Abstract:
A method and apparatus for directionally coupling two technically different microwave transmission lines, one being a waveguide and the other a stripline, in which a central conductor of the stripline is plunged into waveguide through a slot in a wall of the waveguide and loaded by capacitors or inductors. The width of the central conductor and values of the capacitors and inductors determines the desired coupling coefficient.

Description:
BACKGROUND OF THE INVENTION 
     I. Field Of The Invention 
     The present invention relates to apparatus and method for directionally coupling a stripline or microstrip to a waveguide-type transmission line. The invention has application to the &#34;slab&#34; type of plane microwave antennas and to the control of microwave power. 
     II. Description Of The Prior Art 
     Even though the power tolerance of striplines has much increased due to the feasibility of bonding the various components, it still remains relatively modest. Therefore, with regard to high power antennas, it is sometimes necessary to use a main power distributor having waveguide design capable of tolerating high power and capable of feeding the remainder of the antenna having a stripline design. The performance of the junction between the various antenna parts in such cases is a source of power loss. 
     Many limitations are placed on various solutions presently used to achieve waveguide-to-stripline coupling. For example, with regard to FIG. 1, we note that the structure of a conventional waveguide-to-stripline coupler consists of a stripline 2 which includes two plates 5, 6 and a central conductor 4 located flat on one of the sides 5 of a waveguide 1. Coupling is ensured by one or more radiating slots 3 coupled to the conducting core of the microstrip 4. 
     Even though efficient, this form of coupling does not result in coupling coefficients in excess of 0.1, i.e., 20 dB. 
     In relation to FIG. 2, a conventional coaxial waveguide coupler is shown consisting of a metal bar 7 of a length nearly that of a half-wave and located within a waveguide 8. This type of coupler results in substantial coupling coefficients, but is relatively selective, with the operational frequency band being narrow. Also, this type of coupler demands a connection system 9 appropriate to the particular ultimate stripline to be used. 
     The present invention wholly averts the drawbacks cited above. 
     The present invention makes possible coupling coefficients varying over a wide range from 6 dB to more than 30 dB. 
     The present invention permits a wide operational frequency band, 7.5% on either side of the nominal operating frequency, without thereby modifying the characteristics. 
     The present invention requires no intermediary component, the coupler being used being an integral part of the stripline, and the technical connection problems being eliminated. 
     Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
     To that end and in conformity with the invention, a directional microwave coupler to couple microwave energy from a waveguide transmission line to a stripline transmission line is provided which comprises a longitudinal slot fashioned in a side of said waveguide; a central conductor of said stripline extending through said slot and into said waveguide; and phase-shifting means for equalizing the phase velocity of the microwaves in said conductor and the phase velocity of the microwaves in said waveguide adjacent to said conductor. Advantageously, these phase-shifting means are one or more capacitors in series with the central conductor or else one or more inductors in parallel to each other and connected between the central conductor and a wall of waveguide. 
     Preferably, the coupler includes a stripline wherein the central conductor is within the waveguide for a length of about one fourth of the length of the guided wave. 
     The invention also relates to a method for coupling a waveguide transmission line and a stripline transmission line, where this method is characterized by placing a central stripline conductor within the waveguide through a slot in a wall of the waveguide and wherein the speed of propagation of the wave in the stripline is altered, and preferably accelerated by such phase-shifting means as one or more capacitors in series with the central conductor or one or more inductors in parallel to one another and connected between the central conductor and a wall of the waveguide, so as to substantially equalize the phase velocities in the two transmission lines in the area where the central stripline conductor is within the waveguide. 
    
    
     The invention and its implementation are shown in greater elucidation in relation to the description below and the attached drawings, wherein: 
     FIGS. 1 and 2 schematically show two types of conventional couplers; 
     FIG. 3 schematically shows a perspective of a coupler of the invention; 
     FIGS. 4 and 5 are electric diagrams relating to modifications of the coupler illustrated in FIG. 3; 
     FIG. 6 is a schematic perspective of an alternative embodiment of the coupler of the invention; and 
     FIG. 7 is a schematic perspective of still another embodiment of the coupler of the invention. 
    
