Bidirectional overmoded THz coupler
A low-loss bidirectional HE 11 coupler is disclosed with substantially higher directivity than prior HE 11 THz couplers for overmoded waveguides operating in a frequency band centered about f 0 , where f 0 is between 0.03 to 1.5 THz, with mid-band free-space wavelength λ 0 . The coupler comprises a main hollow-metal waveguide, aligned along the z axis, of inner radius r 1 , where r 1 is greater than 2λ 0 , a first end of the main waveguide is identified as port-1 and its second end is identified as port-2. A first transverse lofted transition waveguide couples a port-3 of radius r 3 , where r 3 <r 1 , to a substantially elliptical opening of mean radius greater than r 3 in one side of the main waveguide. A similar second tapered lofted transition waveguide, aligned on the same axis as the first lofted transition, identified as the y axis, couples a port-4 to a substantially elliptical opening in the opposite side of the main waveguide. A substantially elliptical dielectric mirror, centered with respect to the intersection of the axes of the main waveguide and the transverse coupling waveguides, rotated at approximately 45° with respect to the xy plane about the x axis for HE 11 E-field polarization in the y direction, supported on its +x and −x sides, has major and minor transverse dimensions sufficient to intersect a substantial fraction of the cross-sectional area of the beam in the main waveguide. The mirror is further characterized as having thickness greater than λ m /10 and less than λ m , where λ m is the wavelength in the mirror at f 0 . The mirror dielectric is further characterized as typically having dielectric constant ε m less than 3.7 and loss tangent less than 0.02. The coupler is further characterized as having portions of the inside surfaces of waveguides near their intersections coated with a microwave absorptive material that is characterized as having loss tangent greater than 0.01 and thickness greater than λ a /5, where λ a is the wavelength in the absorptive coating at f 0 . Other inner surfaces of the waveguides may be bare metal, or corrugated, or laminate lined. A polarizer may be included at port-1 and/or port-2 to partially block s-polarization, here polarization with E field in the x direction. The polarizer may be formed by etching an array of narrow traces on copper-clad low-loss laminate of substrate thickness approximately λ p /2, where λ p is the wavelength in the polarizer substrate at f 0 . The traces are further characterized as having periodicity approximately λ 0 /4, trace width less than λ 0 /12, and being aligned in the x direction.
1 . A mirror-based THz coupler for diverting a portion of a substantially HE 11 main microwave beam of nominal frequency f 0 and nominal free-space wavelength λ 0 to a coupled beam, said coupler comprising:
a hollow round main waveguide of inner radius r 1 aligned along a z axis, where said r 1 is further characterized as being greater than 2λ 0 ;
a port-1 at a first end of said main waveguide and a port-2 at a second end of said main waveguide, said first end identified as the negative z end;
a first substantially compound elliptical opening on a first side of said main waveguide, said opening centered on an orthogonal y axis intersecting said z axis at an intersection point between said port-1 and said port-2, said first side identified as the negative y side, said intersection point also identified as the origin in the exemplary Cartesian coordinate system;
a second substantially compound elliptical opening on the positive y side of said main waveguide, said second elliptical opening centered on said y axis;
a first coupling waveguide on said y axis for coupling said first elliptical opening to a port-4 at a negative y location on said y axis;
a second coupling waveguide on said y axis for coupling said second elliptical opening to a port-3 at a positive y location on said y axis;
a dielectric mirror of substantially elliptical shape located such that its center approximately coincides with said intersection point;
said mirror further characterized as lying in mirror angle plane, said angle plane being at greater than 30° and less than 55° with respect to the plane in which z equals zero;
said mirror further characterized as having major and minor transverse dimensions sufficient to intersect a substantial fraction of said main beam;
said mirror further characterized as being of a material of dielectric constant ε m , where ε m is less than 3.7;
said mirror further characterized as having thickness t m , where t m is further characterized as being greater than λ m /16, where λ m is further characterized as the wavelength at nominal frequency f 0 inside said mirror;
said waveguides further characterized as having metallic inner surfaces with an absorptive dielectric coating over portions of their inner surfaces near said elliptical openings;
said absorptive coating further characterized as having dielectric loss tangent δ a greater than 0.01 at said frequency f 0 .
2 . The coupler of claim 1 in which said ε m is further characterized as being less than 2.6.
3 . The coupler of claim 1 in which said first coupling waveguide is further characterized as including a lofted section.
4 . The coupler of claim 1 in which said mirror is further characterized as being supported on its +x and −x sides.
5 . The coupler of claim 1 in which said first coupling waveguide is further characterized as comprising a round coupling section, a substantially elliptical coupling section, and a lofted section between said round coupling section and said elliptical coupling section.
