WO2000069021A1

WO2000069021A1 - Concealed radio antenna system

Info

Publication number: WO2000069021A1
Application number: PCT/GB2000/001547
Authority: WO
Inventors: Ian Benjamin Hopley
Original assignee: ASG TECHNOLOGY Ltd
Current assignee: ASG TECHNOLOGY Ltd
Priority date: 1999-05-05
Filing date: 2000-05-04
Publication date: 2000-11-16
Anticipated expiration: 2001-11-05
Also published as: GB2352334B; GB9910246D0; GB0010842D0; AU4419700A; GB2352334A

Abstract

A concealed antenna system for a vehicle radio communication system consists of a slot or notch cut in a conductive panel which is adapted to form part of or be fitted to the structure of a vehicle with which the radio communication system is to be used. In order to produce vertically polarised radiation the antenna is positioned with the slot or notch horizontal and in order to ensure that the antenna operates with a non-resonant feed line matched to the antenna impedance a coaxial cable is connected to the edges of the slot by insulated leads linked to the magnetic field in a portion of the slot. Alternatively a non-magnetic electrically conductive element is positioned spanning a part of the slot and insulated therefrom.

Description

CONCEALED RADIO ANTENNA SYSTEM

The present invention relates to vehicle radio communication antenna systems. One known such system operates in the range 425 MHz to 460 MHz (nominally 450 MHz)

and at present, vehicle mobile radio communication systems operating at these

frequencies tend to use a quarter wavelength whip antenna mounted above a ground plane which is typically provided by a surface of the body of the vehicle concerned. Such an antenna arrangement is not satisfactory for a number of reasons and attempts

have been made to incorporate a so-called "slot" antenna into a vehicle.

One known such system is described in DE 40 00 381. A problem encountered in the mass production of such antennae, however, is the requirement for impedance matching of the slot, formed in an electrically conductive panel, with the feed line from

the radio equipment. Without proper matching standing waves can be set up in the

feed line which seriously limit the transmission power and renders the antenna

effectively inoperable due to limited range. In DE 40 00 381 reference is made to the

presence of an impedance matcher without any discussion of what form this may take.

According to the present invention there is provided a slot antenna system for use in

a vehicle radio communication system, comprising an electrically conductive panel

adapted to be carried by or to form part of the structure of the vehicle, the panel having an elongate slot in it the effective length of which is an integral sub-multiple of the

wavelength of the radiation with which the antenna system is to be used, a coaxial feed

cable the central conductor of which is connected to one longitudinal edge of the slot and the screen of which is connected to the other longitudinal edge of the slot opposite

the first connection, characterised in that the feed cable connection to the slot is made in such a way that the feed line is non-resonant at the operating frequency and is

matched to the impedance of the antenna independently of the length of the feed cable.

The slot in the panel may be in the form of a parallel sided slot, a dumb bell shaped slot, or an elongate notch cut into an edge of the panel. If the non-conductive region of the panel is in the form of a parallel sided slot then that slot may have a folded configuration.

The feed cable is preferably connected to the slot at the longitudinal edges thereof by insulated leads which are positioned such as to be linked to the magnetic field pattern

produced by the slot in use thereof. Such linkage may be achieved, for example by positioning the leads to lie generally parallel to the length of the slot and between its

long edges.

Alternatively, there may be provided an electrically conductive non-magnetic element

located in the magnetic field produced by the slot in use and insulated from the conductive panel. This element may span the slot and may be in the form of an

aluminum foil with a dielectric between itself and the conductive panel. The connector

leads may be secured to the insulating panel on the side thereof remote from the

conductive panel.

The radiation of signals inwardly of the vehicle may be prevented by providing an electrical screen which may be, for example, in the form of a conductive panel spaced from an attached to one surface of the electrically conductive panel

The antenna may be mounted so that the radiating region thereof is behind a non- conductive surface panel of the vehicle such as a non-metallic radiator grille or number

plate, or the antenna may be incorporated in an outer panel of the body of the vehicle In this case the slot antenna may be rendered invisible by filling with a non-metallic body filler of the type used for repairs to vehicle bodywork, which may be painted to match the rest of the vehicle This filler may be finished flush with the panel to provide

an entirely invisible antenna Various embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which

Figure 1 is a schematic front view of a slot antenna formed as an embodiment of the invention;

Figure 2 is a front view of a slot antenna of alternative form to that of

Figure 1 ; sometimes referred to as a notch antenna,

Figure 3 is a schematic perspective view of the slot antenna system of

Figure 2 seen from one side,

Figure 4 is a perspective view of the embodiment of the invention of Figure 3 seen from the opposite side,

