Patent Abstract:
A spectral control antenna apparatus includes a feed region or feed gap and a surrounding space or medium. A signal path between a feed region and a surrounding space or medium is characterized by a length dependent impedance with a plurality of extrema whereby the antenna apparatus exhibits a desired spectral response. The invention is well-suited for application to planar antennas, particularly planar antennas characterized by a slot type transmission line structure. If such a transmission line structure is an offset slot line, then by overlapping sections of the offset slot line relatively low impedances are possible, thus enabling the large variations in impedance necessary for effective filtering behavior.

Full Description:
This application claims benefit of prior filed now abandoned Provisional Patent Application Ser. No. 60/512,872 filed Oct. 20, 2003. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to antennas and more specifically to a system and method for spectral control of same. 
   2. Description of the Prior Art 
   Practitioners of the antenna arts have long realized that a tapered antenna feed leads to an improved broadband match. Early examples of such antennas include those of Carter [U.S. Pat. No. 2,181,870], and Brillouin [U.S. Pat. No. 2,454,766]. These concepts have been applied to planar antennas as well, notably by Nester [U.S. Pat. No. 4,500,887] who taught a tapered microstrip horn. Antenna radiating elements have been similarly tapered. For instance, Barnes [U.S. Pat. Nos. 6,091,374; 6,400,329 and 6,621,462] disclosed a tapered slot antenna and the inventor disclosed a semi-coaxial horn with a tapered horn element [U.S. Pat. No. 6,538,615]. 
   In some cases, a tapered feed and tapered radiating element have been combined in the same antenna structure. For example, Lindenblad [U.S. Pat. No. 2,239,724], invented a wideband antenna with a tapered feed connected to a tapered bulbous radiating element. More recently the inventor implemented a planar antenna with a tapered feed structure smoothly flowing into elliptically tapered planar dipole elements [U.S. Pat. No. 6,512,488 and 6,642,903]. 
   This prior art is characterized by generally monotonic variations in impedance with distance along a signal path traversing an antenna feed structure, radiating elements, and surrounding medium or space. These monotonic variations in impedance are generally considered desirable because they help to optimize a broad band match between an antenna and a transmission line. These monotonic variations may be discontinuous (as in a Klopfenstein taper) or have points of inflection (as in an Exponential taper). 
   Wavy shaped or corrugated antenna structures have been adopted for diffraction control or to increase impedance [Kraus, Antennas 2 nd  ed., New York: McGraw-Hill, pp. 657–9]. McCorkle [U.S. Pat. No. 6,590,545] discloses (FIG. 21) a planar UWB antenna with a wavy shaped slot. McCorkle suggests that a band stop transfer function might be possible by adjusting the width of the tapered clearance, however neither the drawings nor the detailed description provide any guidance to one skilled in the art as to how such adjustment gives rise to band stop behavior. In practice, the small periodic variations in tapered clearance shown by McCorkle are largely ineffective in giving rise to significant manipulation of an antenna transfer function, particularly since the disclosed variations maintain a continuous increase in width. 
   The inventor [U.S. Pat. No. 6,774,859] discovered that a practical means for implementing band stop or frequency notch filters in an otherwise ultra-wideband antenna is to incorporate a discrete narrow band resonant structure. 
   An alternate filtering technique, stepped impedance low pass filtering is also known in the art [David M. Pozar, Microwave Engineering, 2 nd  ed., New York: John Wiley &amp; Sons, 1998, pp. 470–473]. This technique has not been applied to control impedance of antennas and implement desired transfer functions in antennas, however. 
   The extreme bandwidths of ultra-wideband antennas leave them especially vulnerable to interferers. It is a challenge to design an RF-front end to provide sufficient rejection to adjacent interferers just above an antennas operating band without adversely impacting performance in a desired band. For instance, it is desirable to have an ultra-wideband antenna responsive to the 3.1–5.0 GHz band without being responsive to interferers operating above 5.0 GHz. An electrically small UWB antenna is naturally unresponsive to signals lying below its operational band. Making such an antenna unresponsive to higher frequency signals is a greater challenge. 