6 . The coupler of claim 1 in which said t m is further characterized as being less than approximately λ m .
7 . The coupler of claim 1 in which said δ a is further characterized as being greater than 0.05 at said f 0 .
8 . The coupler of claim 1 in which said absorptive coating is further characterized as having dielectric constant ε a , said absorptive coating further characterized as having thickness t a , said t a further characterized as approximately equal to kλ a , where k is selected from the set containing 0.25 and 0.75, and where λ a is further characterized as the wavelength at nominal frequency f 0 inside said coating.
9 . The coupler of claim 1 in which said first compound elliptical opening is further characterized as having minor diametrical dimension d eo , said d eo further characterized as being approximately 1.3r 1 .
10 . The coupler of claim 1 in which said t m is further characterized as being approximately λ m /4.
11 . The coupler of claim 1 in which said t m is further characterized as being approximately λ m /4.
12 . The coupler of claim 1 in which said absorptive coating is further characterized as extending a distance approximately r 1 on the inside surface of said main waveguide from the plane in which z equals zero.
13 . The coupler of claim 1 in which said absorptive coating is further characterized as extending a distance approximately 1.5r 1 from said main waveguide into said second coupling waveguide.
14 . The coupler of claim 1 in which said mirror angle is further characterized as being greater than 40° and less than 47° with respect to the plane in which z equals zero.
15 . The coupler of claim 1 in which said first compound elliptical opening is further characterized as having minor diametrical dimension d eo and said mirror is further characterized as have projected length z m along the z axis, said z m further characterized as being less than 1.5r 1 , and said d eo is further characterized as being greater than said z m .
16 . The coupler of 1 further characterized as including a polarizer in said main waveguide.
17 . The coupler of claim 16 in which said polarizer is further characterized as comprising copper traces on a polarizer substrate of dielectric constant less than 2.6, said substrate having thickness t p , where t p is further characterized as being approximately equal to λ p /2, where λ p is further characterized as the wavelength at nominal frequency f 0 inside said polarizer substrate, said traces are further characterized as having width less than λ 0 /12 and having center-to-center spacing approximately equal to λ 0 /4.
18 . A wire-array-based THz coupler for diverting a portion of a substantially HE 11 main microwave beam of nominal frequency f 0 and nominal free-space wavelength λ 0 to a coupled beam, said coupler comprising:
a hollow round main waveguide of inner radius r 1 aligned along a z axis, where said r 1 is further characterized as being greater than 2λ 0 ;
a port-1 at a first end of said main waveguide and a port-2 at a second end of said main waveguide, said first end identified as the negative z end;
a first substantially compound elliptical opening on a first side of said main waveguide, said opening centered on an orthogonal y axis intersecting said z axis at an intersection point between said port-1 and said port-2, said first side identified as the negative y side, said intersection point also identified as the origin in the exemplary Cartesian coordinate system;
a second substantially compound elliptical opening on the positive y side of said main waveguide, said second elliptical opening centered on said y axis;
a first coupling waveguide on said y axis for coupling said first elliptical opening to a port-4 at a negative y location on said y axis;
a second coupling waveguide on said y axis for coupling said second elliptical opening to a port-3 at a positive y location on said y axis;
a wire array located such that its center approximately coincides with said intersection point;
said wire array further characterized as lying in beam splitter plane, said splitter plane being at greater than 30° and less than 55° with respect to the plane in which z equals zero;
said waveguides further characterized as having metallic inner surfaces with an absorptive dielectric coating over portions of their inner surfaces near said elliptical openings;
said absorptive coating further characterized as having dielectric loss tangent δ a greater than 0.01 at said frequency f 0 ;
said absorptive coating further characterized as having dielectric constant ε a , said absorptive coating further characterized as having thickness t a , said t a further characterized as being greater than the quantity λ a /10, where λ a is further characterized as the wavelength at nominal frequency f 0 inside said coating.
19 . The coupler of claim 18 in which said t a is further characterized as approximately equal to kλ a , where k is selected from the set containing 0.25 and 0.75.
20 . The coupler of claim 18 in which said δ a is further characterized as being greater than 0.05 at said f 0 .
21 . The coupler of claim 18 in which said first coupling waveguide is further characterized as comprising a round coupling section, a substantially elliptical coupling section, and a lofted section between said round coupling section and said elliptical coupling section.
22 . The coupler of claim 18 in which said absorptive coating is further characterized as extending a distance approximately r 1 on the inside surface of said main waveguide from the plane in which z equals zero.
23 . The coupler of claim 18 in which said absorptive coating is further characterized as extending a distance approximately 1.5r 1 from said main waveguide into said second coupling waveguide.