Figure 5 is a perspective view of an alternative embodiment of the invention,

Figure 6 is a cross section through an embodiment of the invention having a screening enclosure,

Figure 7 a front view of a further embodiment of the invention,

Figure 8 illustrates the mounting of a slot antenna in a motor vehicle body panel, and

Figure 9 illustrates the mounting of a slot antenna in a wind deflector attached

to the roof of the driver's cab of an articulated vehicle

Referring to Figure 1, an antenna for producing a vertically polarised electromagnetic

wave consists of a parallel sided slot 2 cut into a sheet 3 of copper or other material of high electrical conductivity The length of the slot 2 is equal to approximately half the wavelength of the electromagnetic radiation with which the antenna is to be used Preferably the radiation has a nominal transmission frequency of 450 MHz The antenna 1 is fed by a coaxial cable 4 the inner conductor 5 of which is connected to one

longitudinal edge of the slot 2 at connection point 7 The screen 6 of the coaxial cable

4 is connected to the other longitudinal edge at a point of connection 8 opposite that of the inner conductor 5 of the coaxial cable 4 The connection of the screen of the coaxial cable 4 to the edge at point 8 of the slot 2 can be made with or without a balun,

as deemed desirable in a particular case

According to the invention the impedance of such an antenna 1 may be matched to that

of the coaxial cable 4 by means which will be described in more detail in relation to Figures 3 to 9

The standard surface mounted monopole antenna currently in use has no DC connection between the central conductor and screen of the supply cable to the

antenna The antenna construction described above, however, presents negligible DC

resistance between the inner conductor 5 and the screen of the coaxial cable 4 In order to make the antenna of the present invention interchangeable with a conventional monopole antenna, a small fixed capacitor 9, of negligible impedance at the frequency of the radiation used, is included in the connection between one of the conductors, for

example the central conductor 5 of the cable 4 and the slot 2.

One practical way in which a parallel sided slot may be formed is by removing material between two circular openings. This ensures that there are no sharp discontinuities at

which cracks may initiate. This results in a parallel sided slot of dumbbell shape (not shown). This will reduce slightly the resonant frequency of the slot, so obtaining a more compact antenna system for a given radiation wave length. Alternatively, if a

parallel sided slot is used this can be of a folded configuration (not shown). In this case, the connections between the coaxial cable 4 and slot are made at or near the point of the fold in the slot.

Referring now to Figure 2, there is shown a second embodiment in which the slot

region of the panel is formed as a notch 30 in one edge 31 of a conductive panel 32.

In this case, the notch 30 has a length of approximately one quarter of the wavelength of the radiation with which the antenna is intended to be used. As before, the antenna

is fed by a coaxial cable 33, with connections to the approximate centres of the

longitudinal edges 34 and 35 of the notch 30 opposite each other.

Figures 3 and 4 illustrate the practical structure of a slot antenna system such as that

illustrated in Figure 2 in more detail. In these drawings the conductive panel 32 is

shown secured in face-to-face relationship with a dielectric panel 72 to which it is secured in any convenient manner, such as by adhesive or by individual fixing elements

(not shown). Two holes 73, 74 are formed in the longitudinal edges 34, 35 of the slot 30 by which to make electrical connection with two insulated leads 75, 76 respectively one of which is connected to a capacitor 9 connected to the inner core 5 of the coaxial

cable 33 which constitutes the feed line to the slot 30. The feed line 33 terminates closely adjacent the closed end 77 of the slot 30, on the side of the insulating sheet 72 remote from the conductor panel 32 and the two insulated leads 75, 76 lie substantially parallel to the length of the slot 30, diverging slightly to pass through the openings 73, 74 allowing the bared ends of these wires to be connected, for example by solder, to

the conductive panel 32 at the positions adjacent the edges 34, 35 illustrated in Figure

3.

It is important that the material chosen for the dielectric panel 72 has a low power

factor at the operating frequency. This configuration of connections ensures that the

antenna operates with a non-resonant feed line matched to the antenna impedance. Provided that this condition is met, there will be no standing wave pattern in the

coaxial feed cable 33 and consequently the matching will be independent of the length of this feed cable. In order to ensure that the leads 75, 76 remain in the selected

position on the dielectric 72 they can be held in place by a suitable adhesive and,

likewise, a suitable adhesive may be used to secure the coaxial cable 33 to the insulating panel 72. The divergent leads 75, 76 which pass from the end of the coaxial

cable to the anchorage holes 73, 74 for the connections to the slot 30 are thus linked

to the magnetic field in a portion of the slot which, by adjusting the precise paths of the

leads 75, 76 varies the area of the approximately triangular region between them (shown cross hatched in Figure 4 and identified with the reference numeral 78). It is

thus possible to vary the magnetic linkage in order to obtain the optimum impedance match to the antenna feed cable. Once this position has been established repeated production can be achieved utilising the same dimensions, for which purpose the blank

dielectric panels 72 can be suitably marked to enable operatives correctly to position the wires. In an automated system, of course, appropriate jigs or the like will be used to ensure that the leads 75, 76 are correctly positioned in relation to the holes 73, 74 and thus the slot 30.

