Abstract:
This application relates to antennas including, for example, an antenna for operation at a frequency in excess of 200 MHz comprising: an insulative substrate having a central axis, an axial passage extending therethrough and an outer substrate surface which extends around the axis; a three-dimensional antenna element structure including at least one pair of axially coextensive elongate conductive antenna elements on or adjacent the outer substrate surface; and an axial feeder structure which extends through the passage and comprises an elongate laminate board wherein the laminate board proximal end portion includes lateral extensions projecting in opposite lateral directions, and wherein, adjacent the laminate board proximal end portion, the substrate has recesses on opposite sides of the axis which receive at least edge parts of the said lateral extensions of the laminate board proximal end portion.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/564,227, filed on Nov. 28, 2011, and entitled “ANTENNA”, and also claims priority to United Kingdom Patent Application 1120466.6, filed on Nov. 25, 2011, and entitled “AN ANTENNA”, both of which are hereby incorporated herein by reference. 
     
    
     FIELD 
       [0002]    This application relates to an antenna for operation at a frequency in excess of 200 MHz, particularly to such an antenna having an axial feed structure comprising an elongate laminate board extending through a passage in an insulative substrate with antenna elements on or adjacent an outer surface of the substrate. The disclosed technology also includes a method of making a multiple-band antenna. 
       BACKGROUND 
       [0003]    A dielectrically-loaded antenna with a laminate board axial feed structure is disclosed in U.S. Published Patent Application No. 2011/0221650 (U.S. application Ser. No. 13/014,962, filed Jan. 27, 2011). Included in the antennas disclosed in this document is a quadrifilar helical backfire antenna having a cylindrical dielectric core, conductive helical radiating elements plated on the outer cylindrical core surface portion, which elements are fed from a distal end of an axially extending elongate laminate board feed structure. The feed structure comprises an elongate transmission line section acting as a feed line which extends through an axial passage in the core from a proximal core surface portion to a distal core surface portion, and an antenna connection section in the form of an integrally formed proximal extension of the transmission line section the width of which, in the plane of the laminate board, is greater than the width of the passage. The antenna elements are coupled to the transmission feed line via an impedance matching section. The contents of this published application are expressly incorporated in the present application by reference. 
       SUMMARY 
       [0004]    It is an object of certain embodiments of the disclosed technology to provide an improved antenna with a laminate board feed structure. 
         [0005]    According to one aspect of the disclosed technology, an antenna for operation at a frequency in excess of 200 MHz comprises: an insulative substrate having a central axis, an axial passage extending therethrough and an outer substrate surface which extends around the axis; a three-dimensional antenna element structure including at least one pair of axially coextensive elongate conductive antenna elements on or adjacent the outer substrate surface; and an axial feeder structure which extends through the passage and comprises an elongate laminate board having a proximal end portion for connection to host equipment circuitry, an intermediate portion including a transmission line, and a distal end portion coupled to the antenna elements; wherein the laminate board proximal end portion is wider than the intermediate portion in that it includes lateral extensions projecting in opposite lateral directions, and wherein, adjacent the laminate board proximal end portion, the substrate has recesses on opposite sides of the axis which receive at least edge parts of the lateral extensions of the laminate board proximal end portion. In a preferred antenna embodiment, the substrate comprises a dielectric core of solid material which has a relative dielectric constant greater than 5 and occupies the major part of the interior volume defined by the core outer surface. The core outer surface preferably comprises oppositely directed distal and proximal outer surface portions and a side outer surface portion which extends between the distal and proximal outer surface portions, with the axial passage extending through the core from the distal surface portion to the proximal surface portion. In this preferred embodiment, the recesses are grooves in the proximal outer surface portion of the core. 
