Abstract:
The present invention relates to a device for transmitting and/or receiving electromagnetic waves comprising at least one antenna with at least one radiating element transmitting and/or receiving signals of given polarization and a feed array produced in microstrip technology consisting of lines devised so as to give parasitic radiation. In this case, the feed array is devised and dimensioned in such a way that the parasitic radiation has the same direction and the same polarization as the radiation of the antenna and combines in-phase with the said radiation of the antenna.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a device for transmitting and/or receiving electromagnetic waves, more particularly to an antenna known by the expression “printed antenna” fed from an array produced in mircostrip technology. 
     Hereinbelow, the expression “printed antenna” (or “microstrip antenna”) will refer to an antenna produced in so-called “microstrip” technology, comprising a radiating element, typically a “patch”, a slot, a dipole, etc., or an array of such elements, the number of elements depending on the desired gain. This type of antenna is used as primary source at the focus of a lens or of a parabola or as a planar array antenna. 
     BACKGROUND OF THE INVENTION 
     In printed antennas, the radiating elements, be they unitary or grouped into an array, are fed from a feed array formed of microstrip lines. In general, this feed array radiates, to a greater or lesser extent, undesired radiation or parasitic radiation which disturbs the main radiation of the antenna. The principal effects resulting from this parasitic radiation are a rise in the cross-polarization of the printed antenna. Other undesirable effects, which are more or less significant, may also result from this parasitic radiation, namely: 
     an impairment of the radiation pattern of the antenna with a rise in the side lobes and/or a deformation of the main lobe, 
     an impairment of the efficiency of the antenna, namely radiation losses. 
     Current solutions attempt to limit or minimize the parasitic radiation: 
     through a judicious choice of the parameters of the dielectric substrate such as the thickness, permittivity, etc., 
     by optimizing the line widths, 
     or by minimizing the discontinuities from which the parasitic radiations stem. 
     However, all the solutions proposed hitherto require compromises which limit their effectiveness. For example, a slender substrate exhibiting a high dielectric permittivity minimizes the radiation of the feed lines but also reduces the effectiveness of the radiation of the radiating elements and hence the efficiency of the antenna. Likewise, the use of narrow lines reduces the parasitic radiation but the smaller the widths of the lines, the larger the ohmic losses. 
     BRIEF SUMMARY OF THE INVENTION 
     Consequently, the aim of the present invention is to propose a solution which, instead of reducing the harmful effects of the parasitic radiation, uses them to contribute to the main radiation of the antenna. 
     A subject of the present invention is therefore a device for transmitting and/or receiving electromagnetic waves comprising an antenna with at least one radiating element transmitting and/or receiving signals of given polarization and a feed array produced in microstrip technology consisting of lines devised so as to give parasitic radiation, characterized in that the feed array is devised and dimensioned in such a way that the parasitic radiation has the same direction and the same polarization as the radiation of the antenna and combines in-phase with the said radiation of the antenna. 
     In a known manner the parasitic radiation is generated by discontinuities in the lines of the feed array, such as elbows, T circuits, line width variations. 
     In accordance with one embodiment of the present invention, the relative phase of the source of parasitic radiation is determined by the length of the lines of the feed array. Preferably, the feed array is a symmetrical array. 
     In the case of a linearly polarized antenna, the lengths of lines Li on each side of an elbow are given by the following equations: 
     
       
         L 1 =λ{fraction ( 1 / 2 )}+ k 1λ1  k 1=0,1,2, . . .  
       
     
     
       
         L 2 = k 2λ2  k 2=0,1,2, . . .  
       
     
     where λi represents the wavelength guided in the line of the feed array of length Li with: 
     λi=30/(f{square root over (εr eff)}) [in cm] 
     with f: working frequency [in GHz] 
     εr eff: effective permittivity of the material for the portion of line of length Li. 
     Moreover, in the case of a circularly polarized antenna comprising at least two radiating elements, the lengths of lines Li of the feed array formed of a T circuit with two elbows are given by the following equations: 
     
       
         L′ 2 =L 2 + k 1λ{fraction (2/4)} k 1=1,2,3 
       
     
     where L′ 2  and L 2  are the two branches of the T. 
     
