US3835421A

US3835421A - Microwave transmission line and devices using multiple coplanar conductors

Info

Publication number: US3835421A
Application number: US00401554A
Authority: US
Inventors: Brecht R De; L Napoli
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1972-12-14
Filing date: 1973-09-27
Publication date: 1974-09-10
Anticipated expiration: 1991-09-10

Abstract

Three coplanar conductive surfaces on the top surface of a dielectric substrate form a microwave transmission line having first and second transmission modes used in the construction and operation of various microwave devices, such as amplifiers unbalanced-to balanced transmission line transformers and directional couplers.

Description

7 United States Patent [191 De Brecht et al.

[ Sept. 10, 1974 MICROWAVE TRANSMISSION LINE AND DEVICES USING MULTIPLE COPLANAR CONDUCTORS [75] Inventors: Robert Eugene De Brecht,

Cranbury; Louis Sebastian Napoli, Hamilton Square, both of NJ.

[73] Assignee: RCA Corporation, New York, NY.

[22] Filed: Sept. 27, 1973 211 App]. No.: 401,554

Related US. Application Data [62] Division of Ser. No. 315,087, Dec. 14, 1972.

[52] US. Cl. 333/26, 333/84 M [51] Int. Cl.- I-I0lp 5/10, H03h 7/42 [58] Field of Search 333/26, 84 M [56] References Cited OTHER PUBLICATIONS Nishide et al., Balance-to-Unbalance Transformers,

BALANCED TRANSMISSION UNE\\ OUTPUT TERMINALS Monogr. Res. Inst. Appl. Elec. (Japan), No. 18, (1970), pp. 69-73 relied on.

Primary ExaminerPaul L. Gensler Attorney, Agent, or Firm-Edward J. Norton; Joseph D. Lazar; Donald E. Mahoney [5 7] ABSTRACT Three coplanar conductive surfaces on the top surface of a dielectric substrate form a microwave transmission line having first and second transmission modes used in the construction and operation of various microwave devices, such as amplifiers unbalanced-to balanced transmission line transformers and directional couplers.

2 Claims, 8 Drawing Figures UNBALANCED TRANSMISSION LINE ERMINAL PAIENIEBSEPIOIQH v 3885.421 Y sum 1 BF 5' PRIOR ART PAIENIED E W 3.835.421

SHEEI 3 0F 5 loo ODD MODE IMPEDANCE (RAHO) EVEN MDDE IMPEDANCE PAIENTED SE? 1 0 i974 3; 835.42 1. sum u or 5 A v' BALANCED TRANSMISSION UN OUTPUT TERMINALS PAH-1mm SEP 1 019m SHEET 5 [IF 5 OUTPUT SIGNAL PORT This is a division of application Ser. No. 315,087, filed Dec. 14, 1972, now Pat. No. 3,798,575.

DESCRIPTION OF THE PRIOR ART Existing microwave transmission lines suitable for microwave integrated circuits employ a strip like conductor on the top surface of a dielectric substrate and a ground planar conductor on the bottom surface of the dielectric substrate. Microwave energy is confined substantially within the dielectric substrate and is transmitted from an input port to an output port in the TEM (transverse electromagnetic) mode. Some microwave integrated circuits employ two adjacent and coplanar strip-like conductors on the top surface of the dielectric substrate. The microwave transmission characteristics of these circuits are dependent on how the electric fields of the microwave energy are distributed between conductive surfaces on both sides of the substrate.

For certain devices, it is inconvenient and impractical to use a transmission line having a ground planar conductor on the bottom surface of a dielectric substrate and one or two coplanar striplike conductors on the top surface of the dielectric substrate. A prior art transmission line described in U.S. Pat. No. 3,560,893 issued to Cheng Paul Wen on Feb. 2, 1971, describes the use of three coplanar and parallel strip-like conductors on the top surface of a dielectric substrate. Microwave energy is transmitted along the three conductor transmission linesin a first transmission mode that confines the electric field of the applied microwave energy between the center conductor and the two outer ground potential conductors. Certain microwave de vices require not only the first transmission mode but a new second transmission mode that confines the field between the two outer conductors for efficient operation.

