EP1184930B1 - Frequency selective surface waveguide filter - Google Patents

Abstract

A waveguide filter is hereby presented for separating electromagnetic waves of differing wavelengths by means of transmission through, or reflection from, a two-dimensional frequency selective surface. Electromagnetic energy consisting of any arbitrary wavelength enters a section of waveguide. A two-dimensional array of thin metallic film, either self-supporting or supported by a dielectric film, is transversely located at an arbitrary cross section within the waveguide. The film consists of one or more patterns so replicated and arranged as to permit the transmission of defined wavelengths of electromagnetic energy, and to reflect other wavelengths. By this means, selected wavelengths can be separated from a broad spectrum, and transmitted further along the waveguide. This invention is not limited to any defined cross-section of waveguide, and can be applied to any arbitrary shape. This invention is also not limited to any one pattern of metallic film. Also, a multiplicity of such two-dimensional films may be located longitudinally in the waveguide to increase the filtering effect. Further, such frequency selective surfaces can be combined with coupling means to effect transmission between microstrip and waveguide structures. <IMAGE>

Description

FIELD
• This invention relates generally to the separation of different wavelengths of electromagnetic waves. More specifically, the invention relates to the separation of electromagnetic waves utilizing a waveguide incorporating a two-dimensional frequency selective surface combined with a waveguide to microstrip transition.
BACKGROUND OF THE INVENTION
• Microwave energy can be propagated in a number of different modes, and in a number of physical structures. The microwave energy propagated through such structures can exist at any arbitrary frequency or spectrum of frequencies. In general, for a given application only specific frequencies from the spectrum are utilized. Therefore, devices known as filters are commonly placed in the structure to separate the broad spectrum of microwave frequencies into specific frequencies.
• One of the structures commonly employed for the transmission of electromagnetic energy is the waveguide. Waveguides offer very low loss to the passage of such waves, and further, confine the energy within the waveguide. One of the functions waveguides can be used for is the above-mentioned filtering, i.e. selection or suppression of a specific band of frequencies from a broad spectrum of frequencies.
• Conventional waveguide filters for separating different frequencies or wavelengths generally rely on three-dimensional structures that simulate, in electromagnetic wave form, the well-known filter elements encountered at lower frequencies, such as inductors, capacitors, and combinations of same to form resonant and anti-resonant circuits. These filter elements may consist of posts, irises, and other physical shapes located both transversely and longitudinally along the waveguide. Along the longitudinal axis, the filter elements are separated by defined fractions of electromagnetic wavelengths. These fractions of wavelengths are defined by well known mathematical relationships between the frequency of the band of electromagnetic energy being transmitted and the dimensions of the waveguide. The more filter elements are used along the longitudinal axis of the waveguide, the greater the filtering effect and, unfortunately, the size of the waveguide. Thus, due to the nature of the elements used, conventional waveguide filters are relatively complex as well as large in size, thereby proving disadvantageous in applications where low cost and small physical size are essential.
• In some cases however, a single element positioned transversely in the waveguide may be sufficient for the desired degree of filtering. An example of this is a simple band-stop filter described in "Bandstop Iris for Rectangular Waveguide" by N.G. Patterson and I. Anderson, in Electronics Letters, 28th October 1976, Vol. 12, No.22. The filter element is in the form of an iris in which the metallic pattern simulates an inductor and capacitor in series, (i.e. a bandstop structure). If the filtering effect is sufficient for a given requirement, such a simple structure does reduce the length of the waveguide. However, in addition to the iris, for excitation purposes the waveguide may have to be equipped with a transition to another medium (e.g. a coaxial cable or a microstrip circuit) that requires an additional length of waveguide.
• Another structure commonly used for the transmission of electromagnetic energy is the microstrip. Correspondingly, filters may be constructed using microstrip circuits. A microstrip circuit consists of a thin-film metallic trace deposited on a dielectric substrate. Conventional microstrip filters consist of planar conductive elements, which simulate inductors, capacitors, and resonant elements. A problem that arises when microstrip filters are employed to filter electromagnetic energy is that microwave energy outside of the frequency band of interest may bypass the microstrip circuitry, thus causing degradation of the filtering effect. In addition to lower filtering performance compared to waveguide filters, conventional microstrip filters present a higher unwanted loss to the selected desired signal than waveguide filters.
• Another structure used to filter electromagnetic signals is known as a Frequency Selective Surface (FSS).
• FSS's, although planar like microstrips, act on waves propagating in free space. They are used in certain antenna configurations to pass one band of frequencies and reflect another. They may be realized either by using a metallic plate with cutouts (e.g. holes of a certain size) or by a metallic pattern on a dielectric substrate, similar to a microstrip circuit with resonant circuits. It is possible to insert such planar structures into a waveguide to achieve a filtering effect. An important characteristic, for the purpose of comparison with other, FSS-like structures, is the fact that every such planar element can function as a filter on its own. The placement of more such elements along the longitudinal axis of the waveguide with proper spacings intensifies the filtering action.