         [0006]    Certain embodiments of the disclosed technology are particularly applicable to an antenna for receiving and/or transmitting circularly polarised waves. The core is preferably cylindrical, the above-mentioned antenna elements comprising helical elements on the cylindrical side outer surface of the core. As in the above-referenced U.S. Published Application No. 2011/0221650, the core side surface portion also carries a plated proximal sleeve linking proximal ends of the helical elements, and the proximal outer surface portion and the grooves have a conductive coating connected to the sleeve. The feeder structure transmission line includes a conductor connected to this proximal surface portion conductive coating via electrical interconnection of a conductive layer on at least one of the laminate board lateral extensions as well as the conductive layer in the respective groove housing that lateral extension. Locating the lateral extensions of the laminate board proximal end portion in the grooves fixes the rotational position of the feed structure laminate board about the substrate central axis and, therefore, with respect to the antenna element structure. With this property in mind, it is preferable that the width of at least one of the recesses matches the thickness of the laminate board proximal end portion. 
         [0007]    In the preferred feed structure, the intermediate portion of the laminate board comprises an inner conductive layer forming an elongate inner conductor of the transmission line and, on opposite sides of the inner conductive layer, the intermediate portion has interconnected shield conductor layers forming elongate shield conductors which are axially coextensive with the inner conductor so as to form a shield around the inner conductor. 
         [0008]    As part of the antenna element structure, there may be an annular interconnecting conductor on or adjacent the core outer surface (e.g. in the form of the above sleeve) that links the proximal ends of the elongate conductive elements. The feeder shield conductors are connected to the annular interconnecting conductor at an axial position corresponding to that of the base of the respective recess. Connecting the annular interconnecting conductor to the feeder using a conductor in the base of the recess rather than at the axial position of the proximal outer surface portion of the core has the effect of shortening the conductive path lengths between the proximal ends of the elongate conductive antenna elements and their connection to the feeder and, additionally, the operative length of an outer surface of the feeder between that connection and its distal connection to the antenna elements. This raises the frequency of resonant modes of the antenna associated with a composite conductive path including such conductors. 
         [0009]    In the case of the substrate comprising a high dielectric constant solid core, the dimensions of the axial passage extending through the core from the distal surface portion to the proximal surface portion, and those of the shield conductors of the feeder are such that the shield conductors are spaced from the wall of the passage. This also reduces the relevant electrical length of the feeder and increases the frequencies of the associated resonant modes. 
         [0010]    In the preferred embodiment, the antenna has first and second operating frequencies in excess of 200 MHz respectively associated with first and second modes of resonance. The first mode is characterised by currents passing around the annular interconnecting conductor and rotational phasing of the currents in the elongate conductive antenna elements around the antenna axis, producing a rotating dipole in the electric field. The second mode is a coaxial monopole mode in which currents in the elongate elements are spatially in phase with each other. 
         [0011]    The frequency of the first mode of resonance is determined primarily by the electrical lengths of the elongate, preferably helical, antenna elements, whereas that of the second mode of resonance is determined by the electrical lengths of the elongate antenna elements and that of the conductive path formed between the proximal ends of the elongate elements and the distal end of the transmission line, which includes the feeder shield. In the preferred antenna, the first mode of resonance is associated with axially directed circularly polarised waves and the second mode of resonance is associated with linearly polarised waves, the plane of polarisation containing the antenna axis. The second operating frequency is higher than the first. 
         [0012]    Such an antenna is suited, for instance, to dual-service operation at the GPS L 1  frequency, 1575 MHz and at the Wireless LAN and Bluetooth frequency in the region of 2450 MHz. 
         [0013]    Typically, the axial depth of the substrate is greater than its lateral extent so that, in the case of a cylindrical substrate, the ratio of the substrate length to its diameter is greater than 1 and, preferably, between 1.2 and 2.5. It is preferred that the position of the annular conductive path linking the proximal ends of the elongate conductive elements, whether in the form of a simple metallised ring of the rim of a conductive sleeve, should be at a distance of between 15% to 30% of the overall axial length of the substrate from the proximal periphery of the outer substrate surface. Typically, the depth of the slots or recesses is less than 50% of this distance. A slot or recess depth of greater than 0.5 mm is preferred. 