SUMMARY OF THE INVENTION According to the present invention a transmission line for electromagnetic energy comprising first, second and third coplanar strip-like conductors having predetermined widths adjacent to one surface of a dielectric substrate confine the electric fields of the electromagnetic energy substantially within the dielectric substrate in first and second transmission modes. The first conductor has a first edge separated from an adjacent edge of the second conductor at a first relative electric potential by a first predetermined gap. The first conductor also has a second edge, opposite the first edge, that is separated from an adjacent edge of the third conductor at a second relative electric potential by a second predetermined gap. The first, second and third conductor widths, the dielectric constant of the dielectric substrate, and the first and second predetermined gaps are arranged to confine the electric fields of the electromagnetic energy substantially within the dielectric substrate between the first and second conductors and between the first and third conductors in a first transmission mode and between the second and third conductors in a second transmission mode.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a prior art electromagnetic energy transmission line.

FIG. 2 is a perspective view of a transmission line in accordance with one embodiment of the present invention.

FIG. 3 is a plot of the even mode impedance of the transmission line shown in FIG. 2 as a function of the ratio of the transmission line dimensions, a lb the dielectric constant of the dielectric substrate and the ratio of the transmission line dimensions c -b /2a FIG. 4 is a plot of the ratio of odd mode impedance to even mode impedance as a function of the ratio of the transmission line dimensions a lb the dielectric constant of the dielectric substrate and the ratio of the transmission line dimensions c -b,/2a,.

FIG. 5 is a perspective view of a coplanar conductor directional coupler in accordance with another embodiment of the present invention.

FIG. 6 is a perspective view of an unbalanced-tobalanced transmission line transformer in accordance with another embodiment of the present invention.

FIG. 7 is a schematic representation of the unbalanced-to-balanced transmission line transformer illustrated in FIG. 6.

FIG. 8 is a top view of a microwave transistor pushpull amplifier in accordance with a still further embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Referring to FIG. 1, there is shown a perspective view of a prior art electromagnetic energy transmission line. The transmission line comprises three coplanar

striplike conductors

11, 15 and 17 on the top surface 22 of a dielectric substrate 13. The prior art transmission line, illustrated in FIG. 1, is described in U.S. Pat. No. 3,560,893 issued to Cheng Paul Wen on Feb. 2, 1971. A single, thin relatively narrow strip-like conductor 11 is separated by predetermined gaps from two relatively wide strip-like conductors l5 and 17 both at the same RF and DC. ground potential. The minimum width of the first relatively wide strip-like ground conductor 15 is more than twice as wide as narrow strip-like conductor l1 and is spaced near to and parallel with coplanar narrow strip-like conductor 11. The minimum width of the second relatively wide strip-like ground conductor 17 is likewise more than twice as wide as narrow striplike conductor 11 and is spaced near to and parallel with coplanar narrow strip-like conductor 11 but on the opposite side of narrow strip like conductor 11 relative to ground conductor 15. The top surface 22 of dielectric substrate 13 having the three coplanar

striplike conductors

11, 15 and 17 thereon is open to free space. The bottom surface 16 of dielectric substrate 13 is likewise open to free space.

The distribution of the electric field of electromagnetic energy coupled to the transmission line from a source, not shown, is represented by dashed electric field lines 19. Electric field lines 19 are distributed only between narrow strip-like conductor 11 and wider strip-

like conductors

15 and 17 along their entire lengths. Electric field lines 19 are contained mainly within the dielectric substrate 13 between narrow striplike conductor 11 and

ground conductors

15 and 17. However, some electric field lines 19, not shown, are

distributed between conductor 11 and conductors l and 17 in the free space region. The intensity of the electric field within dielectric substrate 13 is dependent on the magnitude of the dielectric constant of dielectric substrate 13. A discontinuity in displacement current density at the interface between dielectric substrate 13 and free space is produced by that portion of electric field 19 tangential to the air-dielectric boundary on dielectric surface 22. The discontinuity in displacement current on dielectric surface 22 produces an axial component of magnetic field, represented by dashed lines 20, associated with electric field 19. A portion of the axial component of the magnetic field 20 at the interface between dielectric substrate 13 and free space on surface 22 is in the direction of propagation. The magnetic field 20 extends along both sides of narrow conductor 11 and passes under narrow conductor 11. The distance, d, between conductors and 17 is preferably less than one-half wavelength at the operating frequency in order to prevent the transmission of electromagnetic energy in undesired modes.

