Abstract:
The present invention features a reconfigurable resonant cavity specifically for use with a slot radiator. A series of internal planes with frequency-selective materials disposed on their top surfaces, in conjunction with switchable shorting pins, is used to reconfigure the cavity&#39;s resonant frequency. PIN diodes, MEMS or other photonically or electrical activated switching devices may be used to selectively “activate” shorting pins. A single resonant cavity may be electrically reconfigured to operate at two, three, or even more different frequency bands.

Description:
FIELD OF THE INVENTION  
         [0001]    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.  
         BACKGROUND OF THE INVENTION  
         [0002]    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.  
           [0003]    Using a resonant cavity behind a slot is the primary solution for maximizing gain from a slot element.  
           [0004]    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.  
           [0005]    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.  
           [0006]    It is a further object of the invention to provide a reconfigurable resonant cavity which implements “fences” by using selectable shorting pins.  
           [0007]    It is still another object of the invention to provide a reconfigurable resonant cavity which uses frequency-selective materials (FSS) to control the resonant frequency of the cavity.  
         SUMMARY OF THE INVENTION  
         [0008]    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. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    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:  
         [0010]    [0010]FIG. 1 is a schematic, cross-sectional view of the reconfigurable resonant cavity of the invention; and  
         [0011]    [0011]FIG. 2 is a schematic view of a light-activated, switched shorting post for use in the resonant cavity of FIG. 1. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0012]    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.  
         [0013]    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.  
         [0014]    A slot  102  is shown in an upper conductive plane  104 . The slot  102  is configured in accordance with well 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.  
         [0015]    Two dielectric layers  108   a ,  108   b  are disposed in cavity  100 , layers  108   a ,  108   b  also being substantially parallel to both upper conductive plane  104  and lower ground plane  106 . Selectively disposed on the top surface of dielectric layers  108   a ,  108   b  are resonant elements of frequency selective material  110  to form intermittent frequency-selective surfaces (FSS) on dielectric layers  108   a ,  108   b.    
         [0016]    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  108   a ,  108   b  carrying resonant elements of frequency selective material  110  defines a potential alternate bottom ground plane for cavity  100 .  
         [0017]    These alternate bottom ground planes  108   a ,  108   b  must 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 .  
         [0018]    Pairs of posts  112  are located the closest to centerline  118  and extend only between upper conductive plane  104  and a first dielectric layer  108   b . This defines the smallest of the resonant cavity configurations suitable for operation at an arbitrary frequency F hi .  
         [0019]    Similarly pairs of posts  114  are located further away from centerline  118  and connect dielectric layer  108   b  to upper conductive plane  104 . This defines a somewhat larger configuration of a resonant cavity for operation at an arbitrary frequency F mid .  
         [0020]    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 .  
         [0021]    Optimally, shorting posts  116  may be fixed, permanent connections, as well as switched.  
         [0022]    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 .  
         [0023]    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  108   a ,  108   b  and/or conductive upper plane  104  and ground plane  106 . Reliable contact must be made at both ends of the composite  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 .  
         [0024]    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  108   a ,  108   b . 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  100  which may interfere with its operation.  
         [0025]    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.  
         [0026]    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.  
         [0027]    Having thus described the invention, what is desired to be protected by Letters Patent is presented in the subsequently appended claims.