GB2147744A - A radiating device with an improved microstrip structure and its application to an adaptable antenna - Google Patents

Abstract

The invention concerns radiating elements of the microstrip type. Dielectrics (not shown) separate a lower metal coating or earth plane (PM), an intermediate metal slab (P11) and an upper metal slab (P12). Slab (P12) is connected to slab (P11) via short circuit pins (CC12), and slab (P11) is connected to the ground plane via short circuit pins (CC11). A coaxial cable has a screening (CB12) electrically connected to the ground plane (PM), whilst its core (CA12) passes through the dielectrics and without contact with either the ground plane or with the intermediate slab (P11), comes to be connected to the upper slab (P12). Of reduced size, and with a wide angular spread, such an element is capable of operating simultaneously on two frequencies with a very narrow band. It readily facilitates impedance matching at the power level. It is particularly worthwhile for the making of adaptable antennae, with a hemispherical cover, and capable of working in circular polarisation. In a further embodiment (Figs 11 and 12) individual coaxial feeds are connected to the top and intermediate slabs. Four units successively displaced by 90 DEG provide circular polarisation. <IMAGE>

Description

SPECIFICATION A radiating device with an improved microstrip structure and its application to an adaptable antenna The inventiqn concerns radiating devices or antennae.

It is known how to make radiating devices whose elements have a microstrip structure forming a resonator with losses. In view of their resonant character, these structures have a narrow band and the resonance losses which are fairly well localised, make them a worthwhile radiating element. Such radiating devices are described in the work "Microstrip Antenna, Theory and Design" by J.R. James, P.S. Hall and C. Wood, IEE Electromagnetic Waves Series, Volume No. 12, edited by Peter Peregrinus Ltd.

A radiating element of this kind comprises, on the one hand, a microstrip structure forming a resonator with losses and constituted by at least two stacked dielectric plates, with an extensive metallisation forming the ground plane, fixed under the lower plate of a first metallised slab of a chosen configuration placed between the two plates, and of a second metallised slab, also of a chosen configuration, affixed to the upper plate and on the other hand, means allowing an adapted electric connection between the said micro-strip structure and its circuit of utilisation which is to use the antenna element with a view to radio-electric emission or reception.

In certain applications, which may concern data transmissions as well as the determination of bearings, it is desirable to have radiating elements at one's disposal meeting constraining requirements: - the ability to work simultaneously on two frequencies with a very narrow band, of the order of a few percent of the central frequency on each of them, -an angular spread extending as near as possible to 1809, which corresponds with several radiating elements to a quasi-hemispherical cover, - as small dimensions as possible, and - the ability to function in circular polarisations, if required.

These requirements are especially encountered when one wishes to make certain types of adaptable antennae.

The radiating elements of the microstrip type available so far meet the requirements set out above only imperfectly.

On the other hand, the present invention arises to resolve the problem based on satisfying these requirements as a whole, on the basis of a radiating element with an improved microstrip structure.

In accordance with the present invention, there is provided a radiating device of the type comprising at least one radiating element which comprises: on the one hand, a microstrip structure forming a resonator with losses and constituted by at least two stacked dielectric plates with extensive metallisation forming the ground plane fixed under the lower plate, by a first metallised slab with a chosen configuration placed between the two plates, and by a second metallised slab also of a chosen configuration fixed on the upper plate; and on the other hand means allowing an adapted electric connection between the said microstrip structure and an application circuit; wherein the first and the second slabs which are placed in a chosen relative position are interconnected via a short circuit line placed in a chosen position near one edge of the second slab; wherein the first slab is connected to the ground plane via a short circuit line which is also in a chosen position and near one edge of the first slab; and wherein the electric connection means comprise a duct passing through the dielectric plates, the ground plane and the first slab via openings in the first slab, as well as a coaxial cable whose screening is connected to the ground plane whilst its central conductor uses the said duct, without contacting either the ground plane or the first slab, to make electric contact with the second slab.

In particular first embodiment, the two short circuit lines are situated on the same side of the coaxial cable.

There is thus obtained a radiating element capable of functioning on two frequencies, with radiating zones which are reduced to a respective slot for each one of these two frequencies and a common adapted input for the use of the radiating elements on the two frequencies.

In another embodiment, the electric connection means comprise, moreover, a second duct passing through the lower dielectric plate and the earth plane, via openings arranged in the latter as well as a second coaxial cable whose screening is connected with the ground plane, whilst its central conductor uses the second duct without contacting the ground plane, to make electric contact with the first slab. Such a radiating element is capable of functioning on two frequencies with a radiating zone practically reduced to a single slot for each one of these two frequencies, and with a separate electric input adapted to each of the slabs which is associated with a respective one of these two frequencies. Preferably in the second embodiment, the two short circuit lines are placed on either side of the set of the two coaxial cables.