In order to distribute the electric field between narrow strip-like conductor 11 and wider strip-

like conductors

15 and 17, a difference in potential must exist between narrow strip-like conductor 11 and wider strip-

like conductors

15 and 17. Since both wider striplike conductors l5 and 17 are at the same RF. and DC. ground potential, the boundary conditions for establishing an electric field between

conductors

15 and 17 does not exist. The characteristic impedance of the prior art three coplanar strip-like conductor transmission line is dependent on the establishment of an electric field between only narrow strip-like conductor 11 and wider strip-

like conductors

15 and 17.

Referring to FIG. 2, there is shown a perspective view of a transmission line 40 comprising three coplanar strip-

like conductors

21, 25 and 27 on the top surface 35 of a dielectric substrate 23 in accordance with one embodiment of the present invention. Center strip-like conductor 21 is separated by predetermined gaps from first and second outer strip-like conductors, 25 and 27 having predetermined widths. First outer strip-like conductor 25 is spaced near to and parallel with coplanar center strip-like conductor 21. Second outer strip-like conductor 27 is spaced near to and parallel with coplanar center strip-like conductor 21 but on the opposite side of center strip-like conductor 21 relative to first outer conductor 25. The top surface 35 of dielectric substrate 23 having the three coplanar strip-

like conductors

21, 25 and 27 thereon as illustrated in FIG. 2 is open to free space. Unlike the prior art transmission line illustrated in FIG. 1, the minimum width of first and second

outer conductors

25 and 27 according to the present invention is not limited to be at least twice as wide as center conductor 27. Contrary to the prior art arrangements, according to the present invention a difference in RF potential is provided between

outer conductors

25 and 27 in accordance with several arrangements to be described.

Electromagnetic energy from a source, not shown, is coupled to transmission line 40. The distribution of the electric field of the electromagnetic energy coupled to transmission line 40 is represented by dashed electric field lines 29. Electromagnetic energy can be transmitted along transmission line 40 in a first transmission mode that distributes electric field lines 29 between center strip-like conductor 21 and outer strip-

like conductors

25 and 27 along their entire lengths since a difference in RF. potential exists between center striplike conductor 21 and outer strip-

like conductors

25 and 27. Electromagnetic energy can also be transmitted along transmission line 40 in a second transmission mode that distributes electric field lines 29 between only outer strip-likeconductors 25 and 27. Conditions can be established as will be apparent to those skilled in this art that would allow simultaneous transmission of electromagnetic energy in both the first and second transmission modes. It should be understood that the terms first and second transmission modes designate for convenience of description and the appended claims both the arrangements and the modes of operation of transmission line 40 according to the present invention.

A portion of electric field 29 is tangential to the airdielectric boundary on dielectric surface 35 and produces a discontinuity in displacement current density at the interface between dielectric substrate 23 and free space. The discontinuity in displacement current on dielectric surface 35 produces an axial component of magnetic field, represented by dashed lines 31, associated with electric field 29. A portion of the axial component of the magnetic field 31 at the interface between dielectric substrate 23 and free space on surface 35 is in the direction of propagation when the distance, d, between

outer conductors

25 and 27 is small compared to the electrical distance of one wavelength at the operating frequency. Under these conditions, the magnetic field lines 31 extend along both sides of center conductor 21 and eventually pass under center conductor 21 forming a closed magnetic loop having a portion in the direction of electromagnetic transmission. The magnitude of the magnetic field 31 present at the air-dielectric interface on one side of center conductor 21 is not equal to the magnitude of the magnetic field 31 on the other side of center conductor 31. If the magnetic field lines 31 were represented by magnetic field vectors, the vectors would appear at the air-dielectric interface on the top surface of dielectric substrate 23 between center conductor 21 and

outer conductors

25 and 27. The vectors would have a magnitude and direction that would define a condition of circular polarization existing on the top surface of dielectric substrate 23 between center conductor 21 and

outer conductors

25 and 27. The sense of circular polarization (clockwise or counterclockwise) would be the same if viewed on opposite sides of center conductor 21. This polarization condition is significant in the construction of microwave ferrite devices as described by Lax and Button in Chapter 12 of Microwave Ferrites and Ferrimagnetics, McGraw-Hill publication.

