Broadband stripline coupler

Abstract: Microwave apparatus fabricated in stripline is made up of (a) printed circuitry (26U, 26L, 28U, 28L) to form a directional coupler on both sides of a sheet (24) of a dielectric material effectively suspended in space between a pair of ground planes (20U, 20L), the printed circuitry being connected by shorting posts (30) passing through the dielectric material (24); and (b) a matching section dielectric load (34), formed from a dielectric material having a dielectric constant greater than unity, overlying portions of the printed circuitry. The matching section dielectric load (34) reduces the difference between the phase velocities of the even and odd modes of propagation, so that the characteristics of the directional coupler change less with change in frequency.

Description: This invention relates to a directional coupler comprising: a first ground plane, a second ground plane, a dielectric substrate disposed between the ground planes, circuitry disposed on the substrate and having a main line with a main line coupling section and a branch line with a branch line coupling section, the coupling sections being shaped and arranged to couple microwave energy from one line to the other line, a first sheet of dielectric material disposed between the first ground plane and the said circuitry, and a second sheet of dielectric material disposed between the second ground plane and the said circuitry. It is known in the art that directional couplers fabricated in stripline (sometimes referred to hereinafter as "stripline couplers") may be used in many applications in microwave circuitry. For example, when low loss and constant dielectric properties are of paramount importance, a so-called suspended stripline coupler may be used to advantage. Such a coupler differs from a conventional stripline coupler in that a suspended stripline coupler comprises a printed circuit of appropriate shape supported (without a solid spacer having a dielectric constant greater than unity) between two opposing ground planes. The absence of a solid dielectric material reduces, as compared with a conventional stripline coupler, the susceptibleness of a suspended stripline coupler to changes in insertion loss and directivity when the frequency of an impressed microwave signal is changed. Unfortunately, a suspended stripline coupler fabricated in any known manner is satisfactorily operable on microwave energy only at a frequency that is within a relatively narrow band of frequencies encompassing a design frequency. However, many applications require satisfactory operation at any frequency within a relatively wide band of frequencies. For example, in a monopulse radar incorporating frequency diversity as a palliative against electronic countermeasures, it is necessary that a directional coupler operate satisfactorily over an extremely wide frequency band. Satisfactory operation, i.e., flat coupling over a broad frequency range, requires a multi-section coupler in which "even" and "odd" mode propagation of microwave energy occurs. An "even" mode of propagation means that an effective open circuit appears between the main and branch lines of a directional coupler. An "odd" mode means that an effective short circuit appears between the main and branch lines of a directional coupler. Because the cross sectior of suspended stripline is not homogeneous, the phase velocity of propagation in the odd mode is less than that in the even mode, with the result that flat coupling and high directivity of known multi-section couplers may not be satisfactorily achieved throughout a wide band of frequencies. In Patent Abstracts of Japan, volume 11, abstract no. 325 (E-551), published 22nd October 1987, is an abstract of Japanese patent application publication no. JP 62114301 and describes a suspended stripline coupler having first and second parallel ground planes with a pair of coplanar dielectric substrates suspended between these planes. On one of the substrates a main line is formed by conductive strips on the upper and lower surfaces, and on the other substrate a branch line is formed by substantially U-shaped conductive strips on the upper and lower surfaces. The base of the U-shape and a central section of the main line are disposed adjacent one another to act as coupling sections. A slot-like portion of the dielectric substrates is cut out between the coupling sections to provide better directivity by lessening the effect of the substrates on the wavelengths of the microwave energy and to approximately equalise the phase velocities of the odd and even modes. In "Design of an Overlay Directional Coupler by a Full-Wave Analysis" by L.Su, T. Itoh, and J. Rivera, at pages 1017 to 1022 in IEEE Transactions on Microwave Theory and Techniques Volume MTT-31 (1083), No. 12, December 1983, it is proposed that a dielectric overlay be used to improve the isolation of a microstrip coupler by reducing the differences between or equalising the odd and even phase velocities. The proposed structure has a dielectric substrate with a ground plane on its lower surface and a substantially straight main line and a substantially U-shaped branch line on its upper surface, a central section of the main line and the base of the U-shape being disposed adjacent each other as coupling sections. The dielectric overlay is substantially in the form of a truncated lozenge-shaped sheet of dielectric having six sides which is laid transversely across the coupling sections of the main and branch lines. The design of the overlay is by a full-wave analysis which is based on a spectral-domain method that involves solving an eigenvalue problem in the Fourier transform domain. A directional coupler of the kind defined hereinbefore at the beginning is described by O. Isheda, Y. Isota, M. Miyazaki, F. Takeda, and N. Takeuchi in an