GB2251520A

GB2251520A - Orthogonal slot flat microwave antenna for dual polarization

Info

Publication number: GB2251520A
Application number: GB9113328A
Authority: GB
Inventors: Yves Commault
Original assignee: Thomson CSF SA
Current assignee: Thales SA
Priority date: 1990-06-22
Filing date: 1991-06-20
Publication date: 1992-07-08
Anticipated expiration: 2011-06-20
Also published as: FR2677814B1; ITTO910410A1; DE4120521C2; DE4120521A1; ITTO910410A0; FR2677814A1; IT1249889B; GB9113328D0; GB2251520B; SE9101892L

Abstract

The antenna comprises a first and a second conductive plate (1 and 2) placed one above the other and separated by a dielectric volume, and two excitation striplines (3 and 4). The first conductive plate (1) has, formed in it, the pair of orthogonal radiating slots (6 and 7) disposed to form a cross. The second conductive plate (2) plays the role of a reflector. The central conductors of the excitation striplines (3 and 4) pass in a single plane between the first and second conductive plates (1 and 2) and intersect at a point of intersection in the center of the cross formed by the pair of orthogonal radiating slots (6 and 7). The fact that the central conductors of the excitation striplines (3 and 4) are in a single plane and not in two separate parallel planes, allows to simplify the structure of the antenna and to reduce its thickness. The length of stripline beyond the intersection point is arranged to achieve low cross coupling between the two striplines. <IMAGE>

Description

2 2515 2j_1 Flat microwave antenna with two orthogonal polarizations, with

a pair of orthogonal radiating slots

The present invention relates to flat microwave antennas with double polarization, and more particularly to those including as radiating elements a pair of orthogonal crossed slots excited independently of Pach other by two striplines. These antennas allow to implement fiat arrays with two orthogonal polarizations. They have many applications in the communications and radar fields where, due to their small thickness, they can be integrated into ground-based, airborne or satellite equipments. In addition to their use in the area of free-space radiation, they have also applications in closed systems (waveguides and cavity resonators) where it is useful to have two independent orthogonal polarizations in two separate channels.

Microwave antennas of this type are known, for example, from the U-S patent NO 4,. 922, 263.

This type of antenna comprises two conductive plate disposed one above the other and separated by a dielectric volume. The two crossed orthogonal radiating slots are for- med in one of the conductive plates while the other plays the role of a reflector. The central conductors of the excitation striplines pass between the two conductive plates which complete the stripline structure. Each orthogonal radiating slot is excited by a stripline whose central conductor faces its sides. To this end, the central conductors of the two striplines intersect at right angles in the cen- - 2 ter of the cross formed by the radiating slots while being, at these slots, oriented by 450 from the latter. To avoid any contact in the crossing, they are disposed in different planes between the two conductive plates.

In practice, the central conductors of both striplines are implemented by photolithography of a metallization layer on both sides of a thin dielectric film inserted between two other layers of dielectric metallized over their outer side to form the conductive plate carrying the radiating slots and the reflecting conductive plate.

This type of construction has the disadvantage of requiring three layers of dielectric. In addition, the dielectric film carrying the central conductors of the excitation striplines must be aligned with a great accuracy with the dielectric substrate where the crossed radiating slots are etched, the latter point being very critical with centrimetric waves.

A purpose of the present invention is to provide a flat microwave antenna with a double polarization of smaller thickness and simpler construction.

According to the present invention there is provided a flat microwave antenna with two orthogonal polarizations, with a pair of crossed orthogonal radiating slots excited by two striplines, this antenna comprising a first and a second conductive plate placed one above the other and separated by a dielectric volume, said first conductive plate having, formed in it, the pair of crossed orthogonal radiating slots, with said second conductive plate playIng the role of a reflector, and two excitation stripl-in.ejs-, whose central conductors pass between said fir.stand second conductive plates, each facing the-.sides of one of the slots, and intersects in the center of the cross formed by the pair of orthogonal radiating slots while being, at these slots, oriented by 45 0 from the latter, wherein the central conductors of both excitation striplines of this antenna are disposed in a single plane between the two conductive plates and merge at their intersection in the center of the cross formed by the pair of orthogonal radiating slots.