If the distance, d, between

outer conductors

25 and 27 is larger than the electrical distance of one wavelength at the operating frequency, the axial component of magnetic field lines 31 form a closed loop around center conductor 21. Under this condition, the closed loop of magnetic field lines 31 around center conductor 21 is then transverse to the direction of electromagnetic transmission.

The term even mode impedance, Z is used to identify the impedance of transmission line 40 when electromagnetic energy is transmitted in the first transmission mode, viz., when electric field lines 29 are distributed between center strip-like conductor 21 and outer strip-

like conductors

25 and 27. The impedance Z refers to an even mode impedance having a magnitude dependent on the intensity of the electric field distribution between center conductor 21 and outer conductor 27. The impedance Z refers to an even mode'impedance having a magnitude dependent on the intensity of the electric field distribution between center conductor 21 and outer conductor'25.

Referring to FIG. 3, there is shown a graph of even mode impedance, Z in terms of the relative dielectric constant, 6,, of dielectric substrate 23 versus the ratio of the substrate dimensions, a,/b,, shown in FIG. 2. The dimension a, is the distance from the center line of center conductor 21 to the edge of center conductor 21. The dimension b, is the distance from the center line of center conductor 21 to the nearest edge of outer conductor 25. The dimension 0 is the distance from the center line of center conductor 21 to the furthest edge of outer conductor 25. Assuming that the dimensions of strip-

like conductors

21, 25 and 27 comprising transmission line 40 and the relative dielectric constant e, are known, the graph in FIG. 3 is useful for determining even mode impedance Z under the condition that transmission line 40 is symmetrical or Z Z and the substrate thickness, t, greater than 4 X b, In other words, the dimensions b and c determining the width of outer conductor 25 and the gap between center conductor 21 and outer conductor 25 in FIG. 2, also correspond to the dimensions of the width of outer conductor 27 and the gap between center conductor 21 and outer conductor 27. The magnitude of the even mode impedance is independent of the thickness, t, of dielectric substrate 23 in FIG. 2 when thickness, t, exceeds 4 x b,.

The term odd mode impedance, Z is used to identify the impedance of transmission line 40 when it is transmitting electromagnetic energy in the second transmission mode, viz., when electric field lines 29 are distributed between only

outer conductors

25 and 27.

Referring to FIG. 4, there is shown a graph of the ratio of odd mode impedance to 'even mode impedance, Z /Z versus the ratio of the substrate dimensions, a,/b,, shown in FIG. 2. The graph in FIG. 4 is useful for determining odd mode impedance, Z knowing Z and the dimensions of strip-

like conductors

21, 25 and 27 comprising transmission line 40 provided either transmission line 40 is symmetrical or Z Z As explained above, transmission line 40 is symmetrical when the dimensions b and c, determining the width of outer conductor 25 and the gap between center conductor 21 and outer conductor 25 in FIG. 2 also correspond to the dimensions of the width of outer conductor 27 and the gap between center conductor 21 and outer conductor 27. The magnitude of the odd mode impedance is independent of the thickness, t, of the dielectric substrate 23 in FIG. 2 when thickness, t, exceeds 4 X b i FIGS. 3 and 4 illustrate that the even mode and odd mode impedances of transmission line 40 in FIG. 2 vary as a function of the a /b ratio when transmission line 40 has a predetermined ratio of outer conductor width to center conductor width (c,b /2a A transmission line for electromagnetic energy comprising three coplanar strip-like conductors on the top surface of a dielectric substrate permits construction of passive devices requiring a determination of even and odd mode impedances for improved operation. The disclosed tion of active devices between center conductor 21 and outer conductors 25 and27 as well as a similar connection of other passive components.

