MXPA97001299A

MXPA97001299A - An ant

Info

Publication number: MXPA97001299A
Application number: MXPA/A/1997/001299A
Authority: MX
Inventors: Paul Leisten Oliver
Original assignee: Symmetricom Inc
Priority date: 1996-02-23
Filing date: 1997-02-20
Publication date: 1998-04-01

Abstract

The present invention relates to an antenna for operation at frequencies of more than 200 MHz, comprising a substantially cylindrical electrically insulating core of a material having a relative dielectric constant greater than 5, with the core material occupying most of the defined volume by the outer surface of the core, a feeder structure extending axially through the core, a trap in the form of a surrounding part of the core conductive sleeve and having one edge, and first and second pairs of antenna elements each connecting to one end of the feeder structure and at the other end to a binding edge of the sleeve, the antenna elements of the second pair being larger than those of the first pair, wherein the antenna elements of both pairs follow trajectories which are longitudinally extend respective and, said joining edge follows a non-flat path around the core, the eleme Antenna numbers of the first pair being attached to the binding edge at points that are closer to the connections of the elements to the feeder structure which are the points at which the antenna elements of the second pair are joined to the edge of the antenna.

Description

AN ANTENNA This invention relates to an antenna for operation at frequencies of more than 200 MHz, and particularly but not exclusively to an antenna having helical elements on or adjacent to the surface of a dielectric core to receive the circularly polarized signal. Such signals are transmitted by satellites of the "Global Positioning System (GPS)". Such an antenna is described in co-pending British Patent Application No. 9517086.6, the full disclosure of which is incorporated in the present application to form part of the subject matter of this application as presented first. The first application describes a quadrifilar antenna having two pairs of diametrically opposed helical antenna elements, the elements of the second pair following serpentine paths that deviate on each side of a half helical line on an outer cylindrical surface of the core so that the elements of the second pair are larger than those of the first pair that follow helical trajectories without deviation. Such a variation in the lengths of the element makes the antenna suitable for the transmission or reception of circularly polarized signals. Applicants have found that said antenna tends to favor the reception of elliptical signals instead of circularly polarized and it is an object of the present invention to provide the improved reception of circularly polarized signals.

