Abstract:
Embodiments of a MEMS antenna are presented. Additionally, systems incorporating embodiments of a MEMS antenna are presented. Methods of manufacturing a MEMS antenna are also presented. In one embodiment, the MEMS antenna includes a substrate, a metallic layer disposed over the substrate, the metallic layer forming a ground plane, the ground plane having a region defining a gap disposed therein, a protrusion disposed over the substrate within the region defining the gap, the protrusion extending outwardly from the ground plane, the protrusion having a length and a width, the length being greater than the width, and a first electromagnetic radiator element disposed over the protrusion, the first electromagnetic element having a length and a width, the length being greater than the width.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to antennas generally. More specifically, the present invention relates to a MEMS dipole/monopole antenna and a method of manufacture thereof. 
       BACKGROUND OF THE INVENTION 
       [0002]    Microelectromechanical system (MEMS) antennas are known in the art. Such antennas are manufactured by micromachining technology. This technology may be attractive, particularly for integrated antennas, because it enables efficient packaging, high radiation efficiency, and less mutual coupling between antenna elements. MEMS antennas can be classified into at least two main categories: flat MEMS antennas and 3D MEMS antennas. Flat MEMS antennas, such as patches, are realized on a thin membrane, fabricated by etching under said membrane. 3D MEMS antennas, such as horns or waveguides, may be realized by etching grooves in a substrate. The surface of each groove can then be covered with metal. Each groove represents part of the desired 3D structure. These parts are bonded together to realize the complete 3D structure. The volume defined by this 3D structure can be left as air or filled with a dielectric material. 
         [0003]    Similarly, reconfigurable or multimode antennas, operable in both a dipole and monopole mode, are known in the art. In a dipole mode, antennas primarily radiate or cover the angular range around the broadside (i.e., perpendicular to the substrate, or up-and-down). In a monopole mode, antennas primarily radiate or cover the angular range around the endfire (i.e., parallel to the substrate, or side-to-side). Reconfigurable or multimode antennas may be attractive because they allow a single antenna to replace two, where both dipole and monopole modes are needed or advantageous. This reduces packaging size, which is particularly important in the field of consumer electronics. Such reconfigurable or multimode antennas lack, however, many of the aforementioned advantages of MEMS antennas because such reconfigurable or multimode antennas are often manufactured using conventional planar technology. Planar antennas, particularly at high frequencies, suffer from the excitation of surface waves within the substrate because of the excitation of the unwanted slab modes inside the substrate. These modes are excited if the operation frequency exceeds their cutoff frequencies. The surface waves within the substrate increase losses (i.e., reduce radiation efficiency); increase backside radiation (i.e., radiation below the substrate); and increase the mutual coupling between the elements of the antenna array, which deteriorates the array factor. 
         [0004]    Therefore, despite MEMS antennas and reconfigurable or multimode antennas being known in the art, there is still a need for an antenna that efficiently and effectively combines the benefits of such technologies. 
       SUMMARY OF THE INVENTION 
       [0005]    A novel MEMS dipole/monopole antenna is presented. In an embodiment of this invention, the antenna may be comprised of (1) a ring coupler having first and second input ports and first and second output ports and (2) first and second support blocks, each providing support for a vertical arm and a horizontal arm. The vertical and horizontal arms may act as the antenna arms or elements of the antenna. The first output port may connect with the first vertical arm of the first support block, and the second output port may connect with the second vertical arm of the second support block. 
         [0006]    To operate the antenna in a dipole mode, a signal may be applied to the first input port. The ring coupler may adjust the signal such that the signals incident on the first and second output ports are out-of-phase. Correspondingly, the currents on the first and second vertical arms may flow in opposite directions, with their electromagnetic fields destructively interfering with one another. The currents passed to and on each horizontal arm, however, may flow in the same direction because of the bend between the vertical and horizontal arms. These currents may therefore constructively interfere with each other. In this way, the antenna can function in a dipole mode. Similarly, to operate the antenna in a monopole mode, a signal may be applied to the second input port. In this instance, however, the ring coupler will not adjust the signal (due to the spacing of the ports on the ring coupler), and the signals incident at the first and second output ports may remain in-phase. Correspondingly, the currents on the first and second vertical arms may flow in the same direction, constructively interfering with each other. The currents passed to and on the first and second horizontal arms, however, may flow in opposite directions and cancel each other out. In this way, the antenna can function in a monopole mode. 
         [0007]    A manufacturing process for a MEMS dipole/monopole antenna is also presented. The antenna may be fabricated using bulk micromachining. In one embodiment of the process, the antenna may be manufactured using a single silicon wafer, which is coated on both sides with an oxide. The antenna may then be etched from the top and bottom surfaces to define the components of the antenna. Thereafter, a metal may be selectively deposited on the top and bottom surfaces of the wafer to realize the components of the antenna. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0008]      FIG. 1   a  is an angled, top view of an embodiment of the antenna. 
