Abstract:
A coaxial mirror is provided for reflecting an electromagnetic signal. The mirror includes an outer pipe, an inner pipe, and first and second rods. The outer pipe extends between input and output ports, with closed initial and final terminals disposed at their respective ports. The inner pipe extends between a closed fore end and an open aft end. The inner pipe is coaxially disposed between the initial and final terminals within the outer pipe. The first rod, coaxially disposed within the outer pipe, extends from the input port to the fore end. The second rod, coaxially disposed within the inner pipe, extends from downstream of the fore end to the output port. Preferably, the first and second pipes are cylindrical tubes. Preferably, fluoropolymer fills the annular region between the inner and outer pipes, and fluoropolymer foam fills the inner pipe. Preferably, the first pipe has an electrically conductive inner surface, the second pipe has electrically conductive inner and outer surfaces, and the first and second rods have conductive surfaces. A first embodiment includes a conductor, coaxially disposed within the inner pipe, that extends from the fore end to the second rod. In a second embodiment, the second rod is hollow, and is preferably filled with the foam.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described was made in the performance of official duties by one or more employees of the Department of the Navy, and thus, the invention herein may be manufactured, used or licensed by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     BACKGROUND 
     The invention relates generally to coaxial radio frequency (RF) mirrors. In particular, the invention relates to a more convenient design intended for field application. 
     Radar applications incorporate narrow-band filtering under inelastic scattering in which a strong continuous-wave transmission signal at a transmit wavelength encounters a target that returns a faint echo at a return wavelength slightly shifted from the transmit wavelength. (This condition contrasts from elastic scattering that lacks the wavelength shift in return signal.) The radar receiver thus listens for a weak return signal near the frequency of the stronger transmit signal. Narrow-band filtering employs co-axial RF mirrors to reflect the signal through a gain medium to enable detection. 
     SUMMARY OF THE INVENTION 
     A coaxial RF mirror can be employed to provide narrow-band filtering. However, conventional RF mirrors lack qualities that facilitate field use due to design constraints that render these delicate and awkward. 
     Conventional coaxial RF mirrors yield disadvantages addressed by various exemplary embodiments of the present invention. In particular, the exemplary embodiments described herein improve the ability to field such electromagnetic reflectors with increased ruggedness and reduced length. 
     Various exemplary embodiments provide a coaxial mirror for reflecting an electromagnetic signal. The mirror includes an outer pipe, an inner pipe, and first and second rods. The outer pipe extends between input and output ports, with closed initial and final terminals disposed at their respective ports. The inner pipe extends between a closed fore end and an open aft end. The inner pipe is coaxially disposed between the initial and final terminals within the outer pipe. The first rod, coaxially disposed within the outer pipe, extends from the input port to the fore end. The second rod, coaxially disposed within the inner pipe, extends from downstream of the fore end to the output port. 
     Preferably, the outer and inner pipes are cylindrical tubes. Preferably, fluoropolymer fills the annular region between the inner and outer pipes, and fluoropolymer foam fills the inner pipe. Preferably, the outer pipe has an electrically conductive inner surface, the inner pipe has electrically conductive inner and outer surfaces, and the first and second rods have conductive surfaces. In various exemplary embodiments, the mirror includes a conductor, coaxially disposed within the inner pipe, which extends from the fore end to the second rod. In alternate exemplary embodiments, the second rod is hollow, and is preferably filled with the foam. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and various other features and aspects, of various exemplary embodiments will be readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings, in which like or similar numbers are used throughout, and in which: 
         FIG. 1  is an elevation cross-sectional view of an RF mirror; 
         FIG. 2  is a tabular view of dimensions and material properties of sections in the mirror; 
         FIG. 3  is a block diagram of a narrow band-pass filter that employs the mirror; 
         FIG. 4  is an elevation cross-sectional view of an alternate RF mirror; and 
         FIG. 5  is a tabular view of dimensions and material properties; 
         FIG. 6  is an elevation cross-sectional detail view of the cylindrical tubes; and 
         FIG. 7  is an elevation cross-sectional detail view of the wire and tube within an aft foam-filled region. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description of exemplary embodiments of the invention, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific exemplary embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized, and logical, mechanical, and other changes may be made without departing from the spirit or scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims. 
       FIG. 1  shows an elevation cross-section view of a folded coaxial radio frequency (RF) mirror  100  in cross-section with inner and outer coaxial pipes. An outer tube assembly  110  comprises an outer cylindrical tube  112  bounded by an inlet end  114  and an outlet end  116  to define a first cavity  118  that includes a forward section  120  filled with air. In exemplary embodiments, the forward section  120  extends 0.102 meter in length. A fore rod  122  extends along the forward section  120 . An inlet port  124 , with a coaxial input connector  126  (such as series-N), attaches upstream of the inlet end  114 . 