       
         L′ 3 =L 3 + k 2λ¾ k 2=1,2,3 
       
     
     where L 3  and L′ 3  are the lines connecting to the radiating elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other characteristics and advantages of the present invention will become apparent on reading the description of various embodiments, this description being given with reference to the appended drawings in which: 
     FIG. 1 is a diagrammatic plan view of the various discontinuities which the microstrip lines may have, 
     FIG. 2 is a diagrammatic plan view of a feed array with the orientation of the E fields, 
     FIG. 3 is a diagrammatic plan view of a printed antenna and of its feed array exhibiting parasitic radiation, 
     FIG. 4 is a diagrammatic plan view of a feed array according to the present invention in the case of linear polarization, 
     FIG. 5 is a diagrammatic plan view of a feed array according to the present invention in the case of circular polarization, 
     FIGS. 6 a  and  6   b  are diagrammatic plan views of a feed array with four patches respectively with parasitic radiation having the same polarization as the main radiation or having polarization inverse to that of the main radiation, 
     FIG. 7 represents the ellipticity in the case of the arrays of FIGS. 6 a  and  6   b.   
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     To simplify the description, in the figures the same elements bear the same references. 
     Moreover, the present invention will be described whilst referring to a printed antenna whose radiating elements consist of patches. However, it is obvious to the person skilled in the art that the present invention may be applied to any other type of printed antenna whose radiating elements are connected to a feed array produced in microstrip technology. 
     Represented in FIG. 1 are various types of discontinuities which may be produced in a feed array formed by lines according to microstrip technology. The reference  1  represents an elbowed line. The reference  2  represents a widthwise line jump and the reference  3  represents a T. 
     As described in particular in the reference “Handbook of Microstrip Antennas” edited by J. R. James &amp; P. S. Hall, published by Peter Peregrinus Ltd., London, and more particularly in the introduction to Chapter 14 entitled “Microstrip Antenna Feeds”, pages 815 to 817, it is known that the discontinuities in the feed lines such as represented in FIG. 1 give parasitic radiation. In accordance, in particular, with the thesis by M. EL. Haj Sleimen on “Studies of Millimeter Printed Antenna Arrays” carried out at the Laboratoire Antennes et Réseaux de Rennes in 1999, it is possible to give an estimate of the orientation of the main radiation of the discontinuities such as the elbow  1 , the widthwise line jump  2  and the T  3 . This field is referenced E in FIG.  1 . 
     Represented in FIG. 2 is a feed array consisting of microstrip lines exhibiting a conventional structure. More particularly, this feed array comprises a T  10  extended by two branches  11 ,  12  of respective lengths L 1  and L 2 . Each branch is extended by elbows  13 ,  14 . The elbow  13  is extended by a line segment  15  of length L 3  while elbow  14  is extended by a line segment  16  of length L 4 , the two line segments terminating in elbows  17 , 18 . Moreover, the T  10  exhibits an increase in line width over a length L 5  which is equal to λ5(?)/4 in the present case. As represented in FIG. 2, the various discontinuities exhibit parasitic radiation according to the field E 1  for the elbow  13 , the field E 2  for the elbow  14 , the field E 3  for the elbow  17 , the field E 4  for the elbow  18 , the field E 5  for the T and the field E 6  for the line broadening. From the six discontinuities E 1  to E 6  of the feed array identified in FIG. 2, it is possible to calculate the total field E generated by the feed array. Employing an orthonormal reference frame I,J, the unit vector of the fields E 1  to E 5  is therefore:                           
     In this case, for the calculation of total field E, the following parameters will be taken into account, namely: 
     the effectiveness of the radiation of each of the discontinuities, 
     the attenuation of the lines, 
     and the power delivered by the feed at the level of each of the discontinuities. 
