antenna in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a handheld communication unit, in this case, a portable telephone has a telephone body  10  with an inner face  101 , at least part of which is normally placed against the head of the user when used to make a call, so that the earphone  10 E is adjacent the user&#39;s ear. The telephone  10  has an antenna  12  mounted at the end of the telephone body  10  with its central axis  12 A running longitudinally of the body  10  as shown. 
     The antenna  12  is shown in more detail in FIG.  2 . As will be seen, the antenna has two longitudinally extending elements  14 A,  14 B formed as metallic conductor tracks on the cylindrical outer surface of a ceramic core  16 . The core  16  has an axial passage  18  with an inner metallic lining  20 , and the passage houses an axial inner feed conductor  22 . The inner conductor  22  and the lining  20  in this case form a coaxial transmission line through the core for coupling a feed line  23  to the antenna elements  14 A,  14 B at a feed position on the distal end face  16 D of the core. The conductors on the core also include corresponding connecting radial antenna elements  14 AR,  14 BR formed as metallic tracks on the distal end face  16 D, connecting diametrically opposed .ends  14 AE,  14 BE of the respective longitudinally extending elements  14 A,  14 B to the feed line. The junction of these radial elements and the axial transmission line constitutes a balanced feed termination. The other ends  14 AF,  14 BF of the antenna elements  14 A,  14 B are also diametrically opposed and are linked by a cylindrical conductor  24  in the form of a plated sleeve surrounding a proximal end portion of the core  16 . This sleeve is, in turn, connected to the lining  22  of the axial passage  18  by a transversely extending conductive layer  26  on the proximal end face  16 P of the core  16 . The sleeve  24  and the conductive layer  26  together form a open-ended cavity filled with the dielectric material of the core, the open end of the cavity being defined by a rim  24 R lying substantially in a plane perpendicular to the central axis  12 A of the core and the antenna as a whole. 
     Accordingly, the sleeve  24  covers a proximal portion of the antenna core  16 , thereby surrounding the coaxial transmission line formed by the lining  20  and the inner conductor  22 , the material of the core  16  filing the whole of the space between the sleeve  24  and the lining  20 . As described in the above-mentioned co-pending applications, the sleeve  24  and the transverse layer  26  together form a balun so that signals in the feed line are converted between an unbalanced state at the proximal end of the antenna to an at least approximately balanced state at the distal face  16 D. 
     A further effect of the sleeve  24  is that the rim  24 R of the sleeve  24  can effectively constitute an annular current path isolated from the ground represented by the outer conductor of the feed line which means that, in this isolating condition, currents circulating in the elongate helical elements  14 A,  14 B are confined to the rim  24 R so that these elements, the rim, and the radial elements  14 AR,  14 BR together form an isolated loop. 
     In the illustrated antenna, the longitudinally extending helical elements  14 A,  14 B are of equal length, each being in the form of simple helix executing a half turn around the axis  12 A of the core  16  with the distal and proximal ends of the helical elements respectively located in a common plane, as indicated by the chain lines  28  in FIG.  2 . The balanced termination of the transmission line also, clearly, lies in this plane. An effect of this structure is that when the antenna is resonant in a loop mode it has a null in its radiation pattern in a direction transverse to the axis  12 A and perpendicular to the plane  28 . This radiation pattern is, therefore, approximally of a figure-of-8 shape in both the horizontal and vertical planes transverse to the axis  12 A, as shown by FIG.  3 . Orientation of the radiation pattern with respect to the antenna as shown in FIG. 2 is shown by the axis system comprising axes x, y, z shown in FIGS. 1,  2  and  3 . The radiation pattern has two notches, one on each side of the antenna. To orient one of the nulls of the radiation pattern in the direction of the user&#39;s head, the antenna is mounted such that its central axis  12 A and the plane  28  are parallel to the inner face  10 I of the handset  10 , as shown in FIG.  1 . The relative orientations of the antenna, its radiation pattern, and the telephone body  10  are evident by comparing the axis system x, y, z as it is shown in FIG. 2 with the representations of the axis system appearing in FIGS. 1 and 3. 