The independence of the excitation striplines in spite of the connection of their central conductors at their in- tersection is obtained by creating a short-circuit in each line at the point of intersection, either by terminating them as open lines or by extending them from the point of intersection by a length equal to "L A (where ' L is the wavelength in the stripline) or by terminating them as a short-circuited line by an extension from the point of intersection by a length equal to 'L /2 leading to a conductive pin short-circuiting the two conductive plates.

The central conductors of the striplines being disposed in a single plane between the two conductive plates, the antenna can be constructed by means of two dielectric layers instead of three, which simplifies its construction and reduces its thickness.

Other features and advantages of the present invention will become apparent from the following detailed description of preferred embodiments given as a non-limitative example with reference to the accompanying drawings, in which: - Figure 1 is a perspective view of a flat antenna which embodies the invention; - Figure 2 is a dismounted view of the antenna shown in Figure 1, its two dielectric plates being taken apart; 4 - Figure 3 is a plan view of an antenna in which the inven- tion is embodied showing by transparency the of the excita- tion striplines in the vicinity of the radiating slots; - Figure 4 is a dismounted view of an antenna including a single dielectric plate, the second being replaced by a air layer; - Figure 5a is a plan view of another embodiment of the antenna shown in Figure 3 showing by transparency the excitation lines leading to the short-circuiting pins; - Figure 5b is a sectional view along the axis Ox of Figure 5a; - Figure 6 is a plan view of a further embodiment of the antenna shown in Figure 3 having a series of short-circuiting pins disposed around the orthogonal crossed slots to suppress a possible even TEM propagation mode; - Figure 7 shows an array of flat antennas in which the invention is embodied fed by common excitation lines; and - Figure 8 shows an antenna in which the invention is embodied excited with a right-hand or left-hand circular polarization through a 90 0 - hybrid coupler with two input ports and two output portsThe flat antenna shown in Figure 1 is made up of two superposed plates of dielectric 1 and 2. The first plate of dielectric 1 has on its inner side, in contact with the other plate of dielectric 2, metallization strips 3, 4 along both orthogonal axes Ox and Oy of a system of coordinates traced in the plane of the antenna, in the-center 0 of the latter. The outer side of the first plate of dielectric 1 is metallized and forms a metal plane 5 viith two orthogonal radiating slots 6, 7, formed in it, disposed to form a cross centered on the origin 0 of the system of coordinates and oriented by 450 with respect to both orthogonal axes Ox and Oy. The second plate of dielectric 2 also has a metallized outer side. The latter forms a reflecting metal plane B. The excitation lines 3, 4 form, with the metal planes 5 and 8, the central conductors of two striplines. As this is apparent in Figures 2 and 3, they penetrate between the two superposed plates of dielectric 1, 2 up to the region of the radiating slots 6, 7. In the vicinity of the latter, they follow the orthogonal axes Ox, Oy, intersect while joi- ning in the center 0 of the system of coordinates, then part to finally stop after a short distance. Each of them faces the sides of the radiating slots 6, 7 in order to excite them. They are decoupled by their extensions which produce a short-circuit at their junction in the center of the an- tenna, in the origin 0 of the system of coordinates.

The structure of the antenna is very flat since it is limited to the assembly of two dielectric substratesof low thickness which may typically be of 1.58 mm. The choice of the dielectric material of the substrates is unimportant provided it has a low loss in the selected band of frequencies. It is determined by technological considerations (differential thermal expansion with the support of the antenna, cost, etc.) or radio requirements (the permittivity determines the phase velocity of the waves in the striplines and the size of the elements which are a function of the wavelength). The dielectric material may even be air if it is possible to maintain the spacing between the metal strips 3, 4 of the central conductors of the striplines and the metal planes 5, 8, which can be achieved by means of metal spacers and localized dielectric spacers. Finally, the dielectrics of the two plates may be differ&nt in permittivity and in thickness. In the latter case, the striplines are 6 unsymmetrical. A limit case is that of the antenna shown dismounted in Figure 4 where the second plate of dielectric carrying the reflecting metal plane is replaced by a simple metal plate 10 spaced from the first plate of dielectric 9 by 2 layer of air and maintained attached to the latter by a system of spacers not shown. The excitation lines are then close in construction to the conventional microstrip with a metal cover.

