GB2445592A

GB2445592A - Driven and parasitic patch antenna structure with an inclined beam

Info

Publication number: GB2445592A
Application number: GB0700642A
Authority: GB
Inventors: Keith David Newsome; Simon Bower
Original assignee: e2v Technologies Ltd
Current assignee: e2v Technologies Ltd
Priority date: 2007-01-12
Filing date: 2007-01-12
Publication date: 2008-07-16
Anticipated expiration: 2027-01-12
Also published as: GB0700642D0; WO2008084249A1; GB2445592B

Abstract

An antenna structure comprises a first patch antenna 9 with feed means 12 and a second patch antenna 14, which is parasitically driven from the first patch antenna 9. The patch antennas 9, 14 are arranged to generate a beam D which is inclined relative to the plane of the patch antennas. The patches 9, 12 may be on one surface of a substrate 10 with a ground plane 11 formed on the other surface. The offset nature of the beam D, which is inclined in the direction of the spaced patches 9, 14, facilitates more flexible mounting options for the antenna structure. A pair of such structures may be provided, to provide a beam D which is alternately inclined in two directions, or an additional driven patch coupled to the parasitic patch 14 may be provided for the same purpose. The antenna structure may be used in a vehicle intruder alarm system.

Description

I

ANTENNA STRUCTURE

This invention relates to antenna structures.

This invention especially relates to antenna structures with inclined beams. Such antenna structures are suitable for use in vehicle alarms, for example, vehicle intruder alarms, where there are often restrictions on where the antenna structure can be mounted.

Typically, antenna structures comprising one or more patch anteni as have been used for vehicle intruder alarms. -For example, referring to Figure 1, the Applicants' prior patent application no. 0604032.3, discloses an arrangement of a pair 1 of patch antennas centrally mounted on the interior of the roof of a vehicle 2, which generate either lobes A or B to illuminate, respectively, either the rear or front of the vehicle, and to receive radiation therefrom. A suitable antenna arrangement to produce such radiation patterns is shown in Figures 2 and 3. Two patches 3, 4, driven by feeds 5, 6, are mounted on one side of a substrate 7 having a ground plane 8 on the other side. If the patches are driven in phase (Figure 3), a resultant beam of radiation C would be directed perpendicular to the plane of the patches. Referring to Figures 4 and 5, if the drive waveform for one of the patches leads or lags that for the other patch, the resultant beams A,B will be tilted to the left or right. In the antenna arrangement described in the above-mentioned prior patent application, the beam from the patches 3, 4 is toggled between the orientations of Figures 4 and 5 to illuminate either the front (B) of the vehicle, or the rear (A) of the vehicle, and to receive radiation therefrom.

A disadvantage with this arrangement is that the boresight of the beams always lies in a plane perpendicular to the axis of the patches, whether toggled to illuminate the front or the rear of the vehicle. Hence, unless other measures are taken, it is necessary to mount the antenna structure of the alarm along the centre-line of the vehicle (as shown in Figure 1). However, this may present packaging problems. Another disadvantage is the complexities of the switching.

The principle of driving elements of an antenna array at phase.offsets relative to,each..

other has been used in phased array antennas for many years, although generally with many more than two antenna patches.

It has also been proposed to use a parasitic patch antenna on the same substrate surface as a driven patch antenna (Wood C, Improved Bandwidth of Microstrip Antennas using Parasitic Elements, lEE Proc. Vol. 127, Pt H, No 4, August 1980, pages 23 1-234), in order to increase the bandwidth of the antenna.

The invention provides an antenna structure, which comprises a first patch antenna having a feed by means of which the first patch antenna can be driven, and a second patch antenna not having a feed and parasitically coupled to the first patch antenna, wherein the first and second patch antennas are sized and arranged to generate a beam inclined to the plane of the patch antennas.

The inclined beam generated by the antenna structure of the invention provides flexibility from the point of view of location of the antenna structure.

The antenna structure is able to produce the inclined beam, due to the phase difference between the beams radiated by the driven and the parasitic patches. The drive electromagnetic radiation may be rf, microwave or higher frequency.

The antenna structure of the invention is especially suitable for use in a vehicle alarm, for example, a vehicle intruder alarm arranged to detect intrusion into a vehicle. But it could also used for other types of vehicle alarm, such as an alarm mounted on the external walls of a..vehicle in order to detect proximity of persons. This could be used to detect persons, for example, who attempt to spray graffiti on the outside of railway carriages in a siding.

The beam may be toggled, to illuminate two different regions of the vehicle, by changing the frequency of the driving radiation above and below the resonance frequency of the parasitic patch, by provision of another pair of patches with the driven patch of each pair being energised in alternation to provide an offset toggled beam, or by provision of a further driven patch parasitically coupled to the second (parasitic) patch.