           [0009]      FIG. 1   b  is a close-up view of the support blocks of  FIG. 1   a,  showing the vertical arms extending below the top surface of the substrate and the horizontal arms resting on the support blocks. 
           [0010]      FIG. 1   c  shows the surface current distribution on the horizontal and vertical arms of an embodiment of the antenna in the dipole mode of operation at 77 GHz. 
           [0011]      FIG. 1   d  shows the surface current distribution on the horizontal and vertical arms of an embodiment of the antenna in the monopole mode of operation at 77 GHz. 
           [0012]      FIG. 2   a  is a cross section of the starting substrate with an oxide coated on the top and bottom surfaces. 
           [0013]      FIG. 2   b  is a top view, cross section, and bottom view of the substrate after oxide has been etched (1) from both the top and bottom surfaces to define the openings for the vertical arms and (2) from the top surface to define the support blocks. 
           [0014]      FIG. 2   c  is a top view, cross section, and bottom view of the substrate with the vertical arms etched through from the top surface to the bottom surface of the substrate and with the support blocks realized on said top surface. 
           [0015]      FIG. 2   d  is a top view, cross section, and bottom view of the substrate after a metal has been deposited to realize the horizontal and vertical arms, the ground plane, the transmission lines, and any impedance matching devices. 
           [0016]      FIG. 3   a  shows the S-parameters of an embodiment of the antenna versus frequency for the dipole mode of operation. 
           [0017]      FIG. 3   b  shows the 3D radiation pattern of an embodiment of the antenna in the dipole mode of operation at 77 GHz. 
           [0018]      FIG. 3   c  shows the radiation pattern of an embodiment of the antenna in the dipole mode of operation at 77 GHz. 
           [0019]      FIG. 4   a  shows the S-parameters of an embodiment of the antenna versus frequency for the monopole mode of operation. 
           [0020]      FIG. 4   b  shows the 3D radiation pattern of an embodiment of the antenna in the monopole mode of operation at 77 GHz. 
           [0021]      FIG. 4   c  shows the radiation pattern of an embodiment of the antenna in the monopole mode of operation at 77 GHz. 
       
    
    
     DETAILED DESCRIPTION 
     Antenna 
       [0022]      FIG. 1   a  illustrates an embodiment of the present invention. The antenna  2  may comprise, among other things, a ring coupler  20  and first and second support blocks  40 ,  40 ′ situated on a substrate  10 . The substrate  10  may have a top surface  12  and a bottom surface  14  and may be comprised of a dielectric substance, such as high resistivity silicon. The first support block  40  may provide support for a first vertical arm  42  and a first horizontal arm  44 . The second support block  40 ′ may provide support for a second vertical arm  42 ′ and a second horizontal arm  44 ′. The ring coupler  20  may have first and second input ports  22 ,  26  and first and second output ports  24 ,  28 . (The antenna  2  may be used to both transmit and receive signals, and, correspondingly, the adjectives “input” and “output” used to reference the ports  22 ,  24 ,  26 ,  28  are without limitation and for convenience of reference only.) The first output port  24  may connect with the first vertical arm  42 , and the second output port  28  may connect with the second vertical arm  42 ′. 
         [0023]    To operate the antenna  2  in a dipole mode, a signal may be applied to the first input port  22 . When a signal passes through this port  22 , the ring coupler  20  may equally divide power between the output ports  24 ,  28 , but adjust the signal such that the signal incident at the first output port  24  is 180° out of phase with the signal incident at the second output port  28 . (As detailed below, this is attributable to the distances between the first input port  22  and the first and second output ports  24 ,  28 .) Correspondingly, the current on the first vertical arm  42  may flow in a direction opposite to the current on the second vertical arm  42 ′. Because these currents flow in opposite directions, their electromagnetic fields may destructively interfere with one another, canceling each other out. The currents on the first and second horizontal arms  44 ,  44 ′, however, may flow in the same direction, with their electromagnetic fields constructively interfering with each other. Said currents may flow in the same direction on said arms  44 ,  44 ′ because the horizontal arms  44 ,  44 ′ are bent in relation to the vertical arms  42 ,  42 ′ and further extend outwardly, away from one another, in opposite directions. This inverts the direction of the flow of the currents, such that if the currents on the vertical arms  42 ,  42 ′ are in opposite directions, the currents on the horizontal arms are in the same direction  44 ,  44 ′, and vice-versa.  FIG. 1   c  shows the directional flow of current on the arms  42 ,  42 ′,  44 ,  44 ′ of an embodiment of the antenna  2  in the dipole mode of operation, as obtained using Ansoft/HFSS simulator. In this way, the horizontal arms  44 ,  44 ′ can act as an array of two dipoles and the antenna  2  can function in a dipole mode. 