     The antenna shown in FIG. 2 also has resonances due to the sleeve acting as a waveguide. In particular, if the circumference of the sleeve is equal to an integer number of guide wavelengths at a required alternative operating frequency, a ring mode resonance is set up, characterised by at least one voltage dipole oriented diametrically across the cavity opening. The helical elements  14 A,  14 B which, together with the radial connections  14 AR,  14 BR and the transmission line  20 ,  22 , act as a feed system, impart a rotational component to the dipole such that it spins about the central axis  12 A. This effect is shown diagrammatically in the plan view of FIG. 4, in which the dipole is illustrated as extending between two diametrically opposed locations “H” of high voltage amplitude, the arrows indicating the rotational component. Computer simulations of the antenna structure (produced using the microstripes package of Kimberley Communications Consultants Ltd.) reveal that the ring resonance is characterised by current density maxima at diametrically opposed positions “H” not only at the rim  24 R of the sleeve but also extending down the inner surface of the sleeve towards the transverse conductive layer or bottom wall  26 , as shown in FIG.  4 B. The dotted lines in FIG. 4B indicate approximate contours of constant current density on the inner surface of the sleeve. The patterns shown in FIGS. 4A and 4B correspond to a ring resonance occurring when the circumference of the rim  24 R is substantially equal to the wavelengths λ g  at the required alternative operating frequency. Further ring resonances exist when the guide wavelength is an integer sub-multiple of the rim circumference so that, for instance, two or three opposed pairs of current and voltage maxima are present, spaced around the rim  24 R and the inner surface of the sleeve  24 . Thus, in the general case, one or more pairs diametrically opposed current maxima like the pair shown in FIG. 4B may exist at the operating frequency or frequencies. 
     In each case, the ring resonance yields a cardioid radiation pattern for circularly polarised radiation at the respective frequencies, as shown in FIG.  5 . It follows that the antenna is particularly suitable for receiving circularly polarised signals when the antenna is oriented with the open end of the cavity pointing upwards. In this way, satellites in view fall within the upper dome of the cardioid response, substantially irrespective of bearing. 
     The applicants have, therefore, made use of the sleeve  24 , which is used as a balun, also to form a waveguide which is excited in a circular guide mode of resonance. This is achieved without orthogonal phasing antenna element structures such as in the prior quadrifilar antenna disclosed in GB-A-2292638, such a structure being characterised by two orthogonally related pairs of diametrically opposed helical elements arranged such that the elements of one pair form part of a conductive path which is longer than the path containing the elements of the other pair. 
     The spinning dipole referred to above is achieved by virtue of the tangential excitation component imparted by the rim being connected to helical elements of the feed system at diametrically opposite positions. Advantageously, each series combination of helical element  14 A,  14 B and connection element  14 AR,  14 BR has an electrical length equal to a whole number of guide quarter-wavelengths. The preferred embodiment, as illustrated in FIG. 2, has helical and radial element combinations each having an electrical length which is one half of the guide wavelength along those elements, so that current maximum at the balanced feed termination on the distal face  16 D is translated to current maxima at the junctions  14 AF,  14 BF of the helical elements  14 A,  14 B with the rim  24 R. Balance at the termination on the distal end face  16 D is achieved by virtue of the sleeve  24  acting as a balun at the frequency of ring resonance. 
     The antenna described above with reference to FIG. 2 is configured and dimensioned to exhibit a ring resonance in the Globalstar uplink (user to satellite) transmit band of 1610 to 1626.5 MHz and a loop resonance in the European GSM cellular telephone band of 890 to 960 MHz. The first of these bands is also the uplink band for the Iridium satellite telephone system. In this first band, the electrical length of the sleeve rim  24 R is at least approximately equal to the guide wavelength λ g  (i.e. each semicircle between the junctions of the helical elements  14 A,  14 B and the rim  24 R yields a phase shift of about 180° at a frequency within the band. Each helical element  14 A,  14 B and its associated radial connection element  14 AR,  14 BR have an electrical length λ g /2. Although each helical and radial element combination is considerably longer than the rim semicircle beneath, it has a similar electrical length because the effective value for the relative dielectric constant experienced by the two current paths is different such that λ g  along the rim is shorter than λ g  along the helical and radial elements at the same frequency. 
     The loop resonance, in this embodiment in the GSM band, occurs when the looped conductive path represented by the radial and helical elements  14 AR,  14 A, one or other of the semicircles of the rim  24 R, and the other helical and radial elements  14 B,  14 BR, has an electrical length of one wavelength (i.e. a phase transition of 360°). 