In practice, the short circuit lines are advantageously made in the form of a series of short circuit pins, preferably interspaced in a substantially regular manner.

As regards the geometrical configuration of the slabs, the latter is substantially rectangular, in the mode of embodiment which is presently preferred by the applicant. The dimension of the slab in the transverse direction to the short circuit line, which is thus a straight line, is substantially equal to one quarter of the wave length A g, with Ag = Ao/ < where A o is the wave length in air at the central frequency whilst er represents the relative permittivity of the dielectric.

For its part, the other dimension of the rectangle in the direction parallel to the short circuit line, may be chosen between one quarter and half of this same wave length, approximately.

Moreover, it is currently considered desirable for the second slab to be a little smaller than the first that is to say, the projection of the second slab in the plane of the first slab is completely contained in the external contour of the first slab.

The radiating elements thus defined satisfy all of the requirements set out above, with the exception of the fact that for each frequency, being comparable to one radiating slot, their radiation patterns duly open out over an angle close to 180O, but only within one zone. This zone is defined by the immediate proximity of the plane perpendicular to the short circuit lines, and passing through the coaxial cable or cables.

To obtain the hemi-spherical cover in circular polarisation, one uses sets of four radiating elements as defined above, placed near each other, but in different orientations, obtained by rotation of 90". That is to say, that if the angle 0 is given for the direction of one of the structures, the three others will be, in relation to the first, rotated by 90 , 180 and 270".

This may be applied to either of the two particular modes of embodiment set out above. In practice, the set of radiating elements is obtained on the same stacked dielectric plates. Because of this, and taking into account the connection via the coaxial cable coming from below, the set has very small dimensions.

The sets of radiating elements thus obtained are particularly worthwhile for being used in the making of certain types of adaptable antennae, capable of functioning in circular polarisation if required.

More generally, it may be applied to other types of antenna arrays. it is recalled that an adaptable antenna is an antenna constituted by an array of radiating elements which are electrically accessible on an individual basis which makes it possible to adapt the antenna to the signal which is to be received by a coherent weighted and selective summation of the signals relating to each one of the elements and to eliminate, on the other hand, the interfering and/or undesirable signals.

Other characteristics and advantages of the invention will appear on examining the following detailed specification as well as the attached drawings, wherein: Figure 1 illustrates an example of a conventional microstrip element functioning on a single frequency; Figures 2 and 3 illustrate the radiating characteristics of the element of Figure 1; Figure 4 illustrates another example of a conventional microstrip element, comparable to that of Figure 1, whilst Figure 4A is a conventional representation of the microstrip element of Figure 4; Figure 5 illustrates a variant of the microstrip element of Figures 4 and 4A, wherein the feeder line is placed asymmetrically in order to obtain impedance matching; Figure 6 illustrates another example of a conventional microstrip element capable of working on two frequencies;; Figure 7 illustrates yet another example of a conventional microstrip element, also capable of working on two frequencies; Figure 8 illustrates, in the form of a schematic view in perspective, without the dielectric being interposed, an embodiment of the two frequency microstrip elements in accordance with the present invention; Figure 9 is a conventional schematic view of the element of Figure 8, the convention of the representation being the same as for Figure 4A in relation to Figure 4; Figure 70 is a cross sectional view of the device of Figure 8, along the section line X-X; Figure 11 is a schematic conventional view of a second mode of embodiment of the present invention wherein provision is made for two separate electric inputs for the two operating frequencies of the device;; Figure 12 is a cross sectional view along the section line XII-XII of Figure 11; Figure 13 illustrates in the same conventional representation as above, a set of four radiating elements of the type of Figure 8, allowing a substantially hemi-spherical cover with circular polarisation; and Figure 14 illustrates, again with the same convention of representation, a set of four radiating elements according to Figure 11 allowing a substantially hemi-spherical cover.

Figure 1 illustrates the general aspect of a very simple microstrip element in perspective. A dielectric plate D1 is metallised on its lower part covering the whole of the lower surface so as to form a ground plane designated PM. On its upper part, it is metallised with a delimited geometric configuration which here is rectangular, denoted P1. On one of its sides, this rectangular metallisation P1 is extended by a feeder line or tail section LA1. The making of such metallisations on a dielectric substrate may be effected by the techniques of serigraphy or other equivalent techniques known by the specialist in hybrid circuits.

