GB2192757A

GB2192757A - Antenna

Info

Publication number: GB2192757A
Application number: GB08617541A
Authority: GB
Inventors: Michael John Charter
Original assignee: Antiference Ltd
Current assignee: Antiference Ltd
Priority date: 1986-07-17
Filing date: 1986-07-17
Publication date: 1988-01-20
Anticipated expiration: 2006-07-17
Also published as: GB2192757B; GB8617541D0

Abstract

An antenna assembly 1 comprises a mounting member 2 and an antenna member 9. The antenna member 9 includes an elongate antenna conductor 11 and is detachably securable to the mounting member by way of a spring loaded attachment arrangement. The assembly includes an inductive loading coil 13 in series with the antenna conductor 11 and this loading coil comprises a spring which provides the spring loading for the attachment arrangement. <IMAGE>

Description

SPECIFICATION Antenna This invention relates to radio receiver and/or transmitter antennas, and particularly, but not exclusively to an antenna to be mounted on the body of a vehicle.

For optimum signal reception, the length of a straight rod antenna should be approximately one quarter of the wave length of the received signal (A/4).

A A/4 receiving antenna mounted on a vehicle and coupled to a receiver of which the imput impedance is of the order of 100 to 120Q provides adequate performance at 88 to 108 MHz without the need for any matching impedance. Due to favourable propagation factors this means that there is sufficient signal present for satisfactory performance at LW and MW. However in order to be suitable also for- FM reception a 2/4 antenna needs to be 700 to 800 mm long; this is excessive in that an antenna of this length has low mechanical strength and is unstable in the air flow created at high vehicle speeds.

Accordingly, it is desired to use a short antenna less than A/4 in length. Such an antenna has low resistance and high capacitative reactance, and without any matching network the strength of the signal transferred from the antenna to the receiver input at FM frequencies is insufficient. It is known to compensate this capacitative reactance of a short antenna by including an inductive loading coil in the antenna system. Such coil may be inserted at the base of, or part way up the antenna, and it enables a greater proportion of the received signal to be transferred to the receiver.

A compromise has to be made between the antenna length and the coil size, and it has been found that an antenna length of between 0.11 A and 0.15 A gives reasonabie signal level at FM, MW and LW and uses a practical size coil.

Another problem with simple non-retractable vehicle-mounted rod antennas is that they have to be made detachable from the vehicle so as to avoid damage, for example during washing in automatic washers.

The need to provide not only the loading coil but also the arrangement to provide the facility for detaching the antenna tends to complicate the overall structure, thereby reducing the cost advantage in this type of antenna.

The present invention aims to alleviate this problem, and to that end provides an antenna assembly comprising a mounting member and an antenna member which includes an elongate antenna conductor and is detachably secured or securable to the mounting member by way of a spring-loaded attachment arrangement, the assembly including an inductive loading coil in series with the antenna conductor, said loading coil comprising a spring which provides the spring loading for said attachment arrangement.

The attachment arrangement is preferably a bayonet fitting in which a stud member which is provided with a plurality of lateral spigots is insertable against the bias of the spring into a collar formed with shaped slots for receiving the spigots.

The loading coil which acts as the spring is preferably included in a portion of the antenna member. In the disclosed embodiment, the antenna member has a base portion which comprises a non-conductive cylindrical sleeve which accommodates the loading coil, this coil being in the form of a helical spring, the antenna conductor projecting from one end of the sleeve, and the other end of the sleeve accommodating a part of the attachment arrangement. In the case of a bayonet fitting, this part is the slotted collar.

The mounting member is adapted to be mounted on a vehicle body, and for that purpose includes first and second inter-engagable parts to be positioned on opposite sides of a vehicle panel with a coupling element, which may be integrally formed with one or other of the first and second parts, extending through an aperture in the panel. An electrical cable is coupled to the inner part of the mounting member and is electrically coupled through the mounting member with the antenna member.

The antenna member preferably attaches to a pivotable component of the mounting member so that the orientation of the antenna member can be adjusted for optimum reception.

A preferred embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: Fig 1 is an elevational view showing the component parts of an antenna assembly according to the present invention; and Fig 2 is a part-sectional view of the indicated portion of the assembly of fig 1.

In the antenna assembly 1 shown in the drawings a mounting base 2 has upper and lower parts 3 and 4 which are adapted to be positioned on opposite sides of a panel 5 of a vehicle body (e.g. the vehicle roof), and to be fixed together, e.g. by means of a bolt which projects from the lower part 4 outwardly of the body through an aperture in the panel 5, and a nut which threadedly engages the bolt and is accommodated within the upper part 3.

The lower part of the mounting member is adapted to connect the braiding of a coaxial cable 6 to the vehicle body 5, and to provide an electrical connection by way of the bolt between the centre conductor 7 of the cable 8 and a conductive element 8 of the upper part 3. The conductive element 8 of the mounting base is electrically insulated from the vehicle body 5 by an insulating washer 23.