   In view of the foregoing, there is a need for a system and method of modifying an antenna slot or notch to create the large variations in impedance necessary to implement effective distributed filters. There is a further need for a method to implement filtering or a desired transfer function with minimal modifications to an existing antenna design. Additionally, there is a need for an antenna apparatus that implements filtering capability inexpensively without requiring the added expense and board space of a lumped element filter structure in the RF front end of a radio device. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an object of the present invention to provide a means for modifying an antenna slot or notch to create large variation in impedance necessary to implement effect distributed filters. It is a further object of the present invention to provide a desired transfer response to an otherwise broad band antenna. Yet another object of the present invention is to implement filtering capability inexpensively without requiring the added expense and board space of a lumped element filter structure in the RF front end of a radio device. 
   These objects and more are met by the present invention: a spectral control antenna apparatus including a feed region or feed gap and a surrounding space or medium. A signal path between a feed region and a surrounding space or medium is characterized by a length dependent impedance with a plurality of extrema whereby the antenna apparatus exhibits a desired spectral response. The invention is well-suited for application to planar antennas, particularly planar antennas characterized by a slot type transmission line structure. If such a transmission line structure is an offset slot line, then by overlapping sections of the offset slot line relatively low impedances are possible, thus enabling the large variations in impedance necessary for effective filtering behavior. 
   An antenna spectral control system includes an RF device, a feed region, a surrounding space or medium, and a signal path between the feed region and the surrounding space. The present invention teaches using a variation in characteristic impedance along the length of a signal path to give rise to a desired spectral response. Means for varying impedance may include dielectric loading, transmission line geometry variation, or other means for varying impedance. A particularly effective way of varying impedance involves using an offset slot line transmission line structure with overlapping sections. In alternate embodiments, discrete lumped capacitances or inductances may be distributed along a signal path for added spectral control. 
   In alternate embodiments, a spectral control antenna apparatus comprises a dielectric substrate, a first conducting layer, and a second conducting layer. A first conducting layer and a second conducting layer cooperate to form a slot line transmission line structure including a plurality of extrema. A first conducting layer and a second conducting layer may be co-planar on the same side of a dielectric substrate, or may lie on opposite sides of a dielectric substrate. In still further embodiments, a slot line transmission line structure includes a plurality of overlapping sections. 
   Further, a method for spectral control of an antenna comprises providing a signal path between a feed region and a surrounding space or medium having a characteristic impedance with dependence on a length of a signal path; and providing a means for varying impedance whereby an antenna exhibits a desired spectral response. A means for varying impedance may include using lumped elements, dielectric loading, or geometry variations. 
   With these and other objects, advantages, and features of the invention that may become hereinafter apparent, the nature of the invention may be more clearly understood by reference to the detailed description of the invention, the appended claims and to the several drawings herein. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-section of a same-side slot line. 
       FIG. 2  is a cross-section of an overlapping offset slot line. 
       FIG. 3  is a cross-section of a wide offset slot line. 
       FIG. 4  is a schematic diagram depicting a preferred embodiment spectral control UWB magnetic slot antenna according to the teachings of the present invention. 
       FIG. 5  is a circuit diagram showing an equivalent circuit for a preferred embodiment spectral control magnetic slot antenna. 
       FIG. 6  is an exploded view of a preferred embodiment spectral control magnetic slot antenna. 
       FIG. 7  is a plot of an impedance profile of a potential implementation. 
       FIG. 8  is a plot of a spectral response of a potential implementation. 
       FIG. 9  is a schematic diagram of a first alternate embodiment spectral control antenna and a corresponding impedance profile. 
       FIG. 10  is a schematic diagram of a second alternate embodiment spectral control antenna and a corresponding impedance profile. 
       FIG. 11  is a schematic diagram of an elliptical dipole antenna modified according to the teachings of the present invention. 
       FIG. 12  is a schematic diagram of a spiral slot antenna modified according to the teachings of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Overview of the Invention 
   The present invention is directed to a system and method for spectral control of antennas, particularly ultra-wideband antennas. Instead of the monotonic impedance variation taught in the prior art, the present invention teaches that the impedance of an antenna may be controlled so as to create a desired frequency response. 