Referring to FIG. 5, there is shown a perspective view of a directional coupler, according to this invention, comprising three coplanar strip-

like conductors

51, 55 and 57 on the top surface of a dielectric substrate 53. A directional coupler is a passive microwave device used for dividing microwave energy coupled to an input port between two output ports. The propagation of microwave energy transmitted to each output port is dependent on the desired coupling coefficient. Part of the energy reflected at the two output ports is directed to a fourth port usually terminated in an energy absorbing load. The design of a directional coupler in terms of even mode, Z' and odd mode, Z impedances is known being described by Matthaei, Young andJones in Chapter 13 of Microwave Filters Impedance- Matching Networks, and Coupling Structures. The desired characteristics of a directional coupler (coupling coefficient, bandwidth, etc.) are directional coupler design goals described in Chapter 13of the above cited'text and are used to calculate the magnitude of Z and Z The magnitude Zb used in FIGS. 3 and 4 is equivalent to Z' /Z. The magnitude of Z used in FIGS. 3 and 4 is equivalent to 2Z' Thus, FIGS. 3 and 4 can be used to determine the widths of conductors 51,.55 and 57 and the separation between center conductor -51 and

outer conductors

55 and 57 that would allow operation of a directional coupler having desired operating characteristics.

FIG. 5 also illustrates a method of coupling microwave energy to and from a transmission line comprising three coplanar strip-

like conductors

51, 55 and 57 on the top surface 65 of dielectric substrate 53. Coaxial outer conductor 58 of coaxial connector 66 is connected to outer strip-like conductor 57. Coaxial center conductor of connector 66 is connected to the closest end of center strip-like conductor 51. Coaxial outer conductor 59 of coaxial connector 67 is connected to outer strip-like conductor 57. Coaxial center conductor 71 of connector 67 is connected to the closest end of center strip-like conductor 51. A length L of outer strip-like conductor 57 separates coaxial outer conductor 58 of connector 66 from coaxial outer conductor 59 of connector 67. The length L, is equivalent to an electrical length of substantially M4, where )t is the wavelength determined by the equation:

C being the velocity of light 'in a vacuum, f the midband operating frequency and e, the relative dielectric constant of dielectric substrate 53.

Coaxial outer conductor 61 of. coaxial connector 69 is connected to outer strip-like conductor 55. Coaxial center conductor 73 of connector 69 is connected to center strip-like conductor 51 at the same end as coaxial center conductor 70 of coaxial connector 66. Coaxial outer conductor 60 of connector 68 is connected to outer strip-like conductor 55. Coaxial center conductor 72 of connector 68 is connected to center strip-like conductor 51 at the same end as coaxial center conductor 71 of connector 67. A length 1. of outer strip-like conductor SSseparates coaxial outer conductor 61 of connector 69 from coaxial outer conductor 60 of connector 68. The length L is equivalent to an electrical length of substantially M4, where A is the wavelength determined by equation (1).

Outer strip-like conductor 57 may be at the same DC. potential as outer strip-like conductor 55 if either coaxial outer conductors 58 or 59 is at the same DC. potential as either of coaxial outer conductors 60 or 61. However, outer strip-

like conductors

55 and 57 are not at the same R.F. potential when coaxial

outer conductors

58, 59, 60 and 61 are connected to outer strip-

like conductors

55 and 57 as illustrated in FIG. 5. Thus, by arranging the connections as just described, the previously discussed boundary conditions for exciting the even and odd mode impedances in a transmission line comprising three coplanar strip-like conductors are preserved.

The length L of center strip-like conductor 51 is equivalent to an electrical length of M4, where A is the wavelength determined by equation (1). Center striplike conductor 51 is coextensive and parallel with outer strip-like conductor 55 over length L, with outer striplike conductor 57 over length L It is well known that a microwave signal coupled to an input port connector of a directional coupler may be divided into two output signals that are coupled from two output port connectors that are directly opposite the input port connector. For example, if a microwave signal is coupled to input port connector 66, part of the microwave signal is transmitted directly to directly opposite output port connector 67 and part of the microwave signal is coupled to directly opposite output port connector 69. Substantially none of the input microwave signal is coupled to diagonally opposite connector 68.

