DE202004020508U1 - Antenna system for simultaneous operation in different frequency ranges e.g. for submarine, using 2 quadrifilary helix antennae with double coils wound around common longitudinal axis - Google Patents

Abstract

The antenna system has at least 2 quadrifilary helix antennae (3,4) offset from one another by 90 degrees and provided with double coils (R0,R90; T0,T90) having a 90 degree phase shift, both antennae wound about a cooom longitudinal axis and rotated through an angle of 45 degrees to one another, such that the double coils have the same spacing from the longitudinal axis in a given perpendicular plane.

Description

I. Field of application

The The invention relates to an antenna system for simultaneous operation in at least two different frequency ranges, in particular for sending and receiving in full duplex mode, especially for use in submarines.

II. Technical background

at Satellite communication between fixed and moving Users from aviation and shipping, thus including among others Satellite communication of submarines, usually become antenna structures used in a circular polarization, a quasi-hemispheric Radiating diagram, since in this case the need a tracking of the antenna system to the possibly geostationary satellite systems can be avoided.

To The current state of knowledge is the only antenna structure that has these required properties, a resonant quadrifilar Helix antenna. This usually exists from two double helices, which extend around a common longitudinal axis windings are orthogonal to each other and with 90 ° phase shift be stimulated.

There it is in such an antenna system to a resonant working System, the usable bandwidth is limited and it is difficult to adapt to broadband usage or to a use for to reach two different frequencies. In cases where one operation is required in different frequency ranges, therefore, usually on two separate, spatially separate quadrifilar helical antennas are used. this applies especially when operating simultaneously in both frequency ranges possible should be.

An example of such a case is the UHF Satcom satellite communication system, where the transmission and reception of data occurs in different frequency ranges: UHF Satcom reception 240-270 MHz UHF Satcom broadcast 290-320 MHz

When advantageous to look at this, if at the same time and independently can be sent and received (so-called full-duplex operation). there is a low coupling, so a low crosstalk between the transmitting / receiving antennas required. This is usually, as already mentioned above necessary that the transmit and receive antennas as separate elements are formed.

are However, the dimensions of the antenna system by the available Limited space, so can a spatial Separation of transmitting and receiving antennas made only with considerable effort.

An example of this is an antenna system of a submarine. In addition to the above-mentioned frequency ranges for satellite communication in the UHF Satcom system, even more frequency ranges are usually required beyond this, for example: Shortwave reception 1 - 30 MHz VHF reception (marine radio) 155 -160 MHz Line of sight function 225 - 400 MHz GPS 1575.41 / 1227.6 MHz

In addition to the limited space available, there is a further requirement in particular for an antenna system used by submarines that the entire structure of the antenna due to the positioning on the outer skin of the submarine hull and the associated burden of the circulating water as possible must be compact. At the same time, the greatest possible stability of the antenna system with respect to mechanical loads is to be ensured, since short-term horizontal loads of up to 400 G and vertical loads up to the range of 150 G can occur.

III. Presentation of the invention

a) Technical task

It is therefore the object according to the invention, one possible to provide a compact antenna system, which is a preferably simultaneous operation in at least two different frequency bands, in particular in the field of satellite communications using circular polarized waves with a quasi-hemispheric emission characteristic, allows.

in this connection such an antenna system preferably finds use a submarine. However, a user can also operate on conventional Ships over Water or on land benefit from the advantages of the invention.

b) Solution of the task

These The object is achieved by the characterizing features of claim 1 solved. Advantageous embodiments emerge from the dependent claims.

Accordingly, an antenna system for simultaneous operation, in particular for transmitting and receiving in full duplex operation, proposed in at least two different frequency ranges, which in addition to possibly other existing antennas comprises at least two quadrifilar helix antennas, each of which two orthogonal to each other and excited with 90 ° phase shift Double helix has. Here, orthogonal is to be understood that the two double helices of a helical antenna each run with an offset of 90 ° in the direction of rotation about the longitudinal axis of the helical antenna into each other, so that viewed in a radial sectional plane the imaginary connecting lines between the intersections of the respective helices of a double helix the plane orthogonal to each other, such as in 3 is indicated by the dashed lines. According to the invention, the two helical antennas wind around a common longitudinal axis, wherein they are rotated with respect to this longitudinal axis by an angle of 45 ° to each other. The double helixes of both helix antennas each have the same distance to the longitudinal axis in an arbitrary sectional plane transverse to the longitudinal axis.

