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Publication numberUS3659228 A
Publication typeGrant
Publication dateApr 25, 1972
Filing dateJul 30, 1970
Priority dateJul 30, 1970
Also published asCA927938A1, DE2138315A1
Publication numberUS 3659228 A, US 3659228A, US-A-3659228, US3659228 A, US3659228A
InventorsNapoli Louis Sebastian
Original AssigneeRca Corp
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Strip-type directional coupler having elongated aperture in ground plane opposite coupling region
US 3659228 A
Abstract
A compact, microstrip directional coupler is described wherein two narrow strip-like conductors are closely spaced to each other so as to provide a signal coupling region therebetween. The two narrow conductors are spaced from a ground planar conductor by a substrate of dielectric material having a relatively high dielectric constant compared to the adjacent medium wherein the odd mode phase velocity of a signal propagating along the coupler is unequal to the even mode phase velocity thereof. An elongated aperture is located in the ground planar conductor in the coupling region to alter the odd mode phase velocity so that it is equal to the even mode phase velocity.
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Description  (OCR text may contain errors)

United States Patent Napoli 1 51 Apr.25, 1972 [54]; STRIP-TYPE DIRECTIONAL COUPLER HAVING ELONGATED APERTURE IN GROUND PLANE OPPOSITE COUPLING Primary Examiner-Paul L. Gensler Attorney-Edward J. Norton [57] ABSTRACT A compact, microstrip directional coupler is described wherein two narrow strip-like conductors are closely spaced to each other so as to provide a signal coupling region therebetween. The two narrow conductors are spaced from a ground planar conductor by a substrate of dielectric material having a relatively high dielectric constant compared to the adjacent medium wherein the odd mode phase velocity of a signal propagating along the coupler is unequal to the even mode phase velocity thereof. An elongated aperture is located in the ground planar conductor in the coupling region to alter the odd mode phase velocity so that it is equal to the even mode phase velocity.

2 Claims, 6 Drawing Figures STRIP-TYPE DIRECTIONAL COUPLER HAVING ELONGATED APERTURE IN GROUND PLANE OPPOSITE COUPLING REGION This invention relates to directional couplers and more particularly to an asymmetrical strip transmission line directional coupler.

With the coming of miniaturization to microwave systems, there has been the development and use of a form of asymmetrical strip transmission line wherein a narrow strip-like conductor is spaced by a high dielectric constant material from a single broad planar conductor. This form of strip transmission line is often referred to as "microstrip" transmission line. This microstrip" transmission line can be fabricated by the use of low cost printed circuit techniques and when using relatively high dielectric constant material is very compact in size.

Microstrip directional couplers wherein two such narrow strip-like conductors are closely spaced from a common ground planar'conductor and are closely spaced from each other along a given coupling region are known. Microstrip directional couplers using relatively high dielectric constant material are however relatively recent and have been associted with miniaturized microwave devices and systems.

When electromagnetic signals are applied to one of the narrow strip-like conductors of such a directional coupler, there is associated at the coupling region odd and even mode excitation conditions. In a microstrip coupler, a larger portion of the electric field of the signals propagating in the odd mode is located outside of the relatively high dielectric substrate in the ambient air than for the signals propagating in the even mode. Because of this propagation difference for signals in the even and odd mode and because of the dielectric difference between the substrate and the ambient air, the phase velocity of the signal traveling in the odd mode is different from that in the even mode. When the dielectric constant of the substrate material is substantially greater than that of the ambient air, such as above five, in order to reduce the size of the coupler and confine the fields of signals applied thereto, there is a marked difference between the odd and evenmode phase velocity. This marked difference between the odd and even mode phase velocity causes substantial deterioration in the signal directivity.

It is an object of this invention to provide an improved highly directive microstrip directional coupler having a substrate of relatively high dielectric material.

Briefly this object is accomplished by a directional coupler wherein first and second narrow strip-like conductors are spaced from a ground planar conductor. The first and second narrow strip-like conductors are closely spaced to each other andto the ground planar conductor so as to provide an electromagnetic coupling region therebetween. In response to electromagnetic signals applied thereto, these signals propagate in the odd and even mode along the coupling region of the coupler. The spacing between the narrow strip-like conductors and the ground planar conductor is provided by a substrate having a dielectric constant of at least six causing the inherent phase velocity of signals propagating in the odd mode to be substantially different from the signals in the even mode. The odd mode only is perturbed sufficiently to an extent such that the odd mode phase velocity is equal to the even mode phase velocity.

The above-mentioned invention will be better understood by reference to the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a top plan view of a prior art directional coupler,

FIG. 2 is a cross sectional view of FIG. 1 illustrating odd mode propagation,

FIG. 3 is a cross sectional view of FIG. 1 illustrating even mode propagation,

FIG. 4 is a top plan view of a directional coupler in accordance with the principles of the present invention,

FIG. 5 is a cross sectional view of the coupler of FIG. 4 taken along axis 5-5, and

FIG. 6 is a plot of coupling versus frequency for a directional coupler of the type following the principles of the present invention.