     By taking these elements into account, it is known practice to calculate the total field in a conventional manner. Then, the total field having been calculated, it is possible to determine the ellipticity of the parasitic radiation according to known methods which will not be described in the present application. In fact, on the basis of known equations, it may be seen that the relative phases of the parasitic radiation sources of the feed array are determined by the lengths L 1 , L 2 , L 3 , L 4 , L 5 , that their relative amplitudes depend on the nature of the discontinuity and are proportional to the relative power transported by the line experiencing the discontinuity. These radiation sources may be likened to a radiating array and the theory of arrays makes it possible, by knowing the location of the sources, their relative phase and their relative amplitude, to calculate the radiation pattern of this array and to determine, in particular, the polarization of the radiated field. Thus, to cause, in accordance with the present invention, the parasitic radiation to be in the same direction as the main radiation, to have the same polarization as the main radiation, and to combine in-phase with the main radiation, it is necessary for the phase centre of the source equivalent to the feed array to coincide with the phase centre of the array and for the radiation maximum to occur in the direction of the maximum of the main field, and for it to have the same polarization as the latter. 
     Thus, as represented in FIG. 3 which relates to a linearly polarized printed antenna, the parasitic radiation given by the elbows  1 , 2  has a resultant parallel to the main radiation. More specifically, the printed antenna of FIG. 3 consists of N arrays of four patches P 1 , P 2 , P 3 , P 4 , more particularly of eight arrays of four patches. As represented in FIG. 3, the four patches of a first array P 1 , P 2 , P 3 , P 4  are connected symmetrically by a feed array comprising elbows  1 , 2  giving parasitic radiations  1 , 2  and T circuits giving parasitic radiations  3 , 4 . Four arrays of four patches are connected together symmetrically, as represented in the right-hand part of FIG. 3, by T microstrip lines giving a parasitic radiation such as symbolized by the arrows  5 ,  6 ,  7  and  8 . In this case, the main radiation together with the parasitic radiations can be symbolized as represented in the lower part of FIG.  3 . The arrow F represents the main radiation to which is added the radiations of the elbows  1  and  2  which give a radiation F′ in the same direction as the main radiation but of opposite sense, the radiations of the T circuits  3  and  4  which cancel one another out,  5  and  6  which cancel one another out and  7  and  8  which cancel one another out, in such a way as to obtain a resultant radiation parallel to the main radiation F but of lower amplitude. Thus, in the case of the printed antenna in FIG. 3 consisting of eight arrays of four patches symmetrically connected, if the conditions relating to the direction of the parasitic radiation and to the polarization of this parasitic radiation are fulfilled, the condition concerning the phase is not fulfilled. Thus, if the radiation is not controlled in-phase, it may partially or totally oppose the main radiation of the antenna and hence reduce its efficiency. To ensure maximum efficiency of the antenna, in accordance with the present invention, and as represented in FIG. 4, it is necessary to ensure that the parasitic radiation combines in-phase with the main radiation. 
     As represented in FIG. 4, the four patches P′ 1 , P′ 2 , P′ 3 , P′ 4  giving a main radiation Φ 1  are connected by a feed array comprising elbows and T circuits. More specifically, the patches P′ 1  and P′ 2  are linked together by a T feed circuit comprising two branches of identical length L 3  extended by an elbow linked by way of an identical length of line L 4  to the patches P′ 1 , P′ 2 . The patches P′ 3  and P′ 4  are connected in an identical manner, the two T feed circuits being linked together by another T feed circuit comprising two identical branches of length L 1  extended by elbows linked to the point C of the first T elements by line elements of identical length L 2 . 
     To obtain parasitic radiation which combines in-phase with the main radiation in the case of linear polarization, as represented in FIG. 4, the lengths L i  given above must obey the following rules: 
     
