GB2272575A

GB2272575A - Dual band antenna

Info

Publication number: GB2272575A
Application number: GB9222932A
Authority: GB
Inventors: John Welsh
Original assignee: GEC Ferranti Defence Systems Ltd; GEC Marconi Avionics Holdings Ltd
Current assignee: Leonardo MW Ltd
Priority date: 1992-11-02
Filing date: 1992-11-02
Publication date: 1994-05-18
Anticipated expiration: 2012-11-02
Also published as: GB9222932D0; GB2272575B

Abstract

A dual antenna for microwave and lower band frequencies. A microwave patch antenna 2 is directed vertically upward for communication with satellite or aircraft and a monopole, whip antenna 1 is arranged for omnidirectional communication in the azimuth plane. In one embodiment, for microwave and VHF, the monopole antenna is constituted by the outer conductor 9 of a coaxial feed to the patch, the VHF and microwave signals being mutually isolated by their respective skin effects inside and outside the coaxial outer conductor. In another embodiment, the monopole (25, fig 3) is an extension of a coaxial line inner conductor, the outer of which penetrates the patch antenna ground plane (33) and patch (29) and shorts the two together at a voltage node. The patch is fed separately by conventional coaxial feed. <IMAGE>

Description

Dual Antenna Arrangement The present invention relates to radio frequency antennas.

There is frequently the need to transmit and/or receive separately or simultaneously on several unrelated frequencies with minimum mutual interference between the radio channels. In those applications where the frequencies are required to be similarly polarised and/or their radiation coverage patterns are fairly similar, there are several well known means of connecting one or more conductors comprising the antenna system to achieve high antenna effeciency at the required several wavebands.

In applications where the polarisation and the coverage pattern required at the different frequencies are very dissimilar, it has hitherto been necessary to employ a number of separate antennas and, if these are mounted in close proximity, to accept the undesirable coupling and interference between them.

A particularly severe case occurs when a ground based vehicle is required to communicate with a vehicle overhead, (e.g.

aircraft or satellite) and therefore with predominantly horizontal polarisation, (i.e. North-South or East-West or any combination of these including circular polarisation), while at the same time wishing to communicate with another ground base of initially unknown bearing.

One proposed solution to this requirement has been to employ two separate antennas: a broad beamwidth horizontally or circularly polarised antenna with its main beam pointing roughly vertically upwards, such as a "patch" antenna, to communicate with the satellite, (often on a microwave frequency), and a vertical monopole or "whip" aerial giving omnidirectional coverage with vertical polarisation in the azimuth plane with virtually no radiation vertically upwards at some other, (usually lower) frequency to communicate with the other ground based station. While this ground-to-satellite and ground-to-ground situation is perhaps the most likely application, it will be clear that other scenarios are catered for equally, eg air-to-satellite and air-to-air, ai-r-to-air and air-to-ground.

Thus any situation where one station has to communicate by radio with two others in significantly different and preferably orthogonal directions, will lend itself to application of the invention.

For maximum effectiveness in the above ground-satellite ground-ground situation, both antennas require to be mounted on a roughly horizontal electrically conducting "ground plane", as high as conveniently possible to give, as near as possible, an unobstructed line of sight to their respective communicating stations. In practice typical ground planes comprise: motor car roof, tank turret etc.

During transmission by either antenna, when these are in close proximity, some of the transmitted signal is unintentionally received by the other, which can result in interference and/or blocking of the other frequency.

An object of the invention is to combine two such antennas without increasing the coupling between them significantly and, at the same time, achieve one or more of the advantages of space saving, avoidance of vulnerability, improved appearance and simplified installation.

According to one aspect of the invention, a dual antenna arrangement comprises a patch antenna and a monopole antenna, the monopole antenna having maximum gain in multiple directions in a plane, and the patch antenna having maximum gain along an axis normal to said plane, the monopole antenna incorporating a coaxial waveguide the outer conductor of which is directly connected to a ground plane of the patch antenna.

According to a second aspect of the invention, in a dual antenna arrangement comprising a patch antenna and a monopole antenna, the monopole antenna extending in a direction substantially normal to the plane of the patch antenna, the monopole antenna is of coaxial form having an inner conductor connected as a feed to the patch antenna and an outer conductor constituting the monopole radiating element.

The monopole antenna may extend between its own ground plane and a ground plane of the patch antenna, the outer conductor being directly connected to both ground planes.

The capacitance between the patch ground plane and the monopole ground plane may be such as to produce resonance of the monopole antenna at an operating frequency for which the distance between the patch ground plane and the monopole ground plane is less than one quarter wavelength.