         [0024]    To operate the antenna  2  in a monopole mode, a signal may be applied to the second input port  24 . When a signal passes through this port  24 , the ring coupler  20  again may equally divide power between the output ports  24 ,  28 , but in this mode may not adjust the phase of the signal, and, thus, the signals incident at the first and second output ports  24 ,  28  may be in phase. (As detailed below, there may be no phase shift because the distances between the second input port  26  and the first and second output ports  24 ,  28  are the same.) Correspondingly, the currents on the first and second vertical arms  42 ,  42 ′ may flow in the same direction, which results in their electromagnetic fields constructively interfering with one another. The currents on the horizontal arms  44 ,  44 ′ may flow in opposite directions, however, and cancel each other out. Again, this is attributable to the bend between the vertical arms  42 ,  42 ′ and the horizontal arms  44 ,  44 ′.  FIG. 1   d  shows the directional flow of current on the arms  42 ,  42 ′,  44 ,  44 ′ of an embodiment of the antenna  2  in the monopole mode of operation, as obtained using Ansoft/HFSS simulator. In this way, the antenna  2  can function in a monopole mode. 
         [0025]    There are numerous benefits to an antenna that combines the benefits of a reconfigurable or multimode antenna with the benefits of a MEMS antenna. The benefits of a reconfigurable or multimode antenna, as previously mentioned, may include the use of a single antenna in place of two and better coverage for the entire half-space (as the dipole mode covers the broadside direction and the monopole mode covers the endfire direction). When such features are combined with the benefits of a MEMS antenna, which may include efficient packaging, high radiation efficiency, and less mutual coupling between antenna elements, the result is a more effective and efficient antenna. 
         [0026]    The antenna  2  of the present invention may be used in any application that requires a high operating frequency. In some embodiments, the antenna  2  may be a 3D MEMS antenna designed to operate at 77 GHz, a frequency reserved for automotive systems. In certain of such embodiments, the antenna  2  may, for example, be used as part of the radar for a cruise control system. Although a high operating frequency is not required, the dimensions of the antenna  2  will become much larger at lower frequencies and, thereby, the benefits of MEMS technology (such as high radiation efficiency) will be diminished. 
         [0027]    Returning to  FIG. 1   a,  in some embodiments, the ring coupler  20  may comprise a ring-shaped transmission line positioned on the bottom surface  14  of the substrate  10 . The ring-shaped transmission line may be a microstrip, a coplanar waveguide (CPW) line, a coupled microstrip line, or any other type of planar transmission line. In certain embodiments, the ring-shaped transmission line may be a microstrip with a width of 88 μm, which corresponds to a characteristic impedance of 70.7Ω (with the ring coupler  20  having a radius of 333 μm and a circumference of 2.092 mm, which corresponds to 1.5λ g  at 77 GHz). (λ g  represents the guided wavelength of the signal.) Generally speaking, the characteristic impedance of a transmission line is the constant ratio between the voltage and current of either the transmitted or reflected waves along the line. The larger the width of the transmission line, the lower its characteristic impedance. Such a characteristic impedance of 70.7Ω may, in certain of such embodiments, ensure that with respect to the ring coupler  20 , the input impedances (i.e., the impedance at both the first and second input ports  22 ,  26 ) match the characteristic impedances of the feeding lines connected at these junctions. Such matching, among other things, maximizes transmission between the input ports  22 ,  26  and output ports  24 ,  28  and minimizes reflections at the input ports  22 ,  26 . Any type of conductive material may be used for the ring-shaped transmission line, including aluminum, silver, gold, and copper. The depth of such material may be varied, depending on the operating frequency of the antenna  2  and the type of material, but generally should be at least five skin depths. This should help to ensure that such material can function properly as, among other things, a shield. In certain embodiments, the ring-shaped transmission line may be a microstrip comprised of copper having a thickness of 3 μm and a conductivity of 58×10 6  S/m (where S/m means 1/(ohm.m)). 
