Radiation diversity antennas

The present invention relates to a radiation diversity antenna consisting of radiating elements of the slot-line type coupled electromagnetically to a feed line, in which the radiating elements have a tree structure, each radiating element having a length equal to kλs/2 where k is an identical or different integer from one element to the next and λs is the guided wavelength in the slot-line constituting the radiating element with at least one radiating element comprising a switching means positioned in the slot-line constituting the said radiating element in such a way as to control the coupling between the said radiating element and the feed line as a function of a command. The invention applies chiefly to wireless transmissions.

This application claims the benefit under 35 U.S.C. § 119 of French application number 0302842, filed Mar. 7, 2003.

FIELD OF THE INVENTION

The present invention relates to the field of radiation diversity antennas. This type of antenna can be used in the field of wireless transmissions, in particular within the context of transmissions in an enclosed or semi-enclosed environment such as domestic environments, gymnasiums, television studios, auditorium or the like.

BACKGROUND OF THE INVENTION

Within the context of transmissions inside enclosed or semi-enclosed environments, the electromagnetic waves undergo fading phenomena related to the multiple paths resulting from numerous reflections of the signal off the walls and off the furniture or other surfaces envisaged in the environment. In order to combat these fading phenomena, a well known technique is the use of space diversity.

In a known manner, this technique consists in using for example a pair of antennas with wide spatial coverage such as two antennas of slot type or of “patch” type that are linked by feed lines to a switch, the choice of antenna being made as a function of the level of the signal received. The use of this type of diversity requires a minimum spacing between the radiating elements so as to ensure sufficient decorrelation of the channel response seen through each radiating element. Therefore, this solution has the drawback of being, among other things, bulky.

To remedy this bulkiness problem, the use of antennas exhibiting radiation diversity has been proposed. This radiation diversity is obtained by switching between radiating elements placed in proximity to one another. This solution makes it possible to reduce the bulkiness of the antenna while ensuring sufficient diversity.

BRIEF SUMMARY OF THE INVENTION

The present invention therefore relates to a novel type of radiation diversity antennas.

According to the invention, the radiation diversity antenna consisting of a radiating element of the slot-line type coupled electromagnetically to a feed line, is characterized in that the radiating element consists of arms in a tree structure, each arm having a length equal to kλs/2 where k is an identical or different integer from one arm to the next and λs is the guided wavelength in the slot-line constituting the arm and in that at least one of the arms comprises a switching means positioned in the slot-line constituting the said arm in such a way as to control the coupling between the said arm and the feed line as a function of a command.

The antenna described above can operate in various modes exhibiting radiation patterns that are complementary as a function of the state of the switching means. With this tree structure, a large number of operating modes is accessible.

According to a preferred embodiment of the invention, each arm comprises a switching means. Moreover, the switching means is positioned in an open-circuit zone of the slot, this switching means possibly consisting of a diode, a transistor arranged as a diode or an MEMS (Micro Electro Mechanical System).

According to a further characteristic of the present invention, the length of each arm is delimited by an insert positioned in a short-circuit plane, the insert being placed at the level of the junctions between arms.

Moreover, the tree structure may exhibit an H or Y shape or one which is an association of these shapes.

According to another characteristic of the present invention, the antenna is produced by microstrip technology or by coplanar technology.

DESCRIPTION OF PREFERRED EMBODIMENTS

A preferred embodiment of the present invention will firstly be described with reference toFIGS. 1 to 7. In this case, as represented inFIG. 1, the radiation diversity antenna consists chiefly of a radiating element of the slot-line type formed of arms in an H structure. This structure is produced in a known manner by microstrip technology on a substrate1whose faces have been metallized. More specifically, this structure comprises five radiating arms1,2,3,4,5each consisting of a slot-line etched on the upper face on the substrate10and arranged in an H.

Moreover, as represented inFIG. 1, the slot-lines are fed by electromagnetic coupling according to the theory described by Knorr, via a single feed line6produced on the lower face of the substrate10. Therefore, as represented inFIG. 2, the feed line6is perpendicular to the slot5and extends over a distance Lm of the order of kλm/4 where λm is the guided wavelength in the feed line and λm=λ0/√εreff (with λ0the wavelength in vacuo and εreff the relative permittivity of the line), k being an odd integer. The feed line is extended beyond a distance Lm by a line6′ of length L and of width W which is greater than the width of the line6allowing a 50 Ohm connection. The five radiating arms1,2,3,4,5consist of slot-lines of length Ls in which Ls=kλs/2 with λs=λ0/√εr1eff, εr1eff being the relative permittivity of the slot and k being an integer which may be the same for each arm or different according to the desired tree.

