Transmission line comprised of a center conductor on a printed circuit board disposed within a groove

The invention relates to a transmission line (1) comprising:

FIELD OF THE INVENTION

The present invention concerns a transmission line. Further, it concerns a method for production of a transmission line.

BACKGROUND ART

The use of microstrip lines with air as the dielectric has been used in base station antennas since at least the mid-90s. The advantage is low loss and ease of manufacture in the sense that a flat ground plane can be used for the lines. The disadvantage of such lines is the necessary width, which for example at a 3 mm separation is 13 mm for a 50 ohms line. The absence of a dielectric, other than air, also means that the mutual coupling is rather high.

As another prior art, in respect of the present invention, U.S. Pat. No. 5,652,557 could be mentioned. This patent discloses a transmission line consisting of a longitudinal groove in a dielectric material having longitudinal walls and bottom disposed with a metallisation film. In the groove, a conductor line is formed by a metal film.

However, the conductor of U.S. Pat. No. 5,652,557 is manufactured by disposing a conductor film on a dielectric. Such a production method can be complex and expensive, yielding an expensive transmission line.

SUMMARY OF THE INVENTION

It is an object of the present invention to propose a solution for or a reduction of the problems of prior art. A main object is consequently to propose a transmission line having beneficial electric properties and at the same time is both improved in construction and easy to employ.

According to one aspect of the invention this is accomplished by a transmission line. Using a center conductor comprising at least one conductor formed on a side of a printed circuit board (PCB) provides for a simple and relatively inexpensive construction. Another proposition could be to use a piece of metal as the center conductor. However, compared to such a piece of metal, a PCB offers prospects of better dimensional accuracy, lower weight, and possibly also lower cost than sheet metal.

According to another aspect of the invention, one or more of the above objectives is achieved with a method for production of a transmission line. This is a straightforward method of production that is simple to employ.

DETAILED DESCRIPTION OF THE INVENTION

A conceptual embodiment of the invention is shown inFIG. 1a. It is a transmission line1that comprises:a groove2defined by two parallel conducting walls3and a conducting floor4all electrically connected to each other, together forming a peripheral conductor of the transmission line, anda center conductor5, at least partly submersed in the groove2, the center conductor5being isolated from the conducting walls3and the conducting floor4of the groove2. The center conductor5comprises at least one conductor formed on a side of a printed circuit board6. The transmission line1is operating in near transverse electromagnetic mode (TEM-mode) or quasi TEM-mode. The small discrepancy from a full TEM-mode is due to the different permittivity of air and the dielectric material in the thin PCB dielectric surrounding the center conductor.

Typically, such a conductor formed on a side of a printed circuit board would be a conducting strip formed on the printed circuit board. However, other conductors are, in theory, not ruled out; it is conceivable to have other types of conductors than strips formed on the printed circuit board.

The design of the transmission line as inFIG. 1aensures efficient use of the conducting strip area, since the field is distributed on both sides of the strip due to the surrounding walls and floor of the groove. This means a lower loss compared to a microstrip transmission line. At the same time the open configuration brought about by the groove provides for good accessibility of the line during assembly while still affording mechanical protection of the center conductor. The production of such a transmission line could be configured to be easy and relatively inexpensive.

An alternative to the PCB would be to use a piece of sheet metal. However, compared to such a solution, the PCB may provide better dimensional accuracy, lower weight, and possibly also lower cost than such a piece of sheet metal.

InFIG. 1ba transmission line1is shown having a similar structure to that ofFIG. 1awith corresponding structural elements2,3,4as described above, wherein the center conductor5comprises two conductors on two sides of the printed circuit board6, i.e. both sides of the PCB are used for the center conductor. This solution provides for a slightly lower impedance over a frequency range from 0.50-2.0 GHz according to simulations, see below.

