Impedance Matching Stripline Transition for Microwave Signals

The present invention relates to an impedance matching stripline transition for microwave signals which connect a stripline circuit on a first laminate to a stripline circuit on a second laminate through a number of intermediate laminates.

In the microwave art sometimes so called stripline circuits are used instead of wave guides to conduct microwave signals. Such circuits consist of thin tapes or strips of conducting material, etched on laminates of dielectric material. An applicable area is in producing microwave antennas, described, for example, in U.S. Pat. No. 4,150,383. Such an antenna consists of dielectric slabs with the antenna elements formed as stripline circuits.

In antennas with stripline circuits of the above mentioned kind, the problem arises of impedance matching a stripline circuit on a dielectric slab to a stripline circuit on another slab. The two slabs need not necessarily be located close to each other but a connection is needed, for example, between the outermost of the slabs in the antenna, and a transition then must be provided, passing the intermediate slabs. A possibility would be to provide a connection in the form of a coaxial cable passing the intermediate slabs and to connect the terminals of the coaxial cable to the two stripline circuits. Such a connection will be relatively clumsy and, in addition, problems with the transition coax-stripline circuit will arise.

According to the invention said problem will be solved by a coaxial-like transition on the slabs such as through-plated holes, forming on the one hand a main conductor and on the other an outer conductor (screen) for the microwave signals. In that way a simple connection between the stripline circuits is obtained without any separate coaxial connection and associated transitions.

The invention will be described in detail with reference to the accompanying drawing where

FIG. 1 is a perspective view of four laminate slabs on which the transition according to the invention is provided;

FIG. 2 shows the transition on various sides of the laminate according to FIG. 1;

FIG. 3 shows the transition according to FIG. 1 in cross section; and

FIG. 4 shows an impedance matching on one of the sides according to FIG. 2.

The basic construction of a stripline transition according to the invention comprises, for example, four laminate slabs 1, 2, 3 and 4 coated on all sides with copper according to FIG. 1. The laminate slab 1 is coated on the upper side with a layer 11 of conducting material, for example copper. Through the slab there is a number of holes 12, which are through-plated and on the underside of the laminate 1 a horseshoe-shaped structure 14 of a conducting material (copper) has been formed. On the second laminate 2 a stripline circuit has been formed in known way in which conductor 21 forms an input conductor to the lead-through. The conductor 21 is terminated by a through-plated hole 24 and around the collar 24a of the hole a horseshoe-shaped structure 22 if formed, like the underside of the laminate 1 (side 2 according to FIG. 2), with through-plated holes 23 (FIG. 1). The upper side of the laminate 2 is denoted in FIG. 2 as side 3 and its underside (in FIG. 1 not shown) is denoted as side 4. On side 4 (laminate 2) the through-plated hole 24 ends in a collar 34a which along an open area 32 limited by the circular formed edge of the layer 31 lies isolated from the through-plated holes 33, constituting the continuation of the holes 12 (laminate 1) and holes 23 (laminate 2). The side 4 (laminate 2) is copper-coated like the upper side (side 5) of the laminate 3 and consists of a common ground plane for the stripline transition. On the upper side of the laminate 4 (side 7 in FIG. 1) a stripline circuit, in this embodiment not shown in detail, is arranged whose connecting conductor to the transition is denoted by 41. The conductor 41 is terminated by a collar 44 to fit the collar 35 on the underside (side 6) of the laminate 3. A horseshoe-shaped structure 42 is formed on the upper side of the laminate 4 (side 7) and has through-plated holes 43 through the laminate 4 to the coppercoated underside 45 (side 8) of the laminate 4. In this embodiment, the opening of the horseshoe-shaped structure of the holes 12, 23 and the copper surfaces 14, 22 is turned to the right in FIG. 1, while the opening of the corresponding holes 33, and the surfaces 36, 42 is turned to the left in FIG. 1.

The connection between the two layers of stripline circuits on the upper side (side 3) of the laminate 2 and the upper side (side 7) of the laminate 4 is thus, in principle, performed as a short coaxial line. The outer conductor of the coaxial line is formed by a number of through-plated holes 12, 13, 23, 33, 43 in close circular form (in FIGS. 1, 2 horseshoe-shaped) around a center conductor also consisting of through-plated holes 24, 34. The outer conductor is connected to the ground planes 31, 11 and 45 of the laminates 1-4. At the same time it serves as a suppressor of undesired modes in the microwave signals fed to the transition via the conductor 21.

In FIG. 3 the stripline transition according to FIG. 1 is shown in cross section. On the two laminates 2 and 4 the stripline layers are assembled as a complete multilayer unit, where contact between the through-plated holes of the layers is attained by reason of the construction wherein a collar, for example 24a, which indicates a hole in the center conductor, is pressed against a corresponding collar 34a in the subsequent layer. The same is valid for the outer conductor where, for example, the horseshoe-shaped section 14 is pressed against 22, the section 42 against the section 36 etc, so that the holes form a transition through the laminates. The plated holes are made in each layer, in order to obtain the highest possible flexibility in the construction. The laminate slabs are joined together, for example by gluing.

The construction makes it possible for a great number of circuits to be integrated into one unit, joined as described above, by through-plated holes. It is characteristic that the holes are not plated through the whole structure (as in ordinary multilayer techniques for printed circuits) but provide space for other circuits in the layers where the holes are not present.

For impedance matching of a transition, compensating reactances can be introduced in connecting stripline circuits. In the present construction, however, this can be achieved in a simple way by an inductive matching in the coaxial structure. No extra space is required for this. FIG. 4 shows the lead-through according to FIG. 1 from the underside of the laminate 2 ("side 4"). From the collar 34a there is a stub 31a between the collar and the ground plane 31 which bridges the isolated area 32 and which gives the inductive matching in the coaxial structure.