Ways of carrying out the invention will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a simplified version of the antenna structure of the vehicle intruder alarm disclosed in the Applicants earlier patent application no. 0604032.3, and is a schematic plan view of a vehicle showing the antenna structure together with its lobe patterns; Figure 2 is a plan view of the antenna structure of Figure 1, on an enlarged scale; Figure 3 is an end view of the antenna structure of Figure 2 showing the lobe pattern produced when both patches of the antenna structure of Figure 2 are driven in phase; Figures 4 and 5 are also end views of the antenna structure of Figure 2 showing the lobe patterns produced when there are different phase differences between the drives to the patches 3,4; Figure 6 is a plan view of a first antenna structure of a vehicle intruder alarm in accordance with the invention; Figure 7 is a side view, looking in the direction of the arrows 7-7 in Figure 6, of the antenna structure shown in Figure 6; Figure 8 is a front view, looking along the direction of the arrows 8-8 in Figure 6, of the antenna structure shown in Figure 6, showing the lobe pattern; Figure 9 is a plan view of a second antenna structure of a vehicle intruder alarm in accordance with the invention; Figure 10 is a front view, looking in the direction of the arrows 10-10 in Figure 9, of the antenna structure shown in Figure 9, showing the lobe pattern; Figure II is an end view, looking in the direction of the arrows 11-i 1 in Figure 9, of the antenna structure shown in Figure 9; Figure 12 is a plan view of a third antenna structure of a vehicle intruder alarm in accordance with the invention; Figure 13 is a side view, looking in the direction of the arrows 13-13 in Figure 12, of -the antenna structure shown in Figure 12 showing the lobe pattern; -Figure 14 is a front view, looking along the direction of the arrows 14-14 in Figure 12, of the antenna structure shown in Figure 12, showing the lobe pattern; Figure 15 is a plan view of a fourth antenna structure of a vehicle intruder alarm in accordance with the invention; Figure 16 is a side view, looking in the direction of the arrows 16-16 in Figure 15, of the antenna structure shown in Figure 15; Figure 17 is a front view, looking along the direction of the arrows 17-17 in Figure 15, of the antenna structure shown in Figure 15,. showing the lobe pattern; arid Figure 18 is a schematic plan view of a vehicle showing mounting positions for the intruder alarm antenna structures of the invention and associated lobe patterns.

Referring to Figures 6 to 8, the first antenna structure of the vehicle intruder alarm s comprises a first resonant patch 9 mounted on one side of a substrate 10, the other side of which carries a ground plane 11. The first resonant patch 9 is connected to a feed 12, via which it is driven with electromagnetic energy. The upper edge (as seen in Figure 6) of the patch 9 is connected to the ground plane by a line of vias 13, and the length of the patch in the vertical direction (as seen in Figure 6) is one quarter wavelength of the io driving frequency. The spacing of the feed 12 from the line of vias is such as to produce a desired input impedance, for example, 50 ohms.

A second resonant patch 14 is provided, and this patch does not have a feed. Instead, it is parasitically coupled to the first resonant patch 9. The coupling is a combination of capacitive and inductive, one being more dominant depending on the resonant frequency of the parasitic patch and the drive frequency, but could in theory be almost entirely capacitive only or almost inductive only.

The resonant frequency of the parasitic patch differs from that of the driven patch by less than 3% of the frequency of the driven patch, preferably, by less than 1%.

The upper edge of the parasitic patch 14 (as seen in Figure 6) is also connected by a line of vias 13 to the ground plane, and the length of the patch in the vertical direction as seen in Figure 6 is similar to that of patch 9, and preferably lies within the range of 5% of the length of the patch 9 in the vertical direction. Hence, both patches are close to resonance at the frequency of the driving electromagnetic radiation. The resonant frequency of the patches is affected by factors other than their length, for example, the loading of the driven patch by its feed 12 affects its resonant frequency, and the width, thickness and shape also affects the resonant frequencies of the patches.

The nature of the coupling between the patches (determined by the size and arrangement of the patches) means that there is a phase difference between radiation radiated by the two. The result of this is that the boresight of the resultant beam 1) is inclined to the plane of the patches in the direction in which the patches are spaced, that is, the boresight lies in the plane of Figure 8.

The spacing between the patches 9 and 14 may be less than one quarter wavelength, and is preferably between one tenth and one fiftieth of a wavelength.

The frequency with which the patch 9 can be driven may be from 100MHz to 1 THz, preferably, from 2 to 100 GHz. The resonant frequencies of the driven and parasitic patches could lie within the range 2.40GHZ and 2.500Hz. The driving frequency in such a case could be 2.45GHz.

The substrate may be any insulating material, for example, FR4 (trade mark).

Parameters which affect the angle of inclination of the beam include the spacing between the patches 9 and 10 in the horizontal direction as seen in Figure 6, the width of the patch 14 (that is, the extent in the horizontal direction as seen in Figure 6), and the difference in lengths between the patches (that is, the extent in the vertical direction as seen in Figure 6).