An antenna member 9 is detachably secura ble to a conductive pivot arm 10 which is pivotably connected to the conductive element 8. The antenna member 9 comprises an elongate conductor 11 which projects axially from one end of a generally cylindrical non-conductive sleeve 12. As shown in greater detail in figure 2, the sleeve 12 accommodates a helical spring 13. This spring, which may for example be made of copper or galvanised steel, has at its upper end a plurality of turns which are tightly wound on, and therefore in electrical contact with an enlarged lower end 14 of the conductor 11. The conductor 11 is axially fixed by means of a blob of adhesive 15 at the central aperture formed in a generally conical upper end 18 of the sleeve 12.

As described earlier, this spring 13 provides an inductive loading coil which, when the antenna member 9 is attached to the pivot arm 10 is electrically in series between the conductor 11 and the central conductor of the coaxial cable 6. This spring 13 also provides the spring loading for a bayonet fitting which is illustrated most clearly in figure 2. The pivot arm 10 is formed with a stud member 17 having a pair of diametrically opposed, laterally projecting spigots 18, and this stud is insertable into a collar 19 which is fixed in the base of the sleeve 12, and which is made of electrically conductive material. This collar 19 is formed with a pair of diametrically opposite Lshaped slots 20 for receiving the respective spigots 18.A conductive disc or washer 21 is positioned adjacent the upper end of the collar 19 and is pressed against the collar by the spring 13 which is in a compressed state within the sleeve 12.

In figs 1 and 2 the antenna member is shown detached from the mounting base, and it will be appreciated that to attach the antenna member, the user positions the collar 19 over the end of the stud 17 and twists it while pushing it downwardly, so as to cause the spigots 18 to enter the lower open ends of the slots 20.

As the sleeve is pushed axially onto the stud, the upper end of the stud comes into contact with the disc or washer 21 before the spigots 18 reach the point at which the sleeve can be twisted further, and the antenna member must then be pushed with some force onto the stud against the bias of the spring 13 in order to enable the spigots 18 t6 reach the inner ends of the long limbs 20' of the slots 20. When this point is reached, the sleeve 12 can be rotated slightly, and the pressure of the spring 13 can be allowed to urge the antenna member upwardly to cause the spigots 18 to seat at the closed inner ends of the slots 20, as illustrated at 22.

The spring 13, collar 19 and stud 17 therefore provide a quick-release spring-loaded attachment arrangement between the antenna member 9 and the base support 2. The manner in which the antenna member can be quickly detached from the mounting base will clearly be understood, and does not need to be described in detail.

The component 13 therefore both acts as the inductive loading coil to compensate for the capacitive reactance in the antenna assembly, and also provides the spring loading for the attachment arrangement.

In a modified version (not shown) of the abovedescribed embodiment, the bayonet slots in the collar 19 are formed oblique to the access so that the user employs a twisting action, rather than a pushing action when attaching the antenna member to the mounting base.

The described arrangement therefore provides an antenna assembly which is both simple and cheaply manufactured, and also has the facility for quick detachment and re-attachment of the antenna member when necessary.

Claims

1. An antenna assembly comprising a mounting member and an antenna member which includes an elongate antenna conductor and is detachably secured or securable to the mounting member by way of a spring-loaded attachment arrangement, the assembly including an inductive loading coil in series with the antenna conductor, said loading coil comprising a spring which provides the spring-loading for said attachment arrangement.

2. An assembly as claimed in claim 1, wherein said attachment arrangement comprises a male member insertable within a female member against the bias of said spring.

3. An assembly as claimed in claim 1, wherein said attachment arrangement comprises a bayonet fitting in which a stud member which-is provided with a plurality of lateral spigots is insertable against the bias of the spring into a collar formed with shaped slots for receiving the spigots.

4. An assembly as claimed in any one of the preceding claims, wherein said spring is included in a portion of the antenna member.

5. An assembly as claimed in claim 4, wherein the antenna member has a base portion which comprises a non-conductive cylindrical sleeve, said sleeve accommodating the loading coil which is in the form of a helical spring, the antenna conductor projecting from one end of said sleeve and the other end of said sleeve accommodating a part of the attachment arrangement.

6. An assembly as claimed in claim 5 when claim 4 is appended to claim 3, wherein said part comprises the slotted collar of said bayonet fitting.

7. An assembly as claimed in any one of the preceding claims, wherein said mounting member is adapted to be mounted on a vehicle body and comprises first and second inter-engagable parts adapted to be positioned on opposite sides of a vehicle panel with a coupling element extending through an aperture in said panel.

8. An assembly as claimed in claim 7, wherein said coupling element is integrally formed with one of said first and second parts.

9. An assembly as claimed in claim 7 or 8, wherein the coupling element provides an electrical connection through the mounting member with the antenna member.

10. An assembly as claimed in claim 7, 8 or 9, wherein an electrical cable is coupled to the part of the mounting member which is disposed on the side of the vehicle panel in use remote from the antenna member and is electrically coupled through the mounting member with the antenna member.

11. An assembly as claimed in any one of the preceding claims, wherein the antenna member attaches to a pivotable component of the mounting member so that the orientation of the antenna member can be adjusted.

12. An antenna assembly as claimed in claim 1 and substantially as herein described.

13. An antenna assembly substantially as herein described with reference to the accompanying drawings.