AU776598B2 - Radio antennas - Google Patents

Description

1 1

AUSTRALIA

Patents Act 1990 Standard Communications Pty Ltd

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention Title: Radio antennas The following statement is a full description of this invention including the best method of performing it known to us:- Title Radio Antennas Technical Field This invention concerns radio antennas. In particular, but not exclusively, it concerns the structure of antennas intended to be mounted on moving vehicles. The antennas may also be used in fixed applications.

Background Art The mechanical shape of an omnidirectional antenna with vertical radiation is a long vertical rod. A conventional drive element for antennas with gains in the range 0-5dBi is a coaxial dipole. However, this antenna exhibits very poor rejection of currents flowing over the outside of the feeder "coaxial cable when the diameter of the coaxial dipole is small and approaches 15 that of the feeder cable. The effect of these spurious feeder currents is to adversely affect the radiation pattern, and to make the return loss measurement unstable and dependent on mounting arrangement.

S"Summary of the Invention The invention is an antenna assembly for mounting on vehicles, including: A mounting bracket having at its upper end a first mechanical connector and a first coaxial connector for mechanical and electrical connection respectively to an aerial. At its lower end it has a second 25 mechanical connector for engagement with an aperture in a vehicle's bodywork. The second mechanical connector has a passageway through it to receive a coaxial cable from the interior of the vehicle and the mounting bracket is hollow so a received coaxial cable is able to pass through it to the first coaxial connector; and the aerial having at its lower end a third mechanical connector to attach to the first mechanical connector of the mounting bracket, and a second coaxial connector to make electrical connection with the first coaxial connector, where a metallic sleeve contiguous with the third mechanical connector and in electrical connection with the second coaxial connector is configured to form an earthed coaxial outer and a quarterwave choke to eliminate spurious currents.

Such antennas may operate in the UHF radio spectrum. A range of antennas may be provided with varying gains and physical sizes. The antennas may have an omnidirectional radiation pattern in the H plane.

They may be ground independent. The aerial may be readily detachable, so that different physical lengths may be interchanged on the same mounting bracket. The antennas may be rugged and suitable for off-road conditions on a 4WD vehicle. The antennas may exhibit approximate gains from 2 to 7 dBi, provide a return loss of greater than 10dB and exhibit negligible spurious radiation current. Antennas with lower gains (2-5dB) may be included to allow for those applications where a vehicle can move through a large angle in the vertical plane, without exceeding the beam width of the antenna.

Each antenna may be economical to manufacture, without excessive physical complexity, so that market acceptance and reliability are enhanced.

The first mechanical connector may be threaded with 1/2" BSW, and have a crimped or screw cable entry. The thread strength may be modified to suit the physical size of the antenna.

The second mechanical connector may include an internal and an external cap having threadedly interengageable parts by which they are able to be connected together on either side of the aperture. When so connected the internal cap is inside the vehicle and the external cap is outside the vehicle, and together they define an enclosed passageway for the coaxial cable to pass from the interior of the vehicle to the interior of the mounting :.:.:bracket. Since the cable passes through the centre of the spring or bracket, it 1is protected from physical damage.

S: 25 The mounting bracket may include a coil spring extending between the first and second mechanical connectors.

The coaxial connectors may have a S0239/PL259 interface, but any suitable alternative connector could be used, for example N, TNC, MINI-UHF.

The third mechanical connector, for small antennas, may use a thread cut on the standard PL259 connector to mate with a thread on the first mechanical connector. On larger antennas the threaded portion may be extended so that it bears on a top flange of the mounting bracket, or on the spring. The largest antennas may use a separate coarse threaded section to mate with the first mechanical connector.

The entire S0239 socket may still be present in the first mechanical connector whatever its size, so that a smaller antenna can be mounted to the base assembly. In general different aerials may be interchanged to fit different mounting brackets.

This assembly canll then be used with varying sized springs.

For an antenna gain of 2dBi, a coaxial dipole element may be used. For an antenna gain of approximately 4.5dBi, a coaxial sleeve with a collinear element may be used.

In either case, to achieve a thin coaxial sleeve element without 10 spurious feed currents, a quarterwave choke is necessary. This is configured so that it serves both as a quarterwave choke presenting a high impedance to spurious currents directly at the base of the dipole, and as the earthed outer of the PL259 coaxial connector.