   The present invention will now be described more fully in detail with reference to the accompanying drawings, in which the preferred embodiments of the invention are shown. This invention should not, however, be construed as limited to the embodiments set forth herein; rather, they are provided so that this application will be thorough and complete and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. 
   Transmission Line Structures 
     FIG. 1  is a cross-section  100  of a same-side antenna slot  104 . Same-side slot line  104  comprises a first conducting layer  113 , a second conducting layer  115 , and a dielectric substrate  107 . A first conducting layer  113  and a second conducting layer  115  cooperate to form a transmission line structure constraining fields to a particular signal path. 
     FIG. 2  is a cross-section  200  of an overlapping offset slot line  206 . Overlapping offset slot line  206  comprises a first conducting layer  213 , a second conducting layer  215 , and a dielectric substrate  207 . A first conducting layer  213  and a second conducting layer  215  cooperate to form a transmission line structure constraining fields to a particular signal path. Overlapping offset slot line  206  has a low impedance and is electrically equivalent to a shunt capacitance. 
     FIG. 3  is a cross-section  300  of a wide offset slot line  308 . Wide offset slot line  308  comprises a first conducting layer  313 , a second conducting layer  315 , and a dielectric substrate  307 . A first conducting layer  313  and a second conducting layer  315  cooperate to form a transmission line structure constraining fields to a particular signal path. Wide offset slot line  308  has a high impedance and is electrically equivalent to a series inductance. 
   With shunt capacitance and series inductance, implementation of a low pass filtering response is straightforward. In alternate embodiments, however, other transfer functions like a band stop or even a high pass might be introduced, but at the cost of a larger or more complicated structure than a corresponding low pass filter. 
   PREFERRED EMBODIMENT 
     FIG. 4  is a schematic diagram  400  depicting a preferred embodiment spectral control magnetic slot antenna  461  according to the teachings of the present invention. A first conducting surface  413  on a front side of a dielectric substrate  407  and a second conducting surface  415  on a back side of a dielectric substrate  407  cooperate to form complex tapered slot  417 . Complex taper slot  417  is an example of an offset slot line, in which conducting surfaces (like first conducting surface  413  and second conducting surface  415 ) on opposing sides of a dielectric (like dielectric substrate  407 ) cooperate to form a transmission line structure defining a signal path. A plurality of first vias  419  and a plurality of second vias  421  electrically couple first conducting substrate  413  to second conducting surface  415  in the vicinity of first open termination  409  and second open termination  411 , respectively. In alternate embodiments, first conducting substrate  413  may be electrically coupled using capacitive coupling to second conducting surface  415  by overlapping first conducting substrate  413  and second conducting surface  415 . Preferred embodiment  461  is a closed slot antenna, since complex taper slot  417  is a closed slot (i.e. a closed slot transmission line structure). A closed slot is a slot formed by two conductors (like first conducting surface  413  and second conducting surface  415 ) coupled not only at a feed region but also at a termination region (like first open termination  409  and second open termination  411 ). 
   Complex tapered slot  417  does not vary monotonically from a narrow (low impedance) section in the vicinity of feed gap  405  to a wide (high impedance) first open termination  409  and a wide (high impedance) second open termination  411 . Instead, complex tapered slot  417  differs from conventional prior art slot  401 . Complex tapered slot  417  becomes wider at a first extremum (denoted “α”) resulting in a relatively high impedance. Complex tapered slot  417  becomes narrower and overlaps at a second extremum (denoted “β”), resulting in a relatively low impedance. Complex tapered slot  417  becomes wider at a third extremum (denoted “γ”), resulting in a relatively high impedance. Complex tapered slot  417  becomes narrower and overlaps at a fourth extremum (denoted “δ”), resulting in a relatively low impedance. A narrow or preferentially overlapping section forms a low impedance offset slot (like extrema β and extrema δ) with behavior analogous to a shunt capacitance. Thus extrema β and extrema δ have associated cross sections similar to that of overlapping offset slot line  206 . A wide, high impedance slot (like extrema α and extrema γ) is analogous to a series inductance. Thus extrema α and extrema γ have associated cross sections similar to that of wide offset slot line  308 . A large variation in impedance helps maximize filtering performance in a minimal length. An offset slot line with the ability to include low impedance overlapping sections can support a larger variation in impedance than a corresponding same side slotline. Thus, it is advantageous (although not required) to employ an offset slot line in a spectral control antenna. 
