Abstract:
A waveguide having a pair of quarter-wavelength impedance transformers and adapted to divide and combine microwaves. The pair of quarter-wavelength impedance transformers includes two strip transmission lines connected together at one end, two ground conductors each located adjacent to and spaced from a respective one of said strip transmission lines, each said ground conductor serving as a ground potential at least mutually to the adjacent strip transmission line, and a dielectric substrate supporting thereon said strip transmission lines and said ground conductors. The two strip transmission lines may be connected to each other at numerous positions by resistors.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a microwave dividing and combining waveguide to be used in, for example, microwave communications. 
     2. Description of the Related Art 
     In microwave communications, various types of microwave dividing and combining waveguides are currently known. 
     FIG. 8 of the accompanying drawings shows a typical conventional waveguide. The conventional waveguide generally comprises a common port 90, a pair of distribution ports 91a, 91b, and a pair of quarter-wavelength impedance transformers 92a, 92b located between the distribution ports 91a, 91b. 
     The microwaves inputted from the common port 90 are divided by the two quarter-wavelength impedance transformers 92a, 92b and are outputted from the two distribution ports 91a, 91b. Reversely, the microwaves of the phase inputted from the two distribution ports 91a, 91b are combined by the two quarter-wavelength transformers 92a, 92b and are outputted from the common port 90. 
     This type of waveguide is disclosed in S. B. Cohn &#34;A class of Broad Band Three-part TEM-Mode Hybrid&#34; IEEE Transmission Microwave Theory Technology, MIT-16, No. 2 Feb. 1968), p.p. 110-116. 
     Recently a microstrip line has been developed to reduce the entire waveguide in size and widely utilized in microwave area. 
     In this microstrip line, as shown in FIG. 9, a strip conductor 96a is located on one surface of a dielectric substrate 95a, and a ground conductor 97a is located on the other surface of the dielectric substrate 95a. 
     Since linearly extending quarter-wavelength impedance transformers are used, the first-named conventional waveguide is difficult to reduce in size. 
     Also in the second-named waveguide in the form of a microstrip line, the width of the strip conductor 96a is determined from the dielectric constant and thickness of the dielectric substrate 95a; since the degree of freedom in designing is relatively small, there is a restriction in reducing the size of the waveguide. 
     In some of those types of conventional waveguides, resistors (isolation resistors) are provided to adjust the phase difference produced in the two impedance transformers. For example, as shown in FIG. 8, a plurality of isolation resistors 98 are located at a number of positions between the two strip conductors 92a, 92b. These isolation resistors 98 connect the two strip conductors 92a, 92b with each other at positions thereof to absorb the microwaves which are different in phase from each other. 
     However, if isolation resistors were used in the waveguide in the form of a microstrip line, a divided capacity would have occurred between the isolation resistors and the ground conductors of the microstrip line. Because of this divided capacity, the characteristic impedance of the individual impedance transformer will not reach 50Ω and will thus adversely affect the voltage standing wave ratio characteristics. 
     To this end, a solution has been proposed, as disclosed in Japanese Patent Laid-Open Publication No. Sho 63-246002, in which the capacitive component such as a condenser is located at a branch point of one end of the line serving as the impedance transformer. This construction, however, requires more circuit elements, making the line structure complex. 
     SUMMARY OF THE INVENTION 
     A first object of this invention is to provide a waveguide having an adequate degree of freedom in designing impedance transformers which is hence easy to reduce in size. 
     A second object of the invention is to provide a waveguide in which isolation resistors can be provided without causing any divided capacity, thus guaranteeing excellent voltage standing wave ratio characteristics. 
     The first object can be accomplished by a waveguide for dividing and combining microwaves, comprising a pair of quarter-wavelength impedance transformers, the quarter-wavelength impedance transformers including: two strip transmission lines connected together at one end; two ground conductors each located adjacent to and spaced from a respective one of the strip transmission lines, each the ground conductor serving as a ground potential at least mutually to the adjacent strip transmission line; and a dielectric substrate supporting thereon the strip transmission lines and the ground conductors. 
     The second object can be accomplished by a waveguide for dividing and combining microwaves, comprising a pair of quarter-wavelength impedance transformers, the quarter-wavelength impedance transformers including: two strip transmission lines connected together at one end; two ground conductors each located adjacent to and spaced from a respective one of the strip transmission lines, each the ground conductor serving as a ground potential at least mutually to the adjacent strip transmission line; a dielectric substrate supporting thereon the strip transmission lines and the ground conductors; and a plurality of resistors connecting the two strip transmission lines with each other at a plurality of positions. 