Referring to FIG. 6, there is shown according to this invention a perspective view of an unbalanced-tobalanced transmission line transformer commonly referred to as a balun. The balun provides an impedance transformation from the impedance magnitude of the signal source, not shown, coupled to the unbalanced transmission line input terminal section 74 to the impedance magnitude of a load, not shown, coupled to the balanced transmission line output terminal section 78. The balanced transmission line output terminal section 78 consists of two coplanar strip-

like conductors

79 and 80 on the top surface 95 of dielectric substrate 75. The balun is designed to transmit energy to a

load terminating conductors

79 and 80. The design of balun section 125 determines the characteristic impedance of balanced transmission line 78 and section 125 also pro vides a condition that establishes a phase difference of 180 electrical degrees between

conductors

79 and 80.

Unbalanced transmission line input terminal section 74 consists of an arrangement of three coplanar and parallel strip-

like conductors

81, 85 and 87 in the top surface 95 of dielectric substrate 75 more fully described in US. Pat. No. 3,560,893 issued to C. P. Wen on Feb. 2, 1971. Relatively narrow strip-like center conductor 81 is separated by predetermined gaps from two relatively wide strip-

like conductors

85 and 87 both at the same RF. and DC. ground potential. One method of establishing the same RF. and DC. ground potential at

conductors

85 and 87 is to connect the outer conductor of a coaxial connector, not shown, to

conductors

85 and 87 and the center conductor of the connector to conductor 81. The width, W, of

outer conductors

85 and 87 is at least twice as wide as the width of center conductor 81. A length of mil diameter wire 76 is connected from outer conductor 85 to 5 outer conductor 87. Wire 76 is used to maintain the same RF. and DC. ground potential between outer strip-

like conductors

85 and 87 at the end of unbalanced transmission line input terminal section 74.

Balun section 125 consists of three coplanar and parallel strip-

like conductors

81, 88 and 89 on the top surface 95 of dielectric substrate 75. Center strip-like conductor 81 is separated by predetermined gaps from outer strip-

like conductors

88 and 89. One end of outer strip-like conductor 88 is connected to outer strip-like conductor 87 near the connection point of wire 76. The other end of conductor 88 is connected to one end of strip-like conductor 77. The electrical length of conductor 88 from the connection point of wire 76 to the connection point of conductor 77 is substantially )t/4, where is the wavelength determined by equation (1). The electrical length of conductor 77 is negligible. One end of center strip-like conductor 81 is connected to the other end of strip-like conductor 77. One end of outer strip-like conductor 89 is connected to outer strip-like conductor 85 near the connection point of wire 76.

One end of strip-like conductor 79 of section 78 is connected to conductor 77 anywhere along the length of conductor 77. One end of strip-like conductor 80 of section 78 is illustrated in FIG. 6 as being an extension of outer strip-like conductor 89. Such an arrangement is exemplary only of other possible arrangements. The end of strip-like conductor 80 may be connected to conductor 89 anywhere along the end of conductor 89.

Referring to FIG. 7, there is shown a schematic equivalent of section 125 of the balun illustrated in FIG. 6. The impedance Z, is the load impedance terminating the balanced transmission line output terminal section 78. The schematic representation of section 125 is useful in explaining the determination of odd mode impedance, Z and even mode impedance, Z necessary for the design of a balun operative over a broad frequency band. Section 125 is schematically illustrated as having a first short circuited transmission line stub section 100, having a characteristic impedance ZZ connected in shunt with a transmission line section 102 having a characteristic impedance ZZ A second short circuited transmission line stub section 101 having a characteristic impedance Z is also connected in shunt with transmission line section 102. The electrical length of transmission line section 102 separating the connection points of

sections

100 and 101 to section 102 is substantially M4, where A is the wavelength defmed by equation (I). The electrical length of

sections

100 and 101 from their connection to section 102 to their short circuited ends is substantially AM, where A is the wavelength defined in equation (1).