     FIG. 5  shows equivalent circuit  500  for complex tapered slot  417 . Equivalent circuit  500  behaves like a low pass filter coupled to an antenna  527  or means for transmitting and/or receiving electromagnetic signals. Additional inductance and capacitance may be incorporated in an antenna design using discrete components distributed along complex taper slot  417 . 
   The methods disclosed by the present invention are best suited for creating a low pass filter behavior, however it is also possible to implement other transfer responses in antennas using the teachings of the present invention. Also, although the teachings of the present invention are well suited for application to ultra-wideband antennas, the present invention also has application to broad band or narrow band antennas. 
   Complex taper slot  417  constrains signals to particular signal paths. On a second side of complex tapered slot  417 , radiated signals traverse a signal path from feed gap  405  to second open termination  411  and thence to a surrounding medium or free space intermediate first extremum α, second extremum β, third extremum γ, and fourth extremum δ. On a first side of complex tapered slot  417 , radiated signals traverse a signal path from feed gap  405  to first open termination  409  intermediate similar extrema. An antenna comprises at least one signal path defined by the geometry of the antenna. In many cases an antenna may have more than one signal path, depending on the geometry. 
   For ease of explanation a signal path is described in terms of radiating a signal. A received signal follows an analogous but reversed path. The principles of the present invention apply to both the reception and transmission or radiation of electromagnetic signals. For ease of explanation this application will focus primarily on radiation of signals with the proviso that it is understood that reception of signals is also inherently described. 
   In preferred embodiment  461 , complex tapered slot  417  has four extrema: α, β, γ, δ. In alternate embodiments, complex tapered slot  417  may have more or fewer extrema. Also, complex tapered slot  417  is shown as a symmetric slot with similar taper from feed gap  405  to a wide (high impedance) first open termination  409  and from feed gap  405  to a wide (high impedance) second open termination  411 . In alternate embodiments complex tapered slot  417  may be asymmetric. 
   Preferred embodiment  461  comprises complex tapered slot  417  fed across feed gap  401 . In some embodiments feed gap  401  couples to a feed line  423 . Feed line  423  couples to a connector interface  425 . In still further embodiments, feed line  423  may couple to an RF device  416  via end launcher  410 , connector  412 , and coaxial line  414 . In alternate embodiments, RF device  416  may be located on dielectric substrate  407  and directly coupled to complex taper slot  417  via feed line  423 . 
   Preferred embodiment  461  is a planar antenna system. Planar antennas are advantageous because they tend to be easy and inexpensive to manufacture. If implemented on a flexible or curved substrate, planar antennas may assume a variety of useful form factors. 
     FIG. 6  is an exploded view  600  of preferred embodiment spectral control magnetic slot antenna  461 . Exploded view  600  shows top conducting layer  613 , dielectric substrate  607 , and bottom conducting layer  615 . Terms like “front” and “back” or “top” and “bottom” are used throughout this application to aid the reader in visualizing a particular illustration of an embodiment of the invention and should not be interpreted as limiting or requiring any particular physical orientation or arrangement. 
   DETAILED ANALYSIS OF A POTENTIAL IMPLEMENTATION 
     FIG. 7  is a plot  700  of an impedance profile  741  of a potential implementation. Length along a signal path is plotted on horizontal axis  759  and impedance is plotted along vertical axis  760 . Exponential impedance trace  763  is typical of a prior art monotonically increasing impedance taper. Complex impedance trace  765  is typical of impedance responses taught by the present invention. 
   Note that large variations in impedance are essential to implement a significant filter response in a minimal length signal path. In the potential implementation of impedance profile  741 , the electrical length is 148 degrees measured at 5900 MHz. This is less than a quarter wavelength at 3000 MHz. Impedance variations are over more than a factor of 10 from 9 to 377 ohms. Thus, means for implementing significant variations in impedance are essential for a successful implementation. The table below provides details of this potential implementation by showing the electrical length in phase degrees of a particular impedance section in ohms. 
   
