Frequency selective surfaces

Abstract

A switchable Frequency Selective Surface (FSS) in which the switchable elements are Plasma-shells. Plasma-shells as described herein allow for control or reconfiguration of the FSS electromagnetic (EM) properties.

Claims

1. A plasma discharge frequency selection surface device comprising gas filled plasma-shells, cross-shaped four legged conductor elements, and a conductive ground layer, each plasma-shell being in contact with the legs of two adjacent conductive elements and the conductive ground layer, means for energizing the plasma-shells such that the energized plasma-shells shunt the conductive elements to each other and the conductive ground layer thereby forming a conductive sheet that blocks electromagnetic energy.

2. A plasma discharge frequency selection surface device comprising gas filled plasma-shells and cross-shaped four legged conductor elements, each plasma-shell being in contact with the legs of two adjacent conductive elements, means for energizing the plasma-shells such that the energized plasma-shells shunt the conductive elements to each other to form a conductive sheet that blocks electromagnetic energy.

3. A plasma discharge frequency selection surface device comprising gas filled plasma-shells, cross-shaped four legged conductor elements, and a conductive ground layer, each plasma-shell being in contact with the leg of a conductive element and the conductive ground layer, means for energizing the plasma-shells such that the energized plasma-shells shunt the conductive elements to the conductive ground layer thereby forming a conductive sheet that blocks electromagnetic energy.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a typical two-dimensional composite Frequency Selective Surface (FSS).

(2) FIG. 2A is a physical diagram of a sectioned Plasma-shell with internal plasma.

(3) FIG. 2B shows an equivalent electrical circuit model of a single Plasma-shell used as a microwave tunable element.

(4) FIG. 3 is an FSS with Plasma-shells consistent with this invention.

(5) FIG. 4A is a single FSS unit cell.

(6) FIG. 4B is a single Plasma-shell.

(7) FIG. 4C is an exploded view of the plasma FSS.

(8) FIG. 5 is an assembled view of the plasma FSS connected to a high voltage source.

(9) FIG. 6 is a Plasma-shell FSS test setup.

DETAILED DESCRIPTION OF THE DRAWINGS

(10) FIG. 1 shows a typical two-dimensional Frequency Selective Surface (FSS). Conductive Layer 102 is applied to dielectric substrate 101. Four legged loaded elements 103 are patterned into the conductive layer.

(11) FIG. 2A is a physical diagram of a Plasma-shell 104. Plasma-shells consist of a hollow, impervious dielectric shell of any shape 104 that encapsulates a pressurized gas that can be ionized into conductive plasma 105 independently from the FSS layer. Optional electrodes 106 are shown on the top surface. A differential voltage between electrodes energizes neutral gas molecules into plasma through the process of ionization. Plasma-shells can be produced from a variety of materials including glasses and ceramics. Plasma-shells or shells may be fabricated with multiple material layers to customize shell properties for specific applications. Any shape is possible including cubes and right circular cylinders. Finished external dimensions range from about 0.5 mm to about 10 mm. Plasma-shells are filled with a variety of gases with controlled pressures from about 5 to about 500 Torr. Conductive electrodes of any configuration can be applied to shell surfaces. Large Plasma-shell arrays can be assembled on substrates or conductive layers. In the normal un-energized Plasma-shell condition, the shell material and gas are non-conductive and transparent to RF energy.

(12) FIG. 2B shows an equivalent electrical circuit model for a single Plasma-shell, or shell, used as a tunable microwave element. Conductive electrodes 106 are patterned on the top of the shell 104 to apply an electric (E)-field of sufficient intensity to excite the interior gas 107 into plasma. The shell walls have significant wall capacitance, C.sub.w 106a, in series with a very small capacitance across the gas, C.sub.g 107a. Gas impedance, Z 107b, changes dramatically with the degree of plasma ionization and allows the Plasma-shell to be used as a tunable element.

(13) FIG. 3 is an FSS device with Plasma-shells 307. Plasma-shells 307 connect the cross-shaped four-legged loaded elements 308 covered with a non-conductive layer 309. When energized, the Plasma-shells shunt elements to ground forming a conductive sheet. This blocks microwave transmission, and the plasma-actuated conductive sheet can protect sensitive electronics from electromagnetic pulses (EMP). In addition to the unique switching capability, the plasma-switched (PS) FSS is strong, rugged, lightweight, conformable, and easily retrofitted to any surface. Plasma-shells may be actuated by control electronics connected to the shell or may be actuated by high-power EM energy incident to the surface. The dimensions of the contained plasma are irrelevant to the performance of the FSS.