In the present specification, a metallised surface of a chosen configuration such as P1 will be called a "metallised slab", although the word "slab" is sometimes also used for this metallisation and the dielectric substrate supporting it as a whole.

Figure 2 is a top view of the microstrip element of Figure 1. This top view is shown in a conventional representation, where both the dielectric and the ground plane are omitted so as to simplify the drawing. It will be seen in Figure 2 that the dimension of slab P1 transversely to the feeder line LA1 is W, whilst its dimension parallel to the feeder line LA, is L.

It is known that such a slab constitutes a resonator with losses whose losses are localised at the level of the two edges or sides with dimension W situated one on the side of the feeder line LA, and the other on the opposite side. As is shown in Figure 3, the radiating element of Figures 1 and 2 may thus be likened to two very narrow radiating slots F" and F12 and with length W. Figure 3 also illustrates the direction of the electrical field E on the slot of the element. Figures 1 to 3 are associated with a right angled reference system Oxyz allowing their respective orientations to be defined. In Figures 2 and 3, axisZis perpendicular to the plane of the Figure and is directed towards the observer.

A radiating element such as is shown in Figures 1 to 3 is operating on a single frequency with a very narrow band. However, it does not provide an angular spread extending over approximately 180 because it may be likened to two separate radiating slots F11 and F12 as is shown in Figure 3.

Figure 4 illustrates another type of known microstrip radiating element which has the advantage of having only one radiating slot. It should be observed that the dielectric is not represented in Figure 4, the latter only showing that the ground plane is connected to the metallised slab P4 via a short circuit line CC4 constituted by a plurality of short circuit pins or metallised holes which are here aligned and close to an external edge of the metallised slab P4. In this case, there only remains the radiating slots situated on the side of the feeder line LA4 on the edge perpendicular to the latter. The conventional representation of Figure 4A will be noted in passing wherein neither the dielectric nor the earth plane appear. The short circuit line CC4 is illustrated by a dash line on a top view of the metallisation P4 and of its feeder line LA4.

Such a device would then ensure an angular spread slightly less than 180 on one single frequency. It does, however pose problems with the impedance matching as regards its feeder line LA4. Furthermore, the feeder line LA4 being at least partly comprised in the metallisation plane P1 increases the overall dimensions of the device.

Figure 5 illustrates a variant of Figure 4 wherein the feeder line LA4 is displaced so as to ensure the impedance matching. Independently of the delicate manipulations which are required to find the proper position for line LA4 in each particular case of impedance matching, the fact remains that the line LA4 extends laterally in relation to the metallised slab P4 and thus increases the dimensions of an individual resonating element.

Reference will now be made to Figure 6 which this time concerns a radiating element capable of functioning on two frequencies.

The radiating element now comprises two stacked dielectric plates D1, D2 which after having been given the metallisations described below, may be connected to each other, for instance, by bonding. Under plate D1, there is located the ground plane PM. Between plates D1 and D2, there is located a first metallised slab or the low metallisation slab P6B. On plates D1 and D2 there is located a second metallised slab or high metallised slab P6H. The slab P6B has no supply line. On the other hand, slab P6H has one, as shown at LA6.

The dual frequency radiating element of Figure 6 has the drawbacks of offering two radiating slots on the two respective edges of the two slabs for each one of the two operating frequencies. It is subject to the impedance matching problem described above as far as the exact implantation of the feeder line LA6 is concerned. It is also subject to additional lateral size due to the presence of this feeder line LA6.

In the field of dual frequency radiating elements, there is, moreover, known the embodiment illustrated in cross section in Figure 7. The metallised slabs P7H and P7B are of the same kind as in Figure 6 and may substantially have the same configuration. The feeder mode is, however, different. A feeder device in the form of a coaxial cable arrives from below. Its screening CB7 is connected to the ground plane PM. Its central conductor or core CA7 passes through the ground plane and the low slab P7B without making contact with them, to be connected to a selected point of the high slab P7H.

The embodiment according to Figure 7 resolves the problem of size referred to above. There remains the face, however, that the two slabs P7B and P7H will each radiate along two parallel slots for their respective operating frequencies.

There will now be described the radiating elements in accordance with the present invention.

In a previously unexpected way, and taking into account the unforseeable result which may be produced by any modification made to a microwave device, it has been observed that in certain conditions, provision may be made for a radiating element with two metallised slabs, provided with short circuit in relation to each other and between the intermediate slab and the ground plane, while supplying the upper slab from below, by means of a device of the coaxial cable type whose screening is connected to the earth plane and whose core joins the upper slab.