According to the invention, an antenna for operation at frequencies of more than 200 MHz comprises an electrically insulative substantially cylindrical core of a material having a relative dielectric constant greater than 5, with the core material occupying most of the defined volume by the outer surface of the core, a feeder structure extending axially through the core, a trap in the form of a surrounding part of the conductive sleeve of the core having a ground connection at an edge, and first and second pairs of antenna elements each connected to the feeder structure and at the other end to a binding edge of the sleeve, the antenna elements of the second pair being larger than those of the first pair, where the antenna elements of both pairs follow respective longitudinally extending trajectories and, the joining edge follows a non-planar path around the core, l The antenna elements of the first pair are attached to the joining edge at points that are closest to the connections of the elements to the feeder structure, which are the points at which the antenna elements of the second pair are held at the joining edge. . The longitudinally extending paths are preferably helical paths in which each element subtends the same angle of rotation on the axis of the core, for example, 180 ° or half turn. In this way it is possible to avoid deviations from the larger antenna elements of the respective helical paths, thereby producing more balanced radiation resistances for the antenna elements and the consequent improved performance with the circularly polarized signals. The core may be a cylindrical body that is solid with the exception of a narrow axial passage that houses the feeder structure. Preferably, the volume of the solid material of the core is at least 50% of the internal volume of the shell defined by the antenna elements and the sleeve, with the elements located on an outer cylindrical surface of the core. The elements may comprise metal conductor paths attached to the outer surface of the core, for example by deposition or by etching a previously applied metal coating. For reasons of physical and electrical stability, the core material may be ceramic, for example a ceramic material for microwave, zirconium barium tantalate and neodymium barium titanate, or a combination of these. The preferred relative dielectric constant is above 10 or, in fact, 20 with an amount of 36 which is obtainable using material based on zirconium titanate. 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 by the loss of the core. A particularly preferred embodiment of the invention has a cylindrical core of solid material with an axial degree at least as large as its external diameter and, with the diametral grade of the solid material being at least 50 percent of the external diameter. Thus, the core may be in the form of a tube having a comparatively narrow axial passage of a diameter of at most one-half the overall diameter of the core. The internal passage may have a conductive liner that forms part of the feeder structure or a screen for the feeder structure, thereby narrowly defining the radial spacing between the feeder structure and the antenna elements. This helps to obtain a good repeatability in manufacturing. The helical antenna elements are preferably formed as metal paths on the outer surface of the core that are generally coextensive in the axial direction. Each element is connected to the feeder structure at one of its ends and to the sleeve at its other end, the connections of the feeder structure being made with generally radial conductive elements and, the sleeve being common to all the helical elements. The trap produces a virtual earth for the antenna elements at the binding edge. The radial elements can be placed on an end surface distant from the core. The preferred embodiment has antenna elements with an average electrical length of? / 2, although alternative modalities are viable having electric lengths of, for example,? / 4, 3? / 4,? and other multiples of? / 4, which produce modified radiation patterns. Advantageously, the helical elements extend proximally from the distal end of the core towards the conductive sleeve extending over part of the length of the core from a connection with the feeder structure at the proximal end of the core. In the case of the feeder structure comprising a coaxial line having an internal conductor and an external shield conductor, the conductive sleeve is connected at the proximal end of the core of the external shield conductor of the feeder structure. Using the above-described features it is possible to make an antenna that is extremely strong due to the small size and because the elements are supported on a solid core of the rigid material. Such an antenna can be placed to have a low horizon omnidirectional response with sufficient resistance for use as a replacement for temporary antennas in certain applications. Their reduced size and resistance make them also suitable for non-obstructing vehicle assembly and for use in manipulated devices. It is possible in some cases to mount even directly on a printed circuit board. The longitudinal extension of the antenna elements, i.e. in the axial direction, is generally greater than the average axial length of the conductive sleeve. Typically the average axial length of the antenna element is twice that of the sleeve and the diameters of the elements and the sleeve are equal and are on the scale from 0.15 to 0.25 times the combined length of the antenna elements and the sleeve. Preferably, the average axial length of the sleeve is not less than 0.35 times the average axial length of the antenna elements. The difference in axial length between the antenna elements that averages their average axial length and preferably on the scale from 0.05 to 0.15 times their average length. The antenna can be manufactured by forming the antenna core from dielectric material and metallizing the outer surfaces of the core in accordance with a predetermined pattern. Such metallization may include coating the outer surfaces of the core with a metallic material and then removing the portions of the coating to leave the predetermined pattern oralternatively a mask containing a negative of the predetermined pattern can be formed and the metallic material is then deposited on the outer surfaces of the core while the mask is used to mask portions of the core so that the metallic material is applied in accordance with the Pattern. Other methods may be used to deposit the conductive pattern in the required manner. A particularly advantageous method of producing an antenna having a trap or a symmetrizing sleeve and a plurality of antenna elements forming part of a radiating element structure, comprising the steps of providing a batch of the dielectric material, which they make from the batch at least one test antenna core and, then forming a symmetrizing structure, preferably without any radiating element structure, metallizing on the core a symmetrizing sleeve having a predetermined nominal dimension that affects the resonance frequency of the symmetrizing structure. The resonant frequency of this test resonator is then measured and the measured frequency is used to derive a set value of the symmetrizing sleeve dimension to obtain a resonant frequency of symmetrizing structure. The same measured frequency can be used to derive at least one dimension of the antenna elements to give a frequency characteristic of antenna elements. The antennas manufactured from the same batch of the material are then produced with a sleeve and the antenna elements having the required dimensions. 