Frequency selective surface waveguide filter

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.

Description

FIELD

[0001] 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 two-dimensional frequency selective surface.

BACKGROUND OF THE INVENTION

[0002] Microwave energy can be propagated in a number of different modes, and in a number of physical structures. At higher microwave frequencies, waveguides are commonly employed for the transmission of electromagnetic energy. Waveguides offer very low loss to the passage of such waves, and further, confine the energy within the waveguide.

[0003] The microwave energy propagated through a waveguide can exist at any arbitrary frequency or spectrum of frequencies. In general, only specific frequencies from the spectrum are utilized. Therefore, waveguide filters are commonly placed in the waveguide to separate the broad spectrum of microwave frequencies into specific frequencies.

[0004] 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 transversal elements are separated by defined fractions of electromagnetic wavelengths. These wavelengths are defined by the frequency of the band of electromagnetic energy being transmitted and the dimensions of the waveguide. Devices utilizing these methods are relatively long thereby proving disadvantageous in those applications where small physical size is essential.

[0005] Waveguide filters are commonly constructed using microstrip circuits. The microstrip circuits consist of a thin-film circuit deposited on a substrate. It is frequently necessary to provide a transition between electromagnetic waves existing on a microstrip substrate, typically in the transverse electromagnetic mode, and electromagnetic waves in a waveguide. In practical transitions, it is commonly necessary to select specific frequencies, and to separate these from a broad spectrum of microwave frequencies. Various types of filters are used to accomplish this separation. Such filters 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.

[0006] Waveguide filters, as previously referenced, are physically large, and their use is restricted to those applications where small size is not a required parameter.

[0007] Conventional microstrip filters, on the other hand, consist of planar conductive elements, which simulate inductors, capacitors, and resonant elements. These require a considerable area of the microstrip substrate. Further, a problem can arise when said microstrip filters are employed to filter electromagnetic energy entering the microstrip substrate from the waveguide. Microwave energy outside of the frequency band of interest can propagate in the microstrip circuitry, and cause unwanted effects.

[0008] It is an object of this invention is to provide a means for separating defined frequencies of electromagnetic energy, by means of a two-dimensional film located transversely in a waveguide structure.

[0009] It is a further object of this invention to provide a device capable of separating defined frequencies of electromagnetic energy, and providing a coupling means for electromagnetic energy between a microstrip substrate and a waveguide structure.

SUMMARY OF THE INVENTION

[0010] These and other objects of the invention are provided in a new and improved waveguide filter for use in a waveguide. In general, the waveguide filter of this invention consists of a frequency selective surface having an array of conductor elements. The array of conductor elements is formed by a repeating geometric pattern. The repeating geometric pattern may be a multiplicity of open loop, 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 or attenuating other frequencies. The determination of the frequencies are allowed to pass through the filter and those that are opposed is a result of the width and spacing of the inductive and capacitive elements within the repeating geometric pattern.

[0011] The waveguide and thus the waveguide filter may have any cross-sectional shape including square, rectangular and circular.

[0012] In an embodiment having a waveguide with rectangular cross-section and a rectangular waveguide filter, in operation, the waveguide 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 thin metallic foil which contains the filter elements in the form of discrete open loops. These elements consist of mathematically defined shapes, which represent filter elements realized in a two-dimensional form. Electromagnetic waves of specified frequencies pass through this foil 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.

[0013] An alternate embodiment utilizes a waveguide having a circular cross-section. In this embodiment, the frequency selective surfaces are in the form of concentric circles, and the frequency determining elements on the frequency selective surface may be in the form of concentric circular sections.

[0014] In still a further embodiment, multiple waveguide filters are employed, longitudinally deployed along a waveguide. The separation between such surfaces is determined by the wavelength of the electromagnetic spectrum to be transmitted or reflected.

