FR2565417A1 - Antenna formed from a resonant cavity containing a radiating face - Google Patents

Abstract

The antenna is formed of three parts 1, 2, 3 constituting a resonant cavity equipped with a radiating face 3. According to the invention, four plate-shaped dipole wires 11, 12, 13, 14 which are short-circuited at their end located at the periphery of the antenna and whose length is of the order of lambda 0/4 (where lambda 0 is the antenna's operating wavelength) are arranged on the radiating face 3, and an excitation system 31, 32, 33, 34 linking them to the outside is provided on each wire so that each wire is excited by a wave whose phase is shifted by virtually: 0 DEG , 90 DEG , 180 DEG , 270 DEG respectively by means of a distributor. Application: radio antenna for motor vehicle, arranged on the latter's rear shelf.

Description

l

  ANTENNA FORMED TO ROTATE A RESONANT CAVITY COMPRISING A FACE

RADIANT

  The present invention relates to an antenna formed

  from a resonant cavity provided with a radiating face.

  An antenna of this type is described in Patent Application No. 2,506,525. This antenna is intended to be mounted on the rear shelf of automobiles so that the radiating face is directed upwards. Thus the radiation of the antenna is reflected first of all on the metal parts of the automobile to be

  finally emitted in all directions.

  This known antenna thus provides the same services as those of a whip antenna on the roof of an automobile

  without presenting the disadvantages of vulnerability and fragility.

  However, it should be noted that this antenna has a large footprint: in fact if the radiating face is at the rear location of the automobile, the rest of the body of the antenna is then in the trunk of that- this. Outraged

  this first disadvantage, this known antenna presents a second in-

  Convenient; this drawback lies in the fact that this antenna

  radiates or can only receive light polarization

  which in the case of reception may cause losses

  significant level for any received polarizations.

  The proposed invention aims to overcome these two disadvantages. It should be noted that, of course, an antenna according to the invention can find other applications than this one.

quoted above.

  For this purpose, an antenna of the kind mentioned in the preamble

  bule is remarkable in that on this radiant face are

  four-ply dipole in the form of plates which are short-circuited

  their extremity located on the periphery of the antenna and whose

  length is of the order of <o / 4 (where X0 is the wavelength of

  antenna) and that each strand is provided with a

256 5417

  excitation system connecting them to the outside so that each strand is excited by a wave phase shift practically of: 0, 90, 180,

  270 respectively by means of a distributor.

  The following description, made next to the drawings

  appended, All given as a non-Limiting example, will do well

  understand how the invention can be realized.

  Figure 1 represents, in perspective, and in

  an antenna according to the invention.

  Figure 2 shows an A-view of the antenna of the

gure 1.

  Figure 3 shows a B-view of the antenna of the

gure 1.

  Figure 4 shows a sectional view of the antenna of La

figure 1.

  Figure 5 shows another way of achieving an

tenne according to the invention.

  Figure 6 shows a sectional view of the antenna of the

figure 5.

  Figure 7 shows an example embodiment of a

  tributor particularly well adapted to an antenna according to the in-

  vention. Figure 8 shows a sectional view of the antenna of

  Figures 1 to 4 provided with a distributor on its underside.

  Figure 9 shows how to arrange antennas

  according to the invention on the rear end of an automobile.

  The antenna shown in Figure 1 is formed of three

  Parts 1, 2 and 3. Parts 1 and 2 are all metal.

  The square part I serves as the background for the antenna. This part comes into contact on the perimeter of the part2se presenting in the form of a cylinder of square section of EP thickness. Part 3 is a double-sided printed circuit board. A side A being oriented towards

  the outside of the antenna and a side B being oriented towards the

  which is to say towards the bottom 1. This part 3 also comes into

  touch on the perimeter of part 2 and constitutes the radiant face

of the antenna.

  The invention has four dipole strands 1, 12, 13 and 14 shown in FIG. 1 as being internally discolored on the non-directly visible face 5 of Part 3; these four strands are short-circuited to one of their end with the part 2 by means of screws which tighten them against the part 2. These screws bear the references 21 and 21 'fear the strand 11, 22 and 22' for the Strand 12, 23 and 23 'for The strand

  13 and 24 and 24 'for Strand 14. These strands are fed in quadra-

  phase phase; For this, power points 31, 32, 33 and 34 are provided. The electrical signal has phase shifts of 0, 90, 180 and 270, respectively. These points of supply are

  outside the antenna by an unrepresented fi

  gure, slid in through holes 41, 42, 43 and 44 which are

  The wall around the perimeter of Part 2. These holes

  ge correspond to holes in Part 1, 51, 52 ...

  From the excitation points 31, 32, 33 and 34, the excitation of the different dipole strands 11, 12, 13 and 14 is achieved by a line 61, 62, 63 and 64 of the microstrip type formed on the face.

