WO1993000724A1

WO1993000724A1 - Active integrated microstrip antenna

Info

Publication number: WO1993000724A1
Application number: PCT/CA1992/000258
Authority: WO
Inventors: Apisak Ittipiboon; David John Roscoe; Lotfollah Shafai
Original assignee: Her Majesty The Queen In Right Of Canada As Represented By The Minister Of Communications; The University Of Manitoba
Priority date: 1991-06-20
Filing date: 1992-06-19
Publication date: 1993-01-07
Also published as: CA2089894A1

Abstract

An antenna element suitable for use in a phased array antenna comprises a travelling wave structure (60) formed by a plurality of microstrip transmission line segments (R1-Rm) connected in series by one or more active circuit devices (A1-Am-1). The active circuit devices are controllable to modify the phase and/or magnitude of the signal propagating in the travelling wave structure. The active circuit devices provide the capability to control electronically the radiation pattern of the individual elements by modifying the phase and/or magnitude of the signal in different line segments. Such active antenna elements may be fabricated in various configurations, for example circular, linear and folded. The active circuit devices may comprise phase shifters and/or time delay shifters and/or amplifiers and/or alternators.

Description

ACTIVE INTEGRATED MICROSTRIP ANTENNA

DESCRIPTION TECHNICAL FIELD: This invention relates to antennas, especially phased array antennas, and to antenna elements for use therein.

BACKGROUND ART:

A typical phased array antenna comprises a large number of individual elements, a microwave signal transmitter and a drive circuit with phase shifters for adjusting the phase shift of the signal before it is applied to the elements. The physical configuration of the array and the relative phase of the signals as applied to the elements determines the array pattern. Varying the phase shift of the signal differently for each radiator element causes the radiated beam, comprising the aggregate of the individual emissions from the radiator elements, to change direction and/or pattern.

It is desirable for active phased array antennas to have radiator elements which give both radiation efficiency and wide bandwidth.

Known active phased array antennas typically use microstrip patch or printed dipole antenna elements. In this specification, the term "microstrip" refers to a planar transmission line comprising a strip conductor spaced from a ground plane by a dielectric. US patent number 4,751,513 and US patent number 4,475,108 disclose examples of patch antenna elements with impedance matching devices, namely a PIN diode on the one hand and a varactor on the other. Such resonant structures inherently have narrow impedance bandwidths. Also, such parasitic impedance tuning technigues typically result in larger elements, which is often a disadvantage. Multi¬ layer structures may achieve wider bandwidths, but this is achieved at the expense of fabrication complexity and difficulty.

Travelling wave antenna structures may be used to improve impedance matching and hence antenna bandwidth, but have limitations when used in phased arrays because they have fixed patterns and often have limited grating lobe performance.

US patent number 4,899,163 discloses an antenna comprising a plurality of radiating elements interconnected by printed quarter wave transformers. US patent number 4,529,988 discloses an antenna which includes a passive microstrip array the radiation pattern of which is adjusted by means of the feed arrangements. US patent No. 4,804,965 issued February 14, 1989 discloses a microstrip antenna element comprising a wire or microstrip conductor in the form of a branched loop on one face of a dielectric support. The branched loop extends parallel to a conductive plane disposed on the other face of the dielectric support. Various configurations of branched loop are disclosed including, for example, a hollow cruciform. Each arm of the cruciform comprises a pair of parallel strips with an end portion joining them together. The length of each of these limbs is half the guided wave length in the transmission line of the radiation to be transmitted or received. The length of each end portion preferably is equal to such wave length divided by the number of branches, in this case four. Hence each end portion has a length equal to one quarter of the wave length. The transmission line formed by the perimeter of the cruciform is interrupted in the middle of one of the end portions to enable the signal to be fed into the line. When the other end of the line is terminated with a matching impedance, the antenna element functions as a travelling wave device.

When a plurality of these antenna elements are used in a phased array, the pattern provided by each individual element is fixed by its geometry, which limits changes to the array pattern, i.e. the shape of the radiation lobe for the phased array. Also, the number of antenna elements is limited by their radiation efficiencies. Selecting too few elements results in a large portion of the signal not being radiated but rather being forced to be delivered to the load causing a low antenna efficiency. On the other hand, the array size cannot be made arbitrarily large since the signal power radiates in the first few antenna elements rendering the remaining ones useless.