• US Pat. No. 4,598,262 discloses a structure that, at first glance, appears to bear similarities to a FSS-type waveguide filter. It uses a series of transverse grating elements longitudinally repeated in a waveguide. However, the individual grating elements do not exhibit frequency selectivity of their own. Rather they simply pass a signal with a certain polarization and reflect a signal with polarization orthogonal to that of the former, regardless of frequency. The frequency sensitivity, (i.e. filtering effect) is achieved by applying the signal in two orthogonal modes to the waveguide and orienting the gratings at predefined angles. This, combined with proper spacing of the individual grating elements, results in a series of partial transmissions and reflections which support or cancel each other in the frequency band to be transmitted (or rejected). Thus this type of filter is fundamentally different from those using true resonant FSS structures. It is also bulky as it requires a converter from a single mode to two orthogonal modes on each end of the structure.
• Apart from filtering considerations per se, it is frequently necessary to provide a transition between electromagnetic waves existing in a waveguide and electromagnetic waves in an external medium, such as a microstrip. Where both a filter and a transition from microstrip to waveguide are required, it is common practice to separate these two functions.
• This separation can be in the form of waveguide filters, located within the waveguide, or can be in the form of microstrip structures, located on the microstrip substrate, external to the waveguide. The transition is in yet another physical location in the waveguide. The resulting structure is relatively large and costly even in cases where a single-element waveguide filter would be sufficient for the degree of filtering required.
• Accordingly, it is an object of this invention to apply the FSS principle to a provide a high performance waveguide filter suitable for applications where low cost and small physical size is required.
SUMMARY OF THE INVENTION
• In general, the waveguide filter of this invention consists of a frequency selective surface oriented transversely within the waveguide and having a two-dimensional array of conductor elements supported by a dielectric substrate. The array of conductor elements is formed by a repeating geometric pattern. The repeating geometric pattern may be a multiplicity of open loops, crosses or grids. The arrangement of the repeating geometric pattern results in the formation of a number of inductive and capacitive elements. The interaction of these elements offers little opposition to certain frequencies while blocking other frequencies. The determination of which frequencies are allowed to pass through the filter, and which are opposed, is a function of the shape, width and spacing of the inductive and capacitive elements of the frequency selective surface.
• The waveguide, and thus the waveguide filter, of the present invention may have any cross-sectional shape including square, rectangular and circular.
• In an embodiment having a waveguide with a rectangular cross-section and a rectangular waveguide filter, the signal enters the waveguide at one end, in a transverse mode known as the TE_1,0 mode. The signal is propagated at low loss in a longitudinal direction within the waveguide. Transversely located within the waveguide is a frequency sensitive surface. Electromagnetic waves of specified frequencies pass through the frequency sensitive surface unimpeded, whereas others are reflected by the filter elements. The waves which are selected and transmitted through the film continue to be propagated longitudinally along the waveguide.
• An alternate embodiment utilizes a waveguide having a circular cross-section. In this embodiment, the frequency selective surface is also circular, and the filter elements on the frequency selective surface may be in the form of concentric circular sections.
• In still a further embodiment, multiple frequency selective surfaces are employed, transversely oriented and longitudinally deployed along a waveguide. The required separation between such surfaces is a function of the wavelength of the electromagnetic spectrum to be transmitted or reflected.
• The waveguide filter of the present invention additionally comprises, on a common dielectric substrate, a frequency selective surface combined with a waveguide to microstrip transition in the form of a planar stub, operative to provide efficient coupling between the frequency selective surface and external microstrip circuitry.
BRIEF DESCRIPTION OF THE DRAWINGS
• Many objects and advantages of the present invention will be apparent to those of ordinary skill in the art when this specification is read in conjunction with the attached drawings wherein like reference numerals are applied to like elements and wherein:
• Fig. 1 is a cut-away view of a section of a rectangular waveguide incorporating a frequency selective surface;
• Fig. 2 is a view of a rectangular embodiment of the frequency selective surface of this invention;
• Fig. 3 is a diagram of a circuit equivalent of the frequency selective surface depicted inFig. 2;
• Fig. 4 is a cut-away view of a section of a cylindrical waveguide incorporating a circular frequency selective surface;
• Fig. 5 is a cut-away view of a section of a rectangular waveguide comprising a plurality of frequency selective surfaces;
• Fig. 6 is a cut-away view of a section of a rectangular waveguide comprising a frequency selective surface with a waveguide to microstrip transition; and
• Fig. 7 is a graph illustrating the coupling and filtering performance of the waveguide filter ofFig. 6.
DETAILED DESCRIPTION OF THE INVENTION