         [0028]    As mentioned above, the ring coupler  20  may have first and second input ports  22 ,  26  and first and second output ports  24 ,  28 . These ports  22 ,  24 ,  26 ,  28  represent points of intersection between (1) the ring coupler  20  and (2) the transmission lines  60 ,  70  (discussed below) and the first and second vertical arms  42 ,  42 ′. The ring coupler  20  adjusts the phase of a signal by the spacing of ports  22 ,  24 ,  26 ,  28 . The first input port  22  may be a distance of λ g /4 from the first output port  24 . The first output port  24  may be a distance of λ g /4 from the second input port  26 . The second input port  26  may be a distance of λ g /4 from the second output port  28 . And the second output port  28  may be a distance of 3λ g /4 from the first input port  22 . When a signal is received at the second input port  26 , the distances it must travel to the first output port  24  and the second output port  28  are the same. Because of this, the signals incident at said output ports  24 ,  28  will be in phase. By contrast, the distance a signal must travel from the first input port  22  to the second output port  28  is three times that which it must travel to the first output port  24 . Because of this, the signals incident at said output ports  24 ,  28  will be 180° out of phase with one another. If the distances between ports  22 ,  24 ,  26 ,  28  are changed, the antenna may not function properly in some embodiments. For example, in certain of such embodiments, if the ports  22 ,  24 ,  26 ,  28  were spaced in the same order, but were equidistant from one another, and a signal was passed through the first input port  22 , the signals incident at the output ports  24 ,  28  would be in phase. In such a case, the currents on the vertical arms  42 ,  42 ′ might not cancel one another out because they would also be in phase (rather than 180° out of phase), and, thus, the antenna  2  might not function properly in a dipole mode. 
         [0029]    In some embodiments, the antenna  2  may also comprise first and second support blocks  40 ,  40 ′ (each with a top surface and a side surface) positioned on the top surface  12  of the substrate  10 , the first support block  40  having a first vertical arm  42  and a first horizontal arm  44  and the second support block  40 ′ having a second vertical arm  42 ′ and a second horizontal arm  44 ′.  FIG. 1   b  shows a close-up view of the support blocks  40 ,  40 ′, along with the vertical and horizontal arms  42 ,  42 ′,  44 ,  44 ′, of the embodiment of the antenna  2  shown in  FIG. 1   a.  The support blocks  40 ,  40 ′ may provide support for the vertical arms  42 ,  42 ′ and the horizontal arms  44 ,  44 ′. In certain embodiments, the first and second support blocks  40 ,  40 ′ may each have a length of 973 μm, height of 470 μm, and width of 70 μm (with said length and height respectively corresponding to λ g /2 and λ g /4 at the operating frequency of 77 GHz). The arms  42 ,  42 ′,  44 ,  44 ′ may be affixed to the support blocks  40 ,  40 ′ by the adhesion of the conductive material of said arms  42 ,  42 ′,  44 ,  44 ′ to said blocks  40 ,  40 ′, such as by sputtering, platting, or pulse laser deposition. The support blocks  40 ,  40 ′ may be positioned such that the side surface the first support block  40  (where the first vertical arm may be located  42 ) is squarely facing the side surface of the second support block  40 ′ (where the second vertical arm may be located  42 ′). In certain embodiments, the distance between the side surface of the first support block  40  and the side surface of the second support block  40 ′ may be 0.494 mm. 
         [0030]    The arms  42 ,  42 ′,  44 ,  44 ′ may act as the antenna arms or elements of the antenna  2 . The horizontal arms  44 ,  44 ′ may rest on the top surface of the support blocks  40 ,  40 ′. Each vertical arm  42 ,  42 ′ may rest on the side surface of its support block  40 ,  40 ′ or reside in a hollow cavity in said support block  40 ,  40 ′ near said side surface. Each horizontal arm  44 ,  44 ′ may be positioned at a right angle to its respective vertical arm  42 ,  42 ′, and further each horizontal arm  44 ,  44 ′ may extend outward, pointing away from the other. The arms  42 ,  42 ′,  44 ,  44 ′ need not be connected. In some embodiments, there may be a gap of 30 μm between each vertical arm  42 ,  42 ′ and horizontal arm  44 ,  44 ′. In such embodiments, signals may pass from the vertical arms  42 ,  42 ′ to the horizontal arms  44 ,  44 ′ through electromagnetic field coupling. Said gaps may result in there existing standing or stationary waves on the horizontal arms  44 ,  44 ′. These waves may be terminated by the current nulls at the points of disconnection between the arms  42 ,  42 ′,  44 ,  44 ′. Said gaps may enhance the radiation pattern from the horizontal arms  44 ,  44 ′. In other embodiments, said arms  42 ,  42 ′,  44 ,  44 ′ may be connected to one another. Said arms  42 ,  42 ′,  44 ,  44 ′ may be fastened to one another by the adhesion of metal to metal. In certain of such other embodiments, the arms  42 ,  42 ′,  44 ,  44 ′ may be connected by virtue of simply being formed together during the fabrication process (such as by, for example, the depositing of the metal onto the top surface  12  of the substrate  10 ). 
