Matching element for mobile antenna

Abstract: A coupling unit forming a termination circuit having capacitance and inductance distributed in such a manner as to match and balance a screen mounted antenna comprises a circuit board substrate (1) provided on both sides with patterns of conductive cladding that are configured to provide a capacitor plate (2) for capacitative coupling to a base mounting of the antenna and further elements (3,9,10,12,14,17) that provide the required inductive and capacitative characteristics to enable the termination circuit to match and balance the antenna.

Description: This invention relates to a matching element for a mobile antenna, more especially an antenna of the kind intended to be attached to a glass screen of a motor vehicle. Antennas of the kind mentioned above are well-known and generally comprise a radiating antenna element adhesively secured to the exterior of a glass screen, and a coupling pad adhesively secured to the interior of the screen in facing relationship to the antenna element. Capacitative coupling between the external antenna and the internal pad is provided by overlapping conductive plates arranged externally and internally of the glass screen. The internal coupling pad should also incorporate an impedance matching circuit for matching the antenna to the conventional 50 ohm coaxial cable. In hitherto known arrangements, the impedance matching circuit may be formed by an electrically conductive shell of the coupling pad which forms a ground plane in relation to the internal plate of the coupling capacitor. By connecting the external conductor of the coaxial cable to the ground plane and by connecting tappings from the central conductor of the coaxial cable to the ground plane as well as to the internal plate of the coupling capacitor, an inductive and capacitive network can be established that will provide the desired matching. However, careful adjustment of the position of the ground plane relatively to the capacitor plate and of the lengths of the tappings from the central conductor is required in order to obtain an appropriately tuned circuit, and it is difficult to manufacture such units in mass production whilst maintaining a consistent performance of the coupling units. Furthermore because air forms the dielectric medium of the tuned capacitance, the impedance matching circuit can be rendered ineffective by the presence of moisture in the air. This is a particular problem because of the tendency of moisture to form by condensation near the internal surface of a motor vehicle window. There is also known, see U.S.-A 4882592, a coupling unit having the features of the preamble of Claim 1. It is an object of the present invention to provide such a coupling unit with a matching circuit having an improved circuit configuration. This object is achieved by the features of Claim 1. The invention is illustrated by way of example in the accompanying drawings, in which: Figure 1 is a sectional side view of a coupling unit in accordance with the invention, Figure 2 is a view of the circuit board of the unit of Fig. 1, from the screen-side of the board, Figure 3 is a view of the opposite side of the circuit board, and Figure 4 is an equivalent circuit diagram of the coupling unit shown in Figs. 1 to 3. Referring to the drawings a coupling unit in accordance with the invention comprises a circuit board substrate 1 of any suitable insulating material, provided on one side with a conductive cladding element 2 forming a capacitor plate and an element 14 which contributes to the matching and balancing of the antenna. On the opposite side of the circuit board 1 are provided further conductive elements 3 and 12 completing the matching and balancing circuit. A coaxial cable 4 to be coupled to an antenna element has a central conductor 5 that is soldered both to the element 2 at point 6 and to the element 3 at point 7. The outer screening braid of the coaxial cable 4 is also soldered to the element 12 at points 8 and 13. Conductive element 17 joins elements 3 and 12. It will be moted that the element 3 is in the form of generally ring-shaped conductor portion 9 having internally directed web portions 10 and 19. Web portion 10 provides a terminal connection for the central conductor 5 of the coaxial cable 4. The coaxial cable 4 is soldered to the terminal portions 6,7,8 and 13 in such manner that the conductive paths provided between the terminals 6 and 7 and the terminals 7 and 8 each have a defined length, and this in conjunction with the illustrated shape and arrangement of the elements 2 and 3 is effective to provide an inductive coupling between the coaxial cable 4 and the antenna in a network that also incorporates distributed inductance and capacitance as generally illustrated in the equivalent circuit of Fig. 4. The outer conductor of coaxial cable 4 is also electrically connected to element 14 at points 15 and 16. As illustrated in Fig. 1, the screen-side of the circuit board 1 is covered with an adhesive pad 11 capable of being secured to a glass screen, whereas the reverse side of the circuit board and the coaxial cable 4 are encapsulated in a synthetic plastics moulding 18. The arrangement illustrated is capable of being manufactured economically in mass production whilst maintaining close tolerances of the values of the components of the equivalent circuit of Fig. 4. In Fig. 4 the reference numerals C₁ , C₂, C₃, C₄and C₅ illustrate distributed capacity, the reference numerals L₁, L₂, L₃ and L₄ represent distributed inductance, and the reference L T acts as a 'choke' to prevent current existing on the outer conductor of the coaxial cable 4. The remaining reference numerals illustrate elements already described with reference to Fig.s 1 to 3. The manner in which the construction of Figs. 1-3 provides the equivalent circuit of Fig.4 will be understood by reference to the table below wherein the circuit elements of Fig.4 are related to the physical elements of Fig. 1-3: L1, L4 - The distributed inductance of elements 12 and 14 C1, C2 - The distributed capacitance caused by elements 12 and 17 overlapping element 14. Lt - Transformation obtained due to the physical shape and dimensions of elements 3. L2 - The distributed inductance of the wire joining points 7 and 6 through the board. C4 - The distributed capacitance caused by element 10 overlapping element 2. C3 - The distributed capacitance caused by element 9 partially overlapping element 2. L3 - The distributed inductance of element 9. C5 - The distributed capacitance caused by the proximity of element 10 to element 17. It will be understood that various alterations and modifications may be made to the arrangement as illustrated without departing from the scope of the invention. For example, although the matching and balancing circuit, element 3, is shown as having a ring-shaped portion 9, it may not be strictly necessary for the portion 9 to form a complete ring, provided that the conductive connection between terminals 7 and 8 and 13 is maintained. Also, a coaxial connector of any suitable type can be electrically connected to the matching and balancing circuit in place of the coaxial cable 4 as long as the inner and outer conductor connections are properly made as shown in Figs. 1, 2 and 3 in the case of cable connection. Sections 12 and 14 can be adjusted to vary the frequency of operation thus allowing the arrangement to be used in several applications of differing frequency. As well as direct connections to the matching and balancing circuit via a coaxial connector or coaxial cable, it is possible to replace these with any suitable two port antenna with one port electrically connected to points 8 and 16 or 13 and 15 and the other port electrically connected to points 6 and 7. This configuration allows the arrangement to be used as a window mounted passive repeater to improve the service obtained when using hand portable transceivers inside vehicles.