An object of the present invention is to provide a phased array antenna element the radiation pattern of which can be readily changed, facilitating the changing of the radiation pattern of a phased array antenna formed by a plurality of such antenna elements.

DISCLOSURE OF INVENTION:

According to one aspect of the present invention an antenna element comprises a travelling wave structure comprising a plurality of transmission line segments connected in series by at least one active circuit device, each said segment comprising a strip conductor spaced from a ground plane by a dielectric, said active circuit device interconnecting respective conductors of a pair of adjacent segments and being operable to vary at least one of the phase and magnitude of a radio frequency signal propagating in the travelling wave structure as said signal passes from one to the other of said pair of segments.

The active circuitry provides the capability to control electronically the radiation pattern of the individual elements by modifying the phase and/or magnitude of the signal in different segments or radiator elements.

In a preferred embodiment of the invention, each active circuit device has an input impedance which matches the characteristic impedance of a preceding transmission line segment. In use, the first transmission line segment will be connected to a drive means including a signal source. The drive means may also include means for controlling the active circuit. The final transmission line segment will usually be connected to a matching impedance.

According to a second aspect of the invention there is provided an antenna comprising an array of antenna elements, each comprising a travelling wave structure comprising a plurality of transmission line segments connected in series by at least one active circuit device, each said segment comprising a strip conductor spaced from a ground plane by a dielectric, said active circuit device interconnecting respective conductors of a pair of adjacent segments and being operable to vary at least one of the phase and magnitude of a radio frequency signal propagating in the travelling wave structure as said signal passes from one to the other of said pair of segments.

Incorporating the active circuitry into each individual antenna element affords improved antenna array performance making it possible to enhance not only received or transmitted power but also reconfiguration of the antenna pattern to suit different requirements.

Preferably each active circuit device comprises a separate phase shifter and an amplifier, the phase shifters being controllable by a phase control circuit to vary the degree of phase shift introduced. Alternatively, an amplifier might be used alone, providing that its inherent phase shift is sufficient to provide the desired phase progression around the antenna element. When a plurality of the antenna elements are employed in a phased array, the antenna radiation pattern, i.e. the pattern or shape of the radiation lobe for the phased array antenna, is the aggregate of the array pattern and the individual element patterns which usually, but not necessarily, will be the same for all of the antenna elements in the array. The phase shift required to change the a oresaid antenna radiation pattern may be produced entirely by the active circuits internal to the individual antenna elements. Alternatively, the drive means for the phased array may comprise phase shifters disposed externally of the antenna elements to vary the phase shift of the signal before its application to the antenna element, in the normal way. In such a case, the drive means could include means for coordinating the relative phase shifts internally and externally of the individual antenna element. BRIEF DESCRIPTION OF DRAWINGS:

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of a planar antenna element comprising a travelling wave structure formed by a plurality of arcuate radiator segments interconnected by active phase and amplitude control circuits;

Figure 2 is a block schematic diagram of two of the antenna elements of Figure 1 connected to a common drive circuit;

Figures 3A and 3B are representations of the radiation pattern of the antenna element for different phase shifts;

Figure 4 is a schematic diagram of four antenna elements connected in parallel as part of a phased array antenna;

Figure 5 is a schematic diagram of four antenna elements connected in series of part of a phased array antenna;

Figure 6 is a schematic diagram of two travelling wave structures, one inside the other, connected in series; Figure 7 is a schematic diagram of an elliptical antenna element;

Figures 8 to 12 are schematic diagrams of various other geometries all of which are linear; and

Figure 13 is a schematic diagram of a folded zig-zag antenna element.

MODE(S) FOR CARRYING OUT THE INVENTION:

Referring to Figures 1 and 2, a planar travelling wave antenna element comprises a generally circular microstrip travelling wave structure formed by M arcuate microstrip transmission line segments Rj to R_M lying on the circumference of a circle and spaced equidistantly around it. Line portions 11 and 12 extend radially inwardly from the extremities of the strip conductor of each of the radiator elements j to R_M. Inner ends of line portions 11 and 12 of adjacent microstrip line segments are interconnected by active circuits Aj to A-..._J which comprise monolithic microwave integrated circuit devices. The radial line segment 11 of the first radiator element _! is connected to a drive means comprising a drive circuit 13 to receive a microwave signal from a microwave signal source (not shown) within the drive circuit 13. The radial line segment 12 of the final radiator element R_M is connected to ground by way of a matched load 14.