         [0031]    The vertical arms  42 ,  42 ′ may extend below the top surface  12  to the bottom surface  14  of the substrate  10 , such that the first vertical arm  42  connects to the first output port  24  and the second vertical arm  42 ′ connects to the second output port  28 . In some embodiments, the first and second vertical arms  42 ,  42 ′ may each be a hollow pillar, each pillar having four side surfaces forming the sidewalls of each such pillar, said side surfaces each being a metal strip. In certain of such embodiments, each said side surface or strip may have a length of 0.67 mm, with 0.47 mm located above the top surface  12  of the substrate  10  and 0.2 mm located below said top surface  12  (which lengths correspond, respectively, to λ g /4 and λ g /8 at the operating frequency of 77 GHz); a width of 50 μm; and a thickness of 3 μm. Correspondingly, the vertical arms  42 ,  42 ′ may each have a 50 μm×50 μm square cross-section. At the operating frequency of 77 GHz such dimensions of said vertical arms  42 ,  42 ′ may result in the portions of the vertical arms  42 ,  42 ′ residing above the top surface  12  of the substrate  10  being two λ g /4 monopoles. In accordance with the image method, each λ g /4 monopole may have an “image” having the same current direction as the source λ g /4 monopole. Each source λ g /4 monopole, when combined with its “image,” may act as a monopole having a length of λ g /2. In this way, the vertical arms  42 ,  42 ′ may act as an array of two λ g /2 vertical dipoles. It should be noted that such dimensions may also result in the portions of the vertical arms  42 ,  42 ′ residing below the top surface  12  of the substrate  10  acting as two λ g /8 monopoles (or an array of two λ g /4 vertical dipoles), which may radiate below said top surface  12 . The radiation below the top surface  12  will be less significant than the radiation above said top surface  12 , however, because the portions of the vertical arms  42 ,  42 ′ above the top surface  12  will be longer those portions below said surface  12 . With respect to the first and second horizontal arms  44 ,  44 ′, in some embodiments, these arms  44 ,  44 ′ may each have a length of 860 μm (which length corresponds to λ g /2 at the operating frequency of 77 GHz); a width of 40 μm; and a depth of 3 μm. In certain of such embodiments, said horizontal arms  44 ,  44 ′ will thereby act as an array of λ g /2 dipoles and further be located a distance of λ g /4 from the ground plane  16  resting on the top surface  12  of the substrate  10 . The arms  42 ,  42 ′,  44 ,  44 ′ may be comprised of any type conductive material, including aluminum, silver, gold, and copper. In certain embodiments, said arms  42 ,  42 ′,  44 ,  44 ′ may be comprised of copper having a conductivity of 58×10 6  S/m. 
         [0032]    The first input port  22  of the ring coupler  20  may be fed by a first transmission line  60  having first and second ends  62 ,  64 . The first end  62  may intersect with the first input port  22  of the ring coupler  20  and the second end  64  with a first excitation port  66 . The first excitation port  66  (and second excitation port  76 , which is discussed below) are the locations at which the antenna  2  receives or delivers power to or from an external circuit. As mentioned above, the first excitation port  66  may be used to operate the antenna  2  in dipole mode. The first transmission line  60  may be a microstrip, a coplanar waveguide (CPW) line, a coupled microstrip line, or any other type of planar transmission line. Any type of conductive material may be used for said transmission line  60 , including aluminum, silver, gold, and copper, and the depth of such material may be varied. In some embodiments, the first transmission line  60  may be a microstrip with a width of 200 μm, which corresponds to a characteristic impedance of 50Ω. Also in some embodiments, the first transmission line  60  may be a microstrip comprised of copper having a thickness of 3 μm and a conductivity of 58×10 6  S/m. 
         [0033]    The second input port  26  of the ring coupler  20  may be fed by a second transmission line  70  having first and second ends  72 ,  74 . The first end  72  may intersect with the second input port  26  of the ring coupler  20 , and the second end  74  may intersect with a second excitation port  76 . As mentioned above, the second excitation port  76  may be used to operate the antenna  2  in a monopole mode. The second transmission line  70  may be a microstrip, a coplanar waveguide (CPW) line, a coupled microstrip line, or any other type of planar transmission line. Any type of conductive material may be used for said transmission line  70 , including aluminum, silver, gold, and copper, and the depth of such material may be varied. In some embodiments, the second transmission line  70  may be a microstrip with a width of 200 μm, which corresponds to a characteristic impedance of 50Ω. Also in some embodiments, the second transmission line  70  may be a microstrip comprised of copper having a thickness of 3 μm and a conductivity of 58×10 6  S/m. 
         [0034]    Although the first and second excitation ports  66 ,  76  are used to dictate whether the antenna  2  will transmit signals in a monopole mode or a dipole mode, the antenna  2  is similarly capable of receiving signals in either such mode. This is in part attributable to the reciprocity principle, which provides that the directive properties of a given antenna will be the same whether it is used for transmitting or receiving. In the monopole mode, the antenna  2  primarily radiates in the endfire direction. In the dipole mode, the antenna  2  primarily radiates in the broadside direction. (Such radiation directionality is discussed further below.) Correspondingly, in accordance with the reciprocity principle, if the antenna  2  receives a signal in the endfire direction, it can operate in a monopole mode. Similarly, if the antenna  2  receives a signal in the broadside direction, it can operate in a dipole mode. 