article entitled "An Asymmetrical Suspended Stripline Directional Coupler" at pages 333 to 334 in the Transactions of the IECE of Japan, Vol. E69, No. 4, April 1986. The main line consists of substantially straight strips on the upper and lower surfaces of the dielectric substrate, and the branch line consists of substantially U-shaped strips on the upper and lower surfaces of the substrate. The bases of the U-shapes and the central sections of the main line strips are adjacent each other to serve as coupling sections. The base section of the upper branch strip is made wider than its arm portions, and the central portion of the upper main line strip is made narrower than its end portions. Conversely, the base section of the lower branch strip is made narrower than its arm portions, and the central portion of the lower main line strip is made wider than its end portions. The wide base section of the upper branch strip and the wide central section of the lower main line strip are arranged to partially overlap each other. The purpose of this asymmetrical arrangement is to provide an intermediate coupling value, i.e. 5 to 9 dB. It is also observed there that, to improve the directivity, the first and second sheets of dielectric material are provided respectively on the first and second ground planes. US-A-3 581 243 describes a strip-type transmission line directional coupler comprising upper and lower ground planes, circuitry consisting of a substantially straight main line strip and a substantially U-shaped branch line strip arranged with its base section adjacent a central section of the main line strip, first and second sheets of a primary dielectric medium arranged respectively between the circuitry and the upper and lower ground planes, and a pair of inserts of a different dielectric medium, which are disposed either above and below the central coupling section of the main line strip, or above and below the base section of the branch line strip. It is stated that, when dielectric inserts are above and below the central section of the main line strip, the velocity of propagation of a wave in the main line is controlled by the dielectric medium of the inserts, and the velocity of propagation in the branch line is controlled by the dielectric medium of the first and second sheets. The directivity of the coupler is defined as the ratio of a response to a wave travelling in a favoured direction to a response to a wave travelling in a backward direction. It is stated that by controlling the velocity of propagation of the wave travelling on the main line with respect to the velocity of propagation of the wave travelling on the branch line, the response to the wave travelling in the favoured direction is not greatly changed but the response to the wave travelling in the backward direction is minimized, whereby higher directivity is obtained. It is further stated that when it is desired that the coupling be nearly constant over a very wide frequency range, the branch line may be made of a number of sections, such that a three section or a five section coupler is formed. With the foregoing background in mind, it is a primary object of this invention to provide a directional coupler in stripline, such coupler being operable over a wide band of freqencies. Another object of this invention is to provide a multi-section directional coupler in which the phase velocities of microwave energy propagated in either the odd or even mode of propagation are substantially the same throughout a wide band of operating frequencies. According to the present invention, a directional coupler of the kind defined hereinbefore at the beginning is characterised by a third sheet of different dielectric material so shaped and arranged to correspond to and overlap partially the main line coupling section as to equalise phase velocities of microwave energy in both the odd and even modes of propagation passing in operation through the main line coupling section, and a fourth sheet of the different dielectric material so shaped and arranged to correspond to and overlap partially the branch line coupling section as to equalise phase velocities of microwave energy in both the odd and even modes of propagation passing in operation through the branch line coupling section, the third sheet being disposed between the main line coupling section and the first sheet, and the fourth sheet being disposed between the branch line coupling section and the first sheet, and in that the first and second sheets have a dielectric constant of approximately unity. A preferred embodiment of this invention takes the form of a four-port suspended stripline coupler adapted to divide microwave power applied at a first port into unequal amounts at a second and third port, with substantially no power at a fourth port, such coupler being characterized by a matching arrangement to match the phase velocities within such couple of microwave energy propagating in the odd and even modes. Brief Description of the Drawings For a more complete understanding of this invention, reference is now made to the following description of the accompanying drawings in which: FIG. 1 is a sketch showing how the contemplated directional coupler may be incorporated in a system; and FIG. 2 is an isometric view, partially cut away and exploded, illustrating a preferred embodiment of the comtemplated directional coupler. Description of the Preferred Embodiment Referring now to FIG. 1, it may be seen that, in an exemplary application of the contemplated directional coupler, a small portion of microwave energy being transmitted may be sampled, thereby to permit monitoring of the level of the transmitted microwave energy. Thus, a transmitter 10 is connected to a first port (port #1) of a directional coupler 12, and an antenna 14 is connected to a second port (port #2) of such coupler. A