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 according to the invention; and Figure 2 is a diagrammatic axial cross section of the antenna. Referring to the drawings, a quadrifilar antenna in accordance with the invention has an antenna element structure with four longitudinally extending antenna elements 10A, 10B, 10C and 10D formed as metallic conductor paths on the cylindrical outer surface of an antenna element. ceramic core 12. The core has an axial passage 14 with an internal metallic lining 16 and, the passage houses an axial feeder conductor 18. The internal conductor 18 and the liner 16 in this case form a feeder structure to connect a feeder line towards the antenna elements 10A-10D. The structure of the antenna element includes corresponding antenna elements 10AR, 10BR, 10CR, 10DR formed as metal paths on a distal end face 12D of the core 12 connecting ends of the respective longitudinally extending members 10A-10D towards the feeder structure. The other ends of the antenna elements 10A-10D are connected to a common virtual 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 skin 16 of the axial passage 14 by plating 22 on the proximal end face 12P of the core. As will be seen from Figure 1, the four longitudinally extending elements 10A-10D are of different lengths, two of the elements 10B, 10D being longer than the other two 10A, 10C by virtue of their being extended closer from the proximal end of the core 12. The elements of each pair 10A, 10C 10B, 10D are diametrically opposed to each other on opposite sides of the core axis. In order to maintain approximately uniform radiation resistance for the helical elements 10A-10D. each element follows a simple helical path. Since each of elements 10A-10D subtends the same angle of rotation on the axis of the core, here 180 ° or a half turn, the advance of the screw of the large elements 10B, 10D is more stepped than that of the short elements 10A , 10C. The upper lining edge 20U of the sleeve 20 is of variable height (i.e., the variable distance from the proximal end face 12P) to provide the connection points for the large and short elements respectively. Thus, in this embodiment, the lining edge 20U follows a zigzag path around the core 12, having two peaks 20P and two depressions 20T where it meets the short elements 10A, 10C and the large elements 10B, 10D respectively. Each pair of corresponding radially extending longitudinal and radial elements (e.g. 10A, 10AR) constitutes a conductor having a predetermined electrical length. In the present embodiment, it is positioned so that the total length of each of the pairs of elements 10A, 10AR; 10C, 10CR having the shortest length corresponding to a transmission delay of about 135 ° in the operating wavelength, while each of the pairs of elements 10B, 10BR; 10D, 10DR produces a greater delay, which corresponds substantially to 225 °. Therefore, the average transmission delay is 180 °, equivalent to an electrical length of? / 2 in the operating wavelength. The different lengths produce the phase change conditions required for a quadrifilar helical antenna for circularly polarized signals specified in Kilgus, "Resonant Quadrifilar Helix Design", The Microwave Journal, Díc. 1970, pages 49-54. Two of the pairs of elements 10C, 10CR; 10D, 10DR (ie a large element pair and a short element pair) are connected at the inner ends of the radial elements 10CR, 10DR to the inner conductor 18 of the feeder structure at the distal end of the core 12, while that the radial elements of the other two pairs of element 10A, 10AR; 10B, 10BR are connected to the feeder screen formed by the metal liner 16. At the far end of the feeder structure, the signals present on the internal conductor 18 and the feeder screen 16 are approximately balanced so that the antenna elements are connected to an approximately balanced source or load, as will be explained below. With the direction of direction to the left of the helical portions of the longitudinally extending elements 10A-10D, the antenna has its highest gain for the circularly polarized signals to the right. If the antenna is used instead of the circularly polarized signals to the left, the direction of the propellers is reversed and the connection pattern of the radial elements is rotated 90 °. In the case of the antenna suitable for receiving circularly polarized signals both to the left and to the right, the longitudinally extending elements may be positioned to follow trajectories that are generally parallel to the axis. The conductive sleeve 20 covers a proximal portion of the antenna core 12, thereby enclosing the feeder structure 16, 18 with the material of the core 12 filling the entire space between the sleeve 20 and the metal liner 16 of the axial passage 14. The sleeve form a cylinder having an average axial length / B as shown in Figure 2 and is connected to the liner 16 by plating 22 of the proximal end face 12P of the core 12. The combination of the sleeve 20 and the plywood 22 forms a symmetrizer of So that the 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 state approximately balanced in the axial position generally at the same distance from the proximal end as the upper lining edge 20U of the sleeve 20. To obtain this effect, the average sleeve length / B is such that, in the presence of a material of Underlying core of relatively high relative dielectric constant, the symmetrizer has an average electrical length of? / 4 at the operating frequency of the antenna. Since the core material of the antenna has a reducing effect and, the annular space surrounding the inner conductor 18 is filled with an insulating dielectric material 17 having a relatively small dielectric constant, the feeder structure remote from the handle has an electrical effect Accordingly, the signals at the far end of the feeder structure 16, 18 are at least approximately balanced. (The dielectric constant of the insulation in a semi-rigid cable is typically much smaller than that of the aforementioned insulating core material, eg, dielectric constant et of PTFE is about 2.2). The applicants have found that the variation in the length of the sleeve 20 from the average electrical length of? / 4 has a comparatively insignificant effect on the performance of the antenna. The trap formed by the sleeve 20 provides an annular path along the joining edge 20U for the currents between the elements 10A-10D, effectively forming two curls, the first with the short elements 10A, 10C and the second with the large elements 10B, 10D To the cudrifilar resonance the maximum current exists at the ends of the elements 10A-10D and at the joining edge 20U and the maximum voltage at a level approximately halfway between the edge 20U or the distal end of the the antenna. Edge 20U is effectively isolated from the ground connector at its proximal edge due to the approximate quarter-wavelength trap produced by sleeve 20. The antenna has a main resonance frequency of 500 MHz or greater, the resonant frequency being determined by the respective electrical lengths of the antenna elements and, to a lesser degree, by its width. The lengths of the elements, for a given resonance frequency, also depend on the relative dielectric constant of the core material, the dimensions of the antenna being substantially reduced with respect to a similarly constructed air core antenna. The preferred material for core 12 is zirconium-titanate based material. This material has the aforementioned dielectric constant of 36 and it is also observed that for its dimensional and electrical stability with variable temperature. The dielectric loss is negligible. The core can be produced by extrusion or pressing. The antenna elements 10A-10D, 10AR-10DR are metal conductor paths bonded to the outer and outer cylindrical surfaces of the core 12, each path being at least four times its thickness over its operational length. The paths can be formed initially by plating the surfaces of the core 12 with a metal layer and then selectively recording away from the layer to expose the core in accordance with the pattern applied on a photographic layer similar to that used to engrave printed circuit boards . Alternatively, the metallic material can be applied by selective deposition or by printing techniques. In all cases, the formation of the trajectories 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 dielectric constant substantially higher than that of air, for example, et = 36, an antenna as described above for the reception of L-band GPS at 1575 MHz typically has a core diameter of about 5 mm and the longitudinally extending antenna elements 10A-10D have an average longitudinal extent (ie, parallel to the central axis) of approximately 10A, 10C. The width of the elements 10A-10D is approximately 0.3 mm. At 1575 MHz, the length of the sleeve 22 is typically in the region of 8 mm. The precise dimensions of the antenna elements 10A-10D can be determined in the design stage on a trial and error basis by carrying out specific value delay measurements until the required phase difference is obtained.

