Abstract:
A transmission line feed for a surface wave medium having a dielectric substrate with an array of electrically conductive patches formed thereon. The transmission line feed includes a microstrip substrate, the microstrip substrate having a first permittivity which is lower than a second permittivity of the dielectric substrate of the surface wave medium, the microstrip substrate abutting against the dielectric substrate of the surface wave medium; a tapered microstrip disposed on the microstrip substrate, the tapered microstrip tapering from a relatively narrow end to a relatively wide end, the relative wide end terminating where the microstrip substrate abuts against the surface wave substrate; and an adapter for coupling a transmission line to the relatively narrow end of the tapered microstrip.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    U.S. patent application Ser. No. ______, filed on the same date as this application and entitled “Conformal Antennas for Mitigation of Structural Blockage” is hereby incorporated herein by reference. 
         [0002]    U.S. Pat. No. 7,307,589 to Daniel Gregoire et al. entitled “Large-Scale Adaptive Surface Sensor Arrays” 
     
    
     TECHNICAL FIELD 
       [0003]    A conformal surface wave feed provides a transition from a coaxial line or other transmission line to surface wave transmission that can be used to launch a surface wave onto surface-wave media. 
       BACKGROUND 
       [0004]    A Conformal Surface Wave Feed (CSWF) is believed to be unknown in the art. The closest prior art may be a low-profile waveguide (LPWG) surface-wave coupler (see  FIG. 1   b ) that has been used to feed previous conformal Artificial Impedance Surface (AIS) antennas. 
         [0005]    Disadvantages of this prior art are believed to be that: (1) It is not conformal. As seen in the  FIG. 1   b  below, the LPWG protrudes from the antenna surface. (2) Its insertion loss is much higher than the presently described conformal surface wave feed. (3) It radiates power away from the surface into free space. (4) Its bandwidth is lower than the presently described conformal surface wave feed. 
       Brief Description of the Invention 
       [0006]    The present invention relates to CSWF that can be used to feed an AIS antenna or in other applications. The CSWF provides a transition from a coaxial line or other transmission line to surface wave transmission that can be used to launch a surface wave onto surface-wave media of an AIS antenna, for example. 
         [0007]    In the CSWF, a wave is launched from a transmission line (typically a 50Ω coax-to-microstrip adaptor) into a tapered microstrip (MS) line that spreads the wave energy out into a broad phase front, and then into a surface-wave medium (SWM). The MS is tapered such that the insertion loss is preferably minimized from one end of the taper to the other. The permittivity of the MS substrate is lower than the permittivity of the SWM substrate in order to match the wave speeds between the MS and the surface wave, thus minimizing insertion loss from the MS to the SWM. 
         [0008]    In one aspect the present invention provides a transmission line feed for a surface wave medium having a dielectric substrate with an array of electrically conductive patches formed thereon. The transmission line feed includes: (a) a microstrip substrate, the microstrip substrate having a first permittivity which is lower than a second permittivity of the dielectric substrate of the surface wave medium, the microstrip substrate abutting against the dielectric substrate of the surface wave medium; (b) a tapered microstrip disposed on the microstrip substrate, the tapered microstrip tapering from a relatively narrow end to a relatively wide end, the relative wide end terminating where the microstrip substrate abuts against the surface wave substrate; and (c) an adapter for coupling a transmission line to the relatively narrow end of the tapered microstrip. 
         [0009]    In another aspect the present invention provides a method of feeding RF energy to a surface wave medium having a dielectric substrate with an array of electrically conductive patches formed thereon, the RF energy being fed to said surface via a coaxial transmission line feed. The method includes: providing a microstrip substrate having a first permittivity which is lower than a second permittivity of the dielectric substrate of the surface wave medium; butting the microstrip substrate against the dielectric substrate of the surface wave medium; forming a tapered microstrip on the microstrip substrate, the tapered microstrip tapering from a relatively narrow end to a relatively wide end, the relative wide end terminating where the microstrip substrate abuts against the surface wave substrate; and providing an adapter for coupling the coaxial transmission line to the relatively narrow end of the tapered microstrip. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1   a  depicts an embodiment of a CSWF; the CSWF  10  includes a microstrip whose width tapers from a relatively narrow end at a coax-to-MS adaptor (not shown in  FIG. 1   a ) to relatively wider end at a surface-wave medium (SWM—not shown in  FIG. 1   a ). The CSWF launches a surface wave with a broad phase front into the surface-wave medium and at least a portion of which may be an AIS antenna (also not shown in  FIG. 1   a ). 