The monopole antenna may be inductively loaded to produce resonance of the monopole antenna at an operating frequency for which the overall length of the monpole antenna is less than one quarter wavelength. The inductive loading may be provided by a loading coil connected in series with the outer conductor at a position closer to the monopole ground plane than to the patch ground plane.

Alternatively, the inductive loading may be provided by forming the coaxial monopole antenna in a helical form.

A connection is preferably provided between a tapping point on the outer conductor and the inner terminal of a coaxial connector mounted on the monopole ground plane, the tapping point being spaced from the monopole ground plane by such distance as to provide a predetermined monopole antenna impedance at the coaxial connector.

The monopole ground plane may be mounted on or be constituted by a conductive vehicle roof so that in operation the monopole antenna is substantially vertical and has a gain decreasing with elevation from the patch antenna plane.

The monopole antenna may branch prior to connection with the patch antenna so that the inner conductor feeds the patch at two locations and the arrangement is such that, in conjunction with a quarter wavelength path disparity between the branch lengths, the patch antenna is receptive to circularly polarised radiation.

There is preferably a protective housing transparent to radio frequency radiation, the housing extending over the patch and monopole antennas and being secured to the vehicle roof.

The monopole antenna may be operable at VHF frequencies and the patch antenna at microwave frequencies.

According to a third aspect of the invention, a dual antenna arrangement comprises a monopole antenna and a patch antenna, the patch antenna comprising a conductive patch and a conductive ground plane separated by a dielectric plate, the monopole antenna extending in a direction substantially normal to the plane of the patch antenna and comprising a radiating conductor extending from the inner conductor of a coaxial feed, the coaxial feed having an outer conductor which extends through the ground plane and dielectric plate to the patch and being connected to both the ground plane and the patch, the position at which the monopole antenna extends from the patch being one at which in operation the patch is at substantially the same potential as the ground plane.

The outer conductor of the coaxial feed may comprise a metallic tube extending through the patch antenna and being connected and secured to the patch, to the ground plane and to a coaxial connector.

The monopole antenna preferably protrudes from said metallic tube by about one quarter wavelength at the operating frequency.

The tube preferably does not protrude from the patch on the face from which the monopole extends.

The conductive ground plane may extend beyond the patch in order to maximise the gain at low elevations relative to said ground plane.

The patch antenna may be fed at two locations by a branched coaxial feed the two locations being such, in conjunction with a quarter wavelength path disparity between the branch lengths, that the patch antenna is receptive to circularly polarised radiation.

The patch ground plane may be mounted on or be constituted by a conductive vehicle roof so that in operation the monopulse antenna is substantially vertical and has a gain decreasing with elevation from the plane of the patch antenna.

The monopole antenna may be operable at UHF frequencies and the patch antenna at microwave frequencies.

Two dual antenna arrangements according to the invention will now be described, by way of example only, with reference to the accompanying drawings, of which: Figure 1 is a diagrammatic side view of the first embodiment, being a combined VHF/microwave antenna arrangement; Figure 2 is a cross section of the feed to the microwave (patch) antenna of Figure 1; Figure 3 is a diagrammatic perspective view of the second embodiment, being a combined UHF/microwave antenna arrangement; and Figure 4 is a cross section of the feed to the UHF antenna of Figure 3.

Referring to Figures 1 and 2, this shows the dual antenna arrangement mounted on a vehicle roof, e.g. a car roof. The antenna arrangement comprises a VHF monopole 1 operating at 200MHz having a few turns to provide a distributed inductive loading, and a microwave patch antenna 2 operating at 1.6GHz comprising a rectangular conductive patch 3 separated from a ground plane 5 by a dielectric substrate 7 in known manner.

The monopole antenna 1 is provided by the outer conductor 9 of a coaxial line, the signal on this outer conductor 9 being isolated, as will be explained, from a signal carried between the inner and outer conductors 11 and 9.

The coaxial outer conductor 9 is connected by, for example, a coaxial connector or by simple soldering to the ground plane 5, the inner conductor 11 extending through the ground plane 5 and through the dielectric layer 7 to be connected by soldering to a point on the conductive patch 3. The coaxial line is branched near the patch antenna, the two branches differing in length by one quarter wavelength at the operating (microwave) frequency, 1.6GHz. The two branches are similarly connected to the patch 3 and the ground plane 5. The branch feed points 13 are located off-centre on the main axes of the patch 3 to make the patch receptive to orthogonal signal components. The quarter wavelength path disparity between the branches then makes the patch operable with circularly polarised signals.

At the lower end of the monopole the roof 4 is fitted with two coaxial connectors 15 and 17. The coaxial line is connected to one of them (15) for feeding the patch antenna. The outer conductor 9 of the coaxial line is connected to the inner of the other connector 17 by a simple wire 19 at a point on the outer conductor 9 such that the impedance at the connector 17 is 50 ohms to match the equipment to be connected to the monopole 1.

