Hybrid Coupler Having Interlaced Coupling Conductors

This invention relates generally to radio frequency circuits and more particularly to radio frequency (r.f.) hybrid couplers which combine or divide signals fed thereto among different ports.

As is known in the art, it is often desirable to combine a pair of r.f. signals originating from two devices and deliver such combined r.f. signal to a third device, or alternatively, to split an input r.f. signal from one device and deliver such split components of such r.f. signal to two output devices. One class of couplers includes couplers having radio frequency transmission lines formed on a substrate. In general, one component of such signal is directly coupled between one of a pair of ports and an output port, and a second one of such signals is electromagnetically coupled between a second one of such pair of ports and the output port. An approach used in the prior art to electromagnetically couple a component of such signals has been to couple an electromagnetic field between the edges of a pair of planar, dielectrically spaced strip conductors adjacently formed on a common substrate with end portions of each one of the strip conductors providing a port connection for the coupler. With this prior art approach, the strength of coupling is related to the total area of the edge and the separation between the edges of the strip conductors. As it is also known in the art, the coupler generally provides a characteristic impedance which is compatible with the circuit application of the coupler. Since coupling strength in part is related to total edge area, generally, the total edge area is increased to provide for an increase in coupling strength. One approach used in the prior art to increase the total edge area involves the technique of interdigitating a plurality of narrow strip conductors to thereby increase the total edge area and hence the coupling strength. When designing such couplers, particular attention is given to the characteristic impedance of the coupler since the coupler should provide a characteristic impedance which is compatible with the devices to which it is connected. As is also known in the art, the characteristic impedance of a transmission line, such as a microstrip transmission line, is related to substrate thickness, dielectric constant, and conductor width. Thus, the width, spacing and number of such interdigitated strip conductors are generally selected to provide the coupler with the desired coupling factor and predetermined characteristic impedance. That is, the width and number of such narrow strip conductors and their spacing are generally selected to be sufficiently narrow to provide a coupler with a desired coupling factor, and the width and number of such narrow strip conductors are likewise chosen to provide the coupler with the predetermined characteristic impedance. One problem associated with such a structure is that, as increased coupling strength is required, the conductor widths and spacing therebetween decrease providing a difficult circuit to fabricate with acceptable yields in order to achieve the desired coupling strength and to maintain the predetermined characteristic impedance. Also, as the conductor width decreases, conductor resistivity increases and hence conductor losses and therefor coupler insertion loss increases.

In accordance with the present invention, a radio frequency circuit for coupling an r.f. signal between an input port and a pair of output ports with any reflected power being coupled to an isolated port, includes a pair of strip conductors dielectrically spaced from a ground plane conductor. Each strip conductor has portions disposed in two different planes. Each end of each dielectrically spaced strip conductor provides a corresponding one of such ports. With such an arrangement, a coupler is provided for coupling signals between an input port and a pair of output ports by directly feeding a first component of such signal from the input port to a first one of the pair of ports, and by electromagnetically coupling a second component of such signal between adjacent top and bottom surfaces of such transmission lines coupled between the input port and a second one of such pair of ports. By electromagnetically coupling a signal between adjacent top and bottom surfaces of such transmission lines, an increase in the coupling surface area is provided, providing increased coupling strength without the increase in insertion loss generally associated with prior art structures. Further, such a structure is easier to fabricate than the closely spaced thin conductors generally associated with interdigitated couplers.

In accordance with one embodiment of the invention, the coupler includes a first plurality of successively spaced strip conductor portions disposed a first predetermined distance from a ground plane conductor, and a second like plurality of strip conductor portions disposed a second different predetermined distance from the ground plane conductor dielectrically spaced from the first plurality of strip conductor portions. A surface of each one of the first strip conductor portions is electromagnetically coupled to a surface of a corresponding one of the second strip conductor portions. Each intermediate one of the first and second strip conductor portions is connected to a pair of strip conductor portions flanking the strip conductor portion electromagnetically coupled thereto, to provide a pair of dielectrically separated interlaced strip conductors. With such an arrangement, a symmetric coupler is provided since each strip conductor may be disposed at the same average distance from the ground plane conductor by disposing portions of each strip conductor at a first distance and second portions of each strip conductor at a second distance. Thus, each strip conductor in combination with the dielectric and the ground plane provides a pair of transmission lines having substantially the same electromagnetic characteristics. Further, since a surface of each one of the strip conductor portions is electromagnetically coupled to a surface of a second one of the strip conductor portions, the coupling therebetween is stronger than prior art techniques.