To obtain an antenna with an H structure as represented inFIGS. 1 and 2, making it possible to obtain radiation diversity, switching means are positioned in the slot-line constituting the arm in such a way as to control the electromagnetic coupling between the said arm and the feed line. More specifically, diodes d1, d2, d3, d4, are positioned in each slot-line1,2,3,4in an open-circuit plane of the slot-line. As the slot-lines exhibit a length Ls=kλs/2, more particularly λs/2, the diodes are placed in the middle of each slot-line1,2,3,4. In the embodiment represented, a diode is placed in each of the slots. However, it is obvious to the person skilled in the art that a radiation diversity antenna would already be obtained with a single diode placed in one of the slots.

Moreover, according to another characteristic of the invention, metal inserts are placed in short-circuit zones of the arms of slot-line type, namely at the level of the junctions of the arms, as is represented inFIG. 2. The inserts being located in a short-circuit zone therefore do not modify the operation of the structure when none of the diodes d1, d2, d3or d4is active but they impose a zero-current apportionment in the slot-line when the corresponding diode is active.

Moreover, as will be explained in greater detail hereinbelow, when one of the diodes d1, d2, d3or d4is active, it imposes a short-circuit condition in the open-circuit zone of the corresponding arm of slot-line type, thereby preventing the radiation of an electromagnetic field in this element.

The manner of operation of the structure represented inFIG. 2as a function of the state of the diodes d1, d2, d3, d4will now be explained in greater detail with reference toFIGS. 1 to 7.

1) None of the diodes d1, d2, d3, d4is active: when the H structure is energized, a radiation pattern is obtained such as represented inFIG. 3afor a 3D representation orFIG. 3bfor a 2D representation. In this case, according to the 3D representation ofFIG. 3a, a quasi-omnidirectional radiation pattern is obtained with, in particular, two omnidirectional planes, one at φ=45° and the other at φ=135°. This is confirmed by the 2D pattern ofFIG. 3brepresenting a section through the planes φ=46° and φ=134°. Moreover, the curve ofFIG. 3bshows a maximum oscillation of the 3 db gain for the sectional planes.

2) Just one of the diodes is active, out of the four diodes d1, d2, d3, d4. Four modes of operation can therefore be defined. In this case, for each of these modes, the radiation pattern will possess a quasi-omnidirectional sectional plane. If, as represented inFIGS. 4aand4b, the diode d1positioned in the slot-line1is active, the plane φ=135° is a quasi-omnidirectional sectional plane, as represented in the 3D radiation pattern ofFIG. 4c.

In Table 1 below will be given the direction of the quasi-omnidirectional sectional plane in the case where each of the diodes d1, d2, d3or d4is active in turn as well as the variation in the gain in this plane.

3) Two diodes are active: the case where the diodes are active pairwise in the structure ofFIG. 2will now be described with reference toFIGS. 5a,5band5c. In this case it is possible to define modes of operation exhibiting a U, Z, or T structure as well as their dual modes. The structures have been simulated in the manner represented inFIG. 5band the radiation patterns obtained have shown that each of the modes exhibited a plane for which the radiation pattern is quasi-omnidirectional. Thus, when the diodes d2and d4are active, a U structure with a quasi-omnidirectional radiation pattern for a 90° sectional plane (FIG. 5c1) is obtained, as represented inFIG. 5a1. When the diodes d2and d3are active, a Z structure is obtained, as represented inFIG. 5a. In this case, the quasi-omnidirectional radiation pattern is obtained for a plane such that φ=67.5° (FIG. 5c2). For the dual Z slot obtained when the diodes d1and d4are active, the quasi-omnidirectional plane is obtained for φ=112.5°. When the diodes d3and d4are active, a T structure is obtained, as represented inFIG. 5a3. In this case, the quasi-omnidirectional radiation pattern is obtained for a sectional plane such that φ=0° (FIG. 5c3).

All the results are given in Table 2.

4)FIG. 6diagrammatically represents the case where three diodes are active. In this case, four modes of operation can be defined. For each of these modes, the radiation pattern possesses a quasi-omnidirectional sectional plane. The relation between the active diodes and the quasi-omnidirectional plane is given in Table 3 below.

According toFIG. 7which gives the SWR as a function of frequency, good matching is observed over a sizeable frequency band for the various modes, as a function of the number of active diodes.

By way of indication, the results given above, in particular the patterns, are the results of electromagnetic simulations carried out with the aid of the Ansoft HFSS software on an antenna exhibiting an H structure, such as is represented inFIG. 2, the structure having the following dimensions:

Slots1,2,3,4,5: Ls=20.4 mm, Ws=0.4 mm and i=0.6 mm (i representing the width of a metal insert across the slot simulating an active diode).