The antenna structures are particularly effective as short-range sensors, say, for detection for ranges up to 10 metres.

One way of designing such an antenna structure is to adjust the driving frequency through a range until there is coupling between the parasitic patch and the driven patch.

The optimum operation is when the parasitic patch is coupled one side of resonance such that the coupling is either dominantly capacitive or inductive. The off resonance driven parasitic patch has a phase difference due to the reactive coupling. The phase difference then affects the beam shape. Another way of designing such a structure is to adjust the size of the resonant patch keeping the driving frequency constant, until coupling is achieved.

In use, a transmitter connected to the antenna structure of the intruder alarm illuminates the interior of the vehicle with electromagnetic radiation, and the return signals are detected by a receiver connected to the antenna structure, and an alarm is set off when a moving object is detected. The antenna structure only generates a single beam, so two such antenna structures would be needed to illuminate both the front and rear of a vehicle. Thus, referring to Figure 18, in which reference numerals 18 and 19 denote the rear and front windscreen, respectively, two such structures could be mounted together in the centre of the roof lining in the position 15 to produce lobes Dl and D2, or the structures could be mounted separately at the edges of the roof lining, for example at positions 16, 17, each to illuminate one half of the vehicle only.

The inclination of the beam renders it particularly suitable for use as part of a vehicle intruder alarm. Thus, it is possible to mount the antenna structures at the side of the roof lining, instead of on the centre-line, where the need to accommodate other components may make this awkward.

Variations may be made to the antenna structure described without departing from the scope of the invention. Thus, the drive frequency of the input signal may be alternated -between frequencies above and below the resonance of the parasitic patch in order to change the beam pattern. The detection region of the antenna can be changed by choosing frequencies above and below the resonance of the parasitic patch to steer the beam. One detection region could be as illustrated in Figure 8. Another region could have the boresight in the same plane, but this time inclined towards rather than away from the feed 12 (it depending on whether the driving frequency is above or below the resonance frequency of the parasitic patch).

Instead of the arrangement illustrated in Figure 6, the top edge of the patch 14 could be offset upwards or downwards relative to the top edge of the patch 9. The width of the patch 14 (in the direction parallel to the feed 12) may be the same as that of the patch 9, but it does not have to be. Equally, the length of the patch 14 in the direction extending from the line of vias is the same as that as the patch 9 (the resonant frequencies nevertheless differing because of the loading effect of the feed 12), but it does not have to be. While the length of the driven patch 9 in the direction extending from the vias is one quarter wavelength, the length can be a half wavelength (but there would be no vias in such a case) or any other integral number of quarter wavelengths. Although a single parasitic patch 14 has been described, a second parasitic patch, or a series of parasitic patches, each spaced from the preceding one in the manner in which the patch 14 is spaced from patch 9, may be provided.

Referring to Figures 9 to 11, the second antenna structure is identical to the first antenna structure except in that the parasitic patch 114 is directly coupled to the driven patch 109. A similar lobe pattern results. There is a phase difference between the energy radiated by the patches, so that that the boresight of the asymmetric beam E is inclined to the plane of the patches in the direction the patches are spaced.

Both the first and second antenna structures produce an inclined beam whose boresight extends in one direction only. The third and fourth antenna structures can generate beams inclined in either of two directions to the plane of the patches. Thus, referring to Figures 12 to 14, a first pair of resonant patches 209, 214, identical to the patches 9, 14 of the first antenna structure is accompanied by an identical second pair of resonant patches 309, 314. The patches are mounted on an insulating substrate 210, and are connected to a ground plane 211 on the other side by two lines of vias 213, which extend through the inner vertical edges of the patches 209, 214, 309, 314 (as seen in Figure 12).

The patches 209, 309 and associated feeds 212, 312 are similar to the patches 3,4 and feeds 5,6 of the Applicants prior patent application no. 0604032.3, and are driven so that the patches are driven in phase quadrature with the patch 209 alternately leading and the lagging the patch 309. This is accomplished by driving the resonant patches via alternately short and long feed lines. The result of this is that either one of two radiation lobes F, G is produced, as shown in Figures 13 and 14.

Each of the patches 209, 309 has an associated parasitic patch 214, 314, in the manner of the parasitic patch accompanying the driven patch 9 in Figure 6, so that each of the alternating beams F, C is asymmetric and inclined to the plane of the patches in the direction of spacing of the parasitic patches from the driven ones (Figure 13).

This then enables the front (lobe G) and rear (lobe F) of a vehicle (Figure 18) to be alternately illuminated, but with the antenna structure 20 mounted at the side, not at the centre, of the roof lining.