In the implementation, it is indispensable that the in- tersection point of the radiating slots 6, 7 coincides with that of the excitation lines 3, 4 (Figure 3). This is a condition of decoupling between the orthogonal polarizations. To this end, it is desirable to implement the etching of the radiating slots 6, 7 and of their excitation lines on the same substrate, for the positioning marks on both sides of a substrate can easily be brought into coincidence. The second substrate with its ground plane serves only to close the structure.

The length L F of the radiating slots 3, 4 (Figure 3) is determined by the permittivity of the selected material. As the first resonance of the slot is used, the length of the latter is close to l 6eq /2, where C- e q is the wavelength taking into account a permittivity equivalent to that of the dielectric interface. This length may, in practice, be slightly modified during impedance matching with the excitation lines 3, 4.

The dimensions of each excitation stripline 3, 4 are determined, for one thing, by the choice of the dielectric material (thickness and permittivity) and, for another, by the desired characteristic impedance. The latter is typi cally 50 ohms but may be between a few tens and a few hundred ohms. Its selection depends essentially on the type 7 of connection of the elemental antenna or on the desired distribution of power between several elemental antennas. Once the dielectric material is selected, it is the width of the metallized strip of the central conductor which defines the characteristic impedance of an excitation line. The determination of this width is performed in accordance with a method known by those skilled in the art, for example with reference to the book of H. Howe Jr entitled I'Stripline circuit design", Artech House, 1974.

Another condition of decoupling between the two orthogonal polarizations is that of -the decoupling between the excitation lines in spite of their junction in the center of the antenna. This decoupling between the excitation lines is obtained thanks to the extension L T of the lines beyond their intersection point, which create a short-circuit in the latter. These extensions are terminated either in open circuit, or in closed circuit by a short-circuit. When they terminate in open circuit, as shown in Figure 3, they have a length of about L A (where L is the wavelength of the wave propagating in the line). When they are terminated by a short-circuit formed by a plated-through hole 20 connec ting them to both metal planes 5 and 8 of the antenna, as shown in Figures 5a and 5b, they have 2 length of about L /2.

Referring to Figure 6, there is shown a further preferred embodiment of the antenna in which the present invention is embodied in which there is a number of plated-through holes 25 which connect the two ground metal planes of the antenna and which are disposed symmetrically about the slots. These plated-through holes 25, which may be replaced by added me tal pins, serve to eliminate the presence of the even TEM pro a ation mode ossibly caused by the dissymmetry formed on one of the metal planes.

p 9 p by the radiating slots 8 Furthermore, changes in the width of the excitation lines 3, 4 may be envisaged as impedance matching elements. They are then identical in the two orthogonal excitation lines in order to retain the symmetry of excitation of the two radiating slots. These changes in width May be either localized and, in this case, assimilable to shunt reactive elements, or extended and, in this case, be used as impedance matching sections (of the quarter-wave type). They may be located before or after the intersection point of the excitation lines.

The principle of connection of the excitation striplines while ensuring their decoupling is as follows:

- an open line (i.e., loaded by an infinite impedance) crea tes a short-circuit at a distance equal to L A. In other words, at L /4 upstream, the standing wave pattern is cha racterized by a current antinode and a voltage node; - connecting any impedance to this point of short-circuit does not cause any modification in the regimes of the cur rents and voltages since the impedance is not supplied with current; - it is then possible to connect to this short-circuit point a second line also open at a length L /4 downstream the connection point. The two lines are decoupled, for they are interconnected at zero-potential points.

This property depends obviously on the frequency. This restricts the operation of the antenna to a limited band (typically a few percents), which anyway is the case of a radiating element of the slot type with its associated feed.

Strictly speaking, an open line is-terminated by a reactive load of the capacitive type, which translate in practice into a deviation from L A of the len-gth of the downstream section. Furthermore, the cross connection of two - 9 lines has a complex equivalent circuit. In a simplified manner, it is possible to say that the cross connection can be modelled by connecting a reactive term in parallel at the point of intersection.