The dual antenna is then covered by an RF transparent housing 21 which may be used to provide mechanical support for the arrangement if the patch substrate 5 and ground plane 7 are fixed to it. Some dexterity would then be needed to make the antenna feed connections at 15 and 17. The housing 21 is then fixed to the roof by screws or other convenient means.

The antenna 1 is thus in the form of a shortened monopole above a ground plane. Such shortening may be accomplished by inductively loading part or all of the length of the antenna or by capacitively loading its free end and in the example shown in Figure 1 both methods are used: inductive loading by coiling the length of the antenna 1 (whose outer surface radiates while its inner surface conveys signals to the patch antenna), and capacitive loading by using the patch 3 and its associated ground plane 5 as one plate of an electrical capacitor whose other plate is the conducting surface upon which the whole combined antenna is mounted, ie the car roof 4.

The two antennas operate virtually independently of one another with low mutual coupling and each with high rejection of signals transmitted by the other even though they share the same conducting surfaces. This is achieved by making use of the fact that electrical conductors have a characteristic "skin depth" whose thickness is an inverse function of the radio frequency. Provided that the conductors are thick compared with the skin depth, opposite surfaces of such conductors are effectively isolated from each other.

In the present application the isolated surfaces are the inside and outside of the conducting cylinder forming the sheath 9 of the coaxial waveguide 9/11.

In the embodiment shown in Figure 1, while the inner surface of this sheath conductor 9 together with the inner conductor 11 convey radio frequency signals to the microwave patch 2, the outer surface of the sheath conductor 9 forms part of the radiating structure of the monopole antenna 1.

Referring now to Figures 3 and 4 the lower frequency antenna is in this case a UHF band device operating at 900 MHz. A quarter-wave monopole is then only about 75 millimetres long and can be used at its full length without being too obtrusive or vulnerable.

There is no need for inductive or capacitive loading to reduce the length, as employed in Figure 1.

In this embodiment the UHF monopole extends upwards from the microwave patch as an extension of the inner conductor of a coaxial feed. The monopole antenna 25 is in the form of a straight rod protruding through a hole 27 in the patch 29 of the microwave antenna.

The patch 29 is mounted on a substrate 31 which in turn is mounted on a conductive ground plane 33.

Referring to Figure 4 particularly, a conductive tube 35 protrudes through the ground plane 33, substrate 31 and patch 29 and is soldered both to the patch 29 and the ground plane 33 thus effectively shorting the two together. However, the tube 35 is fitted to the patch/ground plane combination at a position on the patch where the patch/ground plane potential difference is substantially zero.

Thus the operation of the patch antenna is largely unaffected.

The rod monopole 25 is supported in the tube 35 by a rigid dielectric filling 37 and the lower end of the tube 35 is formed as, or is built to incorporate, a coaxial connector 39. The UHF signal is fed between the inner and conductors of this connector.

In addition to the UHF feed there are two microwave feeds to points 41 and 43 on the patch. These are similar to those on the patch 3 in Figure 1 and are provided by a branched coaxial line as before, the inner conductor passing through the ground plane 33 (without of course contacting it) and the substrate 31 and being soldered to the patch 29 at positions to permit operation with circularly polarised signals. The coaxial outer conductor may be soldered to the ground plane.

The combined antenna may then be mounted on a vehicle roof (not shown) which may act as an extension of the ground plane 33. It would also be possible to mount the substrate directly on the vehicle roof although the former design i.e. the use of a self-contained ground plane, simplifies the installation of the antenna.

As mentioned above, at any of one or more voltage nodes the patch 29 is connected to its ground plane 33 by the short conducting tube 35 which then has virtually no effect on the performance of the patch antenna. The inside of this conducting tube 35 acts as a path for radio frequency signals to the upper surface of the patch 29 which now, together with any adjacent conducting surfaces to which it may be directly or indirectly connected, e.g. a car roof, acts as the local ground plane for the monopole antenna.

While the VHF antenna described above was shortened by inductive and capacitive loading and the UHF antenna was not, it will be apparent that the shortening feature is optional for both antennas and will be employed or not according to the particular circumstances.

Again, while the two embodiments have been described as for operation in the VHF and UHF bands it will be apparent that there is no limitation to these bands or within them.

Claims

1. A dual antenna arrangement comprising a patch antenna and a monopole antenna, the monopole antenna having maximum gain in multiple directions in a plane, and the patch antenna having maximum gain along an axis normal to said plane, the monopole antenna incorporating a coaxial waveguide the outer conductor of which is directly connected to a ground plane of the patch antenna.