In accordance with an alternate embodiment of the present invention, a coupler circuit includes a first pair of dielectrically spaced planar strip conductors and a second pair of dielectrically spaced planar strip conductors, dielectrically spaced in a different plane from the first pair, and with each strip conductor of the second pair aligned over a corresponding strip conductor of the first pair and electromagnetically coupled thereto. Each strip conductor of the first pair is alternatively connected to the one strip conductor of such dielectrically spaced conductors of the second pair not electromagnetically coupled thereto, at a plurality of locations along the length of such lines. With such an arrangement, a coupler having a high degree of coupling strength is provided. Further, the propagation of a signal through the transmission lines provided in combination with each of such strip conductor lines is relatively uniform since a first portion of the signal will propagate between the input port and each one of the pair of output ports along a first transmission line having a strip conductor formed on the substrate, and a second portion of the signal will propagate along a second transmission line having a strip conductor formed on the dielectric layer. Further, such connections of diagonally spaced non-electromagnetically coupled ones of such strip conductors of the transmission line will provide equal potential excitation of energy propagating along the transmission lines.

The foregoing features of this invention, as well as the invention itself, may be more fully understood from the detailed description read together with the accompanying drawings, in which:

FIG. 1 is a block diagram of a double balanced amplifier using hybrid couplers in accordance with the invention;

FIGS. 2-4 are a series of plan views showing steps in the construction of a radio frequency circuit in accordance with the invention;

FIG. 3A is a cross-sectional view of FIG. 3 taken along line 3A--3A;

FIG. 3B is a cross-sectional view of FIG. 3 taken along line 3B--3B showing masking steps used to provide an air bridge;

FIG. 4A is a cross-sectional view of FIG. 4 taken along line 4A--4A showing in cross section a first one of a pair of twisted strip conductors;

FIG. 4B is a cross-sectional view of FIG. 4 taken along line 4B--4B showing in cross section a second one of a pair of twisted strip conductors;

FIG. 5 is a plan view taken along line 5--5 of the circuit shown in FIG. 4;

FIG. 5A is an isometric, partially broken away view taken along line 5A--5A of the circuit shown in FIG. 4;

FIG. 5B is a diagrammatical, isometric view of FIG. 5A;

FIGS. 6-8 are a series of plan views showing steps in the construction of an alternate embodiment of the invention; and

FIGS. 6A, 6B and 7A, 7B are cross-sectional views taken along lines 6A--6A, 6B--6B, 7A--7A and 7B--7B of FIGS. 6 and 7 respectively;

FIGS. 8A-8D are cross-sectional views taken along lines 8A--8A, 8B--8B, 8C--8C and 8D--8D of FIG. 8 showing certain details of construction.

A hybrid quadrature coupler 10 (FIG. 3) for coupling a radio frequency (r.f.) signal between an input port and a pair of output ports with an isolated port being provided for any reflected r.f. signal from such ports will initially be described in conjunction with FIGS. 1-4. Referring first to FIG. 1, a double balanced amplifier 70 is shown to include a first hybrid coupler 10' here configured as a signal divider, a pair of conventional match amplifiers 72, 74, and the second hybrid coupler 10 here configured as a signal combiner connected together, as shown.

Referring now to FIG. 2, a plurality of segmented strip conductor portions 14a-14g are shown formed on a first surface of a dielectric substrate 12 here semi-insulating gallium arsenide (GaAs) having an initial thickness of 15 mils. Such strip conductor portions 14a-14g, (here a conventional metallization system including a first layer of titanium and a second layer of gold) are formed using conventional photolithographic masking and metal evaporating techniques. The strip conductor portions 14a-14g are here evaporated to a thickness of approximately 1 .mu.m and have a width w of here 50 .mu.m. The dielectric substrate 12 has formed on a second surface opposite such first surface a ground plane conductor 16. The ground plane conductor 16 is formed on the substrate 12 after the substrate 12 is thinned to a predetermined thickness, here 4 mils. Each one of such strip conductor portions 14a-14g here an odd number of segments are spaced from one another by a distance d here approximately 15 .mu.m. Each strip conductor portion 14a-14g is here approximately shaped as a parallelogram having an acute vertex angle .theta. between a slanted side 14b' of the segment 14b for example, and a horizontal side 14b" thereof which is selected to be in the range of 0.degree. to 90.degree.. Here the acute vertex angle .theta. is chosen to be approximately 15.degree.. A pair of strip conductor portions 15b and 15d are formed on the substrate 12 adjacent strip conductor portions 14a and 14g, respectively, as shown. Such strip conductor portions 15b, 15d are here used to provide a conductive contact for strip conductors (FIG. 4) to two of such aforementioned ports, here ports B and D (FIG. 4). The length of each horizontal side of such strip conductor portions 14a-14g, the number of such portions 14a-14g and the spacing (d) therebetween are chosen to provide in combination a length here substantially equal to a quarter wavelength .lambda./4 where .lambda. is the wavelength of the corresponding centerband operating frequency of the circuit.