  Part 3. The location of these Lines is located on

  dianes of the square that forms the front face. These different lines

  both of the different points 41, 42, 43 and 44 meet at a common point 70 grounded by means of a pin 75 raised on the bottom 1 by its lower end and brought into contact with the point 70 by its upper end. (a space being provided for this purpose in the dielectric support constituting the printed circuit board of La

part 3).

  The excitation points 31 to 34 can be connected to any known distributor providing the respective phase shifts.

0, 90, 180 and 270.

  FIG. 2 clearly shows how the excitation lines placed on the side A are performed. These identical lines between them are constituted by two sections 61a, 61b; 62a, 62b; 63a, 63b and

  64a, 64b having different widths "La" and "Lb" and Lon-

"La" and "Lb".

  Figure 3 shows the shape of the dipole strands

256541?

  11, 12, 13 and 14. These strands are constituted by a first and second part. The first part has a constant width equal to W on a length HD to which we must deduce the thickness EP of the part 2 to have the length actually participating in the radiation. The second part of the strands has a width gradually decreasing from LD to result in a width

  lD. The total length of the strand is equal to LLD.

  The plate on which are engraved The dipole strands on one side and the excitation lines on the other side is a square whose side has a vaLeurCR; The center door bears the reference O. FIG. 4 which represents the section along C (FIG.

  1) shows that the cavity formed by parts 1, 2 and 3 has a

  foundry H counted from the bottom 1 to the dipole strand

  printed circuit board constituting Part 3. Dimension El

  presents the thickness of the printed circuit board and the dimension

  SD practically the total thickness of the antenna.

  The electrical behavior of the dipole strands constitutes

  actually killing a half-doublet was studied in particular in the references

following

  G. DUBOST "Flat Radiating Dipoles and Applications to Arrays".

  R.S.P. John Wiley and Sons LTD 1981.

  G. DUBOST "Short or Open-circuited Dipole Parallel to Perfect Reflector Plane and embedded in Substrate and Acting

at Resonance ".

  Elect. Letters 26th Nov. 1981, vol. 17, No. 24, pp. 914-916.

  The bandwidth B (in%) of each half-dipole is given by the expression: H.

(R.O.S.-1) 64.2 W. H) 2

3 (Wi -

B = (1)

_R.O.S. (1)

  oo o R.O.S. is the standing wave ratio

  At the wavelength in the vacuum.

  The radiation resistance Rr in ohms of each half

  doublet plate is given by the expression

R = 22.5 1 (2)

  r r 2 (W / M) 2 (1/4-H / Xo) 2 256541t For an antenna operating at a frequency of 920 MHz (λ = 32, mm with H = 9.6 mm and W = 54 mm, RR is found) : RR = 1200 ohms B = 6.8% with ROS = 3 We then see that a quarter-wave transformer with a characteristic resistance of 245 ohms accommodates the radiation impedance of

  each half-doublet substantially at 50 ohms.

  In the horizontal plane that identifies with the plane

  mass, the electric field is vertically polarized.

  The expression of the field radiated in this plane is given to a coefficient of proportionality near by the expression: sin (k - sin a) sin (k -cosa) sin (kod cos a). o 2 + jsin (k dsina). o2 (3) W k cosa ko sina o 2 cos a o d is the distance from the short-circuit to the center O of the antenna (see figure 3); W the width of each half-doublet;

  ko the wave number of the void and j = / ^ 'T.

  With d = 74 mm W = 64 mm X 0 = 326 mm

  The lack of omnidirectionality, expressed by the variability

  the modulus of the expression (3) remains less than 2.1 dB.

  In the direction perpendicular to the plane of the antenna

  (zenith if it is arranged horizontally), the wave is pola-

  ridiculed. The directivity pattern measured in the "E" plane of each pair of opposing half-doubts is substantially constant to the effects of

diffraction near.

  To summarize and specify, we give below the value of the different ribs of the antenna for a frequency of

920 MHz operation.

  CR = 168 mm LD = 20 mm SD = 12.5 mm HD = 47 mm H = 9.6 mm W = 55 mm El = 1.6 mm the X 0.3 mm EP = 10 mm W = 14 mm W = 64 mm lbs. = 4.8 mm LLD = 74 mm d = 74 mm For an operating frequency of around 920 MHz, an R.0.S. <2 from 880 MHz to 960 MHz - an omnidirectionnal defect of less than 2.5 dB - a 3 dB opening of radiation equal to 160 in the E plane. Figure 5 shows another way to perform the excitation of each coil according to the invention. Each dipole strand (only strand 14 is shown in this figure5) is fed with self-transformation, that is to say that experimentally ondéptace the point of excitation80sur the axedeymétrieddebrindedipôle until one finds a impédanceconvenable (that is- to say adapted)

  between the point and Lamassepriseparamples on the merits 1.

  FIG. 6 shows the C-section of the antenna made as shown in FIG. 5. The part 3 can be made from a single-sided printed circuit board whose face

  insulation is directed outwards.