In the alternative embodiment shown in figure 5 the same reference numerals have been used to identify the same or corresponding components to those in Figures 3 and 4.

Here it is assumed that the coaxial cable 33 and the leads 75, 76 cannot for some reason be positioned in contact with the dielectric panel 72 as in the embodiment of

Figures 3 and 4. An appropriate impedance matching can, nevertheless, be achieved

by positioning an element 79 of aluminum foil spanning the slot 30 on the face of the

dielectric 72 remote from the conductive panel 32. Again, at the design stage the

precise dimensions and location of the foil element 79 will be adjusted until the

optimum impedance match is achieved, and thereafter production can follow with the

same dimensions.

An antenna of the type described above will radiate equally on both sides of the sheet

of conductive material 32. In general, this is undesirable and in order to confine the emission of radiation to one side of the panel 32 only, a screening arrangement in the

form of a conducting enclosure 10 encompassing the slot 2 can be attached to one side of the panel 3 as shown in Figure 6. If this is positioned at about one quarter of the wavelength of the radiation signal, as shown in Figure 6, the screen will act as a reflector, which augments the signal strength in the desired direction The screen 10

is preferably of greater dimensions than the conductive panel 32

There are several possible locations on a vehicle for an antenna such as that described above, at which the antenna will be invisible, at least without a close inspection of the vehicle. For example, providing the radiator grille is not metallic the antenna 1 can be

mounted behind the radiator grille This also has benefits because the metal radiator,

even though not a plane sheet, acts as a reflector if the spacing is correct as mentioned above. Another possible position is behind a number plate, again with the proviso that the construction of the number plate should be of a non-metallic type

Alternatively, the antenna may be mounted in or directly behind an aperture formed by

cutting a body panel of the vehicle, the aperture being filled with non-metallic filler of the type usually used for vehicle body repairs and painted to match the remainder of

the vehicle. If the antenna is installed during the construction of the vehicle, then the

panel may be one of the wing or door panels, again, with the slot 2 filled and painted

to match the remainder of the vehicle.

Figure 8 shows diagrammatically a slot antenna 51, 52 mounted in a panel 50 of a motor vehicle (only part of which is illustrated)

Large articulated commercial vehicles often have a wind deflector mounted on the roof of the driver's cab of the vehicle. These deflectors usually are made of glass fibre reinforced plastics materials (GRP) and have a front surface panel which is supported inclined at an angle to the vertical by means of triangular side panels The form of antenna described with reference to Figure 2 is particularly suitable for incorporation

in such a wind deflector and such a mounting is shown schematically in Figure 9

If there are no components within such wind deflectors which would significantly alternate the signal, the screening/reflector arrangement described in relation to the first

embodiment of the invention can be omitted, enabling the antenna to radiate to either

side of the vehicle Alternatively, two directional antennae may be used, one mounted on each side of the wind deflector

The antenna described above has the disadvantage that it is fairly large For example, for use with radiation of a frequency 450 MHz, the slot 2 of the first embodiment of

the invention needs to be approximately 33 cms long, and the notch 30 in the second

embodiment of the invention needs to be made approximately 17cms long The

antenna shown in Figure 7 is intended for use in a confined space such as a front or

rear light cluster of the vehicle It is, in effect, a miniaturised version of that described

with reference to Figure 2 In this case, the length of the notch 40 is much smaller in relation to the wave length of the radiation used, for example, it may be as small as

l/25th of the wave length of the radiation In order to make the antenna resonant at the radiation transmission frequency, a trimmer capacitor 41 may be connected across

the free-ends 42, 43 of the notch 40 and adjusted as required to establish a resonant

condition in the antenna Connections are made to the antenna in the same manner as previously described.

The antennae described above have been considered in transmission mode only. They are, of course, equally suited for use with a transceiver unit, so permitting two-way radio communication between a vehicle and a base station.