Referring now to FIGS. 3, 3A and 3B, a first masking layer 20, here of photoresist is provided over the strip conductor portions 14a-14g and substrate 12, as shown. Using conventional masking and etching techniques, a plurality of here triangularly shaped apertures 22a to 22h and 22a' to 22h' are formed in such masking layer 20, aligned with and exposing selective underlying portions of the strip conductor portions 14a-14g and portions of the strip conductor 15b, 15d. Apertures 22a-22h, 22a'-22h' are here provided to form plating holes through the masking layer 20 to selectively interconnect the strip conductor portions 14a-14g in a manner to be described. A second plurality of apertures 25a-25d are provided in the masking layer 20, exposing selective underlying portions of the substrate 12 and underlying portions of the strip conductors 15b, 15d. As shown in FIG. 3B, a portion of layer 20 is provided over strip conductor 15b so that when a strip conductor is formed in aperture 25a, said strip conductor will bridge strip conductor 15b. Apertures 25a-25d are formed in the masking layer 20 to define an area where strip conductors for ports A-D of the coupler will be provided, in a manner to be described in conjunction with FIG. 4. For example, apertures 25b and 25d which selectively expose a portion of the substrate 12 and the strip conductor portion 15b, 15d (FIG. 3A) respectively provide areas where the port B, and port D strip conductors (FIG. 4) are formed to couple such strip conductors (FIG. 4) to segments 14b, 14f (FIG. 3A). Upon masking layer 20 is provided a layer 26a of here evaporated titanium 600 A thick and a layer 26b of evaporated gold 2000 A thick forming in combination a composite layer 26. A second layer 20' of photoresist is deposited on composite layer 26 and is patterned in the same areas as the first layer 20 of photoresist, and is patterned to provide a third plurality of apertures 27a-27i in masking layer 20' for plating strip conductor portions now to be described.

Referring now to FIGS. 4, 4A and 4B, a second like plurality of successively spaced strip conductor portions 30a-30g is formed in the masking layer 20, through apertures 27a-27g (FIG. 3) and plated on composite layer 26 (not shown) to a thickness of 3 .mu.m. Strip conductor portions 30a-30g are formed dielectrically spaced from and in alignment with strip conductor portions 14a-14g, such that each one of such first strip conductor portions 14a-14g is electromagnetically coupled to a corresponding one of such second plurality of strip conductor portions 30a-30g. Further, strip conductor portions 30a-30g are formed in a criss-cross relation with strip conductor portions 14a-14g (FIG. 2), as shown. Strip conductor portions 30a-30g here are shaped as parallelograms having acute vertex angles of .theta..about.15.degree., as previously described. Segments 30a-30g are also formed in masking layer 20 aligned with apertures 22a-22h, 22a'-22h' so that when formed the strip conductor portions 30a-30g are selectively connected with selected strip conductor portions 14a-14g formed under the masking layer 20. Thus, selective interconnection of portions 30a-30g with corresponding ones of segments 14a-14g provide in combination a pair of interlaced, twisted or interwoven strip conductors 38, 39. Such transmission lines 38, 39 are formed by interconnecting such strip conductor portions 14a-14g and 30a-30g so that strip conductor portions 30a-30g provide air bridges over selected ones of strip conductor portions 14a-14g. Such air bridges or overlays are provided by plating such top strip conductor portions 30a-30g in the apertures 22a-22h, 22a'-22h' (FIG. 3). Each intermediate one of each first and second plurality of strip conductor portions 30a-30g is connected by the air bridges formed therefrom to a pair of strip conductor portions 14a-14g flanking the one of the strip conductor portions 14a-14g electromagnetically coupled thereto.