The integrated circuit devices Aj to A^ form with the transmission line segments R_t to R_M an integral part of the travelling wave structure. Variation of the phase shift of each of these devices A_t to ^ enables the radiation pattern of the antenna element to be changed, as illustrated in Figures 3A and 3B.

The length of each radial line segment 11 or 12 is such that it yields a phase shift φ, The phase shift φ^^ of each of the integrated circuit devices Aj to

is such that

+ 2ø, equals 360°. For a broadside radiation pattern, as illustrated in Figure 3A, where the total phase progression around the circumference will be 27r, the length ό^" of each arcuate microstrip section will be equal to 2π/M. To change the radiation pattern to, say, a toroidal shape as shown in Figure 3B, the phase progression is increased in multiples of 2π. As the progression increases, the beam moves closer to the "horizon", i.e. to the plane of the antenna element. The radiation patterns illustrated in Figures 3A and 3B are for an operating frequency of 3.1 GHz, though higher operating frequencies are also envisaged.

The ability to change the pattern of the individual antenna element electronically is particularly useful when the antenna element is incorporated into a phased array antenna since it permits the transmitted or received power of the phase array antenna to be improved while allowing much greater flexibility in reconfiguring the radiation pattern for the phased array antenna. In a conventional passive phased array antenna where the individual antenna elements do not permit any control of the amplitude and phase distribution within the element, the radiation pattern of the element is fixed. Any changing of the radiation pattern of the phased array antenna is achieved by changing the phase shift of the signal externally, i.e. before application to the antenna elements. When "active" antenna elements embodying the present invention are used in a phased array antenna, as illustrated in Figure 2, the changing of the overall radiation pattern can be achieved entirely by means of the active circuits Aj to A.,.,, within the antenna elements. The drive means for the antenna may comprise additional, external phase shifting circuits 15 provided in the signal feed and controlled by the drive circuit 13, as illustrated in broken lines in Figure 2. In the latter case, the drive circuit 13 will be modified to coordinate the control of the phase shifters 15 and active elements Aj to A_^i to provide the required radiation pattern, which will be the aggregate of the individual element patterns and the array pattern.

Incorporation of the active circuitry into the travelling wave structure facilitates wide beam scanning or low elevation angle beam steering with smaller antenna arrays, or permits arrays with lower grating lobes to be produced more effectively. Also, since the transmission line end of each segment is matched in impedance to the following active circuitry, the remaining signal power is fully received by the circuit and, after amplification or phase shift, delivered to the subsequent segment. This results in full use of the signal power, thus improving the array efficiency and gain and enabling the extension of the array size without a loss of segment effectiveness near the array end.

The monolithic microwave integrated circuit devices A, to A-,,.₁ and all the other components of the microstrip transmission line may be fabricated on a semiconductor wafer. For information about fabricating microwave circuit devices with integral antennas, the reader is directed to US patent No. 4,490,721, issued December 25, 1984.

When the "active" antenna elements are incorporated into a phased array, they can be interconnected in various configurations. Figures 4 and 5 illustrate examples of 2 x 2 sub-arrays of four "active" antenna elements. In Figure 4, the microwave signal inputs of all four antenna elements 40, 41, 42 and 43 are connected in parallel, as shown by broken lines, to a drive circuit 44, and their respective outputs are connected to matching impedances as before. In Figure 5, four "active" antenna elements 50, 51, 52 and 53 are shown connected in series i.e. the input of antenna element 50 is connected to a drive circuit 54 and its output is connected to the input of antenna element 51. The other elements 51, 52, 53 are "daisy chained" and the output of antenna element 53 is connected to a matching impedance as before. In both examples, the active circuit devices are connected in common to the respective drive circuit.