Referring to Figures 15 to 17, an extra driven patch can be used with the first antenna structure to produce the same effect as the toggling centre-mounted antenna structure of the Applicants prior patent application shown in Figures 1 to 5, but without the necessity for driving the resonant patches via alternately short and long feed lines to produce the alternating phase drive.

Thus, patch 409 is driven via feed 412 and is identical to driven patch 9 of the first antenna structure shown in Figure 6. Patch 414 is identical to patch 14 of the first antenna structure, and does not have a feed and is not driven, but is parasitically coupled to patch 409, like patch 14 of Figure 6. These two patches produce inclined lobe H, similar to lobe D in Figure 8. Resonant patch 509 (identical to patch 409) has feed 512, and is also parasitically coupled to undriven patch 414. This produces lobe I when drive 512 is energised, the same as lobe H, but inclined to the left as seen in Figure 17.

Thus, the outward beam of the intruder alarm is toggled between lobe patterns H and I, by alternately driving patches 409 and 414, and the returns from the vehicle interior are received by the respective pairs of driven and parasitic patch antennas. Such an antenna structure can be used in place of that described in the Applicants prior patent application, with the advantage that only one feed 412, 512 need be driven at any time, whereas both patches 3,4 were previously driven, with a consequently more complex switching arrangement. The inclined beam gives great flexibility in positioning the antenna structure in the vehicle, for example, peripheral positions in the roof lining are possible, and further flexibility may be gained by further parasitic patch loading or other beam steering techniques such as lenses and reflectors.

Although the antenna structure has been described hereinbefore for use in vehicle intruder alarms, in fact the alarms are suitable for other purposes, such as vehicle alarms of other types, such as a presence detector to sense whether a seat is occupied, or as an anti-vandal vehicle alarm to be mounted on the outside of a vehicle, such as a railway carriage, in order to detect movement in the presence of the carriage, for the purpose, for example, of detecting graffiti sprayers on carriages left overnight. Other possible uses are for vehicle reversing alarms, or medical uses such as sensing heartbeats.

Claims

I. An antenna structure, which comprises a first patch antenna having a feed by means of which the first patch antenna can be driven, and a second patch antenna not having a feed and parasitically coupled to the first patch antenna, wherein the first and second patch antennas are sized and arranged to generate a beam inclined to the plane of the patch antennas.
2. An antenna structure as claimed in claim I, in which the resonant frequency of the second patch antenna differs from that of the first patch antenna by less than 3% of the frequency of the first patch antenna.
3. An antenna structure as claimed in claim 2, in which the resonant frequency of the second patch antenna differs from that of the first patch antenna by less than 1% of the frequency of the first patch antenna
4. An antenna structure as claimed in any one of claims Ito 3, in which the inclined beam lies in a plane perpendicular to the plane of the patches and extending in the direction in which the patches are spaced.
5. An antenna structure as claimed in any one of claims 1 to 4, in which the frequency of the driving electromagnetic radiation can be varied between being above and below the resonant frequency of the parasitic patch.
6. An antenna structure as claimed in any one of claims I to 4, including a feed by means of which a third patch antenna can be driven, a fourth patch antenna not having a feed and parasitically coupled to the third patch antenna, whereby the beam generated by the third and fourth patches is offset from the perpendicular to the plane of the third and fourth patch antennas.
7. An antenna structure as claimed in claim 6, including means to drive alternately the first and third patches, to produce alternately the offset beams from the first and second, and third and fourth patches.
8. An antenna structure as claimed in any one of claims I to 4, including a further patch antenna having a feed by means of which the further patch antenna can be driven, the further patch antenna being parasitically coupled to the second patch antenna, whereby two beams can be alternately generated offset from the perpendicular to the plane of the patch antennas, depending on whether the first or the further patch antenna is driven.
9. An antenna structure as claimed in any of claims 1 to 8, in which the frequency at which the antenna is driven lies from 100MHZ to I THz.
10. An antenna structure as claimed in any of claims I to 8, in which the frequency at which the antenna is driven lies from 2GHz to 100GHz.
11. An antenna structure as claimed in any of claims 1 to 8, in which the frequency at which the antenna is driven lies from 2.40GHZ to 2.50GHz.
12. An antenna structure as claimed in any one of claims I to 4, in which the spacing between the patches is less than one quarter of the wavelength of the driving radiation.
13. An antenna structure as claimed in claim 12, in which the spacing between the patches is between one tenth and one fiftieth of a wavelength of the driving radiation.
14. An antenna structure substantially as herein described with reference to Figures 6 to 18 of the accompanying drawings.
IS. A vehicle alarm having an antenna structure as claimed in any one of claims I to 14.
16. A vehicle alarm as claimed in claim 15, in which the range is less than 10 metres.
17. A vehicle alarm as claimed in claim 16, in which the alarm is a vehicle intruder alarm.
18. A vehicle equipped with an intruder alarm as claimed in any one of claims 1 to 17