RF/Microwave Plasma-Shell Frequency Selective Surfaces

(14) FSSs are EM structures that interact with EM energy propagating in free space. The microwave frequency range, loosely defined as 0.3-30 gigahertz (GHz), is used for applications including radar, communication, instrumentation, and power transfer. An FSS is a periodic surface with a RF response that varies with frequency. A frequency selective surface layer is composed of arrays of elements that can be of any shape including, but not limited to, dipoles, circular dipoles, helicals, circular or square or other spirals, biconicals, hexagons, tripods, Jerusalem crosses, plus-sign crosses, annular rings, gang buster type antennas, tripole elements, anchor elements, star or spoked elements, alpha elements, gamma elements, MK elements, and/or combinations thereof. FSS layers can be implemented as conductive patterns or non-conductive regions on otherwise conductive sheets. One or more elements are composed into a unit cell that serves as a template that is regularly applied over a flat or curved conformal surface. The geometry of the unit cell may be varied over the FSS layer surface to accommodate features such as edge treatment, tapering of EM properties, close packing, or to optimize other performance properties. FSS layers may be applied to substrates or remain free standing. Common microwave FSS applications include hybrid radomes, spatial band-pass or band-stop filters, dichroic reflectors and subreflectors, absorbers, and polarizers.

(15) Plasma components may be integrated with FSS structures to create a plasma FSS device with desirable properties such as direct EM energy-plasma interaction, high-power limiting to limit power density allowed to transmit through the device, and controllable frequency response (e.g., controllable operating frequency and/or transmission/absorption/reflection properties). The plasma FSS improves the conventional FSS design by modifying the unit cell to include FSS elements in combination with plasma, resulting in plasma-controlled EM properties.

(16) Plasma-shells are gas encapsulating structures that hermetically contain a single gas or gas mixture at controlled pressure independently of the FSS layer. When used as switchable plasma elements, the inner wall is resistant to direct contact with plasma. Plasma-shells are switchable plasma elements because the degree of internal gas ionization can be controlled by application of electrical or RF energy.

(17) The energy required to create and sustain plasma can be supplied externally by high-power incident RF energy, or from a high-voltage power supply connected to conductive elements that are part of the plasma FSS structure. The FSS structure can be used to distribute energy from the power supply to Plasma-shells. Using a power supply to provide Plasma-shell drive voltage is also called biasing.

(18) Disclosed herein is a device to create a large-area plasma-controlled surface that interacts with propagating EM energy. Electrical energy is required for creating and sustaining the FSS device and is provided by an external voltage source or the incident RF wave, and RF-plasma interaction may occur over the entire microwave frequency range with EM energy of any polarization and any power level.

(19) In this invention, a Plasma-shell is sandwiched between two conductor sheets, with the conductor sheets being patterned so as to be transparent to a particular band of radio frequencies (RF).

(20) FIG. 4A shows a single FSS unit cell composed of four etched 402 slots in a conductive sheet 401. The element pattern, called an MK element, is one pattern of many that has the properties of band-pass characteristic, free-standing (does not have unconnected floating elements), and large conductor area coverage. This structure is beneficial for an extremely lightweight structure that effectively produces a volume of plasma within an array of Plasma-shells.

(21) The FSS element pattern is repeated on the conductive sheet on a closely-spaced regular grid. The conductive sheets may be fabricated as a single etched or electroformed metal sheet or a single-sided printed circuit board (PCB). At X-band frequencies (8-12 GHz), the FSS element is slightly larger than a typical Plasma-shell.

(22) FIG. 4B is a single, hollow ceramic Plasma-shell 403, hermetically sealed and filled with any gas.

(23) FIG. 4C shows a material stackup of the Plasma-shell FSS, where two outer patterned conductive layers 404, 405 are laminated onto an array of Plasma-shells 406 to form composite material 407.

(24) As shown in FIG. 5, the composite material shown in FIG. 4C as 407 can be driven with a high-voltage AC power source 502, nominally at frequency of about 1 megahertz (MHz). The high voltage across the Plasma-shell array creates a volume of plasma inside the shells. The plasma can directly interact with propagating EM energy through the outer conductive layers that are effectively transparent to X-band energy.

(25) The frequency response of the Plasma-shell FSS is measured with the test setup in FIG. 6 that shows a vector network analyzer (VNA) 601 that measures FSS scatter (S)-parameters with different plasma states and antenna 602, 603 polarizations on the composite shell 501.

(26) The Plasma-shells are shown as cubes or cuboids with all flat sides. However, other volumetric shapes may be used such as spheres, discs, and domes. The cross-sectional shapes of the Plasma-shell may be square, rectangular, circular, elliptical, triangular, and so forth. Various geometric shapes of gas-filled Plasma-shells including the manufacture of Plasma-shells are disclosed in U.S. Pat. No. 8,368,303 (Wedding et al.), U.S. Pat. No. 8,299,696 (Wedding et al.), U.S. Pat. No. 8,106,586 (Wedding et al.), and U.S. Pat. No. 7,978,154 (Strbik, III et al.), and U.S. Design Pat. D670,238 (Wedding et al.), all incorporated herein by reference.

(27) The foregoing description of various preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments discussed were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims to be interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.