           [0011]      FIG. 1   b  depicts a prior art device for launching surface waves which utilizes a low-profile waveguide coupler (LPWG) which protrudes from the antenna surface. 
           [0012]      FIG. 2   a  is a plan view very similar to  FIG. 1   a , but depicted in a larger scale and with indicia identifying certain elements and features thereof, and with the SWM and AIS depicted. 
           [0013]      FIG. 2   b  is a section view taken through the CSWF of  FIG. 2   a.    
           [0014]      FIGS. 3   a  and  3   b  depict a simulation of the CSWF in plan ( FIG. 3   a ) and side elevation ( FIG. 3   b ) views. The MS taper is fed by the coaxial adaptor on the right. The wave propagates along the MS taper, spreading out into a broad phase front as the MS width increases. At the end of the MS taper, a surface wave is launched into the surface-wave medium (SWM) with insertion loss &lt;−25 dB if the wave speeds are closely matched. In power transmission applications, the surface wave is incident on the CSWF from the left. The broad phase front of the surface wave is funneled through the MS taper to the narrow end of the MS taper where it is collected at a coaxial adaptor. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 1   a  depicts an embodiment of a CSWF  10 . This embodiment of CSWF  10  is integrated with a 24 GHz conformal AIS antenna  20  on a 25-mil substrate. The CSWF  10  is a microstrip whose width tapers from 0.6 mm wide at a coax-to-MS adaptor (not shown in  FIG. 1   a , see element  16  in  FIG. 2   a ) to 30 mm wide at the surface-wave medium in this embodiment. The CSWF launches a surface wave with a broad phase front into a surface-wave medium (SWM) at least a portion of which may be an AIS antenna (See  FIG. 2   a  for a representation of the AIS antenna  20 ). 
         [0016]    The CSWF  10  includes a metallic microstrip  13  whose width tapers from a narrow end  11  at a transmission line  15  (typically a 50 ohm coaxial cable) to microstrip adaptor  16  (not shown in  FIG. 1   a , but commercially available as model 292-04A-5 from Southwest microwave) to a wide end  12  at the surface-wave medium  22 . The CSWF  10  launches a surface wave with a broad phase front into the aforementioned AIS antenna. The AIS  20  antenna is represented by a block in  FIG. 2   a.    
         [0017]    The CSWF  10  need not be coupled to an AIS antenna as the CSWF  10  can be used to interface with SWMs used in devices other than AIS antennas. An SWM is a “surface wave medium”. It is anything that supports surface electromagnetic waves. It is a type of artificial impedance surface (AIS). Not all AIS are SWMs as not all AIS support surface waves—on the contrary, some AIS are designed to inhibit surface waves. However, since an AISA (an AIS antenna) works by purposefully leaking surface waves from it, it is an SWM by definition. 
         [0018]    The CSWF  10  has a microstrip taper formed by a metallic layer  13  on a thin dielectric substrate  14  (typically having a thickness in the range of 25-50 mils) with relatively low relative permittivity En (preferably in a range of 2-4). The relative permittivity of layer  14  is low compared to the AIS substrate&#39;s  22  relative permittivity ∈ r2  which is typically around ˜10. The thickness of the substrates scale inversely to the frequency of operation. For example, 50 mil substrates  14 ,  22  are preferred for 8 to 14 GHz AIS, 25 mil substrates  14 ,  22  for 18 to 30 GHz AIS, and 1″ thick substrates  14 , 22  for 100 to 500 MHz AIS. 
         [0019]    The narrow end  11  of the taper preferably interfaces to a standard transmission line connector  30  such as the aforementioned microstrip to coaxial connector. The width of the microstrip at the narrow end is chosen to match its impedance to the 50 ohm adaptor  16  according to well known technology. The wider end  12  of the taper interfaces to a surface-wave medium formed by metallic patches  26  on substrate  22  that supports the desired surface wave. 
         [0020]    The taper in the tapered microstrip  13  minimizes insertion loss. Insertion losses of less than −25 dB have been experienced when following the design guidance suggested herein. A surface-wave impedance matching region  24  may be used if needed, which is formed by an array of metallic patches  26  on a dielectric substrate  22  whose permittivity is higher than the substrate  14  under the microstrip taper  13 . 