• Referring toFig. 1, a section of arectangular waveguide10 is shown having a frequencyselective surface12 inserted within. The frequencyselective surface12 may have any of a number of patterns, so chosen as to perform the desired filtering action.
• Referring toFig. 2, an embodiment of the frequencyselective surface12 is shown. The frequencyselective surface12 consists of a two-dimensional pattern produced on a metallic film supported by a dielectric substrate. The two-dimensional pattern produced on the metallic film consists of an array ofouter conductor elements14 andinner conductor elements16. The outer andinner conductors elements14 and16 are formed from an electrically conductive metal. The conductive metal may be copper, gold, platinum or any material capable of conducting electricity and suitable for deposition on a dielectric substrate. Theouter conductor elements14 andinner conductor elements16 are separated by spaces which may be formed by selective etching of thin metallic foil. The outerconductive elements14 each consist of discrete open center loops. The loop of each outerconductive element14 begins and terminates on the inner surface of thewaveguide10. The outerconductive elements14 form a symmetric, repeating pattern that may be repeated any number of replications required to perform a specific application.
• The innerconductive elements16 are arranged in a periodic array. Theportions18 of the innerconductive elements16 that are parallel to the electric field component of the microwaves within the waveguide act as inductive elements (the direction of the electric field component is indicated by arrowA ofFig. 1).
• Referring toFigs. 2 and3,portions18 of the innerconductive elements16 create inductive elements within the waveguide that are functionally similar to inductor27 (L1), in the discrete elementequivalent circuit26. Theedges20 of the innerconductive element16 that are perpendicular to the electric field component within the waveguide, in conjunction with theedges22 of the outerconductive elements14, that are perpendicular to the electric field component within the waveguide, act as a capacitive element. This capacitive element is functionally similar to capacitor30 (C), as shown inFig. 3. The portions of the outerconductive elements14 that are parallel to the electric field component within the waveguide are functionally similar to inductor28 (L2), also shown inFig. 3.
• Referring toFig. 4, an alternate embodiment of the waveguide filter of this invention is depicted as having a circular frequency selective surface. The frequency selective surface is formed by the selective etching of a thin metallic foil yieldingconductor elements36 and38. The resulting circular frequencyselective surface34 is utilized within acylindrical waveguide32.
• Referring toFig. 5, multiple frequencyselective surfaces42, 44 and46 are shown deployed in a section ofwaveguide40, where the waveguide has a rectangular cross-section. The use of multiple circular waveguide filters may also be deployed within a cylindrical waveguide.
• Fig. 6 shows a waveguide filter in which a frequencyselective surface50 has been combined with a waveguide tomicrostrip transition52. Essentially, the transition is a planar stub.
• The transition is co-located with the frequency selective surface on a supporting substrate. The substrate with the waveguide tomicrostrip transition52 and the frequencyselective surface50 is transversally inserted into therectangular waveguide 48. Anexit port54 is located in the wall of thewaveguide48 to enable waveguide tomicrostrip transition52 to be coupled to external microstrip circuitry (not shown).
• The frequencyselective surface50 consists of a planar surface formed from a dielectric substrate coated with an electrically conductive metallic film. Specific portions of the metallic film have been removed to form circuit elements. The residual metallic film comprises an array of frequency selectivesurface filtering elements56 and a waveguide tomicrostrip transition52. Thefiltering elements56 consist of a multiplicity of discrete open center square loops. The sizes of these loops are mathematically determined from the frequencies to be reflected or transmitted. While this invention is not limited to any one pattern, a specific pattern is shown in order to explain the functioning of the invention.
• Again with reference toFigure 6, the microstrip towaveguide transition52 is approximately centrally located with respect to the frequencyselective surface50. The dimensions of the waveguide tomicrostrip transition52 are determined by the impedance of the connecting microstrip circuit (not shown) which is external to thewaveguide48, and the dimensions of thewaveguide48. In order to obtain an efficient coupling between thewaveguide48 and the external microstrip circuit, either the dimensions of the waveguide tomicrostrip transition52 can be varied, or impedance matching circuitry can be employed on the external microstrip circuit.
• The dielectric substrate supporting the frequencyselective surface50 and waveguide-to-microstrip transition 52 is located one-quarter of one electrical wavelength away from the short-circuitedend58 of thewaveguide48, said wavelength being that of the signal at the center of the frequency band desired to be efficiently transmitted through the structure. This ensures that the waveguide tomicrostrip transition52 is located at a point of maximum electric field strength, which in turn enables optimum coupling between the waveguide and the external microstrip circuit.
• With reference toFig. 7, the performance of the integrated microstrip to waveguide transition and filter inserted in a waveguide, can be seen to consist of a passband in which efficient and symmetrical transmission exists between the waveguide and the external microstrip circuit. Also, the filtering action is demonstrated by the suppression of the signal at either extremity of the band pass portion. This invention is not intended to be limited to the filtering and coupling characteristics illustrated inFig. 7. Said characteristics are representative of one embodiment only, to illustrate by means of example, the results obtainable by means of this invention.

