A POLARISATION DISCRIMINATOR FOR ELECTROMAGNETIC WAVES WITH CIRCULAR POLARISATION AND DICHROIC SURFACES INCLUDING PLURALITIES OF SUCH DISCRIMINATORS

Abstract: The discriminator comprises two substantially identical elements (A, B) of electrically conductive material lying in respective perpendicular planes, the distance between their centres being substantially equal to an odd number of quarter-wavelengths, preferably one quarter-wavelength. Each element is constituted by a loop (A, B) the perimeter of which is substantially equal to a whole number of wavelengths, preferably one wavelength. Each loop (A, B) has a short gap in its part facing the other loop. The ends of one loop (A, B) are connected to the corresponding ends of the other loop.

Description: A polarisation discriminator for electromagnetic waves with circular polarisat-on and dichroic surfaces including pluralities of 'iuch discriminators The present invention relates to a polarisation discriminator for electromagnetic waves with circular polarisation, usable in particular for forming dichroic surfaces. The particular subject of the invention is a polarisation discriminator comprising two substantially identical, electrically conductive elements lying in respective substantially perpendicular planes, the distances between their centres being substantially equal to an odd number of quarter-wavelengths, preferably one quarter-wavelength. A polarisation discriminator of this type is described in the article "A polarization-selective surface for circular polarisation" by W.V. Tilston and T. Tralman, 1988, IEEE Trans. AP-33, p. 762. This known device includes two electrically conductive elements, which in fact constitute two half-wave resonant dipoles, arranged perpendicular to each other and separated by a distance of one quarter-wavelength. The two dipoles are interconnected at their supply points (the centres of the dipoles) by a transmission line whose electrical length is equal to one half-wavelength. This known device has the disadvantage of having a fairly complicated structure and moreover of being able to operate satisfactorily only in a fairly narrow frequency band because it has three dimensions with which resonance conditions may be associated (the lengths of the dipoles, their distance apart and the length of the transmission line which joins them) . The object of the present invention is to provide a discriminator for circular polarisation which does not have the disadvantages of the discriminator of the prior-art described above. According to the invention, this object is achieved by a discriminator of the type specified above, the main characteristic of which lies in the fact that each conductor element is constituted by a loop the perimeter of which is substantially equal to a whole number of wavelengths, preferably one wavelength, each loop having a short gap in its part facing the other loop and each end of one loop being connected directly to " a corresponding end of the other loop. The loops of the polarisation discriminator may be of generally arbitrary shape but are preferably substantially circular or square and, in the latter case, their sides are one quarter-wavelength long. Further characteristics and advantages of the invention will become clear from the detailed description which follows, with reference to the appended drawings, provided purely by way of non-limiting example, in which: Figure 1 is a perspective view of a first discriminator for circular polarisation according to the invention, Figure 2 is a perspective view of a second embodiment of the polarisation discriminator according to the invention, and Figure 3 is a perspective view of a portion of a dichroic surface formed with the use of a plurality of polarisation discriminators according to the invention, With reference to Figure 1, a discriminator for circular polarisation according to the invention comprises two elements of an electrically conductive material (typically metal) , indicated A and B, in the form of identical, substantially circular loops lying in mutually perpendicular planes. The circumference of each loop A or B is preferably substantially one wavelength but, in general, may be a whole multiple of that length. Each loop A or B has a respective gap (which is extremely narrow compared with the wavelength, for example, one tenth of the wavelength or less) and the ends of each loop defining to the gap are connected directly to the corresponding ends of the other loop. The distance between the centres of the loops A and B is preferably one quarter-wavelength but, in general this distance may be an odd number of quarter-wavelengths. As can be seen in Figure 1, the loops A and B are close to and almost touching each other and are open adjacent their contact points, their ends being connected in pairs. In use, the polarisation discriminator is oriented so that an electromagnetic wave with circular polarisation strikes in a direction corresponding to the line of intersection of the two