[0015] In still a further embodiment, the waveguide filter is combined with a planar stub for the purpose of providing efficient coupling between said frequency selective surface and external microstrip circuitry.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] 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 waveguide filter;

Fig. 2 is a perspective view of an embodiment of the waveguide filter of this invention;

Fig. 3 is a circuit diagram performing an equivalent function to the embodiment of the waveguide filter depicted in Fig. 2;

Fig. 4 is a cut-away view of a section of a cylindrical waveguide incorporating a circular waveguide filter;

Fig. 5 is a cut-away view of a section of a rectangular waveguide revealing the deployment of a plurality of waveguide filters;

Fig. 6 is a cut-away view of a section of a rectangular waveguide depicting a waveguide to microstrip transition: and

Fig. 7 is a graph illustrating the coupling and filtering performance of the embodiment of the waveguide filter of Fig. 6.

DETAILED DESCRIPTION OF THE INVENTION

[0017] Referring to Fig. 1, a section of a rectangular waveguide 10 is shown having a waveguide filter 11 inserted within. The waveguide filter 11 may have any arbitrary pattern, so chosen as to perform the desired filtering action.

[0018] Referring to Fig. 2, an embodiment of the waveguide filter 11 is shown having a frequency selective surface. The frequency selective surface consists of a two-dimensional pattern produced on a metallic film where specific portions 24 of the metallic film have been removed. The two-dimensional pattern produced on the metallic film consists of an array of outer conductor elements 14 and inner conductor elements 16. The outer and inner conductors elements 14 and 16 respectively, are formed from an electrically conductive metal. The conductive metal may be copper, gold, platinum or any material capable of conducting electricity. The outer conductor elements 14 and inner conductor elements 16 may be formed by the selective etching of a thin metallic foil. The outer conductive elements 14 each consist of a discrete open center loops. The loop of each outer conductive element 14 begins and terminates on the inner surface of the waveguide 10. The outer conductive elements 14 are arranged such that the inner walls of the waveguide 10 act as reflective surfaces for the electromagnetic energy impinging on these walls. The outer conductive elements 14 form a symmetric, repeating pattern that may be repeated any number of replications required to perform a specific application.

[0019] The inner conductive elements 16 are arranged in a periodic array. The portions 18 of the inner conductive elements 16 that are parallel to the electric field component within the waveguide acts as an inductive element (the direction of the electric field component is indicated by arrow A of Fig. 1).

[0020] Referring to Fig. 3, the inductive elements created within the waveguide is functionally similar to inductor 27 (L1), in the discrete element equivalent circuit 26. The edges 20 of the inner conductive element 18 that are perpendicular to the electric field component within the waveguide, in conjunction with the edges 22 of the outer conductive elements 14, that are perpendicular to the electric field component within the waveguide, act as a capacitive element. This capacitive element is functionally similar to capacitor 30 (C), as shown in Fig. 3. The portions of the outer conductive elements 14 that are parallel to the electric field component within the waveguide are functionally similar to inductor 28 (L2), also shown in Fig. 3.

[0021] Referring to Fig. 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 yielding conductor elements 36 and 38. The resulting circular waveguide filter 34 is utilized within a cylindrical waveguide 32.

[0022] Referring to Fig. 5, the use of multiple waveguide filters 42, 44 and 46 is shown deployed in a section of waveguide 40, where the waveguide has a rectangular cross-section. The use of multiple circular waveguide filters may also be deployed within a cylindrical waveguide.

[0023] Referring to Fig. 6, in a further alternate embodiment, a waveguide filter 50 has been combined with a waveguide 48 to microstrip (not shown) transition. The integrated waveguide filter 50 and microstrip to waveguide 48 transition can be seen to consist of a rectangular waveguide 48 into which a waveguide filter 50 is transversally inserted. An exit port 54 is located in the wall of the waveguide 48 to enable a coupling stub 52 to be coupled to external microstrip circuitry.

[0024] The waveguide filter 50 consists of a frequency selective surface formed from a dielectric substrate coated with an electrically conductive metallic film. Specific portions of the metallic film have been removed to form the circuit elements. The residual metallic film consists of an array of frequency selective surface filtering elements 34 and a coupling stub 32. The filtering elements 34 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.