Claims: A coupling unit for use with a screen-mounted antenna, comprising a circuit board substrate (1) of suitable dielectric material having patterns of conductive cladding material on both surfaces thereof, the said pattern on one surface of the board that is intended for attachment in facing relationship to a surface of a screen comprising a capacitor plate (2) for providing capacitative coupling to the base mounting of an antenna that is to be attached to the opposite surface of the screen, the said patterns of conductive cladding material being so shaped and arranged as to form in cooperation with said capacitor plate (2) a termination circuit for coupling said antenna to a coaxial cable, and the said patterns further including terminal portions connected to coaxial conductors of, or to be coupled to, said coaxial cable, characterised in that the said one surface of the circuit board comprises, in addition to a first area (2) of said conductive cladding forming said capacitor plate, a second area (14) of conductive cladding shaped to form an inductance; that a first one of said coaxial conductors (5) is directly connected to a first terminal portion (6) of said first area (2) of cladding whilst a second coaxial conductor (4) is directly connected to a second terminal portion (15,16) of said second area (14) of conductive cladding; that the other surface of the circuit board includes third (12) and fourth (9,10) areas of conductive cladding each of which is shaped to form an inductance; that the third area (12) of cladding is arranged to overlie said second area (14) of cladding and includes a third terminal portion (8,13) also directly connected to said second coaxial conductor (4); and that the fourth area (9,10) of conductive cladding includes a fourth terminal portion (7) connected to said first coaxial conductor (5), whereby said second (14) and third (12) areas of conductive cladding in combination and said first (2) and fourth (9,10) areas of conductive cladding in combination respectively provide distributed inductance and capacitance acting in a matching circuit providing a matched coupling between the coaxial cable and the said capacitor plate. A unit according to Claim 1, wherein the said fourth area (9,10) of conductive cladding is substantially in the form of a ring (9) with an internally directed web (10) the periphery of said ring (9) being coupled to said third terminal portion (8,13) and the said web (10) incorporates said fourth terminal portion (7). A unit according to Claim 2, wherein the said second and third areas of cladding (14, 12) comprise U-shaped conductive areas that respectively embrace said capacitor plate (2) and said ring (9). A unit according to any one of Claims 1-3 wherein the outer coaxial conductor (4) is coupled to said second and third terminal portions (15,16;8,13) and the central conductor (5) is coupled to the first and fourth terminal portions (6,7).

Inventors: JESMAN ANDREW, JESMAN CHRISTOPHER, JESMAN, ANDREW, JESMAN, CHRISTOPHER
Applicants: PANORAMA ANTENNAS LTD, PANORAMA ANTENNAS LIMITED
Application year: 1991
Main classification: H03H7
IPC codes: H01Q1, H03H7