Patent Publication Number: US-6707349-B1

Title: Directional coupler for microwave cavities

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention relates to a device for measuring the microwave power inside a waveguide. More particularly, the invention discloses the structure of a micro strip directional coupler for measuring microwave power in different directions inside a waveguide. 
     2. Related Art 
     The microwave power needed in a typical microwave heating system is usually hundreds of watts. If one uses a normal microwave power meter to measure the microwave source, the large power output may easily damage the power meter. Therefore, a microwave coupler is often used in this case. The hundreds of watts of microwave power output is coupled with the microwave coupler in such a way that the measured power is only of mW order. The signal coupled by the microwave coupler can be directly read out by microwave power meters. After a systematic conversion, one can obtain the output power of the microwave source. 
     The microwave heating system is mainly comprised of a power supply, a magnetro, a waveguide, a controller and a cavity. The magnetro is one type of microwave generators. It provides such information as temperature, humidity and weight through detectors, and its output is controlled by a time switch or a feedback controller. The waveguide specifically refers to all kinds of hollow metal waveguides and surface-wave waveguides. Taking a microwave oven as an example, the energy of the microwave generated by the magnetro is transferred in the form of waves through a hollow metal tube to a heating cavity. The microwave coupler is a measuring device for measuring the microwave power. 
     There are many known microwave couplers. They are consisted of appropriate coupling structures between a main transmission line and an auxiliary transmission line. The transmission line that a directional coupler uses to transmit microwave coupling signals can be a coaxial line, a strip line, a micro strip line, a metal waveguide, or a medium waveguide. The coupling structure can be a coupling hole, a coupling branch line, and a continuous structural coupling. 
     The techniques disclosed in the U.S. Pat. Nos. 4,297,658, 4,792,770, 5,043,684, and 5,185,046 are mainly waveguide directional couplers that utilize two parallel waveguides. The coupling structure is achieved using several coupling holes or coupling windows or slits. The directional coupler using a metal waveguide often has a high conductivity (e.g. copper, aluminum or stainless steel) in order to minimize the energy loss during the transmission process. Furthermore, the inner walls are as smooth as possible and the metal connecting places are made as few as possible. The cross section of the waveguide can be rectangular or circular. They differ in microwave transmission effects, structural designs, and properties of objects to be heated. They often require more delicate machining and are more difficult in manufacturing. 
     The technique disclosed in the U.S. Pat. No. 3,721,921 is a directional coupler using a coupling branch structure. Conventional directional couplers that use strip lines or micro strip lines are of two types. One is a rotational design. It rotates the direction ,of a transmission line so that the transmission line and the coupling hole on the waveguide reach an optimal relative position for coupling. The other is a fixed design. Once the coupler and the waveguide are combined and fixed, the relative position and angle between the transmission line and the coupling hole on the waveguide are unchangeable. The drawback of the rotation-type coupler is in that the structure may become loose and affect the coupling effect. 
     SUMMARY OF THE INVENTION 
     An objective of the invention is to provide a new, simple structure of a directional coupler for microwave coupling cavities that is easy in manufacturing. 
     The manufacturing process of the disclosed directional coupler only involves the steps of making print circuit boards, assembly and electroplating. The step of making print circuit boards is to make the first carrier and the second carrier that contain micro strip lines. The first carrier is implemented by forming a plane copper foil on a fiber substrate. The assembly is to combine the first carrier, the second carrier, and two signal connectors together. The electroplating step covers the surfaces other than the two signal connectors and the position reserved for a coupling hole on the first carrier by a conductive metal (e.g. copper or gold), forming a metal shell. 
     The micro strip type directional coupler of the invention does not use a hollow metal waveguide structure. Only connectors such as screws are needed to fix the directional coupler on an outer side of the coupling hole of the waveguide. Therefore, the invention is easy to make and install and does not occupy too much space. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will become more fully understood from the detailed description given hereinbelow illustration only, and thus are not limitative of the present invention, and wherein: 
     FIG. 1 shows a three-dimensional structure of the invention; 
     FIG. 2 is an exploded view of the invention; 
     FIG. 3 shows a plane structure of the transmission line in the directional coupler; 
     FIG. 4 is a cross-sectional view of the first carrier; and 
     FIG. 5 is a cross-sectional view of the directional coupler. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in FIGS. 1 and 2, the disclosed directional coupler  10  is installed on one outer side of the coupling hole  21  on a waveguide  20 . With reference to FIG. 3, the detailed structure of the directional coupler  10  includes a first carrier  30 , a second carrier  40 , a metal layer  50 , and a connecting means. The first carrier  30  has a metal transmission line  31  on one of its side surfaces. This transmission line  31  may be a micro strip line or a strip line. According to its function, the transmission line  31  can be divided into a coupling section  310 , and a first output section  311   a  and a second output section  311   b  extending from the coupling section  310  to the edge of the first carrier  30 . The other side surface of the first carrier  30  has a predetermined rectangular coupling area  32 . The coupling area  32  is under the central position of the coupling section  310  and overlaps with the position of the coupling hole  21  on the waveguide  20 . The coupling section  310  of the transmission line  31  undergoes microwave signal coupling with the coupling hole  21  of the waveguide  20  through exactly this coupling area  32 . The second carrier  40  overlaps and combines with the first carrier  30  and completely covers the micro strip line  31 . The metal layer covers the surfaces of the combined first carrier  30  and second carrier  40  except for the coupling area  32 . The connecting means fixes the combined first carrier  30  and second carrier  40  on one side of the waveguide  20 . 