     Typically, these resonances are seen when the relative dielectric constant ∈ r  of the ceramic core  16  is 90, the diameter of the core  16  is 10 mm, the axial extent of the balun sleeve  24  is 4 mm, and the axial length of the helical elements  14 A,  14 B (i.e. parallel to the axis  12 A) is about 14.85 mm. In other respects, the antenna structure is as described in the above prior published patent applications, the disclosure is which is incorporated in this specification by reference. The particular material used for the core  16  in the preferred embodiment in the present application is barium titanate or barium-neobidium titanate. 
     Alternative antennas giving different combinations of resonances to suit different services can be designed by, for instance, first establishing suitable dimensions for the twisted loop as described in the above-mentioned GB-A-2309592 to suit one of the required operating frequencies, and then manipulating the diameter of the sleeve to produce the required whole number of guide wavelengths to suit the other of the required operating frequencies. The above-mentioned simulation package can be used to view current and field densities in a software model of the antenna or parts of the antenna. The ring resonance has particular recognisable characteristics as described above with reference to FIG. 4B. A variety of frequency combinations are available not only by choosing different dielectric constants and dimensions, but also by allowing the electrical lengths of the rim, the helical elements and their radial connections and the depth of the balun to be equivalent to integral multiples of the guide wavelengths or quarter guide wavelengths as appropriate. The depth of the balun together with the radius of the transverse conductive layer or bottom wall of the cavity are typically in the region of λ g /4 to achieve balance at the distal face  16 D of the core. Odd number multiples of λ g  or λ g /4 may be used instead. 
     In addition, the ring resonance may be combined with other resonances of the structure described in the above-mentioned prior published applications, including a quasi-monopole resonance characterised by a single-ended mode in which the radial connections  14 AR,  14  BR, the helical elements  14 A,  14 B, and the sleeve  24  combine to form linear paths from the feed termination of the distal face  16 D through to the junction of the transverse conductive layer  26  with the outer screen  20  of the transmission line. 
     In other embodiments of the invention, the ring resonance may be used by itself. An alternative structure which dispenses with the loop mode of resonance is illustrated in FIG.  7 . In this case, each helical element  14 A,  14 B is a quarter-turn element (as opposed to a half-turn element in the embodiment of FIG.  2 ), the electrical length of each helical element and its associated radial connection  14 AR,  14 BR being generally equal to λ g /4, yielding a complete 360° electrical loop at the frequency of ring resonance (each semicircle of the rim  24 R having an electrical length of λ g /2). 
     In multiple-band embodiments of the antenna, signals may pass between the antenna and the respective portions of a radio frequency (RF) front end stage of the connected radio communication equipment via a coupling stage as shown in FIG.  6 . The equipment may be a handheld telephone unit  10  having an antenna  12  as described above with reference to FIG. 2, and RF front end stage portions  30 A,  30 B forming separate RF channels constructed to receive and/or transmit signals in respective operating frequency bands. These front end portions  30 A,  30 B are connected to the antenna  12  by a coupling stage  32  having a common signal line  32 A for the antenna feed line and two signal lines  32 B,  32 C for the respective front end portions  30 A,  30 B. The above-mentioned prior-published GB-A-2311675 discloses a coupling stage in the form of a diplexer, the principle of which may be used where simultaneous use of the antenna  12  in different frequency bands is required. Alternatively, referring to FIG. 8, the simple combination of an impedance matching section  34  and a two-way RF switch  36  (typically a p.i.n. diode device) may be used. Depending of the state of the switch  36 , the common line  32 A is coupled to one or other of the two further signals lines or ports  32 B,  32 C, to which the different front end portions may be connected. It will be appreciated by those skilled in the art that the antenna  12  may be used with communication equipment which is split between separate physical units rather than in a single unit  10  as shown in FIG.  6 .
A dielectric-loaded antenna for circularly polarized radiation has a generally cylindrical solid dielectric body with a relative dielectric constant greater than 5, upon which body is plated a conductive sleeve encircling the body and a conductive end layer which, together with the body, form an open-ended cavity substantially filled with the ceramic material of the body. The electrical length of the cavity rim is a whole number of guide wavelengths corresponding to the antenna operating frequency less than 5 GHz. A rotating standing wave is excited around the cavity rim by a feeder structure including two helical conductor tracks on the cylindrical surface of the body which are coupled between the cavity rim and a coaxial feeder passing axially through the body.