     
       
             
             
             
           
             
             
             
           
         
             
                 
                 
             
             
                 
               Phase Angle 
                 
             
             
                 
               (deg) 
               Z(ohms) 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
                 
               6.9 
               8.62 
             
             
                 
               36.8 
               377 
             
             
                 
               11.4 
               14.8 
             
             
                 
               78.8 
               377 
             
             
                 
               14.1 
               52.4 
             
             
                 
                 
             
           
        
       
     
   
     FIG. 8  is a plot of a spectral response  800  corresponding to impedance profile  741  of a potential implementation. Spectral response plot  800  depicts frequency in MHZ on horizontal axis  827 ; scattering parameter magnitude in dB on primary vertical axis  829 ; and group delay in nanoseconds on secondary vertical axis  831 . Spectral response plot  800  shows return loss (S 11 ) response  835 , through (S 21 ) response  833 , and group delay response  837 . 
   Return loss (S 11 ) response  835 , is comfortably −12 dB or below between 2500 MHZ and 4500 MHz, rising to −3 dB at about 5000 MHz. Through (S 21 ) response  733  shows negligible loss between 2500 MHZ and 4500 MHz, falling off smoothly to −3 dB around 5000 MHz. Group delay response  837  shows only a modest increase around 4800 MHz. Thus, spectral response  800  is not dispersive and is thus well-suited for an antenna. Although many possible numeric and analytic techniques may be applied to develop an impedance taper corresponding to a desired transfer function (or filter response), the inventor has found that readily available analysis software such as Eagleware is an easy and quick way to accomplish this task. 
   ALTERNATE EMBODIMENTS 
     FIG. 9  is a schematic diagram  900  of a first alternate embodiment spectral control antenna, a variable dielectric horn  939  and a corresponding impedance profile  941 . Variable dielectric horn  939  comprises a first radiating element  943 , a second radiating element  945 , and dielectric loading  957 . First radiating element  943  and second radiating element  945  cooperate to form a parallel plate waveguide transmission line structure defining a signal path between feed structure  905  and a surrounding medium or space. Dielectric loading  957  comprises a first dielectric section  947  (denoted “α”), a second dielectric section  949  (denoted “β”), a third dielectric section  951  (denoted “γ”), a fourth dielectric section  953  (denoted “δ”), and a fifth dielectric section  955  (denoted “ε”). For purpose of illustration and not limitation, dielectric loading  957  comprises five discrete sections with fixed dielectric constant. In alternate embodiments, dielectric loading  957  may include more than five or fewer than five sections. In still further embodiments, dielectric loading  957  may comprise a dielectric material with continuously variable dielectric constant. Dielectric loading  957  results in impedance profile  941 . Impedance profile  941  depicts length along horizontal axis  959  and impedance along vertical axis  960 . Impedance profile  941  may be tailored to result in a desired antenna transfer function. First alternate embodiment  939  illustrates how variable dielectric loading may be employed for spectral control of an antenna. The geometry variations illustrated in first alternate embodiment  939  may be applied to any antenna structure in which variation in dielectric constant leads to variation in impedance along a signal path. 
     FIG. 10  is a schematic diagram  1000  of a second alternate embodiment spectral control antenna: a variable geometry horn  1039  and a corresponding impedance profile  1041 . Variable geometry horn  1039  comprises a first radiating element  1043  and a second radiating element  1045 . First radiating element  1043  and second radiating element  1045  cooperate to form a parallel plate waveguide transmission line structure defining a signal path from feed region  1005  to a surrounding space or medium. 
   Variable geometry horn  1039  becomes wider at a first extremum (denoted “α”) resulting in a relatively low impedance. Variable geometry horn  1039  becomes narrower at a second extremum (denoted “β”), resulting in a relatively high impedance. Variable geometry horn  1039  becomes wider at a third extremum (denoted “γ”), resulting in a relatively low impedance. Variable geometry horn  1039  becomes narrower at a fourth extremum (denoted “δ”), resulting in a relatively high impedance. 