Moreover, represented diagrammatically inFIG. 8is the principle of the arranging of a diode in the slot-line, in accordance with the present invention. In this case, the diode used is an HP489B diode in an SOT323package. It is placed across the slot-line F in such a way that one of its ends, namely the anode, is connected to the earth plane P2produced by the metallization of the substrate and the other end, namely the cathode, is connected across a hole V to a control line L produced on the lower face of the substrate, as symbolized by the dashes, the hole V being produced in an element detached from the earth plane P1. The control line L is linked to a supervising circuit (not represented) enabling the diode to be turned on or off. This technique is known to the person skilled in the art and has been described, for example, in the article “A planar VHF Reconfigurable slot antenna” D. Peroulis, K. Sarabandi & LPB. Katechi, IEEE Antennas and Propagation Symposium Digest 2001, Vol. 1 pp 154-157.

The radiation diversity antenna described above exhibits a high diversity of radiation patterns that allows, in particular, its use in systems corresponding to the HIPERLAN2 standard. This antenna has the advantage of being easy to produce using a printed structure on a multilayer substrate. Moreover, the switching system is easy to implement. It can consists of a diode, as represented in the embodiment above but also of any other switching system such as diode-arranged transistors or MEMS (“Micro Electro Mechanical Systems”).

Represented inFIG. 9is a structure similar to that ofFIGS. 1 and 2but produced by coplanar technology. In this case, the feed line is produced on the same face of the substrate as the earth, as symbolized by the element7surrounded by etchings7a,7bwhich cut the slot-line5perpendicularly in its middle. The other elements of the radiation diversity antenna, namely the arms1,2,3,4produced by etching the earth plane A, so as to form the slot-lines, are identical to those ofFIG. 2. The various dimensions remain identical to those of a structure produced by microstrip technology.

The structure represented inFIG. 9is particularly attractive for circuits requiring transference of components.

Another embodiment of the present invention will now be described with references toFIGS. 10 and 11. InFIG. 10, one of the arms or slot-line1′ of the radiation diversity antenna exhibiting an H structure has a length λs while the other arms2,3,4,5have lengths λs/2. In this embodiment, an insert i is envisaged in the slot-line1at a length λs/2 and two diodes d1, d′1 are envisaged respectively at distances λs/4 and 3λs/4 from the start of the slot-line. Operation of the slot-line1is disabled when the diode d1is active. In this case, when only the diode d′1is active, only the second part of the slot-line1does not operate. We thus get back to the operation of an H structure with slot-lines of length λs/2.

Therefore, the present invention can be produced with structures exhibiting arms of slot-line type having lengths which may, if they are a multiple of λs/2, be identical or different for each arm.

Represented inFIG. 11is a 3D radiation pattern obtained by simulation with the aid of the Ansoft HFSS software for an antenna exhibiting a structure of the type of that represented inFIG. 10but in which all the arms1,2,3,4have a length λs, the diodes in this case being passive.

Moreover, the use of slot-lines having different lengths makes it possible to obtain frequency diversity in addition to radiation diversity. Specifically, the length of a slot-line conditions its resonant frequency. A slot-line is dimensioned so that its length L is such that L=λs/2 where λs is the guided wavelength in the slot. Moreover, the resonant frequency f being related to the guided wavelength,

f=cλ⁢⁢s,
if the dimension L is modified, then the frequency is also modified.

Yet another type of structure that can be used to obtain a radiation diversity antenna in accordance with the present invention will now be described with reference toFIG. 12.

In this case, the arm1is extended by two radiating elements1a,1bin such a way as to have a substantially Y structure. In the embodiment ofFIG. 12, the two radiating arms1aand1bare perpendicular, thereby giving the radiation pattern ofFIG. 12a. However, the angle between the arms1aand1bmay have other values while still giving the sought-after result. InFIG. 12, a slot-line1band a slot-line1ahave been added on the slot-line1so as to enlarge the tree. These two new slot-lines are coupled to the slot-line1in such a way that the slot-lines2and3are coupled to the slot-line4. By analogy with what was seen earlier, the slot-line1is coupled to the slot-lines1aand/or1bas a function of the state of the switching elements placed in these slot-lines1aand1b. This type of tree can also be envisaged on the slot-lines2,3and4, as well as on the added slot-lines, so as to arrive at a complex tree structure. Thus, the number of accessible configurations is increased as is, consequently, the order of diversity that the structure can provide. For a structure with N slot-lines (each of these slot-lines being furnished with a switching means), the order of diversity is 2N.