Frequency selective surface

Details Frequency selective surface Frequency selective surface

1995-06-22

2001-04-17

Blum, Theodore M. (Department: 3662)

Communications: directive radio wave systems and devices (e.g.,

Radar reflector

C343S770000, C343S756000, C343S909000

Reexamination Certificate

active

06218978

ABSTRACT:

This invention relates to a frequency selective surface suitable, particularly, but not exclusively, for use as a narrow bond, angularly stable electromagnetic window.
BACKGROUND OF THE INVENTION
A conventional frequency selective surface comprises a doubly periodic array of identical conducting elements, or apertures in a conducting screen. Such a conventional surface is usually planar and formed by etching the array design from a metal clad dielectric substrate. These conventional frequency selective surfaces behave as filters with respect to incident electromagnetic waves with the particular frequency response being dependent on the array element type, the periodicity of the array and on the electrical properties and geometry of the surrounding dielectric and/or magnetic media. The periodicity is the distance between the centres of adjacent elements or between the centres of adjacent apertures.
Such a conventional frequency selective surface has a wide bandwidth and it is desirable to have a surface with a smaller bandwidth which is more selective and which has a relatively large frequency separation between the passband and onset of grating lobes.
There is a need for a generally improved frequency selective surface.
SUMMARY OF THE INVENTION
According to the present invention there is provided a frequency selective surface, including at least one sheet-like frequency selective layer made up of an array of non-electrically conductively spaced apart electrically conductive elements, at least one electrically conductive sheet-like frequency selective layer having an array of spaced apart non-conductive apertures therethrough overlaying said element layer, and a sheet of dielectric material separating said at least one element layer and said at least one aperture layer, with the element layer being complementary in plan view shape to the aperture layer with the element layer and the aperture layer being rotated through 90 degrees in plan with respect to each other and being substantially parallel to one another and with the element array and the aperture array having the same periodicity.
Preferably the at least one conductive element layer is located transversely displaced with respect to the at least one aperture layer by half the periodicity of said layers.
Conveniently each conductive element has the shape of a closed wire-like loop which is preferably square, in plan view and wherein each aperture is a closed wire-like slot of complementary shape in plan view, which is preferably square in shape.
Alternatively each conductive element has the shape in plan view of a three armed tripole with three wire-like substantially linear arms radiating from a central point at 120 degrees to one another, and each aperture has the shape, in plan view, of a three arm tripole slot with three substantially linear arm-like slots radiating from a central point at 120 degrees to one another.
Alternatively each element in plan view has the shape of a patch, preferably circular, and each aperture is of complementary shape in plan view.
Preferably said at least one conductive element layer and said at least one aperture layer are made of copper foil and said dielectric material is polyester.
Conveniently each layer is substantially planar in form.
According to a further aspect of the present invention there is provided a narrow band, angularly stable, electromagnetic window having a surface incorporating or made of a frequency selective surface as hereinbefore described.


REFERENCES:
patent: 4287520 (1981-09-01), Van Vliet et al.
patent: 5189433 (1993-02-01), Stern et al.
patent: 5349364 (1994-09-01), Bryanos et al.

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