     In each of these arrangements, the two strip transmission lines may be located in a predetermined region on the dielectric substrate, with portions other than the connected ends being spaced from each other. Each the strip transmission line may be a conductive strip having a path longer than the distance between its opposite ends. At least a part of each the strip transmission line may be a meandering conductive strip. 
     The two strip transmission lines may be located on the dielectric substrate in mirror-image symmetry. 
     The ground conductors corresponding to the two strip transmission lines may be located outside the predetermined region on the dielectric substrate. Each the ground conductor may define a substantially half-plane. 
     The resistors may be located one at each position where the two meandering conductive strips are adjacent to each other. Each resistor may have a resistance progressively increasing from one end, where the two strip transmission lines are connected to each other, to the other end. 
     With the arrangement of this invention, since the two strip transmission lines and the two ground conductors are located on a common surface of the dielectric substrate, low-impedance lines can be each in the form of a narrow conductive strip. Therefore an adequate degree of freedom in designing the impedance transformers can be realized so that the strip transmission lines can be curved into a meandering form. As a result, it is possible to locate long impedance transformers on a relatively small substrate so that the waveguide can be reduced in size. 
     According to this invention, both the two strip transmission lines and the two ground conductors are located on a common surface of the dielectric substrate, namely, no ground conductors are located on the other surface of the dielectric substrate, in which case it is possible to prevent any occurrence of divided capacitance if the waveguide is provided with isolation resistors. Therefore the waveguide is free of any lag of impedance characteristics due to this divided capacity, guaranteeing good voltage standing wave ratio characteristics. 
     In this waveguide, when a microwave is inputted to a common connected one end of the two strip transmission lines, divided microwaves are outputted from the respective other ends, which are unconnected, of the two strip transmission lines. Reversely, when microwaves are inputted from the respective unconnected ends of the two strip transmission lines, a combined microwave is obtained from the common connected end of the two strip transmission lines. 
     The larger the area of the isolation resistors, the more the divided capacity will become influential. Therefore this invention is particularly useful when applied to a large-power waveguide that requires a large area to be occupied by isolation resistors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view showing a waveguide embodying this invention; 
     FIG. 2 is a cross-sectional view taken along line A--A of FIG. 1; 
     FIGS. 3A and 3B are graphs showing permeation characteristics and reflection characteristics, respectively; 
     FIGS. 4A and 4B are graphs showing isolation characteristics and reflection characteristics, respectively, at distribution ports; 
     FIG. 5 is a plan view showing a modified waveguide according to another embodiment of the invention; 
     FIG. 6 is a cross-sectional view taken along line C--C of FIG. 5; 
     FIG. 7 is a perspective view showing the basic structure of a non-symmetrical double-strip coplanar waveguide; 
     FIG. 8 is a diagram showing a conventional waveguide; and 
     FIG. 9 is a perspective view showing the basic structure of a conventional microstrip line. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the specification, the terms &#34;right&#34;, &#34;left&#34;, &#34;upper&#34; and &#34;lower&#34; mean respective particular directions in the accompanying drawings. 
     The principles of this invention are particularly useful when applied to a microwave dividing and combining waveguide such as is shown in FIGS. 1 and 2. 
     This waveguide comprises a non-symmetrical double-strip coplanar waveguide, whose basic structure is shown in FIG. 7. 
     The non-symmetrical double-strip coplanar waveguide, unlike an ordinary coplanar waveguide, comprises a dielectric substrate 9, a strip transmission line in the form of a strip of conductor 4 located on one surface of the dielectric substrate 9, and a ground conductor 5 located on the same surface of the dielectric substrate 9 along one edge of the strip conductor 4 with a space S therebetween. It is unnecessary for the width of the strip conductor 4 to equal that of the ground conductor 5. 
     Generally, double-strip coplanar waveguides are disclosed in R. K. Hoffman, &#34;handbook of Microwave Integrated Circuits&#34;, Artech House. 
     In the waveguide, as shown in FIGS. 1 and 2, a common port 1, a pair of distribution ports 2a, 2b, a pair of strip conductors 4a, 4b, a pair of first ground conductors 5a, 5b and a second conductor 6 are formed on one surface of the dielectric substrate 9 by sputtering gold. Between the distribution ports 2a, 2b, isolation resistors 10 are formed in a predetermined pattern by sputtering tantalum nitride. 