Referring to FIGS. 1 and 2, there is illustrated a perspective view of a prior art edge coupled microstrip directional coupler 10. Two narrow strip-like conductors 13 and 15 are fixed to one surface of a dielectric substrate 11. A closely spaced ground planar conductor 12 is located on the opposite surface of the substrate 11. The conductors 13 and 15 are closely spaced to each other over a coupling region 17 which is usually about a quarter of a wavelength along at the center operating frequency of the coupler. Electromagnetic signals coupled across conductors l3 and 12 at one end 16 propagate along conductor 13 toward the opposite end 18. At the com,- mon coupling region 17 where conductors '13 and 15 are closely spaced to each other relative to the ground planar conductor 12, the electromagnetic signal energy propagating along this coupling region 17 propagates in both odd and even mode. i

Referring to the odd mode condition illustrated in FIG. 2. the voltage at one of the conductors 13 is positive with respect to the ground conductor 12, and the'voltage at the other narrow conductor 15 is negative with respect to ground conductor 12. Also the current in the odd mode as indicated by ar- I rows 19 and 21 is in one direction (into the paper of FIG. 2) along conductor 13 and in an opposite direction (out of the paper of FIG. 2) along conductor 15. In addition, as indicated by arrow 22, there is a current that is transverse to the direction of propagation along the ground planar conductor 12. As can be seen, referring to FIG. 2, the electric field of the electromagnetic signal energy, as indicated by the dashed arrows, is between the narrow conductors 13 and 15 with a sub stantial portion thereof being located outside of the substrate 1 1 Referring to FIG. 3, there is illustrated the even mode condition wherein the voltages on the conductors 13 and 15 are both positive with respect to the ground planar conductor 12. The currents travel along the conductors 13 and 15 for the even mode condition, as indicated by arrows 24 and 25 into the paper and arrow 26 out of the paper, in the same direction, and there is no transverse current associated with the even mode condition. Also, the electric field as indicated by the dashed arrows is mainly within the substrate 11.

If the phase velocity for both the odd and even modes is equal, then the electromagnetic signal energy propagating along conductor 13 from end 16 toward end 18 is coupled to a given terminal end 21 of conductor 15. The ratio of the coupled energy to one terminal end 21 'of conductor 15 with respect to the other end 23 of conductor 15 is the directivity. High directivity is dependent on the phase velocities of the even and odd mode being equal. If the even and odd mode phase velocities are substantially different, the directivity deteriorates substantially and becomes lower as the velocity difference increases.

Referring to FIGS. 2 and 3, it can be noted that a larger portion of the electric field is outside the dielectric substrate 11 into the ambient air in the odd mode than for the even mode. When the dielectric constant of the substrate 11 becomes higher than about 5 there is a substantial difference between the odd and even mode phase velocities because of the marked difference in dielectric material medium the signal is propagating. Consequently, because of this difference in the even and odd mode phase velocities, the directivity of such couplers is low and deteriorates even further with higher dielectric materials.

Referring to FIGS. 4 and 5 there is illustrated a coupler 30 comprising a dielectric substrate 31 having a ground planar conductor 32 on one surface and the narrow strip-like conductors 33 and 35 closely spaced at coupling region 37 to each other on the opposite surface of said substrate 31. The narrow the narrow conductors 33 and 35. Referring to the cross sectional view of FIG. 5, this aperture or slot 38 is placed in the ground planar conductor 32 such as to be centered below the coupling region 37. Referring to FIGS. 2 and 3, it is noted that in the even mode excitation, there is no component of current at the ground plane transverse to the direction of propagation of the signals, but there is a component of current for the odd mode excitation that is transverse to the direction of propagation of the signals. By the placement of an elongated aperture or slot 38 in the ground conductor 32 at the center line, the transverse odd mode current as indicated by arrow 22 of FIG. 2 is interrupted, but the even mode current is not.

When the odd mode current is interrupted by an elongated aperture, as illustrated in FIGS. 4 and 5, there is an associated increase in the inductive impedance for signals propagating in the odd mode and there is an attendant decrease in the phase velocity. By adjusting the width of the aperture 38 or parallel elongated apertures or slots in the ground conductor, the odd mode inductance can be adjusted so that the odd and even mode phase velocities are substantially equal. By adjusting the width of the slot 38 to provide the equal phase velocities, the directivity is increased.

In one example of the present invention, a highly directional coupler as illustrated in FIGS. 4 and 5 was operated at a frequency range of from 2 to 6 GHz and had the following dimensions: I

Narrow strip-like conductors 33 and 35 were 0.0145 inch wide and were 2 to 3 microns thick copper,

Ground conductor 32 was 2 to 3 microns thick copper,

Coupling region 37 was about 7.4 mm. long,

Coupling distance between conductors 33 and 35 at region 37 was 0.0075 inch,

Slot 38 was 0.012 to 0.018 inch wide and 13.5 mm. long,

Substrate 31 was 99.5% A1 0 material, dielectric constant of 10.4 and was 0.025 inch thick.