         [0021]    Although the CSWF  10  may be used in a number of applications, one currently preferred application is its use as a feed for an AIS antenna  20 . See the application identified above for more information about AIS antennas. The AIS antenna  20  typically has metallic patches similar to the metallic patches  26  and may be formed on a substrate integral with substrate  22 . The metallic patches of the AIS antenna  20  would typically start out with a uniform size corresponding to the smaller size patches  26  at the end of the surface wave impedance taper region  24  remote from the microstrip taper  13 . Thereafter the sizes of patches in the AIS antenna  20  would be varied as discussed in the US patent application incorporated by reference to form transmission regions where the RF signal being applied via coaxial cable  15  (for example) is launched from the surface waves in the AIS antenna  20 . 
         [0022]    The size of the metal patches  26  varies along the direction of wave propagation denoted by arrow A with the patch size decreasing in size towards the AIS antenna  20 . 
         [0023]    An embodiment of disclosed CSWF  10  can be utilized, for example, to use surface waves to transmit high-rate data (&gt;30 Mbps) or power (&gt;1 W) in a two-dimensional surface-wave AIS antenna  20 .  FIGS. 1   a ,  2   a  and  2   b  show an exemplary embodiment of the CSWF  10  preferably used with a conformal AIS antenna  20  operating, in this embodiment, at 24 GHz. The dimensions of the tapered microstrip  13  in this embodiment are 100 mm long by 30 mm maximum width at end  12  and tapering to a 0.6 mm minimum width at end  11 . The substrate  14 , in this embodiment, is preferably 25-mil thick Rogers  3003  (∈ r1 =3.0). The SWM of the surface wave impedance taper region  24  has 0.8 mm metallic square patches  26  distributed on a grid with a 1 mm period on substrate  22  which is preferably 25-mil thick Rogers  3010  substrate (∈ r2 =10.2) in this embodiment. The impedance taper in region  24  can be realized by decreasing size of patches  26 , or patch period or both. Rules of thumb: 1) impedance increases with patch size for a given patch period; 2) impedance increases with patch period for a given fractional patch size (patch size/period); 3) impedance increases with substrate permittivity, and 4) impedance increases with substrate thickness. Any or all of these rules of thumb can be used to implement the impedance taper in region  24 . 
         [0024]    The disclosed feed will work without the impedance taper  24  (by abutting the tapered microstrip directly to an AIS antenna  20 , for example). But the impedance taper  24  is highly desirable to meet specifications for most applications, especially high power applications, since the return loss tends to be unacceptably high without it. The same material as substrate  22  is also preferably used as the substrate of the AIS antenna  20  and, indeed, substrate  22  is preferably shared by the AIS antenna  20  and the surface wave impedance taper  24  as an integral substrate  22 . 
         [0025]    Conformal artificial impedance surface antennas, which are described in the US patent application which is incorporated by reference, modulate a surface wave and radiate its power into a designed radiation pattern. 
         [0026]    In any surface-wave research work, the surface waves must be interfaced to external instruments that rely on conventional RF transmission line communication methods, such as coaxial cables and related connectors. Artificial Impedance Surface antennas  20 , whether or not they are conformal, need to be connected to transmitters and/or receivers and thus cables  15  are typically connected to such transmitters and/or receivers and those cables  15  need in turn to be connected to the AIS antenna  20 . The disclosed CSWF  10  facilitates that connection. 
         [0027]    An important element of the CSWF  10  is its tapered microstrip  13 , one end  11  of which interfaces to a conventional transmission line impedance (for example a 50Ω coaxial cable  15 ), the other end  12  interfaces to a surface-wave medium which typically is in a surface wave impedance taper  24 . A very desirable element is the surface-wave impedance taper  24 , which matches the wave impedance at the end of the microstrip taper  13  to the surface-wave impedance in the surface-wave medium (SWM) being fed by the CSFW  10 , which may be an AIS antenna  20  as described above. Of course, the SWM may comprise something other than an AIS antenna  20  since this invention is useful in launching surface waves from RF signals available in a conventional feed line, such as coaxial cable  15 , into a SWM which can be used in a number of possible applications other than a AIS antenna  20 . 
