FREQUENCY SELECTIVE SURFACES

Abstract: A frequency selective surface or dichroic structure has an array of elements (1) each of which comprises at least two substantially concentric symmetrical closed figures (2, 3). For example, the element array may be a regular array of identical double-squares. Other polygons, or ellipses, may alternatively be used as the closed figures. The closed figures may be formed as conductive paths on a dielectric substrate or, alternatively, the Babinet complement of such a structure may be used.

Description: FREQUENCY SELECTIVE SURFACES The present invention relates to frequency selective or dichroic surfaces and members, more particularly, although not exclusively, for reflector antennas and optical and infra red filters. A dichroic member is used to reflect electromagnetic signals in one frequency band and to pass or transmit signals in another frequency band. One example of such a dichroic device is disclosed in US Patent 4160254. The latter describes a microwave dichroic plate comprising an array of interlaced elements, each of which has-first and second orthogonal arms of approximately the same length and cro ssing at a point at the middle of the arms. The arms are arranged with their centre lines aligned parallel to the X and Y axes of the array and the arrangement is such that a line between the points of crossing of the arms of the closest adjacent elements has differing component. values relative to the X and Y axes. The elements may be formed as crossed slots in a metal plate, between which slots a metal lattice structure is formed. Alternatively, the dichroic plate may comprise the Babinet complement of the aforementioned structure in which the lattice structure and crossed slots are reversed so that the crossed slots become metallic segments separated by a dielectric. The purpose of the interlacing of the array elements is to reduce energy losses and to increase the bandwidth of signals which are transmitted by the plate. The design of most known dichroic plates or members is directed to alleviating these problems. In contrast, other operating parameters of dichroic members and the ease of design of such members have not been significantly improved. Our copending European Patent Application No. 83303127.1 is directed to enhancing the operating parameters of dichroic members, for example, the proximity and stability of reflection and transmission bands, and the mitigation of cross polarisation phenomena and also aims to facilitate the design of dichroic members. It describes a dichroic member comprising a regular grid having elements disposed within the interstices of the grid, these elements being identical and of sub stantially symmetrical shape. For example, the elements may be in the form of hollow squares. The grid and elements may be formed as conductors or, alternatively, the Babinet complement of such an arrangement may be utilised. It is also a general object of the present invention to enhance-the operating parameters of frequency selective or dichroic surfaces or members and to facilitate the design of dichroic devices. The invention consists in a dichroic member having an array of elements, each of which comprises at least two substantially concentric symmetrical closed figures. Preferably, the array is a substantially regular array of similar or substantially identical elements. The closed figures forming the elements may be polygons, such as triangles, rectangles or hexagons, or may be ellipses. The sides of the figures forming each element may be of different widths and the spacing between adjacent elements of the array may be different from the spacing between the concentric figures of each element. The smallest or innermost figure of each element may be of solid shape, instead of being hollow, as are the outer figures. The closed figures may comprise conductors or conductive paths disposed on a non-conductive substrate or, alternatively, may be the Babinet complements of such arrangements. It has been found that the present invention enables the transition between reflection and transmission frequency bands to be made more rapid. Moreover, the widths of the bands are relatively insensitive to the angle of incidence of electromagnetic waves whilst the cross polarisation performance