24 . The coupler of claim 18 in which said first compound elliptical opening is further characterized as having minor diametrical dimension d eo and said wire array is further characterized as have projected length z w along the z axis, said z w further characterized as being less than 1.5r 1 , and said d eo is further characterized as being greater than said z w .
25 . The coupler of claim 18 further characterized as including a polarizer in said main waveguide.
26 . A method for use with a THz coupler, the THz coupler having a hollow round main waveguide of inner radius r 1 aligned along a z axis, where said r 1 is further characterized as being greater than 2λ 0 ;
a port-1 at a first end of said main waveguide and a port-2 at a second end of said main waveguide, said first end identified as the negative z end;
a first substantially compound elliptical opening on a first side of said main waveguide, said opening centered on an orthogonal y axis intersecting said z axis at an intersection point between said port-1 and said port-2, said first side identified as the negative y side, said intersection point also identified as the origin in the exemplary Cartesian coordinate system;
a second substantially compound elliptical opening on the positive y side of said main waveguide, said second elliptical opening centered on said y axis;
a first coupling waveguide on said y axis for coupling said first elliptical opening to a port-4 at a negative y location on said y axis;
a second coupling waveguide on said y axis for coupling said second elliptical opening to a port-3 at a positive y location on said y axis;
a dielectric mirror of substantially elliptical shape located such that its center approximately coincides with said intersection point;
said mirror further characterized as lying in mirror angle plane, said angle plane being at greater than 30° and less than 55° with respect to the plane in which z equals zero;
said mirror further characterized as having major and minor transverse dimensions sufficient to intersect a substantial fraction of said main beam;
said mirror further characterized as being of a material of dielectric constant ε m , where ε m is less than 3.7;
said mirror further characterized as having thickness t m , where t m is further characterized as being greater than λ m /16, where λ m is further characterized as the wavelength at nominal frequency f 0 inside said mirror;
said waveguides further characterized as having metallic inner surfaces with an absorptive dielectric coating over portions of their inner surfaces near said elliptical openings;
said absorptive coating further characterized as having dielectric loss tangent δ a greater than 0.01 at said frequency f 0 , the method comprising the steps of:
connecting a source of microwave energy to the input port;
connecting a load to the output port;
connecting a reverse power detector to the reverse coupling port;
by means of the source of microwave energy, delivering microwave energy to the input port and thence to the output port and to the load; and
detecting reverse power by means of the reverse power detector.
27 . The method of claim 26 further comprising the step of controlling the impedance of the load in response to detected reverse power.
28 . A method for use with a THz coupler, the THz coupler having a hollow round main waveguide of inner radius r 1 aligned along a z axis, where said r 1 is further characterized as being greater than 2λ 0 ;
a port-1 at a first end of said main waveguide and a port-2 at a second end of said main waveguide, said first end identified as the negative z end;
a first substantially compound elliptical opening on a first side of said main waveguide, said opening centered on an orthogonal y axis intersecting said z axis at an intersection point between said port-1 and said port-2, said first side identified as the negative y side, said intersection point also identified as the origin in the exemplary Cartesian coordinate system;
a second substantially compound elliptical opening on the positive y side of said main waveguide, said second elliptical opening centered on said y axis;
a first coupling waveguide on said y axis for coupling said first elliptical opening to a port-4 at a negative y location on said y axis;
a second coupling waveguide on said y axis for coupling said second elliptical opening to a port-3 at a positive y location on said y axis;
a dielectric mirror of substantially elliptical shape located such that its center approximately coincides with said intersection point;
said mirror further characterized as lying in mirror angle plane, said angle plane being at greater than 30° and less than 55° with respect to the plane in which z equals zero;
said mirror further characterized as having major and minor transverse dimensions sufficient to intersect a substantial fraction of said main beam;
said mirror further characterized as being of a material of dielectric constant ε m , where ε m is less than 3.7;
said mirror further characterized as having thickness t m , where t m is further characterized as being greater than λ m /16, where λ m is further characterized as the wavelength at nominal frequency f 0 inside said mirror;
said waveguides further characterized as having metallic inner surfaces with an absorptive dielectric coating over portions of their inner surfaces near said elliptical openings;
said absorptive coating further characterized as having dielectric loss tangent δ a greater than 0.01 at said frequency f 0 , the method comprising the steps of:
connecting a source of microwave energy to the input port;
connecting a load to the output port;
connecting a forward power detector to the forward coupling port;
by means of the source of microwave energy, delivering microwave energy to the input port and thence to the output port and to the load; and
detecting forward power by means of the forward power detector.
29 . The method of claim 28 further comprising the step of controlling the output of the source of microwave energy in response to detected forward power.