Therefore, transmission line 38 shown in cross section in FIG. 4A here includes the ground plane 16, substrate 12 and a composite strip conductor 38' denoted by arrow 38'. Composite strip conductor 38' includes strip conductor portions 31b and 15b connected together, as shown. The composite strip conductor 38' further includes strip conductor portion 15b connected to strip conductor portion 30a which dielectrically bridges strip conductor portion 14a, as shown. The composite strip conductor 38' further includes strip conductor portion 14b connected between strip conductor portion 30c and 30a with strip conductor portion 30c dielectrically bridging strip conductor 14c. The composite strip conductor 38' further includes strip conductor 14d connected between strip conductor portion 30c and strip conductor portion 30e with strip conductor portion 30e dielectrically bridging strip conductor portion 14e, strip conductor portion 14f connected between strip conductor portion 30e and strip conductor portion 30g with strip conductor portion 30g dielectrically bridging strip conductor portion 14g, and strip conductor portions 15d and 31d connected together, as shown.

Further, transmission line 39 shown in cross section in FIG. 4B here includes the ground plane 16, substrate 12 and a composite strip conductor 39' denoted by arrow 39'. Composite strip conductor 39' includes strip conductor portions 31a and 14a, connected together, as shown. The composite strip conductor 39' further includes strip conductor portion 14a connected to strip conductor portion 30b which electrically bridges strip conductor portion 14b, as shown. The composite strip conductor 39' further includes strip conductor portion 14c connected between strip conductor portion 30b and strip conductor portion 30d with strip conductor portion 30d dielectrically bridging strip conductor 14d. The composite strip conductor 39' further includes strip conductor portion 14e connected between strip conductor portion 30d and strip conductor portion 30f with strip conductor portion 30f dielectrically bridging strip conductor portion 14f, and strip conductor portion 14g connected between strip conductor portion 30f and strip conductor portion 31c, with strip conductor portion 31c dielectrically bridging strip conductor portion 15d.

Unlike prior art structures, where a relatively small edge area of a strip conductor is used to couple radio frequency energy to a second strip conductor along an adjacent edge thereof, here the relatively wide top and bottom surfaces W of the strip conductor portions 14a-14g, 30a-30g are used to couple energy between composite strip conductors 38', 39'. Since the coupling between top and bottom surfaces is stronger than the conventional edge coupling technique, a coupler may be fabricated having relatively wider strip conductors than the strip conductors used in interdigitated couplers, and thus have reduced insertion loss.

The transmission lines are here formed in a "twisted", cork screw, or interlaced configuration in order to provide a symmetric coupler wherein each transmission line is provided with substantially the same characteristic impedance. Thus, each composite strip conductor 38', 39' has portions formed in one of two planes. Since, as shown in FIGS. 4A, 4B, each bottom strip conductor portion 14a-14g is spaced a distance S from the ground plane 16, and each top strip conductor portion is spaced a distance S' from the ground plane 16, each composite strip conductor 38', 39' is spaced an average distance S.sub.a from the ground plane 16. Thus, each composite strip conductor 38', 39' provides in combination with the substrate 12 and ground plane 16 a pair of transmission lines having substantially the same electromagnetic characteristics. Thus, the coupler is here a symmetric coupler since each one of such lines has substantially the same electrical characteristics.

Referring now to FIGS. 1, 5, 5A, 5B, the coupling circuit 10 can be used to combine a pair of radio frequency (r.f.) signals fed from here a pair of amplifiers 72, 74 to a pair of ports B, C of the coupler 10 (FIG. 1) and deliver such r.f. signals to an output port A of the coupler 10 (FIG. 1) to a third amplifier (not shown) with the combined components of such r.f. signal being 90.degree. out of phase with respect to each other. When used as a combiner 10, a pair of r.f. signals are fed to input ports B and C with the combined r.f. signal from such ports being fed to an output port here port A, and with no r.f. signal being fed to port D. When used as a combiner, the r.f. signal incident on ports B and C is coupled to port A, as follows: an r.f. signal fed to port C is coupled directly to port A since they are directly connected together, via braided transmission line 39 (FIG. 4) and such signal is shifted in phase by -90.degree. since the length of such braided transmission line is chosen to have a length substantially equal to a quarter of a wavelength (.lambda./4) where .lambda. is the corresponding wavelength of the midband frequency of the r.f. signal to be coupled. An r.f. signal fed to port B is electromagnetically coupled to port A at air bridge portions of strip conductor portion 30a-30g (regions where the braided transmission lines 38, 39 cross each other), as diagrammatically shown in FIG. 5B. At such air bridge or crossover portions of strip conductor portion 30a-30g, the electromagnetic wave propagating down twisted transmission line 38 from port B, towards the isolated port D, will couple onto the twisted transmission line 39 and propagate on transmission line 39 in a direction opposite from the propagation direction on twisted transmission line 38, and such energy thus will be shifted in phase by -180.degree.. Hence the coupled energy from port B will propagate toward port A with a phase shift of -180.degree.. Thus, the signal delivered at port A will be the vector combination of the signals fed from port B and port C and hence, the signals are combined at port A with a 90.degree. phase shift between the incident input signals. For equal incident signals on port B and port C, any reflected portion of the signals from port A will be coupled to port D, and no reflected portion of the signal will propagate towards port B or port C. As shown in FIG. 1, port D is terminated in an impedance equal to the characteristic impedance of the transmission lines 38, 39, here 50 ohms. Alternatively, the microwave circuit can be used as a signal divider 10' (FIG. 1) when a signal is fed to port A, for example, such signal is divided between port B and port C in a similar manner as explained above. In such a situation, the signal incident at port A is divided between ports B and C in quadrature, that is, such components of signals at ports B and C are shifted in phase by 90.degree..