A first device of this kind is illustrated in the view in perspective of Figure 8 where the dielectric has not been represented with a view to simplification. The ground plane PM will be perceived therein. The first slab P11 is connected to the ground plane PM via a line of short circuit pins CC11 close to one edge of slab P11. The second metallised slab P12 is connected to slab P11 via another short circuit line CC12 which is also close to an edge of slab P12.

In Figures 8 to 10, it will be seen that ducts are arranged in the form of cylindrical holes at OD11 and OD12 in the dielectric layers D1 and D2. Similarly, an opening OP11 is arranged in the lower slab P11. An opening OPM is arranged in the earth plane PM. The screening of the coaxial cable, which screening is designated as CB12 is soldered to the earth plane PM where it stops. The core CA12 of the coaxial cable passes through the above mentioned ducts to come to be soldered at a chosen point of the upper slab P12.

It has been shown that all the dimensions of the radiating element are critical. The configuration of the slabs must be chosen with care, as must the provision and positioning of their short circuit lines. This also applies for the position of the two slabs in relation to each other as well as to the position of the opening OP11 and that of the point where core CA12 joins the upper slab P12.

With the rectangular slabs illustrated in Figures 8 to 10, the position of the core of the coaxial cable may be defined by means of the values a, , y, and 5 illustrated in Figure 9. These values must then substantially satisfy the following relations: P = k (oi + P) 5 k' ( + 5) with k and k' of the order of 0.2 to 0.5, preferably around 1/3.

For their part, the dimensions of the rectangular slabs are related to their respective operating wave lengths by the following relations: widths

where Ao and A6 are the wave lengths in air at the central frequencies whilst er is the relative permittivity of the dielectric. The lenqth of the slab may range from its width to twice this width, for example

The expert will, of course, understand that other configurations may be used for the slabs other than rectangular ones for adjusting experimentally as a result, the position of both the short circuit lines and the passing points of the core of the coaxial cable.

In certain applications, it is desirable for the feed to be effected separately for the two operating frequencies. The present invention, for this purpose, proposes a variant which is illustrated in Figures 11 and 12.

In these Figures, there will be recognised earth plane PM, the two dielectrics D1 and D2, the intermediate metallised slab P21, with a preferably rectangular shape and the slab P22, also of a preferably rectangular shape.

In this mode of embodiment, the short circuit line CC21 between the intermediate slab P21 and the ground plane, is situated on one side (the right hand side of the two Figures). The other short circuit line CC22, between the intermediate slab P21 and the upper slab P22 is situated on the other side, that is to say, on the left hand side of the two figures. For the rest, and as before, the two short circuit lines CC21 and CC22 are respectively close to one edge of the rectangular slabs P21 and P22.

Ducts OD21 and OD22 are formed opposite each other, in the two dielectric layers D1 and D2. The duct OD is screened. Opposite the two ducts, slab P2, comprises an opening OP21 whilst the lower metallisation PM comprises an opening OPM21. The screening CB22 of this first coaxial cable comes to be soldered to the lower metallisation PM around the periphery of opening OPM21. Core CA22 of this same coaxial cable passes through the above mentioned ducts to come to be fixed at a chosen point of the upper slab P22.

At a certain distance, here on the other side of the set of the two slabs P21 and P22, provision is made for a second feeder line by coaxial cable which concerns only slab P21. A vertical tubular opening OD20 is formed in dielectric D1 and, opposite the latter, the metallisation PM comprises another opening OPM20. A second coaxial cable has a screening CB21 connected to the ground plane PM substantially over the periphery of opening OPM20. The core of this second coaxial cable designated CA21 passes through the opening OPM20 and tube OD20 to come to be fixed at a chosen point of the intermediate slab P21.

It has been observed that the embodiment of Figures 11 and 12 authorises a separate and adapted electric supply for the radiating device, respectively for the two operating frequencies of the latter.

Although the intervening phenomena are not completely explained, it seems that the location of the short circuits on either side of the metallisations and the implantation of the cores of the two coaxial cables, near the said short circuits, and in two relatively distant zones from each other, makes it possible to contribute to rendering the said feeder supplies substantially independent from each other.

The interspacing is at present covered by: ' - k' (t' + ') 5' k" (' 5') These values are slightly different from the preceding ones to offset the interferences induced by the new feeder mode. The dimensions of the slabs are the same as before.

It seems, at present, that the embodiment of Figures 11 and 12 cannot be applied generally beyond two frequencies, at least as regards the possibility of separate supply for each frequency.