Higher gain versions use a coaxial collinear configuration. The same 15 approach with a quarterwave choke is used to eliminate spurious feed currents.

Brief Description of the Drawings Examples of the invention will now be described with reference to the 20 accompanying drawings, in which: Fig. 1 is an elevation of a first mounting bracket.

Fig. 2 is an elevation of a first aerial.

Fig. 3 is an elevation of a second mounting bracket.

Fig. 4 is an elevation of a second aerial.

Fig. 5 is an elevation of a third mounting bracket.

Fig. 6 is an elevation of a third aerial.

The same reference numerals have been used throughout the drawings to refer to corresponding features.

Best Modes of the Invention Referring first to Figure 1, mounting bracket 1 comprises a standard coaxial connector 2 at its upper end. The bottom part of the housing of coaxial connector 2 is threaded 3 to engage with the internal thread of an end cap 4. End cap 4 is in turn captured in the upper end of coil spring At the lower end of spring 5 a second mechanical connector 6 is inserted into the spring. A passageway 7 passes vertically through connector 6 to enable coaxial cable 8 to pass through the connector 6 and the interior of the spring 5 to the coaxial connector 2 at the top.

The second mechanical connector 6 comprises an external cap 9 and an internal cap 10. The exterior of internal cap 10 is threaded at 11 to engage with an interior thread on external cap 9. In use caps 9 and 10 are assembled together with a spring washer 12 and the panel of a vehicle (not showing) clamped between them. In order to do this the internal cap 10 is pushed up through a hole in the vehicle panel until the washer 12 is captured against the underside of the hole by cap 10. The spigot on which the thread 11 is cut then extends through the hole so that external cap 9 can be screwed down on top of it.

This arrangement has the advantage that a coaxial cable 8 can be 15 pushed up through passage 7 into the interior of spring 5 and connected to the coaxial connector 2 at the top. The coaxial cable is then protected inside ~the interior of the mounting bracket along the entire length that extends out of the vehicle.

Turning now to Figure 2, an aerial 20 is shown to have a third mechanical connector 21 at its lower end. This mechanical connector 21 contains a coaxial connector 22 inside it. It also has a threaded interior 23 which can be screwed on to the threaded exterior of coaxial connector 2 of bracket 1. When this occurs the coaxial connectors 2 and 22 form a coaxial engagement.

25 The aerial itself comprises a hollow fibreglass rod 24 in the centre of l .which rises a solid fibreglass rod 25 containing the central conductor 26. The outer conductor of the coaxial cable 27 extends outside the solid fibreglass rod 25 and is held in place with sticky patches 28. After the central conductor 26 emerges from the end of the solid fibreglass rod 25 it is soldered at 29 to a conductive coil 30, and the upper end of the coil terminates in a further straight section of conductor 31 collinear with conductor 26. Two foam tape blocks 32 surround collinear conductor 26 and terminating conductor 31 in order to space them from the hollow fibreglass tube 24. The distal end of conductor 31 is embedded in epoxy 33 and an end cap 34 is pressed over the top of hollow fibreglass tube 24.

The upper end of solid fibreglass rod 25 is surrounded by a length of alumininium pipe 35. The interior of aluminium pipe 35 is spaced apart from solid fibreglass rod 25 by'O' rings indicated at 36. A collet 37 is positioned at the upper end of aluminium pipe 35 and the outer conductive braid of the coaxial is clamped between the collet 37 and the pipe 35. Foam tape 38 encircles the upper end of the aluminium pipe in order to secure it against the interior of the hollow rod 24. The aluminium pipe acts as a quarterwave small diameter earth sleeve.

A further aluminium bushing 39 is positioned around the lower end of the aerial and is joined to the coaxial connector so that it is in electrical communication with the outer braid conductor to create a quarterwave inverse choke and coaxial outer.

Turning now to Figure 3, a slightly larger mounting bracket 1' is similar •to the bracket 1 of Figure 1 except that it has a larger upper mechanical connector a larger spring 5' and a larger external cap 9' for the second mechanical connector 6'.

Turning now to Figure 4, a slightly larger aerial 20' is shown than is shown in Figure 2. A hollow fibreglass tube 24' in this case is both longer and greater in diameter than the smaller aerial. The third mechanical connector 21' at the end of the aerial 20' has thicker metal covering and an extension part 40 for stress relief. The external aluminium bushing 39' is also bigger and thicker, but also provides quarterwave inverse choke and a coaxial outer. The outer coaxial braid conductor extends in this case all the way to the end of the antenna passing through the coil 30 and alongside the tip 31 and into the epoxy 33. The tape and sticky patch arrangements are slightly different in this case, but otherwise the structure is the same.