In accordance with the invention, the strength of the coupling of the electromagnetic energy propagating on one of such twisted transmission lines and coupled to the second one of such twisted transmission lines 38, 39 can be selected, by selectively varying the surface area of each segment 14a-14g, and 30a-30g to control the effective coupling surface area, or the portions of the surface areas of each of such conductors crossing over a corresponding one of such conductors, and by varying the vertex angle of each of the segments between 0.degree. and 90.degree. and thus varying the angle .phi. (FIG. 5) at which the segments 14a-14g, 30a-30g cross each other, with maximum coupling occuring at .phi. approaches 0.degree. and minimum coupling occuring at .phi. approaches 90.degree.. Further, by providing an odd number of segments, the microwave circuit 10 is configured such that the output port A is located on the same side of the coupler as the isolated port D.

Alternatively, the strip conductor portions 14a-14g may be spaced from the bridging strip conductor portions 30a-30g by a layer of a dielectric material such as silicon nitride, polyimide, or other suitable material. Further, a combination of air and dielectric material may be used to dielectrically space the strip conductor portions 14a-14g, 30a-30g to provide selected electromagnetic characteristics.

Alternatively, the output port A of a coupler may be provided on the same side of the coupler as one of the input ports, here port (C). In such a case, by providing an even number of segments, the output port A will be on the same side of the coupler 10 as input port C since such ports are here directly connected together by transmission line 39 and by adding an additional segment, for example, to each line, the composite or twisted strip conductors 38', 39' cross each other an additional time, changing the position of the terminal end portions of such lines on the substrate, and hence the location of ports D and B.

Referring now to FIGS. 6-8, an alternate embodiment of the invention is shown. Referring first to FIG. 6, a substrate 42 has formed on a first surface thereof a pair of here spaced parallel strip conductors 40a, 40b, and a ground plane 44 formed on a second surface of the substrate 42 opposite the first. Integrally formed with each strip conductor 40a, 40b are a plurality of bonding pads 46a-46g, respectively. Strip conductors 40a, 40b and bonding pads 46a-46g are patterned on the substrate 42 using conventional masking and evaporation techniques. Strip conductors 40a, 40b and bonding pads 46a-46g are here a composite layer of titanium and gold, with gold evaporated to a thickness of 1 .mu.m.

Referring now to FIG. 7, a dielectric layer 48, here of polyimide is deposited on the strip conductor surface of the substrate 42. Using conventional masking and etching techniques, the dielectric layer is patterned to provide a plurality of apertures 47a-47g in alignment with portions of corresponding ones of such bonding pads 46a-46g. Apertures 47a-47g are provided through the dielectric layer 48 to expose selective end portions of such bonding pads 46a-46g. A second plurality of apertures 49a-49d are provided through the dielectric layer 48. A layer 51a of titanium and a layer 51b of gold are deposited to form a composite layer 51 as previously described for composite layer 26 (FIG. 3). Apertures 49a-49d are here used for forming of strip conductors 52a-52d (FIG. 8) therein, such strip conductors 52a-52d being provided to interconnect the coupler (FIG. 8) to external components. Suffice it here to say, that, such apertures 47a-47g, 49a-49d provide plating holes for interconnection to such strip conductors 40a, 40b of a second pair of strip conductors.