         [0035]    In certain embodiments, there may be an impedance mismatch between the ring coupler  20  and the transmission lines  60 ,  70  because, among other things, the impedance at the first end of either such line  60 ,  70  may not match the characteristic impedance of such line  60 ,  70 . More specifically, the characteristic impedance of such transmission line  60 ,  70  may not match the input impedance of the antenna  2  at the point of connection between ring coupler  20  and such transmission line  60 ,  70 . In some embodiments, this impedance mismatch may be between the ring coupler  20  and the second transmission line  70 . Also in some embodiments, this impedance mismatch may not exist with respect to the first transmission line  60  because the dimensions of the antenna  2  may be adjusted to achieve matching with this line  60 , as the input impedance of the antenna  2  is a function of its geometrical parameters. As previously mentioned, impedance mismatches may be problematic because they can cause signal reflection, which may cause power loss in the antenna  2 . 
         [0036]    To overcome such an impedance mismatch, an impedance matching device may be used. An impedance matching device may be any lumped element, such as any capacitor or inductor. In certain embodiments, the impedance matching devices may be stubs  80 ,  80 ′. In certain of such embodiments, these stubs  80 ,  80 ′ may simply be open-ended transmission lines placed along and connected to the relevant transmission line  60 ,  70 . Stubs  80 ,  80 ′ may cure an impedance mismatch by adding a reactive load in parallel at the point on the transmission line  60 ,  70  at which the resistive part of the input impedance of the antenna  2  equals the characteristic impedance of said transmission line  60 ,  70 . This reactive load cancels the imaginary part of the input impedance at the point of connection, resulting in a pure real input impedance that equals the characteristic impedance of transmission line  60 ,  70 . Varying the length of either stub  80 ,  80 ′ may affect its equivalent reactive load. 
         [0037]    In certain embodiments, where an impedance mismatch does exist vis-a-vis the second transmission line  70 , said line  70  may have first and second edges defining the boundaries of the length of said transmission line  70 . First and second stubs  80 ,  80 ′ may each have an inner edge (or an edge closest to the ring coupler  20 ), a free end (or an end opposite the applicable edge of the transmission line  70 ), and an outer edge (or an edge opposite the inner edge). The first and second stubs  80 ,  80 ′ may be connected in parallel to, respectively, the first and second edges of the transmission line  70 . The free ends of said stubs  80 ,  80 ′ may be left open, such that each stub  80 ,  80 ′ is left open-circuit. The length of each stub  80 ,  80 ′, as measured from the applicable edge of the transmission line  70  to the free end of the stub  80 ,  80 ′, may be 137 μm and the width may be 200 μm. Each stub  80 ,  80 ′ may be positioned such that its inner edge is a distance of 16 μm from the second input port  26 . In the monopole mode of certain of such embodiments, this will enhance the impedance matching between the input impedance of the second transmission line  70  and the characteristic impedance of said line  70 . The stubs  80 ,  80 ′ may be comprised of the same material as the transmission lines  60 ,  70  and, in certain embodiments, may be copper microstrips, each having a thickness of 3 ρm and a conductivity of 58×10 6  S/m. 
         [0038]    In some embodiments, a ground plane  16  may be deposited on the top surface  12  of the substrate  10 . Said ground plane may cover the entire surface  12 , but may have two slots such that the support blocks  40 ,  40 ′ can extend upward from the substrate  10 . Said slots may further create a separation between the ground plane  16  and the arms  42 ,  42 ′,  44 ,  44 ′, such that the material of said ground plane  16  and said arms  42 ,  42 ′,  44 ,  44 ′ does not come into contact. The ground plane  16  may, among other things, assist in the directional radiation of the antenna  2  in the dipole mode, such that the signals transmitted by the antenna  2  in this mode radiate primarily in the broadside direction. The ground plane  16  may also serve as a reference plane for the transmission lines  60 ,  70 , ring coupler  20 , and any impedance matching devices (including stubs  80 ,  80 ′). Said ground plane  16  may be comprised of any type of conductive material, including aluminum, silver, gold, and copper, and the depth of such material may be varied. In some embodiments, the ground may be comprised of copper having a thickness or depth of 3 μm and a conductivity of 58×10 6  S/m. In other embodiments, in accordance with the image method, the ground plane  16  may be replaced with two additional horizontal dipoles that are out-of-phase with and parallel to the horizontal arms  44 ,  44 ′. In certain of such other embodiments, said two additional horizontal dipoles may be located a distance of λ g /2 below the horizontal arms  44 ,  44 ′. In this way, the dipoles of the horizontal arms  44 ,  44 ′ and their images (i.e., the two additional horizontal dipoles) may add to each other constructively in the broadside direction and destructively in the endfire direction. 