2. A dual antenna arrangement comprising a patch antenna and a monopole antenna, the monopole antenna extending in a direction substantially normal to the plane of the patch antenna, wherein the monopole antenna is of coaxial form having an inner conductor connected as a feed to the patch antenna and an outer conductor constituting the monopole radiating element.

3. A dual antenna arrangement according to Claim 2, wherein said monopole antenna extends between its own ground plane and a ground plane of the patch antenna, said outer conductor being directly connected to both ground planes.

4. A dual antenna arrangement according to Claim 3, wherein the eapacitance between the patch ground plane and the monopole ground plane is such as to produce resonance of the monopole antenna at an operating frequency for which the distance between the patch ground plane and the monopole ground plane is less than one quarter wavelength.

5. A dual antenna arrangement according to Claim 3, wherein said monopole antenna is inductively loaded to produce resonance of the monopole antenna at an operating frequency for which the overall length of the monpole antenna is less than one quarter wavelength.

6. A dual antenna arrangement according to Claim 5, wherein said inductive loading is provided by a loading coil connected in series with said outer conductor at a position closer to the monopole ground plane than to the patch ground plane.

7. A dual antenna arrangement according to Claim 5, wherein said inductive loading is provided by forming the coaxial monopole antenna in a helical form.

8. A dual antenna arrangement according to Claim 4 and any of Claims 5, 6 and 7.

9. A dual antenna arrangement according to any of Claims 4 to 8 wherein a connection is provided between a tapping point on said outer conductor and the inner terminal of a coaxial connector mounted on the monopole ground plane, said tapping point being spaced from the monopole ground plane by such distance as to provide a predetermined monopole antenna impedance at said coaxial connector.

10. A dual antenna arrangement according to any of Claims 3 - 9, wherein said monopole ground plane is mounted on or is constituted by a conductive vehicle roof so that in operation the monopole antenna is substantially vertical and has a gain decreasing with elevation from the patch antenna plane.

11. A dual antenna arrangement according to any of Claims 2 - 10, wherein the moriopole antenna branches prior to connection with the patch antenna so that said inner conductor feeds the patch at two locations and the arrangement is such that, in conjunction with a quarter wavelength path disparity between the branch lengths, the patch antenna is receptive to circularly polarised radiation.

12. A dual antenna arrangement according to Claim 10, including a protective housing transparent to radio frequency radiation, the housing extending over the patch and monopole antennas and being secured to the vehicle roof.

13. A dual antenna arrangement according to any preceding Claim, wherein said monopole antenna is operable at VHF frequencies and said patch antenna at microwave frequencies.

14. A dual antenna arrangement substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.

15. A dual antenna arrangement comprising a monopole antenna and a patch antenna, the patch antenna comprising a conductive patch and a conductive ground plane separated by a dielectric plate, the monopole antenna extending in a direction substantially normal to the plane of the patch antenna and comprising a radiating conductor extending from the inner conductor of a coaxial feed, the cbaxial feed having an outer conductor which extends through the ground plane and dielectric plate to the patch and being connected to both the ground plane and the patch, the position at which the monopole antenna extends from the patch being one at which in operation. the patch is at substantially the same potential as the ground plane.

16. A dual antenna arrangement according to Claim 15, wherein said outer conductor of the coaxial feed comprises a metallic tube extending through the patch antenna and being connected and secured to the patch, to the ground plane and to a coaxial connector.

17. A dual antenna arrangement according to Claim 16, wherein the monopole antenna protrudes from said metallic tube by about one quarter wavelength at the operating frequency.

18. A dual antenna arrangement according to Claim 16 or Claim 17, wherein said tube does not protrude from the patch on the face from which the monopole extends.

19. A dual antenna arrangement according to any of Claims 15 - 18, wherein said conductive ground plane extends beyond the patch in order to maximise the gain at low elevations relative said ground plane.

20. A dual antenna arrangement according to any of Claims 15 - 19, wherein said patch antenna is fed at two locations by a branched coaxial feed the two locations being such, in conjunction with a quarter wavelength path disparity between the branch lengths, that the patch antenna is receptive to circularly polarised radiation.

21. A dual antenna arrangement according to any of Claims 15 - 20, wherein the patch ground plane is mounted on or is constituted by a conductive vehicle roof so that in operation the monopulse antenna is substantially vertical and has a gain decreasing with elevation from the plane of the patch antenna.

22. A dual antenna arrangement according to any of Claims 1 and 15 - 21, wherein said monopole antenna is operable at UHF frequencies and said patch antenna at microwave frequencies.

23. A dual antenna arrangement substantially as hereinbefore described with reference to Figures 3 and 4 of the accompanying drawings.