Referring now to FIG. 8, a coupler 50 is shown to further include a second pair of spaced parallel strip conductors 40c, 40d formed on the dielectric layer 48 and here aligned with the corresponding strip conductors 40a, 40b previously formed on the substrate thereunder. Integrally formed with each strip conductor 40c, 40d is a plurality of bonding pads 52a-52g. Each bonding pad 52a-52g is formed aligned with a corresponding aperture 47a-47g which were previously formed in alignment with corresponding ones of the bonding pads 46a-46g integrally formed with strip conductors 40a, 40b. As shown in FIGS. 8A-8B, each strip conductor 40a-40d and associated bonding pad thereof is aligned such that diagonally spaced pairs 40a, 40d and 40b, 40c of such strip conductors 40a-40d are alternately connected together by plating the corresponding top bonding pad 52a-52g. through the corresponding aperture 47a-47g (FIG. 7) to connect to the portion of the corresponding bonding pad 46a-46g (FIG. 6) of the corresponding bottom strip conductors 40a, 40b exposed by such apertures 47a-47g. Thus, as shown in FIG. 8A, strip conductor 40b is shown coupled to strip conductor 40c, via plating hole 47b and bonding pads 46b, 52b. In a similar manner, the next interconnection of strip conductor lines 40a-40d as shown in FIG. 8B has strip conductor 40a coupled to strip conductor 40d, via plating hole 47c and bonding pads 46c, 52c. Successive ones of such pairs of diagonally spaced conductors 40a, 40d, 40b, 40c are interconnected in a similar manner.

Port strip conductors 54a-54d are where formed in apertures 49a-49b, respectively. Port strip conductor 54a is formed in aperture 49a (FIG. 7) which selectively exposes an end portion of strip conductor 40a and the substrate 42. Thus, port strip conductor 54a is plated through aperture 49a to provide a direct connection to strip conductor 40a. Also, port strip conductor 54a is connected with strip conductor 40d (FIG. 8C) by plating bonding pad 52a through aperture 47a to bonding pad 46a. Port strip conductor 54c is formed in aperture 49c (FIG. 7) which selectively exposes an end portion of strip conductor 40a and the substrate 42. Thus, port strip conductor 54c is plated through aperture 49c to provide a direct connection to strip conductor 40a. Also, port strip conductor 54c is integrally formed with strip conductor 40d. Thus, port strip conductors 54a and 54c and hence ports A and C are directly connected together, via strip conductors 40a and 40d.

In a similar manner, port strip conductor 54b is formed in aperture 49b (FIG. 7) which selectively exposes an end portion of strip conductor 40b and the substrate 42. Thus, port strip conductor 54b is plated through aperture 49b to provide a direct connection to strip conductor 40b. Also, port strip conductor 54b is integrally formed with strip conductor 40c and strip conductor 40c here bridges over an underlying portion of strip conductor 40a. Port strip conductor 54d is formed in aperture 49d (FIG. 7) which selectively exposes a portion of the substrate 42. Strip conductor portion 54d here bridges over underlying strip conductor 40a and is directly connected to strip conductor 40b by plating bonding pad 52g (FIG. 8D) through aperture 47g (FIG. 7) to bonding pad 46g (FIG. 6). Also, port strip conductor 54d is integrally formed with strip conductor 40c. Thus, port strip conductors 54b and 54d and hence ports B and D are directly connected together via strip conductors 40b and 40c.

In operation, a signal is coupled between port A, and ports B and C, for example, in a similar manner, as was described. Further, a symmetric structure is here provided by having a first portion of such signal propagate along a top conductor, here one of strip conductors 40c, 40d and a second, portion propagating along a bottom conductor, here one of strip conductors 40a, 40b. The alternate connection of diagonally spaced pairs of such strip conductors, as previously described, is provided to insure an equal potential excitation of the electromagnetic wave which propagates along such conductors in response to a signal fed to such lines. Further, such alternately coupled pairs suppress parasitic transmission modes since the effects of the different dielectric constants of the substrate 42, dielectric layer 48 and air are suppressed by periodically connecting such diagonally spaced lines together to provide a balanced configuration along such lines 40a-40d.

Having described preferred embodiments of the invention, it will now be apparent to one of skill in the art that other embodiments incorporating its concept may be used. It is felt, therefore, that this invention should not be restricted to the disclosed embodiments, but rather should be limited only by the spirit and scope of the appended claims.