On the other hand, it appears that the mode of embodiment described first with reference to Figures 8 to 10 can be applied generally to more than two operating frequencies as long as provision is made for more than two superposed metallised slabs with a suitable number of dielectric plates interposed between them.

As indicated above, the radiating elements in accordance with the invention, meet the whole of the requirements set out above, with the exception of the fact that they may each be likened to a radiating slot whose radiation pattern is spread over an angle of 180G but only within one zone.

The aim of the present invention is also to obtain true hemi-spherical cover and also to improve the capability of the radiating elements to function in circular polarisation.

For this purpose, the invention makes provision for the use of sets of four radiating elements orientated in four different directions (although a single element would suffice for other applications).

Figure 13 illustrates a set of four radiating elements such as described with reference to Figures 8 to 10.

These radiating elements are designated E10 - 1 to E10 - 4. According to the geometrical position of their short circuit line, the expert will understand that they are orientated in respective directions which are interrelated like the angles of 0 , 90 , 180 and 270 .

Figure 14 does in the same way illustrate another set of four radiating elements, the latter now being of the type illustrated with reference to Figures 11 and 12. The radiating elements E20-1 toE20.4 are in angular positions which may be associated with angles 0 , 90 , 180 and 270 as before.

Whether one is concerned with a set of radiating elements of Figure 13 or Figure 14, it should be recalled that these benefit from an independent supply which is either common to the two operating frequencies for a given radiating element, in the case of Figure 13 or separate for a given radiating element, and for the two operating frequencies, in the case of Figure 14.

The expert will understand that by means of such sets of radiating elements with separate feed, one may obtain adaptable antennae functioning on at least two frequencies with a narrow band for each frequency and with a quasi-hemispherical angular spread. It is clear that the dimensions of these antennae will be very small. Finally, the latter will be able to operate suitably in circular polarisation if required. It will, moreover, be observed that the impedance matching of the different feeds for the radiating elements is done in a convenient way by means of coaxial lead-in cables connected to each element.

Claims (11)

1. A radiating device of the type comprising at least one radiating element which comprises: on the one hand, a microstrip structure forming a resonator with losses and constituted by at least two stacked dielectric plates with extensive metallisation forming the ground plane fixed under the lower plate, by a first metallised slab with a chosen configuration placed between the two plates, and by a second metallised slab also of a chosen configuration fixed on the upper plate; and on the other hand means allowing an adapted electric connection between the said microstrip structure and an application circuit; wherein the first and the second slabs which are placed in a chosen relative position are interconnected via a short circuit line placed in a chosen position near one edge of the second slab; wherein the first slab is connected to the ground plane via a short circuit line which is also in a chosen position and near one edge of the first slab; and wherein the electric connection means comprise a duct passing through the dielectric plates, the ground plane and the first slab via openings in the first slab, as well as a coaxial cable whose screening is connected to the ground plane whilst its central conductor uses the said duct, without contacting either the ground plane or the first slab, to make electric contact with the second slab.

2. A device according to claim 1, wherein the electric connection means comprise a second duct passing through the lower dielectric plate and the ground plane via openings arranged in the latter as well as a second coaxial cable whose screening is connected to the ground plane whilst its central conductor uses the said duct, without contacting the ground plane, to make electric contact with the first slab.

3. A device according to claim 1, wherein the short circuit line of the first slab and that of the second slab are situated on the same side of the coaxial cable.

4. A device according to claim 2, wherein the short circuit line of the first slab and that of the second slab are situated on either side of the coaxial cable or cables.

5. A device according to any one of claims 1 to 4, wherein each short circuit line comprises a series of short circuit pins, interspaced in a substantially regular manner.

6. A device according to any one of claims 1 to 5, wherein at least one of the metallised slabs is of a substantially rectangular shape.

7. A device according to any one of claims 1 to 6, wherein the projection of the second slab in the plane of the first slab is entirely contained in the external contour of the first slab.

8. A radiating device according to any one of the preceding claims, and comprising at least one set of four substantially identical radiating elements, mounted side by side in different directions, and displaced in relation to each other by 90 .

9. A device according to claim 8, wherein the or each or four radiating elements is mounted on a respective stacked dielectric plate.

10. An indicating device substantially as hereinbefore described with reference to, and as illustrated in Figures 8 to 10, or Figures 11 and 12, or Figure 13, or Figure 14 of the accompanying drawings.

11. Use of the device according to one of the preceding claims as an adaptable antenna, operating on at least two frequencies, with a narrow band on each frequency, and with a quasihemispherical angular spread.