     The common port 1 is located the left end of the dielectric substrate 9, and to this common port 1 a microwave to be divided by a pair of quarter-wavelength impedance transformers 7a, 7b is inputted. Reversely, from this common port 1 a microwave combined by the two quarter-wavelength impedance transformers 7a, 7b is outputted. The thus divided microwaves are outputted from the distribution ports 2a, 2b located at the upper right end and the lower right end, respectively, of the dielectric substrate 9. Reversely, microwaves to be combined by the two quarter-wavelength impedance transformers 7a, 7b are inputted from the two distribution ports 2a, 2b. 
     Each quarter-wavelength impedance transformer 7a, 7b includes a strip transmission line in the form of strip conductors 4a, 4b located between the common port 1 and the distribution port 2, ground conductors 5a, 5b to be a relative ground potential to the respective adjacent strip conductors 4a, 4b, the isolation resistors 10, and the dielectric substrate 9, having the functions of dividing and combining microwaves. In this embodiment, division takes place uniformly; however, this invention should be no means be limited to such an example. 
     The two strip conductors 4a, 4b are joined at their one ends with the common port 1 in the horizontal center line of the dielectric substrate 9. The portions, of the strip conductors 4a, 4b, other than the connected ends are located in a predetermined region on the dielectric substrate 9 and are spaced apart from each other. One strip conductor 4a extends rightwardly meandering in the upper half region of the surface of the dielectric substrate 9 and is connected to the distribution port 2a. Likewise the other strip conductor 4b extends rightwardly meandering in the lower half region of the surface of the dielectric substrate 9 and is connected to the distribution port 2b. The winding path of meander of each strip conductor 4a, 4b is set up in such a manner that a single section of the displacement is equal to a quarter-wave length. 
     The isolation resistors 10, three in total, are located at respective positions where the two strip conductors 4a, 4b are adjacent to each other, namely, one between each pair of confronting turnovers of the two winding paths, i.e., one at every quarter wavelength. 
     The space S1 between the strip conductor 4a and the first ground conductor 5a and the space S2 between the strip conductor 4b and the first ground conductor 5b are respectively convergent toward the respective distribution ports 2a, 2b. The isolation resistors 10 are different in size, becoming smaller from one another toward the distribution ports 2a, 2b; this is, the farther the resistor 10 is located from the common port 1, the higher the resistance value. 
     The curvature, shape and line width of the strip conductors 4a, 4b as well as the shape, number and size of the isolation resistors 10 should by no means be limited to those of the illustrated examples and may be changed depending on the purpose, use, etc. of the individual elements. 
     Each first ground conductor 5a, 5b is located outside the region where the strip conductor 4a, 4b is located on the dielectric substrate 9, electrically defining a substantially half-plane. The half-plane may be such that the first ground conductor 5a, 5b extends from the portion adjacent to the corresponding to the strip conductor 4a, 4b to an infinite point and is large enough to treat electrically. 
     Specifically, as shown in FIG. 1, the first ground conductor 5a is located in the region on the upperside of the strip conductor 4a, and the first ground conductor 5b is located in the region on the lowerside of the strip conductor 4b. Thus there is no ground conductor between the two strip conductors 4a, 4b. 
     The second ground conductor 6 is located in the region to the right of the distribution ports 2a, 2b. 
     The dielectric substrate 9 comprises alumina and should by no means be limited to this specific example. 
     The distance between the common port 1 and the distribution ports 2a, 2b is about 4 mm; the strip conductors 4a, 4b meander and hence have a path length which is longer than this distance. 
     When a microwave is inputted from the common port 1, this microwave is equally divided by the two quarter-wavelength impedance transformers 7a, 7b, and the divided microwaves are outputted from the respective distribution ports 2a, 2b. 
     Reversely, when microwaves are inputted from the two distribution ports 2a, 2b, these microwaves are combined by the two quarter-wavelength impedance transformers 7a, 7b and the combined microwave is outputted from the common port 1. 
     FIGS. 3A, 3B, 4A and 4B show the distribution characteristics and the synthesis characteristics measured when a microwave was inputted from the common port 1. 
     Specifically, FIGS. 3A and 3B show microwave permeation characteristics and microwave reflection characteristics, respectively, the x axis representing the frequency of an inputted microwave. 