planes in which the loops A and B lie. This line is indicated D in Figure 1. Compared with devices of the prior art, the discriminator according to the invention has the following principal advantages. The structure of the polarisation discriminator is much simpler and does not require the presence of an electrical transmission line. Moreover, in the discriminator in question as a whole, there are only two dimensions with which resonance conditions may be associated, that is, the perimeters of the loops and the distance between their centres. By virtue of this characteristic, the discriminator according to the invention can operate over wider frequency bands. As stated above, the shapes of the loops are essentially arbitrary and this introduces a degree of freedom into the dimensioning of the discriminator, resulting in greater flexibility in the design of the element. Figure 2 shows a further embodiment of a device according to the invention, in which the loops A and B are square with sides preferably of one quarter-wavelength. In general, the coupling of the incident field with an individual loop is optimised in operation because the length of the circumference or perimeter of the loop is equal to a whole number of wavelengths, particularly one wavelength. The distance between the centres of the loops (which is equal to an odd number of quarter-wavelengths, preferably one quarter-wavelength) together with their mutual orientation optimises the coupling of a field with circular polarisation with the polarisation discriminator. Now, if one considers two fields with opposite circular polarisations (clockwise and anticlockwise polarisation respectively) , a vector of a magnetic field which couples with the first loop encountered in a given sense will couple with the next loop in one or other sense according to its polarisation. With one polarisation, the potential differences induced at the ends of the two loops are equal and have the same sign; with the other polarisation, the potential differences are also equal but have opposite signs. In other words, for one of the two possible polarisations, the loops appear electrically "open" whilst, for the other polarisation, they appear short-circuited, with a consequent resonance effect in both loops. The polarisation discriminator according to the invention may conveniently be used for forming selective dichroic surfaces for electromagnetic waves with circular polarisation. A portion of such a dichroic surface is shown in perspective in Figure 3. With reference to this drawing, an orderly array of polarisation discriminators of the type described above is incorporated in a substrate C of dielectric material. The discriminators may be arranged, for example, in a matrix, in a chequered arrangement,, in concentric circles, etc. Such a dichroic surface may conveniently be used to achieve a considerable increase in the communication capacity of an antenna of the type with reflectors, enabling such an antenna to operate with two different polarisations. This may be especially useful for communication systems on satallites, in particular, in view of the drastic reductions in weight, bulk and costs which can be achieved. A surface of the type described above may also be used in the aeronautics field for forming selective radomes. Naturally, the principle of the invention remaining the same, the forms of embodiment and details of construction may be varied widely with respect to those described and illustrated purely by way of non-limiting example, without thereby departing from the scope of the present invention.

Claims: CLAIMS 1. A polarisation discriminator for electromagnetic waves with circular polarisation, usable for forming dichroic surfaces and comprising two substantially identical elements (A, B) of electrically conductive material lying in respective substantially perpendicular planes, the distance between their centres being substantially equal to an odd number of quarter-wavelengths, preferably one quarter-wavelength, characterised in that each element is constituted by a loop (A, B) the perimeter of which is substantially equal to a whole number of wavelengths, preferably one wavelength, each loop (A, B) having a short gap in its part facing the other loop and each end of one loop (A, B) being connected directly to a corresponding end of the other loop. 2. A discriminator according to Claim 1, characterised in that the loops (A, B) are substantially circular. 3. A discriminator according to Claim 1, characterised in that the loops (A, B) are substantially square with sides preferably of substantially one quarter-wavelength. 4. A selective surface for electromagnetic waves with circular polarisation, characterised in that it includes an orderly array of discriminators according to one or more of the claims.

Inventors: BRESCIANI DANIELE, BRESCIANI, DANIELE
Applicants: ALENIA AERITALIA & SELENIA, ALENIA - AERITALIA & SELENIA S.P.A.
Application year: 1991
Main classification: H01Q15
IPC codes: H01Q15