These special arrangement of the two helical antennas is characterized a particularly high symmetry, so that according to the invention, the mutual Influencing the two antennas is minimized.

In a preferred embodiment have the respective double helices of the two helical antennas as well the same slope, causing the symmetry of the system again can be further improved.

Preferably Here are the double helices of the two helical antennas on the lateral surface one to the longitudinal axis rotationally symmetrical body arranged. As a rotationally symmetric body comes here in particular a cylinder in question, but other geometric bodies are like For example, a cone conceivable.

The Infeed of the respective helix antenna takes place in a transverse to longitudinal axis extending plane, which lies at one end of the respective helix.

In This feed-in level is the feed of the respective helical antenna on usual Way, d. H. that the respective double helices with a phase shift be operated by 90 degrees, with the feed of each one Double helix preferably over a suitable balun occurs. Come as baluns all known Symmetrieübertäger with Windings or line elements of the known constructions in Question, for example pots, Balancing loops and pots, Slot exchanger.

In a preferred embodiment the symmetrization of each double helix takes place via a symmetry transformer in Shape of a short-circuit cable with λ / 4-length.

The phase-offset feed for generating the circular polarization is preferably carried out under Use of hybrid couplings or the like.

In a preferred embodiment the two feed levels of the two helix antennas coincide. The common entry level of both helix antennas can hereby preferably at the end facing the radiation direction of the Helix antennas are lying. In this case, the preferably used Symmetry transformers easily in a symmetrical arrangement in the Inside the helix antennas are housed.

Prefers are the double helices of at least one helical antenna on the not excited end open. In this way, over the length of the double helices, the respective Resonance of the helix antenna can be influenced. In this embodiment have the double helices of the two nestled nested Helix antennas have a different length, so the two Helix antennas have different resonance frequencies.

at the two frequency ranges on which the two helical antennas of the inventive antenna system can be operated it may preferably be in the range of 210-300 MHz, in particular 240-270 MHz and from 260 - 350 MHz, in particular 290-320 MHz, act to send and receive in this way in the UHF Satcom System to enable.

Even though the two helix antennas of the antenna system according to the invention preferably can be operated simultaneously d. H. in so-called full-duplex operation, can They are also used in so-called half-duplex operation at which is always only one of the two antennas in operation.

Which The two modes ultimately used depends on the requirements of the user as well as the electronic equipment of the antenna system.

Preferably are the two helix antennas of their geometric dimensions, that is, for example, number of turns or pitch of the turns, but especially of their relationship Diameter to length designed so that the one helical antenna resonates in the field from 210 - 300 MHz, especially 240-270 MHz and the second helix antenna their resonance in the range of 260 - 350 MHz, especially 290 - 320 MHz has and at the same time for circular (preferably right polarized) polarized waves a omnihemisphärische Radiation characteristic is achieved.

According to one Another aspect of the present invention comprises the antenna system according to the invention at least one other antenna coaxial with the two helix antennas, preferably arranged in the interior defined by the two helices is.

at This at least one other antenna may be, for example to act a monopole emitter, in particular a rod antenna. However, other types of antennas are conceivable, as long as a coaxial arrangement, preferably in the interior of the helix antennas possible is.

Around one possible low coupling between the helix antennas on the one hand and the additional Antenna on the other hand, is the most accurate coaxial alignment possible the additional Antenna in the center of the helix desirable. Accordingly, therefore, the radiators of the additional Antenna on a rotationally symmetrical shape.

In a preferred embodiment of the antenna system according to the invention If the other antenna is a rod antenna a roof capacity in the form of a plate, due to its geometric dimensions for reception in the frequency ranges of 0.5-60 MHz and 125-190 MHz, in particular in the frequency ranges from 1 to 30 MHz and 155 - 160 MHz can be used.

As has already been indicated above, is a possible symmetrical arrangement of the antenna system for a decoupling of the individual antennas from each other extremely important. In order to achieve such a symmetrical arrangement also in the environmental conditions, which the antenna is subjected to in use, to ensure is according to another aspect of the present invention a special mechanical structure provided.