Provided that the necessary balanced interconnections are made, more than one antenna can be used disposed around the vehicle, so as to give a quasi-omnidirectional

coverage. This arrangement also has the advantage that if one of the antennae is damaged or screened, either deliberately or accidentally, the system as a whole will continue to operate.

In the embodiment of Figure 8 a slot type antenna 51, 52 such as that described with reference to Figures 1 and 2 is incorporated as an integral part of a front wing 50 of

a motor vehicle. The radiating slot 52 of the antenna is filled with polyester body filler

material which is trimmed and painted to match the rest of the front wing so as to render the antenna invisible.

In Figure 9 the wind deflector 61 which is mounted on the roof of the cab 62 of an

articulated commercial vehicle has a front surface 63 which is inclined at approximately

45° to the horizontal and two substantially vertical side panels 64. The structure is

made of glass fibre reinforced plastics material (GRP) and is therefore substantially

transparent to electromagnetic radiation. Mounted behind at least one of the side

panels 64 is an antenna 65 of the type described in relation to Figure 2. As there are usually no components within the wind deflector 61 which would attenuate the signal there is no need for a screening enclosure such as that of Figure 6, which is

incorporated in the antenna described with reference to Figures 1 to 2, so giving bidirectional transmission (or reception). If the wind deflector itself attenuates the

signal two antennae, one on each side, may be provided. If two antennae are used then it is necessary for there to be balanced interconnections between them. In fact, it is preferable for these to be two antenna on opposite sides or ends of the vehicle to give quasi-omnidirectional radiation patterns.

In another embodiment (not shown) the antenna may be formed as a fin of conductive

material with a slot in one edge and a transverse flange at its foot acting as a ground plane and an attachment plate for fixing, for example, to the roof panel of a motor vehicle having a non-magnetic superstructure such as a plastics sign.

Claims

1. A slot antenna system for use in a vehicle radio communication system,

comprising an electrically conductive panel adapted to be carried by or to form part of the structure of the vehicle, the panel having an elongate slot in it the effective length of which is an integral sub-multiple of the wavelength of the radiation with which the antenna system is to be used, a coaxial feed cable the central conductor of which is connected to one longitudinal edge of the slot and the screen of which is connected to

the other longitudinal edge of the slot opposite the first connection, characterised in that the feed cable connection to the slot is made in such a way that the feed line is non-resonant at the operating frequency and is matched to the impedance of the antenna independently of the length of the feed cable.

2. An antenna system according to Claim 1, characterised in that the slot in the

panel is in the form of a parallel sided slot, a dumb bell slot, or a rectangular notch cut

into an edge of the panel.

3. An antenna system according to Claim 1 or Claim 2, characterised in that the

feed cable is connected to edges of the slot by insulated leads which lie in the magnetic field pattern of the slot.

4. An antenna system according to Claim 1 or Claim 2 characterised in that there

is an electrically conductive non-magnetic element spanning the slot and insulated from

the said electrically conductive panel in which the slot is formed.

5. An antenna system according to any preceding claim, characterised in that the

said electrically conductive panel is attached to an insulating panel through which the feed cable connections pass in forming the connections to the edges of the slot.

6. An antenna system according to Claims 3 and Claim 5, characterised in that the said insulated leads are secured to the said insulating panel on the side thereof remote from the said slot and lying generally parallel thereto.

7. An antenna system according to Claim 4 and Claim 5, characterised in that the electrically conductive non-magnetic element is aluminum foil attached to the said insulating panel on the side thereof remote from the said electrically conductive panel.

8. An antenna system according to any of Claims 1 to 7, characterised in that

there is an electrically conductive screen element located closely adjacent one side of

the conductive panel so as to screen the antenna on that side of the conductive panel.

9. An antenna system according to Claim 8, characterised in that the spacing between

the screen and the conductive panel is of the order of one quarter of the

wavelength of the signal radiated by the antenna such that the screen acts as a

reflector.

10. An antenna system according to any of Claims 1 to 9, characterised in that it

is adapted to be mounted behind a non-conductive surface panel of a vehicle.

11. An antenna system according to any of Claims 1 to 4 10 characterised in that the conductive panel is in itself adapted to form at least part of a panel of a vehicle the

slot region of the panel being filled with a solid non-conductive material at least

substantially flush with the surface of the panel.

12. An antenna system according to any preceding claim, characterised in that the surface panel of the vehicle is part of the wind deflector adapted to form part of or be mounted on the roof the cab of a vehicle.

13. An antenna system according to any preceding claim, characterised in that the

said conductive panel is formed as a fin mounted orthogonally with respect to a ground

plane by one edge thereof.