[0025] Again with reference to Figure 6, the microstrip to waveguide transition is accomplished by means of a stub 32, which is centrally located with respect to the long dimension of the planar circuit 31. The dimensions of the stub 32 are determined by the impedance of the connecting microstrip circuit external to the waveguide 30, and the dimensions of the waveguide 30. In order to obtain an efficient coupling between the waveguide 30 and the external microstrip circuit, either the dimensions of the stub 32 can be varied, or impedance matching circuitry can be employed on the external microstrip circuit.

[0026] The planar circuit 31 is located one-quarter of one electrical wavelength away from the short-circuited end 35 of the waveguide 30. This ensures that the stub 32 is located at a point of maximum electric field strength.

[0027] With reference to Fig. 7, the performance of the integrated microstrip to waveguide transition and filter can be seen to consist of a pass-band, in which efficient and symmetrical transmission exists between the waveguide 48 and the external microstrip circuit. Also, the filtering action is demonstrated by the attenuation at either extremity of the band pass portion. This invention shall not be limited to the filtering and coupling characteristics illustrated in Fig. 7. Said characteristics are illustrative of one chosen implementation only, to illustrate by means of example, the results obtainable by means of this invention.

[0028] It will be understood that the waveguide used throughout this invention may be of any desired cross-sectional design according to the knowledge of those skilled in the art and operate in conventional fashion to achieve the intended result. Furthermore, the waveguide will have a design corresponding to the cross-section design of the waveguide.

[0029] The above-described embodiments should be regarded as illustrative rather than restrictive, and it should be appreciated that variations may be made other than those discussed, by workers of ordinary skill in the art without departing from the scope of the present invention as defined by the following claims.

Claims

1. A waveguide filter apparatus for use in a waveguide, comprising:

(a) an electrically conductive substrate having a surface plane oriented orthogonally to walls of said waveguide;

(b) a plurality of conductive elements lying in said surface plane of said electrically conductive substrate; and

(c) a plurality of channels through said electrically conductive substrate;

wherein said plurality of conductive elements are separated by said plurality of channels to form a two-dimensional pattern on said electrically conductive substrate.

2. The waveguide filter according to claim 1, wherein said electrically conductive substrate is an electrically conductive film.

3. The waveguide filter according to claim 2, wherein said conductive elements include a plurality of inductive elements and a plurality of capacitive elements.

4. The waveguide filter according to claim 2, wherein said electrically conductive substrate is selectively etched to form said plurality of conductive elements.

5. The waveguide filter according to claim 2, wherein said electrically conductive film is a copper film.

6. The waveguide filter according to claim 2, wherein said waveguide has a rectangular cross-section and said surface plane of said waveguide filter is rectangular in shape.

7. The waveguide filter according to claim 2, wherein said waveguide has a circular cross-section and said surface plane of said waveguide filter is circular in shape.

8. A waveguide filter apparatus for use in a waveguide, comprising:

a frequency selective surface having a surface plane oriented orthogonally to walls of said waveguide

   wherein said frequency selective surface is patterned with an array of conductive elements.

9. The waveguide filter according to claim 7, wherein said frequency selective surface is an electrically conductive film.

10. The waveguide filter according to claim 8, wherein said array of conductive elements includes a plurality of inductive elements and a plurality of capacitive elements.

11. The waveguide filter according to claim 8, wherein said array of conductive elements are formed by selectively etching said electrically conductive film.

12. The waveguide filter according to claim 8, wherein said electrically conductive film is a copper film.

13. The waveguide filter according to claim 8, wherein said waveguide has a rectangular cross-section and said frequency selective surface of said waveguide filter is rectangular in shape.

14. The waveguide filter according to claim 8, wherein said waveguide has a circular cross-section and said frequency selective surface of said waveguide filter is circular in shape.

Drawing