Accordingly, the antenna system according to the invention preferably comprises a stable base body, on whose outer surface the double turns of the two helical antennas are fixedly arranged along a longitudinal axis of the base body. The double helices can in this case, for example, the shape of wires have, which are mounted on the outer surface. Preferably, however, are bands of an electrically conductive material, such as copper bands, which are arranged flat on the outer surface of the base body.

Of the body consists of any electrically non-conductive material, the desired ones mechanical properties stability and / or having rigidity. Conceivable here are polymers or other Plastics, for example Kapton, Mylar or the like.

According to one preferred embodiment indicates the basic body of the antenna system in the form of a hollow cylinder. The feed the helical antenna is on a front side of the body, preferably seen from the attachment side of the antenna system at the distal End and thus at the direction of the irradiation facing end.

On the front side of the body The feed of the double helixes takes place from a central point which essentially coincides with the longitudinal axis of the helix antennas and thus the basic body coincides, over radial outward running segments of conductive material on the front are applied and each lead to the mating double helices.

Around to prevent the preferably running inside the main body Feeders and the according to a preferred embodiment respectively associated Balanced transformers in the form of λ / 4 short-circuit cables change its position under mechanical load of the antenna system and thus their necessary for decoupling the two helical antennas, lose symmetrical arrangement, these lines are according to the invention by a sufficiently rigid foam material held stationary at their positions. For example, polyurethane foam, Polystyrene, polyvinyl chloride foam, but also other foams used as long as necessary for holding the lines Stiffness guaranteed is.

Next The necessary stiffness should also be the foam used still have special electrical properties, in particular the radiation characteristics of the helical antennas as possible little to influence. Accordingly, according to the invention has the used Foam preferably a dielectric constant and a loss factor similar to those of air.

in this connection can the entire hollow interior of the body with the foam material be lined, the lines then through corresponding channels in the Foam material out become. However, one or more cylinders are preferred from the corresponding foam material used along the longitudinal axis of the basic body are aligned and on the lateral surfaces suitable grooves for receiving and storage of the corresponding lines are provided. In this Case remains free the remaining hollow interior of the body.

Preferably are the lines through the foam used on the large part the length in the Interior of the body kept at a constant distance. This distance, however, will reduced near the entry point on the face of the body, to get through one as possible small transition between the respective feed line and the associated balancing one preferably to get good symmetrization.

In a preferred embodiment can Of course inside the foam body holes for further Lines for the supply of additional Be provided antennas.

To the Protection of the antenna system against water and the usually prevailing water pressure at the use with a submarine is the entire construction according to the invention with a Hood e.g. surrounded by plastic. This plastic hood has a substantially cylindrical shape, which at its respect to the Attach the antenna system to the submarine hull's distal end using a hemispherical cover is provided and at its proximal end to be attached to the hull Plate is tightly connected. The outer diameter of this plastic hood is determined by the space available during installation. As materials come here, for example, glass fiber reinforced plastic, Epoxy resin or the like in question.

When used on a normal ship or on land, this plastic hood essentially serves to protect against external environmental influences. Since the requirement of resistance to pressure in this case is not given, the thickness of the hood may be lower and / or lower requirements be placed on the material used.

c) embodiments

A embodiment according to the invention is described in more detail below with reference to the figures by way of example. Show it:

1 a sectional view through an embodiment of an antenna system according to the invention with two quadrifilar helix antennas;

2 : a top view of the cover plate on the front of the in 1 shown embodiment, on which lies the common feed plane of the helix antennas;

3 : a plan view of the connection plate of the supply lines of in 1 embodiment shown;

4 : a sectional view of the in 2 shown cover plate and

5 : a detailed representation of a subsection of 1 ,

1 shows a sectional view through an embodiment of an antenna system according to the invention. This includes two quadrifilar helix antennas 3 . 4 on the surface of a body 1 are arranged in the form of a hollow cylinder.

In the cylindrical body 1 is at its respect to the attachment of the antenna system proximal end a half-open mounting cylinder 6 whose closed end points in the direction of the radiation of the antenna system. In the middle of this face 61 of the mounting cylinder 6 is a circular recess for receiving a mounting plate 5 provided with the help of screws 56 on the mounting cylinder 6 is attached.