remains satisfactory and waves are not rotated by the dichroic member to any significant extent. Conversely to the structure described in our copending European application, the present invention provides a dichroic member which has a main reflection band lower than the main transmitting band. Dichroic members are conventionally used with radio frequency signals and are widely used in microwave systems. Dichroic members constructed in accordance with the present invention have applications not only in reflector antennas but also in optical and infra red filters. In order that the present invention may be more readily understood, reference will now be made to the accompanying drawings in which:- Figure 1 illustrates one array of elements for a dichroic member according to the invention, Figure 2 is a graph of transmission loss versus frequency for a dichroic member having the element array shown in Figure 1, Figure 3 is the equivalent circuit derived for the element array shown in Figure 1, Figure 4 is a schematic diagram of an antenna embodying a dichroic member of the construction illustrated in Figure 1, and Figure 5 is a schematic diagram of a diplexer embodying such a dichroic member. Referring to the drawings, Figure 1 illustrates one form of a regular array of identical elements 1 for a dichroic member according to the invention. It is a periodic array in which each element 1 comprises two concentric hollow squares 2,3. These hollow squares may have sides formed by conductors disposed on a nonconductive substrate. For example, the conductive squares may be formed on a dielectric substrate sheet by printed circuit techniques. Alternatively, the array may be the Babinet complement of the aforementioned arrangement. A dichroic structure of the type illustrated in Figure 1 provides a plane wave transmission coefficient plot of the form shown in Figure 2 and has two reflection bands f1, f2 which are relatively insensitive in location and width to the angle of incidence of the electromagnetic waves impinging on the structure. Between the frequency bands fl,f2 fl2 is a band of high transmission fT. In most applications, the latter would be used with bands f or 2 to give, respectively, the transmission and reflection bands required by a dual band system, as is hereinafter more fully described. Variation of the relative dimensions of the various conductors of the array elements 1 enable the widths and separation of the transmission and reflection bands to be adjusted to meet the operating requirements of a particular antenna or other device embodying the dichroic structure. Dichroic arrays having a double resonantfrequency transmission characteristic, as described above, can be used as single-layer frequency-selective surfaces with transmission/reflection band centre ratios in the range 1.3:1 to about 2:1. They are typically designed using either a model analysis or, if one is available, an equivalent circuit model. The latter whilst not offering the comprehensive analysis capability of the former, permits extremely rapid computation of the array transmission characteristics. Array elements according to the present invention have the advantage of lending themselves to design by simple equivalent circuit models and an equivalent circuit model for the array of double squares illustrated in Figrue 1 is shown in Figure 3. Referring firstly to Table I, this sets forth a comparison between the band centre frequencies predicted by the equivalent circuit model of Figure 3 and those measured for 13 different arrays of double square elements. The listed dimensions of the elemenets correspond to the dimensions indicated in Figure 1. The swept frequency transmission measurements were made for a plane-wave front incident on 20x20 cm arrays, printed on 0.027 mm polyester substrates (Er = 3). TABLE I "MEASURED AND PREDICTED BAND CENTRE FREQUENCIES" EMI6.1 <SEP> Dimensions, <SEP> mm <SEP> Frequencies, <SEP> GHz <tb> Array <SEP> f1 <SEP> fT <SEP> f2 <tb> <SEP> No. <SEP> p <SEP> w1 <SEP> d1 <SEP> g1 <SEP> w2 <SEP> d2 <SEP> g2 <SEP> Means'd <SEP> Model <SEP> Meas'd <SEP> Model <SEP> Meas'd <SEP> Model <tb> <SEP> 1 <SEP> 5.0 <SEP> 0.17 <SEP> 4.83 <SEP> 0.17 <SEP> 0.17 <SEP> 3.50 <SEP> 0.50 <SEP> 11.5 <SEP> 11.5 <SEP> 17.6 <SEP> 17.5 <SEP> 24.1 <SEP> 24.5 <tb> <SEP> 2 <SEP> 5.0 <SEP> 0.17 <SEP> 4.83 <SEP> 0.17 <SEP> 0.17 <SEP> 3.00 <SEP> 0.75 <SEP> 11.5 <SEP> 11.5 <SEP> 21.0 <SEP> 20.0 <SEP> 29.4 <SEP> 29.8 <tb> <SEP> 3 <SEP> 5.0 <SEP> 0.17 <SEP> 4.83 <SEP> 0.17 <SEP> 0.17 <SEP> 2.50 <SEP> 1.00 <SEP> 11.5 <SEP> 11.5 <SEP> 25.0 <SEP> 24.0 <SEP> 36.4 <SEP> 35.7 <tb> <SEP> 4 <SEP> 5.0 <SEP> 0.17 <SEP> 4.57 <SEP> 0.43 <SEP> 0.17 <SEP> 3.35 <SEP> 0.44 <SEP> 14.5 <SEP> 14.6 <SEP> 19.8 <SEP> 20.2 <SEP> 26.6 <SEP> 26.8 <tb> <SEP> 5 <SEP> 5.0 <SEP> 0.20 <SEP> 4.60 <SEP> 0.40 <SEP> 0.20 <SEP> 2.40 <SEP> 0.90 <SEP> 14.9 <SEP> 14.7 <SEP> 26.5 <SEP> 26.0 <SEP> 41.0 <SEP> 39.4 <tb> <SEP> 6 <SEP> 5.0 <SEP> 0.20 <SEP> 4.60 <SEP> 0.40 <SEP> 0.20 <SEP> 2.90 <SEP> 0.66 <SEP> 14.6 <SEP> 14.6 <SEP> 23.0 <SEP> 22.8 <SEP> 32.3 <SEP> 32.4 <tb> <SEP> 7 <SEP> 5.0 <SEP> 0.06 <SEP> 4.76 <SEP> 0.24 <SEP> 0.42 <SEP> 3.61 <SEP> 0.52 <SEP> 12.5 <SEP> 12.9 <SEP> 18.5 <SEP> 19.4 <SEP> 29.5 <SEP> 30.1 <tb> <SEP> 8 <SEP> 5.2 <SEP> 0.40 <SEP> 4.85 <SEP> 0.35 <SEP> 0.30 <SEP> 3.15 <SEP> 0.45 <SEP> 15.1 <SEP> 14.8 <SEP> 22.2 <SEP> 22.0 <SEP> 31.8 <SEP> 30.6 <tb> <SEP> 9 <SEP> 5.2 <SEP> 0.39 <SEP> 4.82 <SEP> 0.38 <SEP> 0.28 <SEP> 2.82 <SEP> 0.61 <SEP> 15.3 <SEP> 15.0 <SEP> 24.6 <SEP> 24.0 <SEP> 35.7 <SEP> 34.3 <tb> <SEP> 10 <SEP> 5.2 <SEP> 0.17 <SEP> 4.78 <SEP> 0.42 <SEP> 0.30 <SEP> 3.40 <SEP> 0.52 <SEP> 14.1 <SEP> 14.4 <SEP> 20.4 <SEP> 21.0 <SEP> 29.9 <SEP> 29.7 <tb> <SEP> 11 <SEP> 5.2 <SEP> 0.20 <SEP> 4.82 <SEP> 0.38 <SEP> 0.34 <SEP> 2.65 <SEP> 0.88 <SEP> 14.3 <SEP> 14.3 <SEP> 26.0 <SEP> 25.5 <SEP> 41.5 <SEP> 38.8 <tb> <SEP> 12 <SEP> 5.5 <SEP> 0.25 <SEP> 5.12 <SEP> 0.38 <SEP> 0.25 <SEP> 2.62 <SEP> 1.00 <SEP> 13.3 <SEP> 13.0 <SEP> 24.8 <SEP> 24.0 <SEP> 38.0 <SEP> 36.4 <tb> <SEP> 13 <SEP> 5.5 <SEP> 0.20 <SEP> 5.10 <SEP> 0.40 <SEP> 0.34 <SEP> 2.63 <SEP> 1.04 <SEP> 13.3 <SEP> 13.4 <SEP> 25.8 <SEP> 25.0 <SEP> 41.0 <SEP> 38.5 <tb> It will be seen that the first three arrays listed in the Table have identical geometries apart from the inner square sides d2 which are 3.5, 3.0 and 2.5 mm long, respectively. The first reflection band centre on fl occurs at approximately the same frequency (11.5 GHz) for each of the three arrays. The empirical model shows this resonance to be independent of the inner-square side d2 but dependent on its width w2. The second resonant frequency f2 is determined by the inner-square dimensions only and f2 increases as d2 decreases. The relative spacing of this second resonance with respect to fl controls the transmission frequency band centred on fT. T Hence, the ratio fT/l is 1.5 for array 1, 1.8 for array 2 and 2.2 for array 3. At oblique incidence, it has been found that the upper resonance tends to be sensitive to angle, in contrast to that at f For instance, array 3 at 450 has two high-frequency resonances within our frequency range, a narrow one at 27.5 GHz and a more prominent null at 34.5 0Hz in the H-plane. They occur at 31.5 and 33 GHz, respectively, in the E-plane. In general, the reflection (fl) and transmission bandwidths increase as the band separation increases. For example, the -0.5 dB bandwidths common to all angles of incidence up to 450 for arrays 1, 2 and 3 are respectively 14, 17% and 20% in reflection. But in transmission the situation is less clear, since there appear to be losses of up to 0.5 dB at 450 incidence in the H-plane, reducing the common bnadwidths to below 10%. It is believed that it may be possible to optimise these widths for a given fT/f1 ratio by varying the dimensions of the element. Figure 3 shows the equivalent circuit derived for the array of double squares illustrated in Figure 1. The basic equations for calculating the inductance and capacitance of strip gratings are found in "Waveguide Handbook" by N. Marcuvitz and published by Magraw-Hill in 1951. In general form they are given by: inductance: EMI8.1 capacitance: EMI8.2 where F (p, s, #) = P/# (In cosec #s/2 + G (p, s, #)) p G is the correction term;p is the dimension illustrated on Figure 1: s, for inductance purposes, is the width of a conductor and, for capacitive purposes, is the spacing between conductors; and # is the wavelength. The four circuit elements given in Figure 3, Lf1, Cf1, Lf2 and C f2 are calculated as follows: EMI8.3 where L1=F(p,w1,#) EMI8.4 and L2=F(p,w2,1¯) where C1=4F(p,g1,#) EMI8.5 where