         [0014]    In the case of a dielectrically-loaded antenna in which the substrate is a solid core, the relative dielectric constant of the core material is preferably greater than 20 with a figure in the region of 80 being most preferred. Typically, the diameter of the core, in the case of a cylindrical core, is between 5 and 15 mm. The preferred antenna described herein has a diameter in the region of 7.5 mm and an axial length of about 12 mm. With a relative dielectric constant of around 80, such an antenna is particularly suitable for dual-service operation at the frequencies given above. 
         [0015]    Interconnections between the feed structure and the antenna elements may further comprise a lateral laminate board part connected to the above-mentioned elongate laminate board and extending laterally outwardly from the distal end of the axial passage, conductors on the lateral laminate board part coupling the antenna elements to the transmission line. In particular, the lateral laminate board part may comprise a laminate board oriented perpendicularly to the central axis. Impedance matching between the transmission line and the antenna elements is preferably performed by a network associated with a distal region of the feeder structure. 
         [0016]    According to another aspect of the disclosed technology, a method of making a multiple band antenna for operation at frequencies in excess of 200 MHz comprises: providing a plurality of antenna bodies each of which comprises (i) an insulative antenna substrate having a central axis, an axial passage extending therethrough, and an outer substrate surface extending around the axis, the outer substrate surface having distal periphery and proximal periphery, (ii) a three-dimensional antenna element structure including at least one pair of axially coextensive elongate conductive antenna elements on the outer substrate surface, and (iii) a linking conductor encircling the axis on the outer substrate surface and interconnecting proximal ends of the antenna elements, wherein the substrate has proximal recesses on opposite sides of the axis, the recesses extending into the linking conductor to reduce its effective axial extent, wherein the plurality of antenna bodies have the same axial extent, as determined by the distance between the distal and proximal peripheries, but recesses of different respective depths; providing a plurality of feeder structures each comprising an elongate laminate board having a proximal end portion for connection to host equipment circuitry, an intermediate portion including a transmission line and dimensioned to lie in the substrate passage, and a distal end portion for coupling to antenna elements, the proximal end portion having lateral extensions projecting in opposite lateral directions, wherein the plurality of feeder structures have intermediate portions of different lengths; selecting one of the antenna bodies and one of the feeder structures; inserting the selected feeder structure into the axial passage of the selected antenna body with the lateral extensions of the laminate board proximal end portion seated in the proximal recesses of the antenna body substrate; and forming electrical connections between the antenna elements and the laminate board distal end portion and between the linking conductor and the lateral extensions of the laminate board proximal end portion. The elongate laminate boards of the feeder structures are preferably all of the same length, the proximal end portions being of different axial lengths. In this case, the selection of a feeder structure for each antenna depends on the recess depth of the selected antenna body. In this way, the conductive path length associated with the linearly polarised mode of resonance can be altered without altering the outside dimensions of the assembled antenna and, therefore, without altering the mounting and connection requirements of the antennas. 
         [0017]    The disclosed technology will be described below by way of example with reference to the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    In the drawings: 
           [0019]      FIGS. 1A and 1B  are perspective views of a first antenna in accordance with the disclosed technology viewed, respectively, from below and from one side and above and one side; 
           [0020]      FIGS. 1C and 1D  are perspective views of a second antenna in accordance with the disclosed technology viewed, respectively, from below and one side and from above and one side; 
           [0021]      FIGS. 2A and 2B  are exploded views of components of the antenna of  FIGS. 1A and 1B , viewed respectively from the same directions as in  FIGS. 1A and 1B ; 
           [0022]      FIGS. 2C and 2D  are exploded views of components of the antenna of  FIGS. 1C and 1D  viewed, respectively, from the same directions as in  FIGS. 1C and 1D ; 
           [0023]      FIG. 3  is an exploded perspective view of a multiple-layer laminate board forming part of an antenna feed structure; and 
           [0024]      FIG. 4  is a partly sectioned side detail of the antenna of  FIGS. 1A and 1B . 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    Referring to  FIGS. 1A and 1B , a dielectrically-loaded backfire helical antenna in accordance with an embodiment of the disclosed technology has an antenna element structure with four axially coextensive helical tracks  10 A,  10 B,  10 C,  10 D plated or otherwise metallised on the cylindrical side outer surface portion  12 S of a cylindrical ceramic core  12 . The relative dielectric constant of the ceramic material of the core is typically greater than 20. A barium-samarium-titanate-based material, having a relative dielectric constant of 82 is especially suitable. With a total core length of 12 mm and a diameter of 7.5 mm, the antenna has frequencies of operation at 1575 MHz and 2450 MHz, as will be described below. 