     As shown in FIG. 3A, the permeation characteristics S21 from the common port 1 to the distribution port 2a was substantially identical with the permeation characteristics S31 from the common port 1 to the distribution port 2b along the entire range of measured frequencies (6 GHz to 20.5 GHz), from which it is noted that the microwave was uniformly divided. Regarding the permeation characteristics, attenuation was -3.7 dB at 6 GHz (indicated by P in FIG. 3A) and transmission loss was 0.7 dB. At 18 GHz (indicated by Q in FIG. 3A), attenuation was -3.9 dB and transmission loss was -1.0 dB, which are adequately practical values. In the range of 6 GHz to 18 GHz, these values were substantially constant. 
     As shown in FIG. 3B, when a microwave was inputted from the common port 1, the reflection characteristic S11 was over -13 dB, which is adequately small, in the range of 6 GHz to 18 GHz. 
     FIGS. 4A and 4B show the isolation characteristics and the reflection loss measured when microwaves were inputted from the distribution ports 2a, 2b. 
     The isolation characteristic S23, which shows the amount of leaked microwave from the distribution port 2a to the distribution port 2b, was below -21 dB over the entire range of 6 GHz (indicated by P) to 18 GHz (indicated by Q), thus causing a remarkable isolation effect. 
     FIG. 4B shows the reflection characteristics measured when microwaves were inputted from the distribution ports 2a, 2b, the x axis representing frequencies. 
     As shown in FIG. 4B, the reflection characteristic S22 at the distribution port 2a was substantially identical with the reflection characteristic S33 at the distribution port 2b. Over the entire range of 6 GHz to 18 GHz, reflection loss was below -15 dB so that microwaves can be combined, thus causing an adequate reflection characteristics. 
     With the waveguide of this embodiment, since a nonsymmetrical double-strip coplanar waveguide is used, it is possible to reduce the line width of the strip conductors, thus minimizing the area of the surface of the dielectric substrate occupied by the strip transmission lines. Also, by reducing the line width of the strip conductors, the meandering portion of the strip conductors can be formed without deteriorating the voltage standing wave ratio characteristics. Therefore, it is possible to increase the degree of freedom in designing the line length, line width, line curvature, etc. of the individual strip conductor. 
     Further, since the ground conductors are located outside the region in which the strip conductors are located, it is possible to locate the isolation resistors at optional positions in the region between the two strip conductors. There is no ground conductor on the lower surface of the dielectric substrate; that is, there exists no conductor on the lower surface of the dielectric substrate at positions where the isolation resistors are located, thus causing virtually no occurrence of divided capacitance in the isolation resistors. As a result, it is possible to obtain the target value, e.g. 50Ω, of the impedance transformer, guaranteeing a good voltage standing wave ratio characteristics. 
     According to this embodiment, it is possible to increase the line length to be formed within a unit area, without deteriorating the voltage standing wave ratio characteristics, thus realizing a physically smaller waveguide. 
     Practically, in order to construct a waveguide the same in performance as the illustrated embodiment by using microstrip lines, the distance between the common port and the distribution ports should be about 11 mm. Whereas in this embodiment it requires only less than half of that, i.e. about 4 mm. 
     FIGS. 5 and 6 show a modified waveguide according to the second embodiment of this invention. 
     The waveguide of the second embodiment is identical in basic structure with that of the first embodiment; therefore any repetition of description is avoided here for clarity. 
     The two strip conductors 4a, 4b are connected to each other by a total of six isolation resistors 10 at respective positions where the two strip conductors 4a, 4b are adjacent to each other, namely, one resistor between each pair of confronting turnovers of the two winding paths, i.e., one at every quarter wavelength. 
     The space S1 between the strip conductor 4a and the first ground conductor 5a and the space S2 between the strip conductor 4b and the first ground conductor 5b are respectively convergent toward the respective distribution ports 2a, 2b, and each strip conductor 4a, 4b has a varying line width increasing progressively toward the respective distribution port 2a, 2b. 
     In operation, when a microwave is inputted from the common port 1, this microwave is uniformly divided by the two quarter-wavelength impedance transformers 7a, 7b and the divided microwaves are outputted from the respective distribution ports 2a, 2b. 
     Reversely, when microwaves are inputted respectively from the two distribution ports 2a, 2b, these microwaves are combined by the two quarter-wavelength impedance transformers 7a, 7b and the combined microwave is outputted from the common port 1. 
     The waveguide of this embodiment also has practically adequate characteristics. 
     This waveguide can produce the same result as that of the first embodiment. It is possible to increase the line length to be formed in a unit area, thus realizing a physically smaller waveguide.