Figure 6 illustrates an alternative configuration for a phased array in which two coplanar "active" antenna elements

60 and 61 are "nested", i.e. antenna element 61 is the same shape as antenna element 60 but slightly smaller and fits within antenna element 61. The input of antenna element 61 will be connected to a drive circuit (not shown) and its output connected to the input of antenna element 60. The output of antenna element 60 is connected to the matching impedance or "load". It will be appreciated that a set of more than two antenna elements could be "nested" in such a way. It is also envisaged that the antenna elements 60 and

61 could be connected in parallel rather than in series.

The "nested" elements typically will be used as a single element in a phased array. It will be appreciated, however, that each set of nested elements, with its integrated active circuitry, could itself be considered to be a phased array.

The number of radiator transmission line segments in an antenna element embodying the invention may be two or more. Generally, increasing the number increases efficiency, but at the expense of complexity and increased fabrication costs.

The invention is not limited, of course, to circular antenna elements. Figure 7 illustrates an elliptical antenna element and Figures 8, 9, 10, 11 and 12 illustrate various linear antenna elements incorporating radiator segments interconnected by active circuit devices for controlling the phase and amplitude of the signal propagating through the travelling wave structure. The actual radiation takes place from the discontinuities along the line as indicated by arrows. Figure 8 shows a sinusoidal configuration, Figures 9 and 10 show trapezoidal configurations, Figure 11 shows a rampart configuration and Figure 12 shows a zig-zag configuration. It should be noted, particularly from Figures 9 and 10, that the active circuits are not necessarily provided at regular intervals. Active circuit devices could be used, depending upon the circumstances, to give different^' radiation magnitudes at different discontinuities of the travelling wave structure. For example, the radiation at positions X and Y in Figure 9 could be unequal whereas the corresponding radiation magnitudes X' and Y' in Figure 10 could be equal or unequal due to the presence of the intervening active circuit element A' .

Any of these linear configurations could of course be folded rather than straight. Figure 13 illustrates a folded zig-zag antenna element, but other linear configurations would be folded in like manner. It will be appreciated that many other configurations will be apparent to a person skilled in the art. In the preferred embodiment, the transmission line segments j to R_M all have the same characteristic impedance. Consequently, all of the amplifiers A, to A,,,._! have the same input impedance. It would be possible, if required by the application, for the transmission line segments to have different characteristic impedances. The active circuit devices would then have different input impedances to match the characteristic impedance of the preceding transmission line segment.

In the described embodiments all of the active circuits A_j to A.,.._! are adjusted in common and to the same extent. It would be possible, however, though more complicated, to adjust them independently of each other, and to a different extent.

Where wide beam scanning or low elevation beam steering is desired, embodiments of the invention permit smaller antenna arrays or alternatively arrays with lower grating lobes. Embodiments of the invention are advantageously employed where antennas are to be integrated on a semi¬ conductor wafer where wafer area is at a premium. Since the active circuitry may be fabricated as a Monolithic Microwave Integrated Circuitry (MMIC) both the active circuitry and the radiator elements can be integrated on a single surface leading to lower fabrication costs as compared with prior art devices which employ multilayer structures.

Although in the preferred embodiment the control inputs and signal inputs of the active circuit devices are connected separately to the drive circuit 13, it is envisaged that the control signal could be combined with the microwave signal for radiation by the elements Rj to R_M. The active circuits would then have means for extracting the control signal.

The active circuitry could be any device or combination of devices that provide the required degree of phase and/or magnitude control, for example phase shifters and/or time delay shifters and/or amplifiers and/or attenuators.

It is envisaged that magnitude control might be used to fine tune the antenna element to correct for imperfect circular polarization due to physical limitations of fabrication. Although the specific embodiment of the invention uses microstrip transmission line segments, it should be noted that the invention could also be employed with other kinds of antenna element.

An advantage of embodiments of the invention which permit control of the relative radiated power of each antenna element, is that the array size can be controlled or extended to improve its efficiency, gain and the radiation pattern.

INDUSTRIAL APPLICABILITY Embodiments of the invention find application in large active arrays, for example in radar and satellite communications, where the ability to achieve high performance compact phased array antennas is of value. Antennas embodying the invention are of benefit where very wide angle scanning capability with low grating lobe performance is required. Integrating the active circuitry into the antenna array structure itself should lead to cost savings since high performance requirements can be realised with smaller antennas. It is envisaged that embodiments of the invention could meet MSAT or other portable antennas requirements. One specific such application will be small integrated active EHF antennas for personal communication. Other applications for embodiments for the invention include large sophisticated high performance systems, for example airborne radar and communications, where extreme reconfigurability and steerability are desired. Generally, it is envisaged that embodiments of the invention will find application where the antenna is required to radiate circular or linear polarization with the coverage angle from broadside to endfire, such as in GPS systems and mobile communications.