         [0026]    The core  12  has a central passage  12 B, centred on the axis of the cylinder and in the form of a bore  12 B extending through the core from a distal end surface portion  12 D to a proximal end surface portion  12 P. Both of these end surface portions are planar faces extending transversely and perpendicularly with respect to the core axis. They are oppositely directed in that one is directed distally and the other proximally. 
         [0027]    On the distal end surface portion  12 D of the core, the antenna element structure includes four plated or otherwise metallised radial connection elements  10 AR,  10 BR,  10 CR,  10 DR, each connected to one of the antenna elements  10 A- 10 D. Arcuate interconnections  10 AB,  10 CD interconnect the radial connection elements. 
         [0028]    Encircling a proximal end portion of the core  12  is a plated or otherwise metallised conductive sleeve  20  which is conductively continuous with a plated or otherwise metallised conductive covering of the proximal end surface portion  12 P of the core. The rim  20 U of the sleeve  20  forms an annular interconnection of the proximal ends of the helical antenna elements  10 A- 10 D. 
         [0029]    Housed in the axial bore  12 B of the core is a feeder structure in the form of a laminate board  16  having a plurality of conductive layers and a plurality of insulative layers as will be described below. At the proximal end of the bore  12 B, the laminate board  16  is received in grooves  18  opening out in the proximal end surface portion  12 P. In this example the grooves  18  also intersect the cylindrical outer surface  12 S. At the other, distal end of the bore  12 B, the laminate board  16  projects beyond the distal end surface portion  12 D and is received in a slot  20 S of a disc-shaped lateral laminate board part  20  of the feeder structure. Lateral laminate board part  20  overlies the core distal end surface portion  12 D and is of a lateral extent sufficient to overlie, as well, the arcuate interconnecting conductors  10 AB,  10 CD of the antenna element structure. 
         [0030]    A second antenna embodiment, as shown in  FIGS. 1C and 1D , has the same features as those of the first antenna described above with reference to  FIGS. 1A and 1B . However, in the second antenna, the depth of the grooves  18  is less than in the first antenna, and the laminate board  16  is correspondingly modified, as hereinafter described. 
         [0031]    Further details of both antennas and the differences between them are visible in the exploded views of  FIGS. 2A to 2D . Referring, firstly, to  FIGS. 2A and 2B , the elongate laminate board  16  of the feeder structure has a proximal end portion  16 P for connection to host equipment circuitry, an intermediate portion  16 I which forms a shielded transmission line, and a distal end portion  16 D to be received in the slot  20 S of the lateral laminate board part  20 . 
         [0032]    The elongate laminate board  16  has three conductive layers, only one of which appears in  FIGS. 2A and 2B . This first conductive layer is exposed on an upper surface  16 U of the board  16 . Referring to the exploded view of  FIG. 3 , the first conductive layer  16 - 1  extends the full length of the intermediate portion  16 I and substantially the full width, too. On the proximal end portion  16 P of the board  16 , the conductive layer  16 - 1  forms proximal contact areas  16 C which are electrically continuous with that part of the conductive layer which is on the intermediate portion  16 I. 
