Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B5/00—Optical elements other than lenses
  • G02B5/20—Filters
  • G02B5/204—Filters in which spectral selection is performed by means of a conductive grid or array, e.g. frequency selective surfaces
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q15/00—Devices for reflection, refraction, diffraction or polarisation of waves radiated from an antenna, e.g. quasi-optical devices
  • H01Q15/0006—Devices acting selectively as reflecting surface, as diffracting or as refracting device, e.g. frequency filtering or angular spatial filtering devices
  • H01Q15/0013—Devices acting selectively as reflecting surface, as diffracting or as refracting device, e.g. frequency filtering or angular spatial filtering devices said selective devices working as frequency-selective reflecting surfaces, e.g. FSS, dichroic plates, surfaces being partly transmissive and reflective

Definitions

• 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.
• US Patent 4168254 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 crossing 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.
• 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.
• dichroic member comprising a regular grid having elements disposed within the interstices of the grid, these elements being identical and of substantially symmetrical shape.
• 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.
• the invention consists in a dichroic member having an array of elements, each of which comprises at least two substantially concentric symmetrical closed figures.
• 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.
• 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 expending 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
• Figure 5 is a schemata c diagram of a diplexer embodying such a dichroic member.
• 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 non-conductive substrate.
• the conductive squares may be formed on a dielectric substrate sheet by printed circuit techniques.
• 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 f 1 ,f 2 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 f 1 , f 2 is a band of high transmission f T . In most applications, the latter would be used with bands f 1 or f 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 resonant- frequency transmission characteristic 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.
• 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 first three arrays listed in the Table have identical geometries apart from the inner square sides d 2 which are 3 . 5 , 3 . 0 and 2.5 mm long, respectively.
• the first reflection band centre on f 1 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 d 2 but dependent on its width w 2 .
• the second resonant frequency f 2 is determined by the inner-square dimensions only and f 2 increases as d 2 decreases.
• the relative spacing of this second resonance with respect to f 1 controls the transmission- frequency band centred on f T .
• the ratio f T /f 1 is 1.5 for array 1, 1.8 for array 2 and 2.2 for array 3 .
• array 3 at 45° has two high-frequency resonances within our frequency range, a narrow one at 27.5 GHz and a more prominent null at 34.5 GHz in the H-plane. They occur at 31.5 and 33 GHz, respectively, in the E-plane.
• L f1 , C f1 L f2 and C f2 are calculated as follows:
• Figures 4 and 5 illustrate two applications of dichroic members having element arrays as shown in Figure 1.
• 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
• FIG. 5 illustrates another arrangement enabling two feeds to be used simultaneously to give dual band capability.
• 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.
• 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.

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• Physics & Mathematics (AREA)
• Spectroscopy & Molecular Physics (AREA)
• General Physics & Mathematics (AREA)
• Optics & Photonics (AREA)
• Aerials With Secondary Devices (AREA)
• Variable-Direction Aerials And Aerial Arrays (AREA)

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• 1983
  • 1983-09-22 WO PCT/GB1983/000235 patent/WO1984001242A1/en not_active Application Discontinuation
  • 1983-09-22 EP EP19830903094 patent/EP0119253A1/de not_active Withdrawn

Non-Patent Citations (1)

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