power meter 16 and a matched load 18 are connected to the third and fourth ports (port #3 and port #4). The directional coupler 12 is operative in a known manner: (a) to pass the greater portion of microwave energy from the transmitter 10 to the antenna 14 and to pass the remaining portion of such energy to the power meter 16, and not to the matched load 18; and, (b) to pass any microwave energy traveling from the antenna 14 to the matched load 18 and to the transmitter 10, and not to the power meter 16. As shown in Table I, the directivity, i.e., the degree to which a stripline coupler attains the just mentioned operational objects throughout a band of frequencies, is dependent upon the structural details of any stripline coupler. An acceptable figure for directivity is 20 dB. Thus, as shown in Table I, a conventional stripline coupler based on a design suggested by S.B. Cohn, in an article entitled "Shielded Coupled-Strip Transmission Line," published in October, 1955, in the IRE (Institute of Radio Engineers) Transactions, MTT pages 29-38, is a relatively narrow band device. Similarly, a modified version of the conventional stripline coupler (which modification is accomplished simply by providing an air gap between the main line and the branch lines) is a relatively narrow band device. In contrast, the directivity of a stripline coupler, according to this invention is satisfactory throughout a frequency band having a width in the order of 18 gigahertz. DIRECTIVITY (decibels, dB) FREQUENCY (GIGAHERTZ) CONVENTIONAL DESIGN MODIFIED CONVENTIONAL DESIGN INVENTION 2 13 20 26 4 10 15 30 6 7 12 20 8 5 10 25 10 4 10 60 12 3 8 25 14 2 6 22 16 0 4 20 18 0 2 18 20 0 0 10 Referring now to FIG. 2, it may be seen that, for simplicity of illustration and clarity of explanation, elements not essential to an understanding of the invention have been omitted. For example, it will be apparent to one of skill in the art that appropriately configured connectors and transmission lines would be provided in a production model to permit use of the stripline coupler shown in FIG. 2 in a circuit such as the one shown in FIG. 1. The stripline coupler shown in FIG. 2 comprises printed circuitry (to be described) supported between an upper ground plane 20U and a lower ground plane 20L. Spacers 22U, 22L (formed from a closed cell polyimide foam material having a dielectric constant substantially equal to unity) are provided to position the printed circuitry between the upper ground plane 20U and the lower ground plane 20L. A satisfactory foam material is ROHACELL, (Registered Trade Mark of Rohm GmbH), manufactured by CYRO Industries of Orange, New Jersey. In effect then, the just described elements form a suspended stripline coupler because the printed circuitry is arranged to form a pair of directional couplers on opposing sides of a support 24. The dielectric constant of the material of support 24 is not critical. The directional coupler comprises an upper main line 26U, and a lower main line 26L, each having a five-section coupler (not numbered). Further, the directional coupler comprises an upper branch line 28U and a lower branch line 28L, each also having a five-section coupler (not numbered). Via holes plated through the support 24 form shorting posts electrically connecting (as shown by shorting posts 30) the upper and lower branch lines 28U, 28L. Similarly formed shorting posts (not numbered) are provided to connect the upper and lower main lines 26U, 26L. Finally, wall defining posts, such as those designated wall posts 32, are formed, as shown by plating through via holes adjacent to the printed lines or by inserting electrically conductive pins through vias. The wall defining posts reduce leakage effects to a minimum in a known manner. To complete the structure here contemplated, dielectric loading is provided adjacent to each one of the five-section couplers. Such loading is effective to equalize the phase velocities of the microwave energy passing through the illustrated arrangement in both the odd and even modes of propagation. It has been here realized that: (a) the concentration of the electric field in the gap between the five-section couplers is greater for the odd mode of propagation than for the even mode; (b) the phase velocity of the even mode of propagation is greater than the phase velocity of the odd mode; and, (c) the difference between the phase velocities limits the band-width of a suspended stripline coupler. Therefore, if dielectric loading is effected in such a way as to slow down the phase velocity of microwave energy propagating in the even mode, more than the phase velocity of microwave energy propagating in the odd mode, changes in characteristics with changes in operating frequency may be minimized. To accomplish the foregoing, a dielectric load (such as dielectric load 34) is disposed partially to overlap each one of the five-section couplers. As shown by dielectric load 34, each dielectric load is shaped so as to correspond with the steps in the printed circuitry making up each five-section coupler. The thickness of each dielectric load 34 is less than the spacing between the five-section couplers and the opposing ground planes 20U, 20L. Here the dielectric constant of the material of each dielectric load 34 is approximately 2.2. Such constant may, however, be varied. The position of each dielectric load relative to the associated five-section coupler may best be determined empirically to optimize the flatness of coupling over a relatively broad band of frequencies. Thus dielectric loading is provided in a suspended strip line coupler, such loading being effective to equalise the phase velocities of microwave energy in different modes of operation. The number of sections in the coupler may be changed with a concomitant change in the shape of the dielectric load being made.