     A preferred implementation method of the invention is to make the directional coupler using the print circuit board means. For example, one can take an FR 4  glass fiberboard to be the material for the first carrier  30  and the second carrier  40 . Then the print circuit board technique is employed to make a layer of thin copper foil lines on the surface of the first carrier  30 , forming the transmission lines  31 . Afterwards, the second carrier  40  is pressed onto the surface of the first carrier  30 , sandwiching the transmission lines  31  in between. 
     The metal layer  50  covering the surfaces of the first carrier  30  and the second carrier  40  is preferably implemented using electroplating. A layer of conductive metal such as copper or gold is coated on the surfaces of the first carrier  30  and the second carrier  40  to form the metal layer  50 . However, the predetermined rectangular coupling area  32  is not coated with any metal. When the directional coupler  10  is installed on one outer side of the coupling hole  21  on the waveguide  20 , the coupling area  32  is aligned with the coupling hole  21  on the waveguide  20 . In this way, the microwave inside the waveguide  20  is coupled to the transmission line  31  through the coupling hole  21  and the coupling area  32 . 
     A preferred embodiment of the connecting means is to reserve several through-holes  22  on one side surface of the waveguide  20 . Screws  60  or other equivalent elements are then used to connect the first carrier  30  and the second carrier  40  of the directional coupler  10  to the through-holes  22  of the waveguide  20 . This completes the assembly of the disclosed directional coupler  10 . 
     Since the invention simply uses the print circuit board manufacturing process to make the disclosed directional coupler  10 , it is very easy to prepare. One only needs the normal print circuit board procedure along with surface electroplating packaging. As the material used in the invention is the FR 4  glass fiberboard commonly used for making circuit boards, it does not contain any metal structure. There is no need of any further machining process, greatly reducing the manufacturing cost. 
     In principle, the first output section  311   a  and the second output section  311   b  extending to the edge of the first carrier  30  can be connected to a signal transmission cable by welding. Thus, the coupled microwave signal can be transferred to a microwave power meter (not shown) for power measurement. 
     Another preferred embodiment of the invention is to install on both ends of the directional coupler  10  a first signal connector  11   a  and a second signal connector  11   b  for outputting coupling signals. By connecting these two signal connectors  11   a ,  11   b  to a microwave power meter, the coupled microwave signal can be transferred to the microwave power meter for power measurement. The first signal connector  11   a  is fixed on one side of the combined first carrier  30  and second carrier  40 . The signal pin  110   a  of the first signal connector  11   a  is in contact with the first output section  311   a  of the transmission line  31 . The second signal connector  11   b  is fixed on the other side of the combined first carrier  30  and second carrier  40 . The signal pin  110   b  of the second signal connector  11   b  is in contact with the second output section  311   b  of the transmission line  31 . 
     In a preferred embodiment of the invention, the first signal connector  11   a  and the second signal connector  11   b  are SMA connectors. These two signal connectors  11   a ,  11   b  can be fixed on one side of the combined first carrier  30  and second carrier  40  using screws  12  or other equivalent elements. From FIG. 5, one can see that the signal pin  110   a  of the first signal connector  11   a  is sandwiched between the first carrier  30  and the second carrier  40  and is in contact with the first output section  311   a  of the transmission line  31 . Likewise, the signal pin  110   b  of the second signal connector  11   b  is sandwiched between the first carrier  30  and the second carrier  40  and is in contact with the second output section  311   b  of, the transmission line  31 . 
     The assembly of the invention is to first combine the first carrier  30 , the second carrier  40 , and the first and second signal connectors  11   a ,  11   b  by pressing. The combined element is then electroplated with a metal layer  50 . However, the coupling area  32  predetermined at the bottom surface of the first carrier  30 , the first signal connector  11   a  and the second signal connector  11   b  do not need to be covered by any metal. 
     While the invention has been described by way of example and in terms of the preferred, embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.