         [0028]    The tapered microstrip  13  is designed to feed the surface wave in the SWM over a broad area, and the surface wave end  12  of the tapered microstrip  13  is therefore much wider than the coaxial end feed end  11 . As the width of the tapered microstrip increases along the taper, the wave impedance changes as a function of its width according to well-known formulas governing microstrip design. The width is varied in such a way that the insertion loss between the wide and narrow ends is minimized. In practice, the impedance along the taper preferably matches what is known as a “Klopfenstein” impedance taper. See Klopfenstein, R. W., “A Transmission Line of Improved Design”,  Proceedings of the IRE , pp. 31-35, January 1956. Other types of impedance tapers will work as well. 
         [0029]    As such, the taper shape seen in  FIG. 2   a  is characteristic of the low-insertion loss taper formed by using a Klopfenstein impedance taper for the taper of the tapered microstrip  13 . The length of the tapered microstrip  13  affects the insertion loss; longer tapers lead to lower insertion loss. In practice, a length equal to approximately two wavelengths of the transmitted wave (the RF signal in coaxial cable  15 ) is sufficient. 
         [0030]    Wave speeds should be matched between the surface wave and wave in the tapered microstrip  13  at the boundary between the impedance taper  24  and the tapered microstrip  13  in order to minimize insertion loss between the two regions. In order to match the wave speeds, the substrate  14  permittivity ∈ 1  for the tapered microstrip  13  is lower than the substrate  22  permittivity ∈ 2  in the surface-wave region. The wave speed in the tapered microstrip  13  is approximately c/∈ 1   1/2  over a wide bandwidth, where c is the speed of light and ∈r 1  is the relative permittivity of substrate  14 . Substrate thickness and tapered microstrip  13  width affect the wave speed in a well-known, but involved way not presented here. (See: I. J. Bahl and D. K. Trivedi, “A Designer&#39;s Guide to Microstrip Line”, Microwaves, May 1977, pp. 174-182.) So the wave speed formula given above is just a rough approximation. The surface-wave speed in the surface wave taper region  24  is determined by the wave&#39;s frequency, the substrate permittivity ∈ 2  and its thickness, and the size and shape of the metallic patches  26  on the substrate  22 . In general, the surface-wave speed approaches a lower limit of c/∈r 2   1/2  as the frequency and/or the substrate thickness increase (see C. Simovskii et al, “High-impedance surfaces having stable resonance with respect to polarization and incidence angle”, IEEE Trans. Antennas Prop., vol. 53, 908, 2005, and O. Luukkonen et al, “Simple and accurate analytical model of planar grids and high-impedance surfaces comprising metal strips or patches”, IEEE Trans. Antennas Prop., vol. 56, 1624, 2008). As is turns out, the wave speed in the SWM does not get particularly close to the stated limit for patches  26  of a reasonable size, and therefore the permittivity ∈ 2  of substrate  22  in the surface wave impedance taper  24  region must be greater than the permittivity ∈ 1  of substrate  14  under tapered microstrip  13 . 
         [0031]    In some applications, for example certain AIS antennas, the wave speed of the microstrip-guided waves at the end of the tapered microstrip  13  is lower than desired for that application. In this case, the surface-wave speed is caused to increase as the wave moves away from the tapered microstrip  13  by varying the sizes of the metallic patches in the surface-wave impedance taper region  24 . The shapes are varied in such a way that the surface-wave impedance is varied in a controlled fashion that minimizes insertion loss from one end of the surface-wave impedance taper region  24 . In practice, this is readily accomplished with a Klopfenstein impedance taper in terms of varying the sizes of the patches  26  in surface-wave impedance taper region  24 . An impedance taper, such as the Klopfenstein taper, is a mathematical entity. It describes the impedance as a function of distance along a transmission line that matches the impedances between two transmission lines with different impedances. (The SWM can be considered to be a transmission line for surface waves.) For the taper in the microstrip line  16 , this is realized with a strip that gradually spreads out. For the surface-wave impedance taper in region  24 , the taper is a one-dimensional change in surface-wave impedance with distance. So the patches only have to vary in size along the direction of the propagation as depicted by the arrow of region  24  in  FIG. 2   a.    