         [0033]    The second, intermediate conductive layer  16 - 2  of the laminate board  16 , separated from the first conductive layer by an insulative layer  17 , is formed as a narrow elongate feed line conductor positioned centrally between the edges of the intermediate portion  16 I. The third, lower conductive layer  16 - 3  has a similar configuration to the upper conductive layer  16 - 1  in that it extends the full length of the intermediate portion  16 I and is electrically continuous with contact areas  16 E on the proximal end portion  16 P. It is insulated from the intermediate conductive layer  16 - 2  by a second insulative layer  19 . Adjacent each edge of the board intermediate portion  16 I is a line of plated vias  23  interconnecting the upper conductive layer  16 - 1  and the lower conductive layer  16 - 3  along opposite sides of the inner conductor formed by the intermediate layer  16 - 2 . As a result, the combination of the three conductive layers  16 - 1 ,  16 - 2 ,  16 - 3  form a quasi-coaxial shielded transmission line in the laminate intermediate portion  16 I. In this instance, the characteristic impedance of the transmission line is 50 ohms. 
         [0034]    Plated vias  24  between the contact areas  16 C,  16 E on opposite faces of the board proximal end portion  16 P interconnect these contact areas. 
         [0035]    At each end of the inner conductor formed by intermediate layer  16 - 2 , there is a plated via  25  connecting the inner conductor to proximal and distal feed line connection areas  27 P,  27 D on the upper surface  16 U (see  FIGS. 2A and 2B ) of the elongate laminate board  16 . 
         [0036]    The laminate board shown in  FIG. 3  is a variant inasmuch as it has an impedance matching network in its distal end portion. This is a two-pole network having two shunt capacitors C 1 , C 2  as discrete surface-mount capacitors. The network also contains two series inductances L 1 , L 2  constituted by plated tracks of the conductive layer  16 - 1 . 
         [0037]    Still referring to  FIG. 3 , each longitudinal edge of the intermediate board portion  16 I has spaced-apart nibs  28  which increase the width of the intermediate section at their respective axial locations to match the diameter of the bore  12 B ( FIGS. 2A ,  2 B) so that the intermediate laminate board portion  16 I is an interference fit in the bore with the edges of the elongate shield conductors formed by the upper and lower conductive layers  16 - 1 ,  16 - 3  spaced from the wall of the bore. 
         [0038]    Referring generally to  FIGS. 2A ,  2 B and  3 , it will be noted that the laminate board proximal end portion  16 P is significantly wider than the intermediate portion  16 I in that it includes lateral extensions or ears projecting in opposite lateral directions with respect to the central axis. Each ear has a proximal edge  16 PE on a line perpendicular to the central axis. The upper and lower contact areas  16 C,  16 E on the board proximal end portion  16 P extend right to the proximal edges  16 PE. Referring to  FIG. 2A , both grooves  18  are fully plated inasmuch as both the base  18 B and the side walls  18 S of each groove are conductively coated and electrically continuous with the conductive sleeve  14 . 
         [0039]    Connections between the shielded transmission line formed by the intermediate portion  16 I of the elongate laminate board  16  and the antenna element structure are completed by the lateral laminate board part  20 , shown in  FIG. 2A . The slot  20 S in the lateral laminate board part  20  has elongate side walls  20 SW which are each plated (only one such plated wall  20 SW is visible in  FIG. 2A ), each plated side wall  20 SW being connected to a respective segment-shaped inner plated area  201  on the proximal face  20 PF of the laminate board part  20 . 
         [0040]    On each side of the slot, the lateral laminate board part  20  has arcuate peripheral conductor areas  20 P extending over the side edges of the board part  20 . Embodied in and/or carried by the lateral laminate board part  20  are circuit elements (not shown) interconnecting the conductors associated with the slot side walls  20 SW and the peripheral conductor areas  20 P. In the absence of an impedance matching network on the elongate laminate board  16 , these circuit elements may constitute an impedance matching network of the kind disclosed in U.S. Pat. No. 7,439,934, the entire contents of which are incorporated herein by reference. 
         [0041]    In the assembled antenna, solder joints are formed between the distal connection areas  27 D,  29  of the feed line inner conductor and shield conductors, respectively, the side walls  20 SW of the slot  20 S. Solder joints between the peripheral conductor areas  20 P of the lateral laminate board part  20  and the conductors on the distal end surface portion  12 D of the core, specifically the arcuate interconnections  10 AB,  10 CD, together with the above-described connections between the laminate board  16  and the lateral laminate board part  20 , result in the connection of the shielded transmission line formed by the laminate board intermediate portion  16 I to the antenna element structure. 