2 shows a top view from below of the mounting plate 5 , Next to the screw holes 55 to fixing the plate 5 on the mounting cylinder 6 shows the plate 5 initially four connections 51 . 52 . 53 . 54 for the feed lines of the double helixes T0, R0, T90, R90 of the two helix antennas 3 . 4 to which the respective of the hybrid couplings 62 (please refer 5 ) of the base module of the antenna system coming supply lines can be connected. These cables are conventional coaxial cables.

To each of these four ports 51 . 52 . 53 . 54 is in each case diametrically to the longitudinal axis of the body 1 a connection 51a . 52a . 53a . 54a provided, each of the end of belonging to the corresponding double helix T0, R0, T90, R90 λ / 4 short-circuit line 8th represents. This total of eight ports 51 . 52 . 53 . 54 . 51a . 52a . 53a . 54a are symmetrical at equal angular intervals of 45 degrees on a circular line with a constant distance to the longitudinal axis of the body 1 arranged.

Of the four connections 51 . 52 . 53 . 54 run inside the cylindrical body 1 coaxial cables 7 in the direction of the distal end of the main body 1, there first the cover plate 2 with which the distal end of the main body 1 is closed, pierced in a central area. As in 3 is shown, here are the respective outer conductor of the coaxial cable 7 on the front side of the cover plate 2 with sectors 21 . 22 . 23 . 24 connected. These consist of conductive material and lead from the central region in each case radially outward, to feed from there the first coil of the respective double helix T0, R0, T90, R90. In this case, the sectors exist 21 . 22 . 23 . 24 from a copper foil, on the cover plate 2 made of non-conductive material are applied.

Each diametrically to the sectors 21 . 22 . 23 . 24 , each with the outer conductors of the corresponding feeders 7 are connected, there are sectors 21a . 22a . 23a . 24a , which also consist of copper foil and radially lead radially from the central region to the outside, from there to feed the second coil of the respective double helix T0, R0, T90, R90.

In order to obtain a correct supply of the respective double helices T0, R0, T90, R90, the respective inner conductor becomes 7a the feeders 7 a double helix T0, R0, T90, R90, as in 4 represented, with the respective double helix T0, R0, T90, R90 belonging, diametrically-lying sector 21a . 22a . 23a . 24a and an associated line 8th connected. This line 8th is formed in the present case in each case by the outer conductor of a coaxial cable, which, however, has no inner conductor. The outer conductor of the feed line 7 and the associated line 8th each form a λ / 4 short-circuit line for transforming the unbalanced signal of the feed line 7 in a symmetrical signal at the feed point. The administration 8th extends through the cover plate 2 into the interior of the main body 1 , crosses it and is conductive with the plate 2 connected.

Due to the requirement that this λ / 4 short-circuit line be a quarter of the wavelength of the resonance frequency of the respective helix antenna 3 . 4 must correspond to the location of the plate 5 and thus the geometric dimensions of the mounting cylinder 6 inside the body 1 ,

In the present case, the feeders become 7 and the wires 8th from the plate 5 starting inside the body 1 first of three aligned to the longitudinal axis of the body cylinders 9 made of foam material, as well as between these cylinders 9 arranged cutting discs 10 supported. The cylinders 9 in this case have an identical cross section, wherein the feed lines 7 and the wires 8th in corresponding grooves in the lateral surfaces of the cylinder 9 , as well as in through holes in the cutting discs 10 are guided. That way, the wires become 7 . 8th in its symmetrical arrangement with substantially identical distance to the longitudinal axis of the body 1 held.

Following the third cutting disc 10 become the wires 7 . 8th then in grooves in the lateral surface of a substantially cone-shaped foam body 9a led, which is towards the cover plate 2 rejuvenated. At this cone-shaped foam body 9a closes another cutting disc 10a with smaller radius, which in turn has through holes for the lines to be led through 7 . 8th having. Finally, between the cutting disc 10a and the cover plate 2 another cylinder 11 made of foam, which in turn has grooves in its lateral surface around the lines to be led through 7 . 8th take.

In this way, symmetrically the distance between the belonging to a double helix T0, R0, T90, R90 lines 7 . 8th be reduced, so that a better symmetrization of in 4 illustrated, exposed area of the inner conductor 7a becomes small compared to the wavelength λ.