Referring to FIG. 6, there is illustrated the improved directivity of the above-described coupler for signals within the 2 to 6 gigahertz frequency region applied at terminal 39.

Plot a of F IG. 6 illustrates the coupled power in decibels at the desired terminal43 relative to the input power applied at terminal 39. Plot b of FIG. 6 illustrates the coupled power in decibels at the undesired terminal 45 relative to the input power at terminal 39. By reference to plot a, it is shown that signals within the 2 to 6 gigahertz frequency region applied at terminal 39 of the above coupler are coupled about 10 db down to terminal 43. By reference to plot 12, it is shown that signals within the 2.to 6 gigahertz frequency region applied at terminal 39 of the above coupler are coupled about 30 db down to terminal 45. This means that the directivity, the relative power at terminal 43 relative to terminal 45, was 20 db. This amounts to an improvement on the order of 10 to 15 db for a similar coupler as described above without the aperture in the ground planar conductor.

What is claimed is:

1. In a directional coupler of the type comprising:

a dielectric substrate, a pair of narrow strip-like conductors on one surface of said dielectric substrate, ,a single ground planar conductor on the surface of said substrate opposite said one surface so as to be spaced directly opposite said strip-like conductors, said pair of narrow strip-like conductors being closely spaced to each other and to said ground planar conductor so as to provide an electromagnetic coupling region between said strip-like conductors so that electromagnetic signals applied to said coupler are propagated in odd and even modes along said pair of conductors at said region, said substrate having a dielectric constant of at least six causing a marked difference between the phase velocity of signals propagating in the odd mode and in the even mode, said ground planar conductor having an elongated aperture which extends along said surface opposite said one surface and directly opposite and parallel to said region for perturbing the odd mode sufficiently so that said odd mode phase velocity eqlual said even mode Phase velocity. 2. T e combination of 0 arm 1 wherein said elongated aperture extends at least the full length of the coupling region.

Patent Citations
Cited PatentFiling datePublication dateApplicantTitle
US2951218 *Feb 19, 1957Aug 30, 1960IttDirectional couplings
US3094677 *Dec 19, 1960Jun 18, 1963Bell Telephone Labor IncStrip line wave guide coupler
US3480884 *Aug 26, 1968Nov 25, 1969Hewlett Packard CoElectromagnetic wave energy coupling apparatus comprising an anisotropic dielectric slab
US3512110 *May 6, 1968May 12, 1970Motorola IncMicrostrip-microwave coupler
US3575674 *May 9, 1969Apr 20, 1971Microwave AssMicrostrip iris directional coupler
Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US3961296 *Mar 6, 1975Jun 1, 1976Motorola, Inc.Slotted strip-line
US3970927 *Dec 2, 1974Jul 20, 1976Plessey Handel Und Investments A.G.Method and apparatus including a sensing head with a pair of strip transmission lines for detecting metallic objects
US4211986 *Jul 17, 1978Jul 8, 1980Tokyo Shibaura Denki Kabushiki KaishaStrip line coupler having spaced ground plate for increased coupling characteristic
US4375054 *Feb 4, 1981Feb 22, 1983Rockwell International CorporationSuspended substrate-3 dB microwave quadrature coupler
US4737740 *May 26, 1983Apr 12, 1988The United States Of America As Represented By The Secretary Of The NavyDiscontinuous-taper directional coupler
US4792773 *Sep 17, 1986Dec 20, 1988Thomson-CsfUltra high frequency circuit with low parasite capacities
US5138287 *May 11, 1990Aug 11, 1992Hewlett-Packard CompanyHigh frequency common mode choke
US5446425 *Jun 7, 1994Aug 29, 1995Atr Optical And Radio Communications Research LaboratoriesFloating potential conductor coupled quarter-wavelength coupled line type directional coupler comprising cut portion formed in ground plane conductor
US5561378 *Jul 5, 1994Oct 1, 1996Motorola, Inc.Circuit probe for measuring a differential circuit
US5634208 *Mar 26, 1996May 27, 1997Nippon Telegraph And Telephone CorporationMultilayer transmission line using ground metal with slit, and hybrid using the transmission line
US6323741 *Oct 12, 1999Nov 27, 2001Lg Electronics Inc.Microstrip coupler with a longitudinal recess
EP0415558A2 *Jul 30, 1990Mar 6, 1991NGK Spark Plug Co. Ltd.Bandpass filter and method of trimming response characteristics thereof
EP0735605A1 *Mar 27, 1996Oct 2, 1996Nippon Telegraph And Telephone CorporationMultilayer transmission line using ground metal with slit, and hybrid using the transmission line
Classifications
U.S. Classification333/116
International ClassificationH01P5/16, H01P5/18
Cooperative ClassificationH01P5/185
European ClassificationH01P5/18D1