Figure 5 shows an even larger mounting bracket This bracket is similar to the bracket of Figures 1 and 3 but has an even larger spring and a larger upper mechanical connector The upper mechanical connector includes an upper extension 11 which has an internal thread 12 with which to engage a large externally threaded aerial end.

Figure 6 illustrates a larger aerial 20" similar in construction to aerial as shown in Figure 4 but including an even more massive third mechanical connector 21" which has an external thread 44. The external aluminium bushing 39" is also more massive, but still provides quarterwave inverse choke and a coaxial outer. A rubber plug 45 is used to locate the aluminium tube The coaxial connector 2 used in all the embodiments of the mounting bracket is a standard female connector, whereas the coaxial connector 22 used in the aerials is a standard male connector. It follows that all of the aerials can be fitted to all of the mounting brackets, although in practice this is not desirable, but a smaller aerial can be placed in a larger mounting bracket than is required with no disadvantage.

Where the invention has been described with reference to particular embodiments, it should be appreciated that it may be embodied in other forms. In particular, where the mechanical connectors have been shown as included threaded into engagements they could engage in different ways, for instance with bayonet fittings.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

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Claims (14)

1. An antenna assembly for mounting on vehicles, comprising: a mounting bracket having at its upper end a first mechanical connector and a first coaxial connector for mechanical and electrical connection respectively to an aerial, at its lower end it has a second mechanical connector for engagement with an aperture in a vehicle's bodywork, where the second mechanical connector has a passageway through it to receive a coaxial cable from the interior of the vehicle and the mounting bracket is hollow so a received coaxial cable is able to pass through it to the first coaxial connector; and the aerial having at its lower end a third mechanical connector to attach to the first mechanical connector of the mounting bracket, and a second coaxial connector to make electrical connection with the first coaxial connector, where a metallic sleeve contiguous with the third mechanical 15 connector and in electrical connection with the second coaxial connector is configured to form an earthed coaxial outer and a quarterwave choke to eliminate spurious currents.

An antenna assembly according claim 1, operating in the UHF radio S"spectrum.

3. An antenna assembly according to claim 1 or 2, having an S-omnidirectional radiation pattern in the H plane.

4. An antenna assembly according to claim 1, 2 or 3, being ground independent.

5. An antenna assembly according to any preceding claim, in combination 25 with a detachable aerial.

6. An antenna assembly according to any preceding claim, where the first mechanical connector is threaded, and has a crimped or screw cable entry, the thread strength is modified to suit the physical size of the antenna.

7. An antenna assembly according to any preceding claim, where the second mechanical connector includes an internal and an external cap having threadedly interengageable parts by which they are able to be connected together on either side of the aperture.

8. An antenna assembly according to claim 10, where, when the internal and external caps are so connected, the internal cap is inside the vehicle and the external cap is outside the vehicle, and together they define an enclosed 9 passageway for the coaxial cable to pass from the interior of the vehicle to the interior of the mounting bracket.

9. An antenna assembly according to any preceding claim, where the mounting bracket includes a coil spring extending between the first and second mechanical connectors.

An antenna assembly according to any preceding claim, where the coaxial connectors have a S0239/PL259 interface.

11. An antenna assembly according to claim 2, where the third mechanical connector uses a thread cut on the standard PL259 connector to mate with a thread on the first mechanical connector.

12. An antenna assembly according to claim 1, where the third mechanical connector uses a thread to mate with a thread on the first mechanical connector and the thread is extended so that it bears on a top flange of the mounting bracket, or on the spring.

13. An antenna assembly according to claim 1, further comprising a coarse threaded section to mate with the first mechanical connector.

14. An antenna assembly according to claim 1, further comprising a coaxial dipole element for an antenna gain of 2dBi. An antenna assembly according to claim 1, further comprising a coaxial sleeve with a collinear element for an antenna gain of r 16. An antenna assembly substantially as herein described with reference to the accompanying drawings. :I Dated this twenty-one day of June 2004 Standard Communications Pty Ltd Patent Attorneys for the Applicant: F B RICE CO