The characteristic impedance, Z of balanced transmission line section 78 at mid-band frequency, f,,, is

where Z is the even mode impedance of section 102 and Z is the magnitude of the impedance of the signal source, not shown, coupled to unbalanced transmission line section 74. At mid-band frequency, f the shunt connected short

circuited stub sections

100 and 101 each appear as an open circuit or very high impedance connected in shunt with section 102 and thus do not affect the determination of balanced transmission line characteristic impedance Z At operating frequencies other than the mid-band frequency, f,,, the characteristic impedance 2, of balanced transmission line section 78 is:

where Z is the even mode impedance of section 102, Z is the odd mode impedance of

section

102, 6 is the electrical length of

stub sections

100 and 101 at the operating frequency and Z is the magnitude of the signal source, not shown, coupled to unbalanced transmission line section 74. Thus, the desired characteristic impedance 2,, of balanced output terminal section 78 and its variations over a desired frequency band can be determined from equations (2) and (3). The even mode impedance, Z and the odd mode impedance, Z used in equations (2) and (3) together with the graphs of FIGS. 3 and 4 may be used to determine the width of strip-

like conductors

81, 88 and 89 and the spacing between center conductor 81 and

outer conductors

88 and 89 of the balun illustrated in FIG. 6.

Referring to FIG. 8, there is shown a top view of a microwave transistor push-pull amplifier according to the invention, having all conductive surfaces and transistors on the top surface 95 of a dielectric substrate. The push-pull amplifier uses the balun illustrated in FIG. 6 as push-pull amplifier input transformer 103 and pushpull amplifier output transformer 104. For convenience, the numbers identifying the conductive surfaces of the balun illustrated in F IG. 6 are used to identify the conductive surfaces of input and output

pushpull amplifier transformers

103 and 104. A detailed explanation of push-pull transformer-coupled power amplifiers is disclosed in Section 4.2 of Electronic Designers Handbook by Landee, Davis and Albrecht.

Gate electrode 105 of transistor T, is connected to balanced transmission line terminal 80 of input balun 103 and gate electrode 106 of transistor T is connected to balanced transmission line terminal 79 of input balun 103. Source electrode 107 of transistor T, and source electrode 108 of transistor T are connected to strip-like conductors 109 which are at DC. ground potential. As previously discussed, in the description of the balun illustrated in FIG. 6, the widths of strip-

like conductors

81, 88 and 89 and the separation between inner conductor 81 and

outer conductors

88 and 89 of input balun 103 are determined from FIGS. 3 and 4 when the magnitudes of even mode impedance, Z and odd mode impedance, Z are known. Equations (2) and (3) are used to determine the magnitudes of Z and Z 'necessary for the proper impedance transformation from the known impedance of the input signal source, not shown, to the known input impedance magnitude of transistors T, and T As an example, the impedance of the input signal source is 50 ohms and the magnitude of the combined input impedance of transistors T, and T is substantially 200 ohms. The relative dielectric constant, e,, of the dielectric substrate is 2.2. The width of center strip-like conductor 81 of balun 103 is 0.020 inches. The widths of outer strip-

like conductors

88 and 89 of balun 103 is 0.020 inches. The separation between center strip-like conductor 81 and outer strip-

like conductors

88 and 89 of balun 103 is 0.028 inches.

Drain electrode 110 of transistor T, is connected to balanced transmission line terminal of output balun 104 and drain electrode 126 of transistor T is connected to balanced transmission line terminal 79 of output balun 104. The widths of strip-

like conductors

81, 88 and 89 and the separation between inner conductor 81 and

outer conductors

88 and 89 of output balun 104 are determined from FIGS. 3 and 4 when the magnitudes of even mode impedance, Z and odd mode impedance, Z are known. Equations (2) and (3) are used to determine the magnitudes of Z and Z necessary for the proper impedance transformation from the output impedance magnitude of transistors T, and T to the impedance magnitude of the load, not shown, terminating the output signal port. As an example, the impedance of the terminating output load is 50 ohms and the magnitude of the combined output impedance of transistors T, and T is substantially 450 ohms. The width of center strip-like conductor 81 of balun 104 is .016 inches. The widths of outer strip-

like conductors

88 and 89 of balun 104 is .016 inches. The separation betweencenter strip-like conductor 81 and outer strip-

like conductors

88 and 89 of balun 104 is .035 inches.