    
     The above general description and the following detailed description are merely illustrative of the generic invention and additional modes, advantages, and particulars of this invention can be readily suggested to those skilled in the art without departing from the spirit and scope of the invention. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A coupler of the invention which operates in the 2,700 to 3,300 MHz band is described in an illustrative and non-limiting manner below in relation to FIG. 3. Obviously, this coupler can easily be made to operate in another frequency band. 
     A stripline is a multilayer means consisting of three dielectric layers 10, 11, 12 with a dielectric constant ε=2.22. Ports 3, 4 (circled numerals) referenced as 13, 14 of the stripline are implemented in the form of miniature coaxial connectors 15, 16 of the SMA type. The two outer dielectric layers 10, 12 are 1.6 mm thick and are externally metallized by a copper foil 35 microns thick in order to obtain two metal planes 17, 18. 
     The inside dielectric layer 11 is 0.4 mm thick and supports the central metal conductor 19 of the stripline consisting of an etched copper tape 2.7 mm wide providing a characteristic impedance of 50 ohms to the line. This inside layer extends into the waveguide 20 of the WR284 type with 34 mm×72 mm dimensions through a slot 21 which is 38 mm long and 4 mm wide. 
     Inside the waveguide, the width of the central conductor 19 is increased to 8 mm in order to maintain a characteristic impedance of 50 ohms. 
     In order to achieve the same phase velocities, a phase-shifting cell is used, which consists of two capacitors 22, 23 of 1 picofarad each between which is inserted a metal conducting path 24 1 mm wide and 20 mm long, 35 microns thick and with a characteristic impedance of 130 ohms. The total coupler length is 42 mm for a guided wavelength of 140 mm. 
     The electric characteristics within the 2,700 to 3,300 MHz band are as follows: 
     coupling coefficient: 10 dB+/-0.3 dB; 
     directionality: exceeds 15 dB; 
     directionality in 2800-3200 MHz band: exceeds 20 dB; 
     standing wave ratio in the lower stripline path: less than 1.4; 
     standing wave ratio in the lower waveguide path: less than 1.1; and 
     peak power tolerance: exceeds 100 kw in the waveguide path. 
     By periodically loading the central conductor by means of series capacitors or parallel inductors (FIG. 3 illustrates a case using two capacitors 22, 23), the phase velocity of the stripline is accelerated and made equal to the waveguide phase velocity. In this manner good directional coupling between the waveguide (TE mode) and the stripline (TEM mode) is obtained. The electrical length of the coupler as seen by the input 3 (circled number) of the stripline equals one quarter of the guided wavelength. Thus the problem is reduced to coupling between two identical lines. 
     FIGS. 4 and 5 schematically show two couplers designed on the same principle as that of FIG. 3 with three capacitors C1, C2, C3 in series with the conductor 24 or with four inductors L1, L2, L3, L4 in parallel between the conductor 24 and the adjacent wall of the waveguide 20. Obviously, the number of capacitors or inductors used is not meant to be limiting. The ports 1 and 2 (circled numbers) are the inputs and outputs of the waveguide (TE mode) and the ports 3 and 4 (circled numbers) are the outputs of the stripline (TEM mode). 
     Varying coupling values are obtained when modifying the width and the depth of the conducting path 24. Optimal adjustment and directionality are implemented by means of the capcitors or inductors. 
     When the line is loaded by only two capacitors 22, 23 as illustrated in FIG. 3, a directionality exceeding 20 dB is obtained in a 13% frequency band, namely from 2800 MHz to 3200 MHz. The coupling coefficients may vary from 6 dB to 30 dB depending on the width of the track 24. This solution therefore offers the advantage of achieving compact couplers with a wide frequency band and eliminates any connection problems. 
     Also, by controlling the capacitances, it is possible to further shorten the coupler length, however at the expense of the frequency band. 
     FIG. 6 shows an important application of an embodiment of the coupler of the invention in designing radiation modules by combining a coupler 27 of the type described (FIG. 3), a matched load 28 and a radiating dipole 29. The coupler is an integral part of the transmitting stripline. The radiating element, also designed as a stripline, is without auxiliary connecting means. This coupler also can be used to control power in waveguide apparatus by fashioning a slight longitudinal slot at the center of the large side of the waveguide (where this slot does not radiate and causes no perturbation). 
     To that end, it suffices to plunge the central conductor of the coupler of the invention into the waveguide which is to be controlled through a longitudinal slot, which neither radiates nor perturbs because the slot is made in the center of the large side of the waveguide, and to connect the central conductor to a milliwatt-meter. 
     FIG. 7 illustrates another application, namely an array of plane antenna power distribution plates, where the radiating elements are made of striplines. The distribution array consists of a waveguide 30 on which are associated in series several couplers 31, 32, 33 . . . . The input 1 (circled number) of the distributor can tolerate high powers, and the outputs S1, S2 . . . are independent and mutually decoupled. 
     As it is possible to specify the coupling coefficient of each coupler within the range of 6 to 30 dB, it is also possible to achieve any arbitrarily desired radiated energy distribution. 
     Additional advantages and modifications will readily occur to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicants&#39; general inventive concept.