   Variable geometry horn  1039  results in impedance profile  1041 . Impedance profile  1041  depicts length along a signal path on horizontal axis  1059  and impedance along vertical axis  1061 . Impedance profile  1041  may be tailored to result in a desired antenna transfer function. Second alternate embodiment  1039  illustrates how geometry variation may be employed for spectral control of an antenna. The geometry variation illustrated in second alternate embodiment  1039  may be applied to any antenna structure in which variation in geometry leads to variation in impedance along a signal path. 
     FIG. 11  is a schematic diagram of a planar elliptical dipole antenna modified according to the teachings of the present invention: spectral control elliptical dipole  1163 . Spectral control elliptical dipole  1163  comprises a first radiating element  1113  on a front side of a dielectric substrate  1107 , a second radiating element  1115  on a back side of dielectric substrate  1107 , and a feed region  1105 . First radiating element  1113  and second radiating element  1115  cooperate to form complex tapered slot  1117 . Complex tapered slot  1117  is yet another example of geometry variations may be employed for spectral control of an antenna. Complex taper slot  1117  is also a transmission line structure defining a signal path. 
   Spectral control elliptical dipole  1163  is an open slot antenna, because complex taper slot  1117  is an open slot (i.e. an open slot transmission line structure) formed by two conductors (like first conducting surface  1113  and second conducting surface  1115 ) that are not electrically coupled except at a feed region (like feed region  1105 ). The teachings of the present invention may be applied to either closed or open slot antenna structures. Other examples of open slot antennas include monopole antennas, and planar horn antennas. Open slots may include either offset or same-side slot line structures. 
     FIG. 12  is a schematic diagram of a spiral slot antenna modified according to the teachings of the present invention: spectral control spiral slot antenna  1261 . Spectral control spiral slot antenna  1261  comprises complex tapered spiral slot  1217  in conducting layer  1203  excited across feed gap  1205 . Appropriate selection of a geometry for complex tapered spiral slot  1217  leads to a desired impedance profile and thence to a desired antenna transfer function. Complex tapered spiral slot  1217  is an example of a same side slot line. A same side slot line may be used in conjunction with the present invention, although an offset slot line is preferred for planar antenna implementations. 
   Complex tapered spiral slot  1217  also employs discrete loading. Discrete loading comprises first lumped element set  1271 , second lumped element set  1272 , third lumped element set  1273 , and fourth lumped element set  1274 . A lumped element set may include a single lumped element or more than one lumped element. A plurality of lumped element sets may be employed for discrete loading to give rise to a desired impedance profile and a desired antenna spectral response. 
   Lumped element sets behave electrically like shunt elements. Thus if a lumped element set is an inductor, it can affect a high pass filter characteristic. In particular, if a lumped element set is an inductor in series with a resistor, low frequency components that might otherwise be reflected without radiating may be dissipated instead of contributing to poor matching behavior. If a lumped element set is a capacitor, it can affect a low pass filter characteristic. If a lumped element set is a resistor it can implement an attenuation. More complicated arrangements of lumped elements can give rise to more sophisticated impedance profiles and desired transfer functions. Discrete loading may be used alone or in any combination with geometry variation or dielectric loading. 
   The present application has demonstrated application of spectral control techniques to parallel plate antenna structures (such as variable geometry horn  1039 ), to closed slot type antenna structures (such as spectral control spiral slot antenna  1261 ), and to open slot or notch type antenna structures (such as spectral control elliptical dipole  1161  ). In fact, the teachings of the present invention may be applied to any antenna structure in which variation in geometry leads to variation in impedance along a signal path. The teachings of the present invention may also be applied to any antenna structure in which variation in dielectric loading leads to variation in impedance along a signal path. Further, the present application also relates to any antenna structure in which discrete loading is applied along a signal path to create a desired impedance variation. 
   Specific applications have been presented solely for purposes of illustration to aid the reader in understanding a few of the great many contexts in which the present invention will prove useful. It should also be understood that, while the detailed drawings and specific examples given describe preferred embodiments of the invention, they are for purposes of illustration only, that the system and method of the present invention are not limited to the precise details and conditions disclosed and that various changes may be made therein without departing from the spirit of the invention which is defined by the following claims:

Technology Classification (CPC): 7