The manner in which the antenna is manufactured is described in the aforementioned co-pending application No. 9517086.6.

Claims

1. An antenna for operation at frequencies of more than 200 MHz, comprising an electrically insulative substantially cylindrical core of a material having a relative dielectric constant greater than 5, with the core material occupying most of the volume defined by the external surface of the core, a feeder structure extending axially through the core, a trap in the form of a surrounding part of the core conductive sleeve and having one edge, and first and second pairs of antenna elements each connected to one end of the core. the feeder structure and at the other end to a binding edge of the sleeve, the antenna elements of the second pair being larger than those of the first pair, wherein the antenna elements of both pairs follow respective longitudinally extending paths and, said joining edge follows a non-planar path around the core, the antenna elements of the first pair are connected to the binding edge at points that are closer to the connections of the elements to the feeder structure, which are the points at which the antenna elements of the second pair are joined to the joining edge. An antenna according to claim 1, wherein each longitudinally extending antenna element follows a respective helical path around the axis of the core and, the angle subtended by the respective two ends of each antenna element on the axis of the antenna. core is the same in each case. An antenna according to claim 2, wherein each of the antenna elements executes a half turn around the axis of the core, the connections between the elements and the feeder structure being located in a common plane perpendicular to the axis of the core and, where the advance of the screw of the elements of the first pair is different from that of the elements of the second pair. An antenna according to any preceding claim, wherein the trailing edge of the trap follows a zigzag path around the core with the elements of the first and second pair which are joined in peaks and depressions respectively respectively of the joining edge. 5. An antenna in accordance with any previous claim, wherein the grounding edge of the trap is located in a plane perpendicular to the axis of the core and the average axial length of the sleeve forming the trap is at least approximately? / 4, where? is the operating wavelength at the interface between the air and the dielectric material of the core. 6. An antenna according to any preceding claim, which is quadrifilar, having a first single pair and a second individual pair of antenna elements. An antenna according to any preceding claim, wherein the trap and the antenna elements are integrally formed on the outer cylindrical surface of the core. An antenna according to any preceding claim, wherein the antenna elements of the first and second pairs are connected to the feeder structure by respective radial elements on a flat end surface of the core and, where the ground connection of the trap it is formed by a conductive layer formed on the other end surface of the core. An antenna according to claim 8, wherein the feeder structure is a coaxial transmission line, each of the pairs of antenna element having an element connected to an internal conductor of the feeder structure and an element connected to it. an external conductor of the feeder structure and, wherein the external conductor is attached to said conductive layer. An antenna according to any preceding claim, wherein the average axial length of the antenna elements is greater than the average axial length of the conductive sleeve. 11. An antenna according to claim 10, where the average axial length of the antenna element is. at least approximately, twice the average axial length of the sleeve and, the diameter of the elements and the diameter of the sleeve are equal and is on the scale from 0.15 to 0.25 times the combined length of the antenna elements and the sleeve. 1

2. An antenna according to claim 10, wherein the ratio of the average axial length of the antenna elements to the average axial length of the sleeve is less than or equal to 1: 0.35. An antenna according to any preceding claim, wherein the difference in axial length between the antenna elements of the first pair and those of the second pair is less than half its average length.