  The various antennas of the invention can be advantageously excited by a distributor circuit shown in FIG. 7. The overall dimensions of this distributor are such that it can be fixed on the rear of the antennas without this being necessary. greatly increase their thickness. Although the distributor is described for the antenna shown in FIGS. 1 to 4, it goes without saying that

  skilled in the art by modifying some ribs can adapt this distribution.

  on-air scorer shown in Figures 5 and 6.

  The dispenser is made in a triplate line and La

  Figure 7 shows the drawings of active drivers. The points

  51 ', 52', 53 'and 54' correspond to the passages in

  part 1. These points are connected to the coaxial socket 90 by the

  of a set of impedance tripartite lines

  tick 50 ohms (width 15 = 2.25 mm) or 71 ohms (width 17 = 1.5 mm).

  The length of 55 mm of the different sections corresponds to the quarter-wavelength at the average frequency of 920 MHz for a support whose dielectric constant Er has a value of 2.17. The radiation impedance r of each half-doublet plate bypassed at quarter resonance

  The waveform reduced to 51 ', 52', 53 'and 54' is equal to 50 ohms. The Li-

  71 ohm characteristic imopdance quarter wave transform the impedance from 5C ohms to 100 ohms. The two 100 ohm impedances arranged in paraLLeles at points M, N and O are equivalent to an impedance of 50 ohms. The differences in electrical lengths between lines, starting from the point O, connecting the core of the coaxial base 90 to the outlets 51 ', 52', 53 'and 54' are equal to 90 or

    at the center frequency and ensure the excitation

  with four diameters of dipole so as to obtain polarization

  circular. The figure shows various sections of Li

  LL equal, each, for the center frequency of 920 MHz to 55 mm.

  The lengths of the other sections may be modified by the same value in order to adapt, for example, to the antenna represented in

  FIGS. 5 and 6. Such a distributor coupled to an antenna of L'invention

  and made with 1.6 mm thick printed circuit boards does not bring a significant extra thickness to the antenna; it can

  have a total thickness EPP = 15.4 mm.

  Due to the small size of the antenna of the invention

  tion, it is possible to have two (reference 100 and 110) on the rear shelf of an automobile as shown in

  figure 9. Thus this corresponds to a reception by diversity of es-

  2nd order, the distance between the centers of these antennas must be

  at least equal to Xo / 2 (o to (= 326 mm).

  * 0 In the case of a soL-soL Liaison, particularly in

  920 MHz, propagation conditions are extremely difficult.

  An essential phenomenon is the existence of a large number of multiple paths, the combination of which generates during the movement of the vehicle a fluctuation of the signal level following a normal law (Rayleigh's law), causing significant fading. The use of a

  diversity is practically necessary to improve the situation.

  If, on the other hand, the radiation pattern is

  Once the antenna has been installed in the vehicle, reflections on the bodywork elements above the plane of the antenna will be numerous, so that this diagram will have an extremely irregular shape, without as much as the average value (on the upper half-plane) of the signal

  received - or issued - is reduced because of reflections.

95654'17

  As a result, the average level of the signal received by such an antenna in the case of multipath paths is not going to be

  as well as its statistical structure, since the name

  number of multipath trips is important. This remark is basically

  to explain the possible operation of the antenna

  in such a location and the fact that its performance is

  parables to them of a roof antenna.

  If it is still necessary to reduce the thickness of the antenna, it is possible to fill the cavity formed by the face 1 and the radiating face 3 by a dielectric without for that out of the frame

of the invention.

2S65417

Claims (9)

REVENDICATIO NS:   1. Antenna formed from a resonant cavity provided with   of a radiating face, characterized in that on this face   four plates of dipole in the form of plates which are short-circuited at their extremities situated at the periphery of the antenna and whose length is of the order of XO / 4 (where λ 0 is the wavelength of operation of the antenna) and in that   each strand is provided with an excitation system linking them to the outside.   so that each strand is excited by a practically phase-shifted wave.   0, 90, 180, 270 respectively by means of a   tributor. 2. Antenna according to claim 1, characterized in that the excitation system is constituted for each strand by a microstrip line whose ground plane is constituted by the strand of   dipole to be excited by this line and which comprises a first   first end connected to a point of excitation placed towards the phérie of the antenna.   3. Antenna according to claim 2, characterized in that the second ends of said microstrip lines are connected   in a common point connected to the mass of the antenna.   4. Antenna according to claim 2 or 3, characterized in that the dipole strands and their excitation line are arranged   on both sides of a printed circuit board.   Antenna according to claim 1, characterized in that the excitation system is formed by a point connected to a wire   connecting it directly to the outside.   Antenna according to one of claims 1 to 5, characterized   in that the resonant cavity is filled with dielectric.   Antenna according to one of claims 1 to 6, characterized   in that the distributor is constituted by a triplate line network and in that it is arranged on the face opposite to the radiating face.   Antenna according to one of claims 1 to 7   characterized in that it is arranged on the rear shelf of a vehicle.   Antenna assembly according to one of the claims   1 to 7 provided to operate, in reception, by diversity of space characterized in that it is arranged on the beach rear of a vehicle.