         [0032]    In an AIS antenna  20 , the mean surface-wave impedance is relatively low—it is optimum at about 250 to 300 ohms/sq. The impedance necessary to match wave speeds to an SWM at the end of the tapered microstrip  13  is much higher, approximately 500 to 800 ohms/sq. So, in this case, and pretty much for all AIS antennas  20 , there has to be a transition region  24  between the AIS antenna&#39;s operational surface and the high impedance region where the microstrip  13  terminates and couples to the AIS antenna  20  if a good match is desired. In such a case, an impedance taper in region  24  is essential. In an application where the AIS antenna  20  is just a SWM (like applications with power transfer or data transmission via surface waves), it is admissible to use an AIS (or SWM to be more general) with a high impedance everywhere. Then an impedance taper is not necessary. However, even in these applications, it can be desirable to taper the impedance in region  24  because for example, a lower impedance SWM is easier to make because it uses less metal or is thinner or uses a cheaper dielectric substrate with lower permittivity. These considerations are important when the SWM is very large as for a large scale SWM network. See, for example, U.S. Pat. No. 7,307,589 to Daniel Gregoire et al. entitled “Large-Scale Adaptive Surface Sensor Arrays”. 
         [0033]      FIGS. 3   a  and  3   b  depict the results of a simulation done of the CSWF  10  of  FIGS. 2   a  and  2   b . The tapered microstrip  13  is fed via the coaxial adaptor  16  on the right. The wave propagates along the tapered microstrip  13 , spreading out into a broad phase front as the tapered microstrip  13  width increases. At the end  12  of the taper of the tapered microstrip  13 , a surface wave is launched into the surface-wave medium (SWM) region  24  with insertion loss &lt;−25 dB if the wave speeds are closely matched. 
         [0034]    In power transmission applications, the surface wave is incident on the CSWF  10  from the left. The broad phase front of the surface wave is funneled through the tapered microstrip  13  to the narrow end  11  of the tapered microstrip  13  where it is collected at the coaxial adaptor for downstream RF to DC conversion. Two possible power collection applications are (1) Broadcasting wireless power to a distributed network and (2) broadcasting wireless power from one place to another such as between a satellite and an earth station. With respect to the first possibility, a surface-wave power and communication network distributed across a 1 m 2  SWM (again, see U.S. Pat. No. 7,307,589), with a central hub broadcasting data and RF power across the SWM to multiple nodes which collect the RF power, convert it to DC, and use that power to run on-board CPU/radios that communicate with the central hub via surface waves. In the second possibility, the AISA  20  is used as a receiving antenna in wireless power transfer. In that case, microwave power is beamed from one place to another, e.g between a satellite and the earth station. The receiving antenna is an AISA which collects the microwaves on its surface and focuses it to a single point where it is collected by the CSWF  10  and then converted to DC downstream. The same system can work in reverse where the AISA  20  is the power transmitting antenna. 
         [0035]    When used in the power collection applications, a broad surface-wave phase front is incident on the tapered microstrip  13 , which then funnels the energy in the surface wave phase front down to the coaxial adaptor  16  where it can then be transmitted to an RF-to-DC converter to power devices such as CPUs, varactors, LEDs, etc.  FIGS. 3   a  and  3   b  show the wave propagation from coaxial feed  15  to surface waves in a simulation of the CSWF  10 . The insertion loss for the entire device is less than −25 dB when the wave speeds are matched between the tapered microstrip  13  region and the surface wave region. The overall insertion loss tends to be limited by the coax-to-microstrip adaptor  16 . The grey level change of the fields in  FIG. 3   a  indicates the changing power density along the length of the taper, with a maximum power density occurring at the adaptor  16 . 
         [0036]    In the tapered microstrip  13 , the wave energy is confined to the metallic shape of the microstrip  13 . If the RF energy originates from some device (such as a transmitter) coupled to the RF cable  15 , the wave energy spreads out as the width of the tapered microstrip  13  increases along the length of the taper, where it then transitions into a surface wave with a broad phase front. If the RF energy originates as surface waves (such as from an AIS antenna  20 ), then the wave energy concentrates as the width of the tapered microstrip  13  decreases along the length of the taper towards the adapter  16 , where it then transitions into a the RF cable  15 . 
         [0037]    Having described the invention in connection with certain embodiments thereof, modification will now suggest itself to those skilled in the art. As such, the invention is not to be limited to the disclosed embodiments except as is specifically required by the appended claims.