     An inner tube assembly  130  includes an inner cylindrical tube  132  defined by an inlet end  134  and an open outlet boundary  136  to define a second cavity  138  that leads to an outlet section  140  filled with fluoropolymer foam. The outlet section  140  extends between the outlet boundary  136  and the outlet end  116 . In exemplary embodiments, the assembly  130  and the outlet section  140  respectively extend 0.07 meter and 0.01 meter in length. An electrically conductive pin  142  extends downstream from the inlet end  134  to an aft rod  144  that extends through the outlet end  116  to an outlet port  146  that attaches to an SMA-type output connector  148 . In exemplary embodiments, the pin  142  extends 0.01 meter. 
     An input signal  150  is received through the input connector  126  and into the first cavity  118  travelling along the interior walls of the outer cylindrical tube  112  in a downstream direction  151 . The signal travels along the annular concentric region between the inner wall of the outer cylindrical tube  112  and the outer wall of the inner cylindrical tube  132 . The signal reverses propagation direction  152  upon reaching the outlet boundary  136  and proceeds to travel in the upstream direction  153  along the inner wall of the inner cylindrical tube  132 . The signal reverses propagation direction  154  upon reaching the inlet end  134  and travels in the downstream direction  155  along the exterior of the aft rod  144  until exiting as an output signal  156 . 
     The forward section  120  defines a first region for signal propagation filled with air. An outer annular envelope  160  defines a second region between the cylindrical tubes  112  and  132 , which is enveloped with fluoropolymer, such as polytetrafluoroethylene under tradename Teflon®, such as wrapping with tape of that material. A third region includes an inner annular envelope  162  that defines the second cavity  138 , minus the aft rod  144  contained in the inner cylindrical tube  132 . The second cavity  138  is filled with fluoropolymer foam. The aft section  140 , also filled with fluoropolymer foam, constitutes a terminal region before reaching signal exit. 
       FIG. 2  shows tabular lists  200  of dimensions and properties of the regions. The first tabular list  210  includes diameters (i.e., chord that passes through the tube longitudinal axis equivalent to twice the radius from that centerline) in the three regions, the first pair of columns for the inner values, and the second pair of columns for the outer values. The regions of the mirror  100  in  FIG. 1  correspond to: 
     (1) the forward section  120  that defines the first cavity  118  and includes the outer cylindrical tube  112  with inner diameter of 2×r o1  and fore rod  122  with outer diameter of 2×r i1 ; 
     (2) the outer annular envelope  160  that includes the outer cylindrical tube  112  with inner diameter of 2×r o2  (identical to 2×r o1 ) and inner cylindrical tube  132  with outer diameter of 2×r i2 ; and 
     (3) the inner annular envelope  162  that defines the second cavity  138  and includes inner cylindrical tube  132  with inner diameter of 2×r o3  and aft rod  144  with outer diameter of 2×r i3 . 
     The second tabular list  220  in  FIG. 2  includes the permeability μ and the permittivity ∈ of the cavities of the three regions, with the filling materials identified alongside. The third tabular list  230  in  FIG. 2  includes the impedance Z of each of the three regions. 
     In the first tabular list  210  in  FIG. 2  listing inner and outer diameter boundaries of the three cavity regions, the first row (for the forward section  120  or first region) identifies the outer diameter of the fore rod  122  as 2×r i1 =0.00635 m, and the inner diameter of the outer cylindrical tube  112  as 2×r o1 =0.0340 m. The second row (for the outer envelope  160  or second region) identifies the outer diameter of the inner cylindrical tube  132  as 2×r i2 =0.0338 m, and the inner diameter of the outer cylindrical tube  112  as 2×r o2 =0.0340 m. The third row (for the inner envelope  162  or third region within the cavity  138 ) identifies the outer diameter of the aft rod  144  as 2×r i3 =0.0036 m, and the inner diameter of the inner cylindrical tube  132  as 2×r o3 =0.03048 m. The diameters are denoted in the mirror  100  as double-radii. 