A negative DC. bias voltage of 2 volts is applied to gates 105 and 106 of Gallium Arsenide Schottkybarrier FET (FieldEffect Transistor) transistors T, and T A positive DC. bias voltage of 5 volts is applied to drains 110 and 126 of transistors T, and T The gain of the push-pull amplifier was 1.5 db over a 1.0 GHZ band of frequencies centered at 5.2 GHz. The magnitude of the output power was 20 mw and the efficiency of the amplifier was 13 percent.

What is claimed is:

1. An unbalanced-to-balanced transmission line transformer operative over a desired band of frequencies for transforming a first impedance to a second impedance comprising:

a dielectric substrate having a predetermined dielectric constant;

a transmission line for electromagnetic energy, said transmission line having first, second and third coplanar strip-like conductors each having input and output ends and predetermined widths and lengths adjacent to one surface of said dielectric substrate, said first conductor having one longitudinal edge substantially parallel to and separated from an adjacent longitudinal edge of said second conductor by a first predetermined gap, said first conductor having a second edge opposite and substantially parallel to said one edge, said second edge being substantially parallel to and separated from an adjacent longitudinal edge of said third conductor by a second predetermined gap, said first conductor input end being adjacent to said second and third conductor input ends and said first conductor output end being adjacent to said second and third conductor output ends, said second conductor being arranged to be at a first R.F. potential relative to said first conductor and said third conductor being arranged to be at a second different R.F. potential relative to said first conductor in the presence of said electromagnetic energy intermediate said ends, said first, second and third conductor widths, said dielectric substrate, and said first and second predetermined gaps being arranged to form said transmission line,

means for establishing a predetermined D.C. potential at both said second conductor input end and said third conductor input end, whereby said first, second and third conductor input ends, said dielectric constant and said first and second predetermined gaps form at said first, second and third conductor input ends an unbalanced transmission line input terminal section having said first impedance; and

means for connecting said second conductor output transformer according to claim 1, wherein each of said first, second and third conductor predetermined lengths from said input to output ends is substantially )\/4, where A is the wavelength at the center frequency of said desired frequency band.

Claims

1. An unbalanced-to-balanced transmission line transformer operative over a desired band of frequencies for transforming a first impedance to a second impedance comprising: a dielectric substrate having a predetermined dielectric constant; a transmission line for electromagnetic energy, said transmission line having first, second and third coplanar strip-like conductors each having input and output ends and predetermined widths and lengths adjacent to one surface of said dielectric substrate, said first conductor having one longitudinal edge substantially parallel to and separated from an adjacent longitudinal edge of said second conductor by a first predetermined gap, said first conductor having a second edge opposite and substantially parallel to said one edge, said second eDge being substantially parallel to and separated from an adjacent longitudinal edge of said third conductor by a second predetermined gap, said first conductor input end being adjacent to said second and third conductor input ends and said first conductor output end being adjacent to said second and third conductor output ends, said second conductor being arranged to be at a first R.F. potential relative to said first conductor and said third conductor being arranged to be at a second different R.F. potential relative to said first conductor in the presence of said electromagnetic energy intermediate said ends, said first, second and third conductor widths, said dielectric substrate, and said first and second predetermined gaps being arranged to form said transmission line, means for establishing a predetermined D.C. potential at both said second conductor input end and said third conductor input end, whereby said first, second and third conductor input ends, said dielectric constant and said first and second predetermined gaps form at said first, second and third conductor input ends an unbalanced transmission line input terminal section having said first impedance; and means for connecting said second conductor output end to said first conductor output end, whereby said first and second conductor output connected ends, said third conductor output end, said dielectric constant and said first and second predetermined gaps form at said first, second and third conductor output ends a balanced transmission line input terminal section having said second impedance.

2. An unbalanced-to-balanced transmission line transformer according to claim 1, wherein each of said first, second and third conductor predetermined lengths from said input to output ends is substantially lambda /4, where lambda is the wavelength at the center frequency of said desired frequency band.