In the present case is called foam material for the cylinder 9 , the cone-shaped foam body 9a and the cylinder 11 uses a material which preferably has the following physical properties: Density: <50 kg / m ³ , in particular <35 kg / m ³ dielectric constant: 1.0 - 1.1 for frequencies <26.5 GHz Dielectric loss factor: <0.002 for frequencies <10 GHz

(PMI -) - for this example, a suitable polymethacrylimide comes as a commercially available foam material such as foam rohacell ^® 31 in question.

The cutting discs 10 and 10a consist in the present case of a preferably very rigid material with good breaking strength and suitable dielectric properties. For example, these requirements of polyvinyl chloride (hard PVC) are met.

Furthermore, the cylinders 9 , the cone-shaped foam body 9a , the cylinder 11 as well as the cutting discs 10 and 10a in the present case, a central bore, so that optionally centrally on the longitudinal axis of the body 1 one or more feed lines for additional antennas, for example, on the cover plate 2 can be fixed, can be performed.

The double helices T0, R0, T90, R90 of the two helix antennas 3 . 4 are formed in the present embodiment, in each case by corresponding bands of copper foil, which on the lateral surface of the body 1 are wound. Consequently, in the present case, the respective double helices are open at the ends opposite to the feed. The bands are at the transition area between the cover plate 2 and the lateral surface of the body 1 electrically conductive with the respective sectors 21 . 21a . 22 . 22a . 23 . 23a . 24 . 24a connected to the double helices.

The two helix antennas 3 . 4 of the antenna system according to the invention can be operated in the selected embodiment in the two ranges of 240-270 MHz and 290-320 MHz, respectively Sending and receiving in the UHF Satcom system is possible. Since a resonance at a lower frequency is associated with a greater length of the corresponding double helices, it turns out 1 It can be seen that in the present case, the helical antenna 4 which operates in the range of 240-270 MHz.

Depending on the electronics used, the two helical antennas 3 . 4 in the present embodiment, either in full-duplex mode or in half-duplex mode.

Due to the inventive symmetrical arrangement of the two helical antennas wound in each other 3 . 4 In the present case, an isolation between the two antennas of greater than 15 dB can already be achieved. By additional use of filters, this decoupling can be further enhanced, for example, up to 30 dB.

The typical benefit of the present antenna system with the two helical antennas 3 . 4 is typically 0 dBi, but 2 dBi can be achieved. An omni-hemispheric radiation image is generated in a right-polarized radiation. The antenna impedance is 50 ohms. The maximum operating power 200 W.

5 shows a detailed view of a sub-area 1 and represents the base area of the antenna system with the mounting cylinder 6 dar. Inside the mounting cylinder 6 is housed in the embodiment shown here, an additional antenna.

This is a monopoly in the form of a rod antenna with a roof capacity. This monopoly is in the present case by a copper ring 63 formed by means of spacers 64 in a transverse plane of the main body 1 which is connected directly to the base module of the antenna system via an isolated connection.

The additional Rod antenna is in the present case for reception in the frequency ranges from 1 MHz to 30 MHz and 155 MHz - 160 MHz used.

Furthermore, they are located inside the assembly cylinder 6 the two 90 ° hybrid couplings 62 for generating the phase shift for the feeding of the helix antennas 3 . 4 , These are on one the basic body 1 final cover 66 arranged on the outside of the connections 65 are provided, via which the antenna system is connected to the supply unit, not shown.

The Hood for protecting the antenna system against environmental influences is in the present embodiment formed by a radome of glass fiber reinforced epoxy resin, its wall thickness from use, either by submarine or over water or ashore, depends.

1

body

2

cover plate

3

helix antenna

4

helix antenna

5

mounting plate

6

mounting cylinder

7

feeder

7a

inner conductor

8th

management

9

foam cylinder

9a

foam cone

10

cutting wheel

10a

cutting wheel

11

smaller Schaumstoffzylin

of the

21,21a

sectors

22 22a

sectors

23,23a

sectors

24,24a

sectors

51,51a

connections

52,52a

connections

53,53a

connections

54,54a

connections

55

screw holes

56

screw

61

Front side of 6

62

hybrid couplings

63

copper ring

64

spacer

65

connections

66

cover

T0

double helix

T90

double helix

R0

double helix

R90

double helix

Claims (11)