     In the second tabular list  220  in  FIG. 2 , all regions have substantially similar relative values of magnetic permeability μ, proportional to the vacuum value of 4π×10 −7  N A −2 . Typical materials, ranging from copper and aluminum to water share approximately this unity value treated as μ 1 =μ 2 =μ 3 =1 for the three regions. By contrast, comparative values of relative permittivity vary from aluminum at −1300 to strontium titanate at +310, proportional to the vacuum value of 8.854×10 −12  A 2  s 4  kg −1  m −3 . The relative permittivity for air in the first region is approximately unity as ∈ 1 =1, whereas ∈ 2 =2.1 in the second region represents the corresponding relative permittivity value for Teflon®, and ∈ 3 =1.65 in the third region provides an intermediate value of relative permittivity for a Teflon foam mixture. In the third tabular list  230 , the first and third (and terminal) regions have an impedance of Z o1 =Z o3 =100Ω, and the second region has a lower impedance of Z o2 =0.244Ω. 
       FIG. 3  presents a block diagram  300  of a narrow-band regenerative filter with a gain medium  310  flanked by an input mirror  320  and an output mirror  330 . The mirrors  320 ,  330  are analogous to the coaxial mirror  100  that exhibits low losses. The medium  310  provides a limited gain of 3 dB intended to compensate for attenuation losses while avoiding amplification that causes signal oscillation. The medium  310  extends a half-wavelength (½λ) of the filtered signal. The oscillation behaves as an Airy function, which represents the solution of y=Ai(x) to the differential equation y″−xy=0. The mirrors  320 ,  330  reflect the signal passing through the medium  310  to enable detection of the weak return signal. 
       FIG. 4  shows an elevation cross-section view of a folded coaxial radio frequency (RF) mirror  400  in cross-section with inner and outer coaxial pipes as a secondary embodiment. A communication wire  410  extends coaxially through the inner tube assembly  130  and connects to the output connector  148 . A hollow tube  420  coaxially envelopes the wire  410  across most of its length from the outlet port  146 . The tube  420  opens adjacent and downstream of the inlet end  134  to produce a sixth region  430  filled with fluoropolymer foam through which the signal travels. A detail seventh region  440  provides a cross-section of the wire  410  and the tube  420  within the cavity  138 . 
       FIG. 5  shows tabular lists  500  of dimensions and properties of the first, second, third and sixth regions. The fourth tabular list  510  includes diameters; the fifth tabular list  520  provides the dielectric constants μ and ∈, with the filling materials identified alongside; the sixth tabular list  530  includes the impedances Z. Dimensions of the sixth region  430  are defined by the inner diameter 2×r i4  of the hollow tube  420  (as 0.0034 m), and the material characteristics correspond to fluoropolymer foam. The hollow tube  420  has an outer diameter 2×r i3 =2×r o4  (as 0.0036 m) corresponding to the third region  162  (shown in  FIG. 1 ) with remaining dimensions corresponding to values from the first tabular list  210 . In particular, the fourth tabular list  510  in  FIG. 5  lists inner and outer diameter boundaries of four cavity regions, the first row (for the forward section  120  or first region) identifies the outer diameter of the fore rod  122  as 2×r i1 =0.00635 m, and the inner diameter of the outer cylindrical tube  112  as 2×r o1 =0.0340 m. The second row (for the outer envelope  160  or second region in  FIG. 1 ) identifies the outer diameter of the inner cylindrical tube  132  as 2×r i2 =0.0338 m, and the inner diameter of the outer cylindrical tube  112  as 2×r o2 =0.0340 m. The third row (for the inner envelope  162  in  FIG. 1 , or the third region within the cavity  138 ) identifies the outer diameter of the hollow tube  420  as 2×r i3 =0.0036 m, and the inner diameter of the inner cylindrical tube  132  as 2×r o3 =0.03048 m. The fourth row (for the inner envelope  430 , or the sixth region within the cavity  138 ) identifies the inner diameter of the hollow tube  420  as 2×r i4 =0.0034 m, and the outer diameter of the hollow tube  420  as 2×r o4 =0.0036 m. 
     Values of permeability μ for the four regions listed in  FIG. 5  all correspond approximately to unity, μ 1 =μ 2 =μ 3 =μ 4 =1 respectively for air, Teflon and Teflon foam mixture. Values of permittivity ∈ for these regions include ∈ 1 =1 for air in the first section  120 , ∈ 2 =2.1 for Teflon in the outer envelope  160 , and ∈ 3 =∈ 4 =1.65 for the Teflon foam mixture in the inner envelopes  162  and  430 . Values for impedance for these corresponding regions include Z o1 =Z o3 =Z o4 =100Ω, and Z o2 =0.244Ω. 