In the case where both upstream sections of the excit2tion lines are terminated by short-circuits (plated-through holes traversing both substrates and interconnecting the two metal planes and the central conductive strips of the lines) and having consequently a length close to / L /2, the principle of operation remains the same. The C2p2Citi,,e effect at the end of an open line must only be replaced by an inductive effect due to the loop formed by the central strip of the line of interest and the short-circuiting connection.

Various shapes of slots retaining the symmetry and the orthogonality are usable. The same applies to the plan shape of the central strip of the striplines (variable width). In both cases, the purpose is to increase the impedancematching band, the principle of independence, or decoupling, of the polarizations remaining unchanged.

The flat antennas that have been described above allow to implement various radiating structures of small thickness with two orthogonal polarizations. An example of such an in Figure 7 which shows an array of flat orthogonal radiating slots 30 excited in paindependent stripline feeds, one 31 for hori- arization, and the other 32 for vertical polariweight the power distribution between the va- antenna is giver antennas with rallel by two zontal pol zation, which rious antennas.

With the type of flat antenna with double polarization which has been described above, it is also possible to obtain a radiation with a rotating right-hand or left-hand circular polarization, for example, as shown in Figure 8, - 10 by feeding the citation lines two horizontal and vertical polarization ex of an antenna 40 by means of the signals from the two output ports of a 900-hybrid coupler 43 with two input ports 44, 45, the excitation through one, 44, of the input ports causing a radiation'of the left-hand circular polarization antenna, while the excitation through the other input port, 45, causes a radiation of the antenna with a right-hand circular polarization.

- 11

Claims

1. A flat microwave antenna with two orthogonal polarizations, with a pair of orthogonal radiating slots disposed to ' form a cross and excited by two striplines, comprising a first and a second conductive plate placed one above the other and separated by a dielectric volume, said first conductive plate having, formed in it, said pair of orthogonal radiating slots disposed to form a cross, said second conductive plate playing the role of a reflector; and said two excitation striplines whose central conductors pass between said first and second conductive plates, each facing the sides of one of said slots, and intersect in the center of said cross formed by while being, at said wherein said central said pair of orthogonal radiating slots slots, oriented by 450 from the latter, conductors of the excitation striplines are disposed in a single plane between said two conductive plates and merge at their intersection in the center of said cross formed by said pair of orthogonal radiating slots.

2. An antenna according to claim 1, wherein said central conductors of both excitation striplines terminate beyond their point of intersection as open lines by an extension with a length equal to about L /4 (where i L /4 is the wavelength in the excitation striplines).

3. An antenna according to claim 1, wherein said central conductors of both excitation striplines terminate, beyond their point of intersection, as lines short-circuited by 12 - an extension having a length equal to about A L /2 (where L /2 is the wavelength in the excitation striplines) leading to a conducting pin short-circuiting the two conductive plates.

4. An antenna according to claim 1, including metal pins which connect said two conductive plates and which are disposed symmetrically around said pair of orthogonal radiating slots.

5. An antenna according to claim 1, comprising a first and a second plate of dielectric, said first plate of dielectric having on its inner side facing said second plate of dielectric metallization strips forming the central conductors of said excitation striplines, and on its outer side a metallized plane forming said first conductive plate and having, f9rmed in it, said pair of orthogonal radiating slots and said second plate of dielectric having on its outer side a metallized plane which forms said second conductive plate that plays the role of a reflector.

6. An antenna according to claim 1, comprising a plate of dielectric which carries on its inner side metallization strips forming the central conductors of said striplines, and on its outer side a metallized plane forming said first conductive plate having, formed in it, said pair of orthogonal radiating slots, and which is separated from said second conductive plate by a layer of air.

7. A flat microwave antenna substantially as described hereinbefore with reference to the accompanying drawings and as shown in Figures 1 and 2, Figure 3, or Figure 4, or Figures 5a. and 5b, or in Figure 6, or in Figure 7, or in Figure

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