Claims

CLAIMS :

1. An antenna element comprising a travelling wave structure (60) comprising a plurality of transmission line segments (Rj-R,,.) , each said segment comprising a strip conductor spaced from a ground plane by a dielectric, characterized by at least one active circuit device (A_j-A.,,._!) interconnecting respective conductors of a pair of adjacent segments and operable to vary at least one of the phase and magnitude of a radio frequency signal propagating in the travelling wave structure as said signal passes from one to the other of said pair of segments.

2. An antenna element as claimed in claim 1, and control means (16) , characterized in that said control means (16) is connected to said active circuit device and is operable to control said active circuit device to vary at least one of said phase and magnitude.

3. An antenna element as claimed in claim l, and drive means (13) comprising a source for said signal, characterized in that a first (R_x) of said transmission line segments is connected to said source and a final transmission line segment (R-i,) is connected to a matching impedance (14) .

4. An antenna element as claimed in claim 1, 2 or 3, characterized in that said active circuit device has an input impedance matching the characteristic impedance of a preceding transmission line segment connected thereto.

5. An antenna element as claimed in claim 1, 2 or 3, characterized in that said active circuit device comprises a monolithic microstrip integrated circuit.

6. An antenna element as claimed in claim 1, 2 or 3, further characterized by a second travelling wave structure (61) disposed inside the first-mentioned travelling wave structure (60) and in the same plane.

7. An antenna element as claimed in claim 6, characterized in that the first and second travelling wave structures (60,61) have respective signal inputs connected in parallel and signal outputs connected to respective matching impedances.

8. An antenna element as claimed in claim 6, characterized in that the first and second travelling wave structures (60,61) are connected in series.

9. An antenna comprising an array of antenna elements (40- 43;50-53), each comprising a travelling wave structure (60) comprising a plurality of transmission line segments (Rj-R- , each said segment comprising a strip conductor spaced from a ground plane by a dielectric, said antenna being characterized by a plurality of active circuit devices (Aj-A,,.._!) , each active circuit device interconnecting respective conductors of a pair of adjacent segments and being operable to vary at least one of the phase and magnitude of a radio frequency signal propagating in the travelling wave structure as said signal passes from one to the other of said pair of segments.

10. An antenna as claimed in claim 9, and control means (16) connected to each of the active circuit devices, characterized in that said control means is operable to control said active circuit devices to vary at least one of said phase and magnitude.

11. An antenna as claimed in claim 9, and drive circuit means (44;54) comprising a source for said signal, said antenna characterized bv a plurality of phase shifters (15) coupling said source to respective ones of said antenna elements, said phase shifters being controllable to vary the array pattern.

12. An antenna as claimed in claim 9, 10 or 11, characterized in that each of said antenna elements comprises a second travelling wave structure (61) disposed inside the first travelling wave structure (60) and in the same plane.

13. An antenna as claimed in claim 9, 10 or 11, characterized in that each active circuit device has an input impedance matching the characteristic impedance of a preceding transmission line segment.

14. An antenna as claimed in claim 13, characterized in that, in each said antenna element, a first (R of said transmission line segments is connected to a or said source and a final transmission line segment (R,..) is connected to a matching impedance.

15. An antenna as claimed in claim 9, 10 or 11, characterized in that each said active circuit device comprises a monolithic microstrip integrated circuit.

16. An antenna as claimed in claim 12, characterized in that said first and second travelling wave structures in each antenna element have their respective signal inputs connected in parallel and their respective signal outputs impedance matched.

17. An antenna as claimed in claim 12, characterized in that said first and second travelling wave structures in each said antenna element are connected in series.

18. An antenna as claimed in claim 12, characterized in that said antenna elements are connected in series.

19. An antenna as claimed in claim 16, characterized in that said antenna elements are connected in series.

20. An antenna as claimed in claim 17, characterized in that said antenna elements are connected in series.