         [0042]    Referring to  FIGS. 2A and 2B  in conjunction with  FIGS. 1A and 1B , during assembly of the antenna the elongate laminate board  16  is inserted in the bore  12 B of the antenna core  12  so that the proximal edges  16 PE of the lateral ears abut the bases  18 B of the respective grooves  18  in the proximal end portion of the core  12 . The grooves  18  are centred on a diameter containing the central axis of the antenna and have side walls  18 S which are inclined with respect to the plane containing that diameter and the antenna axis so that the grooves  18  are tapered, i.e. narrower at their base  18 B than at their mouths. The width of the grooves at their bases  18 B matches the thickness of the laminate board  16  so that when the laminate board proximal end portion  16 P is fully inserted in the grooves  18 , the board  16  is secured against rotation relative to the core  12  and, hence, relative to the antenna elements  10 A- 10 D. The distance between the proximal edges  16 PE of the proximal end portion  16 , on the one hand, and the extreme distal end of the board distal end portion  16 D on the other hand is such that, when the proximal end portion  16 P is fully seated in the groove  18 , the distal end portion  16 D projects by an amount approximately equal to the thickness of the lateral laminate board part  20 . 
         [0043]    During manufacture of the antenna, solder paste is deposited in the grooves  18  and on the distal end surface portion  12 D of the core  12  so that, when the assembled components are passed through a reflow oven, the upper and lower conductive layers  16 - 1 ,  16 - 3  ( FIG. 3 ) of the elongate laminate board  16  are electrically connected to the conductive plating in the grooves  18 , including the plated groove base  18 B in each case, and connections are also made between the lateral laminate board part  20  and the arcuate interconnecting conductors  10 AB,  10 CD ( FIG. 2B ) on the core distal end surface portion  12 D. The connections between the elongate laminate board  16  and the lateral laminate board part  20  are also made at this stage. Referring to  FIG. 4 , it is preferred that sufficient solder paste is deposited in the grooves  18  such that, when the assembled antenna is heated, solder  31  fills the grooves on each side of the laminate board proximal end portion  16 P and forms fillets  32  between the contact areas  16 C,  16 E on each side of the board proximal end portion  16 P and the plated proximal end surface portion  12 P of the core  12 . 
         [0044]    Electrically, the antenna behaves as a multifilar backfire helical antenna as described in a number of prior patent publications, including GB2310543, GB2311675 and WO2006/136809, the entire contents of all three of these publications being incorporated in the present specification by reference. As described in the prior publications, the primary mode of resonance of the antenna is a circularly polarised mode in which the sleeve  14  encircling the core  12 , and the plating on the core end surface  12 P, together with the feeder structure, form a quarter-wave balun so that currents flow around the rim  14 R interconnecting the proximal ends of the helical antenna elements  10 A- 10 D to produce a distally directed cardioid radiation pattern suited to reception and/or transmission of satellite signals when the antenna is oriented with its axis generally vertical. In this resonant mode, the resonant frequency is mainly determined by the lengths of the helical elements  10 A- 10 D and the relative dielectric constant of the core material. The sleeve  14 , in conjunction with the plated proximal end surface portion  12 P, has a nominal electrical length equivalent to a quarter wavelength, although operation of the structure as a balun is tolerant of wide variations in this electrical length. Operation of the balun has the effect of balancing the antenna feed at the distal end of the transmission line formed by the intermediate laminate board portion  16 I. 