Antenna system for simultaneous operation in at least two different frequency ranges, in particular for transmitting and receiving in full-duplex mode, with at least two quadrifilar helix antennas ( 3 . 4 ), each of which comprises two double helixes (R0, R90, T0, T90) arranged in the direction of rotation with 90 ° offset from one another and excited by 90 ° phase shift, characterized in that the two helix antennas ( 3 . 4 ) are wound around the same longitudinal axis, wherein they are rotated with respect to the common longitudinal axis by an angle of 45 ° to each other and wherein the double helices (R0, R90, T0, T90) of both helical antennas ( 3 . 4 ) each have the same distance to the longitudinal axis in an arbitrary sectional plane transverse to the longitudinal axis. Antenna system according to claim 1, characterized in that - the double helices (R0, R90, T0, T90) of the two helical antennas ( 3 . 4 ) are arranged on the lateral surface of a body rotationally symmetrical with respect to the longitudinal axis and - in particular that the rotationally symmetrical body is a cylinder. Antenna system according to one of the preceding claims, characterized in that - the feed of the respective helical antenna ( 3 . 4 ) takes place in a plane extending transversely to the longitudinal axis, which is located at one end of the respective helix, - in particular that the feed planes of both helix antennas ( 3 . 4 ) and that - in particular the common feed-in level of both helix antennas ( 3 . 4 ) at the end of the helix antennas facing the emission direction ( 3 . 4 ) lies. Antenna system according to one of the preceding claims, characterized in that - the double helices (R0, R90, T0, T90) of at least one helical antenna ( 3 . 4 ) are open at the unexcited end or - that the double helices (R0, R90, T0, T90) of the two helical antennas ( 3 . 4 ) have the same slope. Antenna system according to one of the preceding claims, characterized in that - the double helices (R0, R90, T0, T90) of the two helical antennas ( 3 . 4 ) have different lengths, so that the helix antennas ( 3 . 4 ) have different resonance frequencies and - in particular the one helix antenna ( 3 . 4 ) its resonance in the range of 210-300 MHz, in particular 240-270 MHz and the second helical antenna ( 3 . 4 ) has its resonance in the range of 260-350 MHz, in particular 290-320 MHz. Antenna system according to one of the preceding claims, characterized characterized in that it is in the at least two frequency ranges among other things around the ranges 210-300 MHz, in particular 240-270 MHz and from 260 - 350 MHz in particular 290-320 MHz acts. Antenna system according to one of the preceding claims, characterized in that the two helix antennas ( 3 . 4 ) are designed with respect to their geometric dimensions, in particular their ratio of length to diameter, so that they have an omni-hemispherical radiation characteristic for right circularly polarized waves. Antenna system according to one of the preceding claims, characterized in that - coaxial with the two helix antennas ( 3 . 4 ) at least one further antenna is present, and - in particular that the at least one further antenna is a monopole radiator, in particular a rod antenna ( 63 ), which due to its geometrical dimensions can be used for reception in the frequency ranges of 0.5-60 MHz and 125-190 MHz, in particular in the frequency ranges of 1-30 MHz and 155-160 MHz. Antenna system according to one of the preceding claims, characterized in that - the symmetrization of the helix antennas ( 3 . 4 ) in each case via short-circuit lines, wherein a short-circuit line from a feed line ( 7 ) and a line ( 8th ) and - in particular that all feeders ( 7 ) and lines ( 8th ) in the longitudinal direction of the antenna system at the same distance from the longitudinal axis of the helix antennas ( 3 . 4 ) are arranged symmetrically, wherein each one feed line ( 7 ) and an associated line ( 8th ) diametrically opposite - and / or that in particular the feed lines ( 7 ) and / or lines ( 8th ) at least over a partial section through a foam material ( 9 . 9a . 11 ) are held stationary. Antenna system according to claim 9, characterized in that - the support of the lines ( 7 . 8th ) in grooves on the lateral surface of separate foam bodies ( 9 . 9a . 11 ) and - that in particular the foam body ( 9 . 9a . 11 ) have the shape of cylinders and cones. Antenna system according to claim 9 or 10, characterized in that - between the individual foam bodies ( 9 . 9a . 11 ) Plates ( 10 . 10a ) are arranged from an inflexible material, wherein the lines to be held ( 7 . 8th ) are guided through openings in these plates and / or - in particular the lines ( 7 . 8th ) are guided substantially at a constant distance from the longitudinal axis and the distance is reduced only in the vicinity of the feed points.