Claims: A directional coupler comprising: a first ground plane (20U), a second ground plane (20L), a dielectric substrate (24) disposed between the ground planes (20U,20L), circuitry (26,28) disposed on the substrate (24) and having a main line (26) with a main line coupling section and a branch line (28) with a branch line coupling section, the coupling sections being shaped and arranged to couple microwave energy from one line to the other line, a first sheet (22U) of dielectric material disposed between the first ground plane (20U) and the said circuitry, and a second sheet (22L) of dielectric material disposed between the second ground plane (20L) and the said circuitry, characterised by a third sheet (34) of different dielectric material so shaped and arranged to correspond to and overlap partially the main line coupling section as to equalise phase velocities of microwave energy in both the odd and even modes of propagation passing in operation through the main line coupling section, and a fourth sheet of the different dielectric material so shaped and arranged to correspond to and overlap partially the branch line coupling section as to equalise phase velocities of microwave energy in both the odd and even modes of propagation passing in operation through the branch line coupling section, the third sheet being disposed between the main line coupling section and the first sheet, and the fourth sheet being disposed between the branch line coupling section and the first sheet, and in that the first and second sheets (22U,22L) have a dielectric constant of approximately unity. A directional coupler according to claim 1, characterised in that each coupling section is a multisection coupler and the respective positions of third and fourth sheets (34) of different dielectric material relative to the respective coupling sections are adjusted to equalize the phase velocities of microwave energy passing through each coupling section. A directional coupler according to claim 2, characterised in that the dielectric substrate (24) is a sheet of dielectric material having a dielectric constant greater than unity, the circuitry being formed on opposite sides of the sheet (24), with electrically conductive posts (30) passing through the sheet (24) to connect the circuitry on the two sides of the sheet (24). A directional coupler according to claim 1, characterised in that each of the first and second dielectric sheets (22U,22L) has a closed cell polyamide foam material with a dielectric constant substantially equal to unity; the dielectric substrate (24) has a first surface and a second surface with the circuitry disposed on each surface, the circuitry comprising: (i) an upper main line strip (26U) and a lower main line strip (26L) disposed on the first surface and the second surface of the dielectric substrate (24) respectively, each having a multi-section coupler, the upper and lower main line strips being connected by via holes plated through the dielectric substrate (24); and (ii) an upper branch line strip (28U) and a lower branch line strip (28L) disposed on the first surface and the second surface of the dielectric substrate (24) respectively, each having a multi-section coupler section disposed in juxtaposition with a multi-section coupler section of a corresponding main line strip to provide a gap, the gaps between the multi-section coupler sections on the first surface and the second surface of the dielectric substrate (24) adjoining a void in the dielectric substrate, and the upper and lower branch line strips (28U,28L) being connected by via holes plated through the dielectric substrate (26). A directional coupler according to claim 4, characterised in that the multi-section coupler sections form a five-section coupler.

Inventors: JOHNSON EDWARD G, LANTAGNE JOHN A, VIROSTKO MICHAEL J, JOHNSON, EDWARD G., LANTAGNE, JOHN A., VIROSTKO, MICHAEL J.
Applicants: RAYTHEON CO, RAYTHEON COMPANY
Application year: 1990
Main classification: H01P5
IPC codes: H01P5