       FIGS. 6 and 7  show respective cross-section views of detail regions  170  ( FIG. 6) and 440  ( FIG. 7 ). As shown in  FIG. 6 , the first such view  600  illustrates an external cylindrical periphery  610  and an internal cylindrical periphery  620  of the outer tube  112 , an outer periphery  630  and inner periphery  640  of the inner tube  132 . The internal and outer peripheries  620  and  630  can be coated with an electrically conductive layer and correspond to the respective diameters 2×r o2  and 2×r i2  from column  210  whose dimensions define an outer annular conduit of the outer annular envelope  160 . The outer periphery  630  also defines the outer boundary of an inner annular region within the cavity  138 . The outer envelope  160  and the cavity  138  are correspondingly filled with Teflon and mixed Teflon foam that have respective impedance values of 0.244Ω and 100Ω reported in column  230 . As shown in  FIG. 7 , the second such view  700  illustrates an outer surface  710  of the wire  410 , an inner surface  720  and an outer surface  730  of the hollow tube  420 . The surfaces  710  and  720  define an inner boundary of the cavity  138 . The surface  730  defines the inner annular region of the envelope  430  whose radial boundaries extend to diameter 2×r i4  from column  510 . These surfaces  710 ,  720  and  730  can be coated with an electrically conductive layer. The cavity  138  (extending radially from diameter 2×r i3  to 2×r o3 ) and the envelope  430  within the hollow tube  420  are filled with Teflon foam as identified in column  520  and impedances from column  530 . 
     The folded coaxial RF mirror  100  is to be used in a field deployable RF Fabry-Perot interferometer used in a RF Brillouin Scattering radar. The mirror  100  reduces the overall size and increases the ruggedness of a more conventional RF mirror. Conventionally, a coaxial RF mirror may be constructed from co-linear concatenated sections of coaxial transmission line alternating between sections with high and low characteristic impedance. Because each section of the mirror is quarter-wavelength (¼λ) long at the center frequency of the mirror&#39;s operation, the conventional co-linear mirror can be quite lengthy at low frequencies. 
     For a mirror made from rigid materials, the need for a dielectric Bragg-mirror to have a high Q-resonation necessitates the construction of the mirror from a metal, such as copper, having very high conductivity. However, copper is a relatively soft metal and prone to bending or crushing, as well as being a difficult material to machine. Conceivably, a coaxial. RF mirror could also be constructed from flexible cable, but such a mirror would have degraded performance. This is because the performance of this mirror although improves as the impedance contrast increases, it can be difficult to obtain a great deal of contrast between the characteristic impedances utilizing commercially available coaxial cable. 
     Multiple coaxial cables, such as assemblies  110  and  130 , are nested within each other to achieve the requisite alternating high and low characteristic impedances. The radii are varied and dielectrics can be carefully selected to achieve the desired characteristic impedance in each section. The mirror  100  demonstrates an exemplary embodiment with three folded sections. However, the design can be easily extendable to an arbitrary number of folded sections. 
     The input side has a section of 50Ω transmission line of arbitrary length terminated with a General Radio Type 874 (GR874) input connector  126  (or type-N) and the output side has a section of 50Ω semi-rigid coax of arbitrary length terminated with an SMA output connector  148 . In the cross-section diagram of mirror  100  and the first table  210 , the notations r i1 , r i2 , r i3  signify the radii of the inner conductor (being outer peripheries of the respective fore rod  122 , inner tube  132  and the aft rod  144 ), and r o1 , r o2 , r o3  the radii of the outer conductor of the respective first, second and third sections of coaxial transmission line (being inner surfaces of the outer tube in the first and second sections and the inner surface of the inner tube). In the cross-section diagram of mirror  400  and the third table  510 , the notations r i4  and r o4  (=r i3 ) respectively signify the inner and outer radii of the hollow tube  420 . This structure for the mirror  100  is thus physically shorter than the conventional design due to the nesting of the coaxial transmission lines. The mirror  100  can be constructed of silver or gold-plated brass to maintain the high Q and improve the ruggedness of the structure. The layers  610 ,  620 ,  630 ,  640 ,  710 ,  720  and  730  can be selectively coated with such electrically conductive metals. 
     Interferometer tests have been conducted with three-and-one-half-wavelength (3½λ) quarter-wave tube of copper with slugs to provide a mirror antenna for ultra-high-frequency (UHF) waves. The phenomenon absorption and release of energy by photons from electron shells via acoustic travel has been demonstrated in the past. This can also be accomplished with radio waves, but with greater power levels because signal resolution from scatter cross-section diminishes as the fourth power of frequency, as ψ 4 , or of the wavelength inverse, as λ −4 . Electromagnetic signals are typically employ much shorter wavelengths than acoustic signals. 
     While certain features of the embodiments of the invention have been illustrated as described herein, many modifications, substitutions, changes and equivalents will now occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the embodiments.