Claims (7)

1. A waveguide filter comprising:

   (a) a waveguide (10; 32; 40; 48);

   (b) a frequency selective surface (12; 34; 42, 44, 46; 50) located within said waveguide (10; 32; 40; 48), said frequency selective surface (12; 34; 42, 44, 46; 50) comprising a planar electrically conductive film supported on a dielectric substrate located within said waveguide (10; 32; 40; 48) and oriented orthogonally to walls of said waveguide (10; 32; 40; 48);characterised by

   (c) a microstrip located externally to said waveguide (10; 32; 40; 48); and

   (d) a waveguide to microstrip transition (52), which couples said waveguide (10; 32; 40; 48) to said microstrip, wherein at least a portion of said waveguide to microstrip transition (52) is located on said substrate.
2. The waveguide filter according to claim 1, wherein said film is selectively etched to form inductive and conductive elements.
3. The waveguide filter according to claim 2, wherein said electrically conductive film is a copper film.
4. The waveguide filter according to claim 1, wherein said waveguide has a rectangular cross-section and said frequency selective surface has a rectangular shape complementary to said cross-section of said waveguide.
5. The waveguide filter of claim 1, wherein said frequency selective surface is located at a distance from a short-circuited end of said waveguide, said distance equal to one quarter of an electrical wavelength of a signal that is at the center of a frequency band desired to be efficiently passed through said waveguide filter.
6. The waveguide filter of claim 1, wherein said waveguide filter comprises a plurality of frequency selective surfaces longitudinally spaced along an interior of said waveguide and orthogonally oriented with respect to said walls of said waveguide.
7. The waveguide filter of claim 1, wherein said microstrip additionally comprises impedance matching circuitry.

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