         [0045]    The antenna has a second mode of resonance also described in the above-mentioned GB2311675, in which currents flowing in the helical antenna elements  10 A- 10 D, instead of being trapped at the sleeve rim  14 R, flow longitudinally through the sleeve  14  and thence directly to the shield conductors of the feeder via the connections of the latter formed in the grooves  18 . These currents flow along the outside of the shield formed by the shield conductors between the grooves  18  and the distal end of the transmission line so that a complete conductive loop is formed (a) through the connections made by the lateral laminate board part  20 , (b) through the helical elements  10 A-LOAD and the sleeve  14 , (c) along the base of each groove  18 , and (d) along the shield conductors of the feeder. The electrical length of this composite conductive path defines the frequency of the second mode of resonance, which is a resonance characterised by linearly polarised radiation, polarised in planes in containing the antenna axis. The associated radiation pattern is generally toroidal, i.e. with an omnidirectional maximum at zero elevation and vertical (axial) nulls. 
         [0046]    The resonances of both resonant modes have associated harmonic resonances as well. 
         [0047]    With regard to the linearly polarised mode of resonance, the electrical length of the composite conductive path defining the resonant frequency is dependent on the depth of the grooves  18  since the effective conductive length between the rim  14 R of the sleeve  14  and the feeder shield decreases at the depth of the groove increases. In addition, as the depth of the groove increases, the effective length of the conductive path formed by the outside of the feeder shield decreases. Given the tolerance of the circularly polarised mode of resonance to changes in the effective length of the sleeve  14 , it is possible to alter the resonant frequency of the linearly polarised mode by varying the depth of the grooves  18 . It is appropriate to vary the axial depth of the lateral extensions or ears of the laminate board proximal end portion  16 P accordingly (by increasing or decreasing the distance between the proximal edges of the proximal end portion  16 P and the distal end of the laminate board  16  so that the axial positions of the distal and proximal ends of the laminate board  16  relative to the proximal and distal end surface portions  12 P,  12 D of the core  12  are maintained constant). 
         [0048]    Accordingly, manufacture of antennas in accordance with embodiments of the disclosed technology is performed by providing a range of antenna bodies, each consisting of a core  12  with the plated antenna structure, in which the groove depth d G  ( FIG. 2A ) is different from antenna body to antenna body, the overall length and diameter of the antenna body remaining constant. Similarly, a corresponding range of elongate laminate boards  16  is provided, having proximal end portions  16 P of different depths d P  ( FIG. 2A ). In other words, the elongate laminate boards  16  are provided with intermediate portions  16 I of different lengths d I . 
         [0049]    To assemble the antenna described above with reference to  FIGS. 1A ,  1 B,  2 A,  2 B and  3 , an antenna body with grooves  18  of a first depth d G  is selected together with a laminate board  16  of a matching proximal end portion depth d P . If an antenna with a linearly polarised resonant mode of lower frequency is required, then an antenna body in which the depth of the groove  18  is less is selected, i.e. depth d G1 , as shown in  FIGS. 1C ,  1 D,  2 C, and  2 D. An elongate laminate board  16  with a longer intermediate portion  16 I (length d II ), and a proximal end portion  16 P of smaller axial extent d P1  is then selected. In this instance, the relevant conductive path length is greater, since the effective depth of the sleeve  14  is greater and the effective length of the outside of the shield conductors is greater, yielding the required lower resonant frequency. 
         [0050]    Maintenance of the other dimensions of the antenna bodies and laminate boards leads to economies both in the production of the antennas and in their mounting in, e.g. equipment sub-assemblies and housings. 
         [0051]    In the preferred embodiments herein described and shown, the resonant frequency of the linearly polarised resonant mode is higher than that of the circularly polarised resonance mode, this relationship being in respect of resonances at the respective fundamental frequencies of resonance. This is achieved in part as a result of the spacing of the feeder shield conductors from the wall of the bore  12 B, thereby reducing the dielectric elongation of the electrical length of the shield conductors. 
         [0052]    The above-described antenna embodiments are quadrifilar helical antennas. Falling within the scope of the disclosed technology are antennas other than quadrifilar helical antennas. For instance, antennas with cuboid-shaped dielectric cores may be used, as well as helical antennas with different numbers of helical elements. Such antennas include hexafilar and octafilar antennas as described in, for instance, GB2445478A, the disclosure of which is incorporated herein by reference.