       Method of Fabrication 
       [0039]    The antenna  2  described herein can be fabricated using various types of bulk micromachining, including, without limitation, deep reactive ion etching, LIGA, and electroforming. A method of fabricating an embodiment of the antenna  2  may comprise the following steps. Although the following method is presented in a specific sequence, other sequences may be used and certain steps omitted or added. It should be noted that the shapes of any etchings, and the dimensions of such shapes, as well as the shapes and depths of any deposited metal, will be dictated by the dimensions and shapes of the antenna  2  and the components thereof. 
         [0040]    As shown in  FIG. 2   a , a substrate  10 , such as a high resistivity silicon wafer, having a top and bottom surface  12 ,  14  is provided. In some embodiments, said substrate  10  may be a 0.67 mm thick high-resistivity silicon wafer with a dielectric constant of 11.9 and conductivity of 0.05 S/m and further having a resistivity of 2000 Ω.cm. The top and bottom surfaces  12 ,  14  of said substrate  10  may be coated with an oxide, such as SiO 2 , to a thickness of at least 4 μm. The oxide layer may act as a mask to protect the substrate  10  during etching. 
         [0041]    As shown in  FIG. 2   b , the oxide on the top and bottom surfaces  12 ,  14  of the substrate  10  may be selectively etched to define the openings for the vertical arms  42 ,  42 ′. In certain embodiments, a square having a width and length of 50 μm may be etched in the top surface  12  and bottom surface  14  of the substrate  10 , with the remaining oxide surrounding such square having a width (from the edge of the square) of  15  The oxide on the top surface  12  of the substrate may be further etched to define a basis for the support blocks  40 ,  40 ′. Any type of etching may be used to remove layers from the substrate, including wet etching and dry etching. The depth of the etching may be to 4 μm, so as to remove the oxide in the selected areas that will define the vertical arms  42 ,  42 ′ and the support blocks  40 ,  40 ′. 
         [0042]    As shown in  FIG. 2   c , the top surface  12  of the substrate  10  that is not protected by oxide (i.e., that has been etched) may be further etched to a depth of 470 μm. Further, the openings for the vertical arms  42 ,  42 ′ on the bottom surface  14  of the substrate  10  may be etched to a depth of 200 μm. The result of this step is the realization of the support blocks  40 ,  40 ′ and the hollow pillars therein that form the basis for the vertical arms  42 ,  42 ′. Deep reactive ion etching may be used in this step. 
         [0043]    As shown in  FIG. 2   d , metal may be deposited by various techniques (including, without limitation, sputtering, platting, and pulse laser deposition) on the top and bottom surfaces  12 ,  14  of the substrate  10  to realize the components of the antenna  2 . In certain embodiments, copper may be deposited to a thickness of 3 μm as follows: (1) on the top surfaces of the support blocks  40 ,  40 ′ to realize the horizontal arms  44 ,  44 ′; (2) in the openings or hollow pillars to realize the vertical arms  42 ,  42 ′; (3) on the top surface  12  of the substrate  10  around the support blocks  40 ,  40 ′ to realize the ground plane  16 ; and (4) on the bottom surface  14  of the substrate  10  to create the ring coupler  20  and transmission lines  60 ,  70 , as well as any impedance matching devices (such as stubs  80 ,  80 ′). In some embodiments, oxide may remain on the top surface  12  of the substrate  10  underneath the arms  42 ,  42 ′,  44 ,  44 ′ to expedite the fabrication process, as such oxide may have no harmful electromagnetic effect on antenna performance. 
