Reconfigurable resonant cavity with frequency-selective surfaces and shorting posts

[0002] This application claims the benefit of provisional application No. 60/190,372 filed on Mar. 17, 2000.

[0003] The present invention relates to resonant cavities and, more particularly, to a reconfigurable resonant cavity for use in conjunction with a slot antenna element to provide broadband operation of the antenna at more than one selected frequency band.

[0004] Slot radiators exhibit increased gain, typically 3 dB, when placed over a resonant cavity. Because the resonant cavity provides a high Q, the operational bandwidth of the system is limited.

[0005] Using a resonant cavity behind a slot is the primary solution for maximizing gain from a slot element.

[0006] It is, therefore, an object of the invention to provide a reconfigurable resonant cavity which results in high gain, broadband performance from an integrated slot radiator.

[0007] It is another object of the invention to provide a reconfigurable resonant cavity which includes movable “fences” which define the effective size of the cavity.

[0008] It is a further object of the invention to provide a reconfigurable resonant cavity which implements “fences” by using selectable shorting pins.

[0009] It is still another object of the invention to provide a reconfigurable resonant cavity which uses frequency- selective surface materials (FSS) to control the resonant frequency of the cavity.

[0010] In accordance with the present invention there is provided a reconfigurable resonant cavity for use with a slot radiator. Selectable, electrically conductive posts, operating in cooperation with FSS material, are used to define movable cavity walls, resulting in multiple, selectable, predetermined resonant frequencies of operation for the cavity. Microelectromechanical switches (MEMS) or other photonically or electrically operated switching devices are used to activate and deactivate the electrically conductive posts so as to effectively move the cavity walls.

[0011] A complete understanding of the present invention may be obtained by reference to the accompanying drawings, when considered in conjunction with the subsequent detailed description, in which:

[0012] FIG. 1 is a schematic, cross-sectional view of the reconfigurable resonant cavity of the invention; and

[0013] FIG. 2 is a schematic view of a light-activated, switched shorting post for use in the resonant cavity of FIG. 1.

[0014] Resonant cavities placed beneath slot radiators are well known for enhancing the gain of slot radiators. Gain enhancements in the range of 3 dB are typical. However, the resonant cavity provides this phenomena over a limited bandwidth and is, therefore, unsuited for broadband applications. The reconfigurable resonant cavity of the present invention overcomes this difficulty.

[0015] Referring now to FIG. 1, there is shown a side, schematic view of the reconfigurable resonant cavity of the invention, generally at reference number 100. For purposes of disclosure, cavity 100 is shown configured for three-band operation. However, it should be obvious that by altering the number of dielectric/FSS layers and the number and/or location of the conductive posts, the inventive cavity may be configured to operate in more than three frequency bands.

[0016] A slot 102 is shown in an upper conductive plane 104. The slot 102 is configured in accordance with welt known principles and forms no part of the instant invention. A reconfigurable slot is ideal for use with the inventive reconfigurable cavity of the present invention. A lower ground plane 106 is located substantially parallel to and spaced apart from upper conductive plane 104, thereby defining the maximum depth of the resonant cavity 100 and, therefore, the lowest frequency of operation.

[0017] Two dielectric layers 108a,108bare disposed in cavity 100, layers 108a,108balso being substantially parallel to both upper conductive plane 104 and lower ground plane 106. Selectively disposed on the top surface of dielectric layers 108a,108bare resonant elements of frequency selective surface material 110 to form intermittent frequency-selective surfaces (FSS) on dielectric layers 108a,108b.

[0018] By using frequency selective materials having different unit cell periodicites, the absorption and reflection characteristics of the surfaces may be controlled. This allows cavity 100 to form a well-behaved resonator at each of the frequency bands to which it may be tuned. In addition, resonant elements of frequency selective material 110 helps control the Q of the resonator. Each dielectric layer 108a,108bcarrying resonant elements of frequency selective surface material 110 defines a potential alternate bottom ground plane for cavity 100.

[0019] These alternate bottom ground planes 108a,108bmust have their respective FSS layers electrically connected to upper conductive plane 104 for them to become effective ground planes. These connections are made by means of conductive posts 112, 114, 116 located on either side of a vertical centerline 118 of slot 102.

[0020] Pairs of posts 112 are located the closest to centerline 118 and extend only between upper conductive plane 104 and a first dielectric layer 108a.This defines the smallest of the resonant cavity configurations suitable for operation at an arbitrary frequency F_hi.

[0021] Similarly pairs of posts 114 are located further away from centerline 118 and connect dielectric layer 108bto upper conductive plane 104. This defines a somewhat larger configuration of a resonant cavity for operation at an arbitrary frequency F_mid.

[0022] Finally, pairs of posts 116 are located still further away from centerline 118 and connect lower ground plane 106 to upper conductive plane 104, thereby defining the largest possible configuration of resonant cavity suitable for operation at an arbitrary frequency F_low.

[0023] Optimally, shorting posts 116 may be fixed, permanent connections, as well as switched.

[0024] As previously mentioned, additional dielectric layers with FSS material could be added along with additional sets of shorting posts to define additional resonant frequencies for cavity 100.

[0025] Referring now also to FIG. 2, there is shown a schematic representation of a light-activated switching arrangement suitable for switching posts 112, 114, 116. Shorting posts 112, 114, 116 may be implemented in a number of ways. Typically, optically activated microelectromechanical switches (MEMS) 152 are used. The MEMS 152 may be mounted on a small substrate (not shown) which is mounted in a small, composite metalized tube 150. An optical control fiber 154 is attached to the MEMS 152 and exits the cavity 100. The tube 150 is mounted vertically between dielectric layers 108a,108band/or conductive upper plane 104 and ground plane 106. Reliable contact must be made at both ends of the composite metalized tube 150. The reliability of this configuration is highly dependent upon the flexibility of the tube 150 and the rigidity of the cavity structure 100 itself. The advantage of optically controlled switches such as MEMS 152 is that only non-metallic fibers 154 enter the cavity. In alternate, electrically activated switching embodiments, metallic conductors (not shown) must enter cavity 100. These metallic conductors may interfere with the operation of the resonant cavity 100 either by de-tuning the cavity 100 or by introducing interfering signals into the cavity 100.

[0026] In alternate embodiments, FET switches, not shown, may be used to connect shorting posts 112, 114, 116 to their respective upper plane 104, ground plane 106 and/or dielectric layers 108a,108b.In still other embodiments, PIN diodes or other optically controlled switches, not shown, may be used for switching posts 114, 116. PIN diodes convert light energy, typically in the 0.75-1 micron wavelength range to electrical signals. The disadvantage of PIN diodes is that they typically require a bias current to form a low-resistance contact. This bias current may be supplied through RF chokes, but this adds complexity and cost and may also introduce components into cavity l00 which may interfere with its operation.

[0027] In another embodiment, the switched shorting posts 112, 114, 116 themselves are formed from semiconductor material. When this semiconductor material is illuminated by laser light of an appropriate wavelength, sufficient free carriers are liberated, making the posts 112, 114, 116 sufficiently conductive at the frequency of interest. The disadvantage of this approach is that posts 112, 114, 116 must be continuously illuminated by the laser in order to remain conductive.

[0028] Since other modifications and changes varied to fit particular operating requirements and environments will be apparent to those skilled in the art, the invention is not considered limited to the example chosen for purposes of disclosure, and covers all changes and modifications which do not constitute departures from the true spirit and scope of this invention.

[0029] Having thus described the invention, what is desired to be protected by Letters Patent is presented in the subsequently appended claims.