L3=P(p,2w2,#) Cf2 = (C1 in series with C2) *d2/p where C2=4F (p,g2#) In each case the capacitance has been calculated using an effective dielectric constant of 1.12. It will be seen from the Table that close agreement is obtained at the first resonant frequency fl. At the upper resonance, the model is less satisfactory but nevertheless predicts f2 to within about 7%. The trans mission frequency fT is adequately predicted (to within 5%) in all 13 cases by the equivalent circuit.. As is usually the situation with equivalent circuit models at present, the results apply to plane wave normal incidence only, but are also good guides to the location of the bands at oblique incidence. Figures 4 and 5 illustrate two applications of dichroic members having element arrays as shown in Figure 1. In Figure 4, which illustrates a Cassegrain antenna, the dichroic member is a curved secondary dichroic mirror 5 mounted in front of the prime focus feed 6 of the antenna and this permits the prime focus feed to be used at frequencies where the dichroic mirror is transmitting, whilst the Cassegrain feed 7 in the centre of the main reflector bowl 8 is used at frequencies where the dichroic mirror is reflective. Figure 5 illustrates another arrangement enabling two feeds to be used simultaneously to give dual band capability. In this diplexer arrangement, the dichroic member is a flat dichroic plate 9 positioned between two feeds 10,11. The feed 10 operates at frequencies for which the plate 9 is reflective, whereas the feed 11 operates at frequencies for which the plate is transmissive. This diplexer arrangement can be used instead of the single band feed in a standard reflector antenna. Such operation has been achieved hitherto by constructing the frequency selective surface from stacks of waveguides or arrays of elements- such as resonant dipoles. The applications shown in Figures 4 and 5 assist in highlighting further advantages of the element array illustrated in Figure 1. These advantages are concerned with applications in which multi-banding is required. Such applications have been difficult to implement using known dichroic members. A major problem in implementing multi-banding applications is the cross polarisation which the known dichroic members exhibit when several are used together. The cross polarisation performance of the element array shown in Figure 1 has been found to be particularly improved in relation to known devices. Whilst particular embodiments have been described, it will be understood that modifications can be made without departing from the scope of the invention, as defined by the appended claims. For example, each element of the array illustrated in Figure 1 may comprise more than two concentric squares. This increases the number of reflection and transmission bands available. The smallest or innermost square of the multi-square element may be solid in shape instead of hollow, as are the or each outer square.

Claims: CLAIMS 1. A dichroic member comprising an array of elements (1) characterised in that each element comprises at least two substantially concentric symmetrical closed figures (2,3). 2. A dichroic member according to claim 1, characterised in that the element array is a substantially regular array of similar or substantially identical elements (1). 3. A dichroic member according to claim 1, characterised in that the closed figures (2,3) forming the elements are polygons or ellipses. 4. A dichroic member according to claim 1, characterised in that the sides of the closed figures (2,3) forming each element are of different widths. 5. A dichroic member according to claim 1, characterised in that the innermost figure (3) of each element is of solid configuration. 6. A dichroic member according to claim 1, characterised in that the closed figures (2,3) are formed as conductive paths on a non-conductive support. 7. A dichroic member according to claim 1, characterised in that the closed figures (2,3) are formed as non-conductive paths on a conductive support.

Inventors: LANGLEY RICHARD JONATHAN, PARKER EDWARD ANDREW, LANGLEY, RICHARD, JONATHAN, PARKER, EDWARD, ANDREW
Applicants: KENT SCIENT IND PROJECTS, KENT SCIENTIFIC AND INDUSTRIAL PROJECTS LIMITED
Application year: 1983
Main classification: H01Q15
IPC codes: G02B5, H01Q15