       Test Results 
     Dipole Mode of Operation 
       [0044]    The S-parameters of an embodiment of the antenna  2  in the dipole mode of operation, specifically S 11  and S 21 , are plotted versus frequency in  FIG. 3   a . The S-parameters were calculated using Ansoft/HFSS simulator. S-parameters are coefficients that show how the antenna  2  is distributing the power it receives. Generally, it is preferable for power to be radiated, rather than reflected, and thus  FIG. 3   a  shows the amount of reflected power in the antenna  2 . Specifically, S 11  is equal to the square-root of the fraction of power that is reflected back to the first input port  22  as a result of exciting said port. (In the dipole mode, as previously mentioned, the antenna  2  may be excited via the first excitation port  66 , which provides a signal to the first input port  22 .) As can be determined from  FIG. 3   a , the impedance bandwidth for which S 11 &lt;−10 dB is 3.8%. (−10 dB is often used in this context in the antenna industry as a threshold below which an antenna&#39;s reflected power is sufficiently low.) As can further be seen in  FIG. 3   a , the antenna&#39;s  2  reflected power is at its lowest when the antenna  2  is operating at 77 GHz. S 21  is the square-root of the fraction of power that is transferred to the second input port  26  due to the excitation of the first input port  22 . Thus, S 21  shows the coupling or power transferred between said ports  22 ,  26 . As can be seen from  FIG. 3   a , the coupling between said ports  22 ,  26  is weak, less than −15 dB over the entire impedance bandwidth. This indicates good isolation between the ports  22 ,  26 . This is in part attributable to the spacing of the first input port  22  and the second input port  26  on the ring coupler  20 . As the distances between said ports  22 ,  26  along the ring coupler  20  are unequal, the signals incident at the non-excited port (i.e., the second input port  26 ) destructively interfere with one another. 
         [0045]    The 3D radiation pattern at 77 GHz of an embodiment of the antenna  2  in the dipole mode of operation is shown in  FIG. 3   b . As can be seen from  FIG. 3   b , in the dipole mode, the antenna  2  is primarily radiating from the top surface  12  of the substrate  10 , or upward; the radiation from the bottom surface  14  of the substrate  10 , or downward, is weak. The antenna  2  so radiates from the top surface  12  because, among other things, the arms  22 ,  24 ,  26 ,  28  are located on said top surface  12  and the ground plane  16  serves as a “shielding” layer between the areas above and below the top surface  12  of the substrate  10 . This is particularly beneficial in applications where radiation is preferably from one side of a substrate. During these simulations, the calculated gain of this embodiment of the antenna  2  was determined to be 8.6 dBi. Based on this and the directivity of the radiation shown in  FIG. 3   b , the radiation efficiency of this embodiment was determined to be 92%. Since the top surface  12  of the substrate  10  is primarily air, energy loss is largely attributable to conductor loss (or, in other words, the finite conductivity of the metal used in the antenna  2 ). Dielectric losses are negligible because the dielectric substance that comprises the substrate  10  is isolated from the antenna  2  by the ground plane  16 . Hence, there likelihood of the excitation of surface waves in the substrate  10  is minimal. 
         [0046]    The radiation patterns in the planes phi=0 and phi=90°, where phi is the angle measured from the x-axis, can be seen in  FIG. 3   c . This figure represents a cross-section of the 3D radiation patterns shown in  FIG. 3   b . As can be determined from this figure, the front-to-back ratio of radiation in the antenna  2  is 18.6 dB. The front-to-back ratio is the ratio between the power density in the broadside direction and the power density in the opposite direction on the other side of the substrate  10 . For applications that require an antenna to radiate primarily from one side, it is important to have a large front-to-back ratio. 
       Monopole Mode of Operation 
       [0047]    The S-parameters of an embodiment of the antenna  2  in the monopole mode of operation are plotted versus frequency in  FIG. 4   a . Specifically, S 22  and S 12  are plotted, where S 22  is equal to the square-root of the fraction of power that is reflected back to the second input port  26  as a result of exciting said port and S 12  is the square-root of the fraction of power that is transferred to the first input port  22  due to the excitation of the second input port  26 . As can be determined from  FIG. 4   a , the impedance bandwidth for which S 22 &lt;−10 dB is 3.9%. This is roughly the same as in the dipole mode of operation because the input impedances of the antenna  2  at both input ports  22 ,  26  are behaving the same way with respect to frequency variation around resonance (i.e., variation of the real and imaginary parts of the input impedance with frequency). As was the case also in the dipole mode, the coupling between the first and second input ports  22 ,  26  in this monopole mode of operation is weak, with S 12 &lt;−15 dB over the entire impedance bandwidth. This is expected because, as previously mentioned, the distances between the first and second input ports  22 ,  26  along the ring coupler  20  are unequal. 
         [0048]    The 3D radiation pattern at 77 GHz of an embodiment of the antenna  2  in the monopole mode of operation is shown in  FIG. 4   b . A radiation null can be seen from this figure, which null shows weak radiation power density in the broadside direction. This is because in the monopole mode of operation, the antenna  2  primarily radiates in the endfire direction. The gain and radiation efficiencies of the antenna  2  in this mode are 5.4 dBi and 93%, respectively. 
         [0049]    The radiation patterns in the planes phi=45° and phi=135°, where phi is again the angle measured from the x-axis, are plotted in  FIG. 4   c . The figure represents a cross-section of the 3D radiation patterns shown in  FIG. 4   b . Again, the radiation null can be seen in this figure in the monopole mode can be seen in this figure. Front-to-back radiation is not important in this mode because the antenna  2  is radiating in the endfire direction.