       
         L 1 =λ 1 /2+ k   1 λ 1    k   1 =0,1,2, . . .  
       
     
     
       
         L 2   =k   2 λ 2   k   2 =0,1,2, . . .  
       
     
     
       
         L 3 =λ 3 /2= k   3 λ 3    k   3 =0,1,2, . . .  
       
     
     
       
         L 4   =k   4 λ 4    k   4 =0,1,2, . . .  
       
     
     where λ i  represents the wavelength guided in the portion of the feed array of length L i ; i.e. λ i  30/f{square root over (ε reff )} (in cm) 
     where f=working frequency (in GHz) 
     (ε reff )=effective permittivity of the material for the line portion of length L i . 
     Taking as phase reference the phase of the wave at the junction point of the first T, if the length L 1  is such that L 1 =λ 1 /2+k 1 λ 1  k 1   =0,1,2, . . . , the phase φ of the wave at the level of the first elbow would be  180° (φ=2πL1/λ 1 =π+2k 1  π) and the field radiated by the elbow (shown dotted in the Figure) would have a sense represented in the figure. Thus, by summing the two elbow discontinuities on either side of the first T, the total field emanating from these two discontinuities adds constructively with the field radiated by the T discontinuity (represented as a continuous line in the figure). If L 1  had been equal k 1 λ 1 , the fields radiated by the elbows would have opposite senses to those represented in the Figure, and their resultant would directly oppose the field radiated by the T, reducing the gain of the antenna, etc. 
     An embodiment of the present invention relating to the case of circular polarization will now be described with reference to FIG.  5 . In this case, the printed antenna consists of an array of four patches P″ 1 , P″ 2 , P″ 3 , P″ 4  connected to a feed array produced in microstrip technology, the feed array consisting of two T circuits linked together. More specifically, the first T circuit comprises two branches of length L 2  and L′ 2 , extended by elbows C 1 ,C 2 , the elbow C 1  being linked respectively to the patch P″ 1  by a length of line L 3  and the elbow C 2  to the patch P″ 2  by a length of line L′ 3 . Likewise, the patches P″ 3  and P″ 4 . Moreover, the two inputs of the T circuits are connected together at a common point A by lengths of line L 1  and L′ 1 . As represented in the bottom part of FIG. 5, the assembly of patches P″ 1 , P″ 2 , P″ 3 , P″ 4  gives circularly polarized main radiation to which is added, on account of the elbows C 1 ,C 2  and of the T circuits  3 , 4 , parasitic radiation, likewise circularly polarized and having the same sense as the polarization of the main radiation. Hence, a total radiation consisting of the main radiation to which the parasitic radiation is added is obtained. In order for the phase relation to be satisfied, the various lengths must be such that: 
     
       
         L 1 =L′ 1   
       
     
     
       
         L′ 2 =L 2+   k   1 λ 2 /4  k   1 =1,2,3, . . .  
       
     
     
       
         L 3 =L′ 3   +k   2λ   3 /4  k   2= 1,2,3, . . .  
       
     
     λ i  representing the wavelength guided in the part of the feed array of length L i , as defined hereinabove. 
     Represented in FIGS. 6 a  and  6   b  is a printed antenna consisting of an array of four patches  10 ,  11 ,  12 ,  13  connected to a feed circuit using the principle of sequential rotation. This antenna can serve for the illumination of a parabolic antenna or of an antenna of the Luneberg lens type. These four patches  10 ,  11 ,  12 ,  13  are fed from a feed array consisting, respectively for FIG. 6 a , of lines of length L 1 , L 2 , L 3 , L 4 , the lines L 1  and L 2  forming the two branches of a T circuit, the line L 1  being connected to the line L 3  by an elbow, the line L 2  being connected to the line L 4  by an elbow, the line L 3  being connected to the two patches  10  and  11  by another elbow and the line L 4  being connected to the two patches  12  and  13  by yet another elbow. The T circuit and the four elbows give parasitic radiation with circular polarization whose sense is identical to that of the polarization of the main radiation. 
     In FIG. 6 b , the feed array has been modified in such a way that the two branches of the T circuit are of length L′ 1  and L′ 2 , so as to give parasitic radiation symbolized by the arrow E which, by adding to the parasitic radiation of the elbows, gives parasitic radiation with circular polarization but of opposite sense to that of the main radiation. In this case, as represented in FIG. 7, the ellipticity (TE) as a function of frequency, obtained for the two arrays, shows one of the advantages of the present invention. For the circuit of FIG. 6 b , the TE is less than 1.74 dB over a frequency band of 630 MHz. For FIG. 6 a , the TE is less than 1.74 dB over two bands, one of 330 MHz centred at 12.1 GHz and the other at 150 MHz centred at 12.7 GHz. It may be seen in the chart that, at equivalent TE level (3 dB), this represents an increase in bandwidth of TE of 40% for the circuit in accordance with the present invention. 
     With the present invention, the following advantages are obtained: 
     improvement in the efficiency of the antenna, 
     no contradictory choices to be made both in respect of the substrate and in respect of the design of the antenna, 
     in the case of circular polarization, in particular, the level of cross-polarization is very low.