Simulations of the attenuation of the transmission lines inFIGS. 1aand1brespectively are shown inFIGS. 2aand2b.FIG. 2acorresponds toFIG. 1aandFIG. 2bcorresponds toFIG. 1b.FIGS. 1a/2aare for a one-sided, 200 mm copper line on a 20 mil (0.508 mm) thick Rogers4003substrate.FIGS. 1b/2bare for a two-sided, 200 mm copper line on a 20 mil (0.508 mm) thick Rogers4003substrate.FIGS. 2aand2bshow attenuation vs. frequency between two ports (waveport1, waveport2). For both cases, total width of the groove2was 8 mm (corresponding to the floor4), depth was 15 mm (corresponding to the walls3). Material of walls and floor was aluminium. Line width was 11 mm, line thickness was 35 μm. As can be seen, attenuation is a bit less for the two-sided case ofFIGS. 1b/2b.

Returning toFIGS. 1aand1bfor awhile, the center conductor5of the transmission line1, could be formed on the printed circuit board6by etching. This means that a high dimensional precision of the center conductor could be accomplished. Such precision could be attained using standard, and therefore inexpensive, etching equipment well known in the electronics industry.

In principle, the printed circuit board6of the transmission line1could be positioned in the groove2with its main surfaces perpendicular to the walls3of the groove2. However, when the printed circuit board6of the transmission line1is positioned in the groove2with its main surfaces in parallel with the walls3of the groove2, a more efficient use of the conducting strip area is achieved, resulting in a lower attenuation of the transmission line.

The walls3and the floor4of the transmission line1according to the invention could be constructed in many different ways. They could be separate parts that are joined together by suitable means or an integral part. Such separate parts or integral part could for instance be an aluminium part, such as an aluminium extrusion. Also, as seen inFIG. 3, at least a part of any of the walls3and the floor4defining the groove could also be a part of an antenna reflector7. This enables a rational construction wherein e.g. the back of an antenna reflector can be reused as a part of the groove, thus providing a dual function.FIG. 3illustrates in a side elevation view an antenna reflector having mounted at the back of it a plurality of transmission lines1. Also shown inFIG. 3is a slidable dielectric8positioned in a space between the center conductor5and at least one of the parallel conducting walls3. Due to the slidable dielectric, the impedance of a part of the transmission line could be controlled, for instance in order to vary/tune the phase of a signal on the transmission line. The dielectric could be slidable in a suitable direction. InFIG. 3that could be in the direction in or out of the paper. Alternatively it could be in a direction in or out of the groove, i.e. left or right in the figure.

AsFIGS. 4a-4cshow, a System10comprising a plurality of transmission lines1according to any of the previously described transmission lines of the invention could be formed, wherein at least two center conductors5(FIGS. 4a,4c) of the plurality of transmission lines are connected to each other with a conducting element9(FIGS. 4b,4c) in order to provide for a dividing of a signal that can enter the system10on one of the plurality of transmission lines into at least one other transmission line. The conducting element9is shown inFIG. 4a-4cgoing through a port between the two grooves of the figure.

FIGS. 4a-4call show the same system10from different angles and in the case ofFIG. 4c, also in some translucency in order to clarify the design and placement of a PCB6with its center conductor5in the system.

In order to provide for a rational and economical production of such a system10, at least two center conductors could be formed on a same printed circuit board6. That would yield at least two conductors using only one PCB, as can be seen inFIGS. 4a-4c. The forming of at least two center conductors on the same PCB also has the further benefit that the conducting element9between them could be a conductor formed on the printed circuit board6. In that way, there would not be any requirements for the soldering of wires between the transmission lines.

A figure similar to that ofFIGS. 4a-4cis shown inFIG. 5with the addition of a slidable dielectric8. This is basically a three dimensional schematic view of the slidable dielectric ofFIG. 3, showing only two transmission lines1though.

With reference toFIG. 6, for the case wherein at least two center conductors5in the system of the invention are on separate printed circuit boards6, it would be beneficial if the conducting element9extends through one of the walls3and the floor4, that is either through one of the walls or through the floor, of the groove of at least one of the transmission lines, preferably both of the lines. This is shown inFIG. 6, where a cut out in the middle wall is effected in order to provide space for the conducting element9. It should be noted that the walls and floors of the at least two transmission lines should share the same voltage potential, in order to avoid any differential modes. In particular, it is important to maintain the same potential of the walls in both transmission lines. It can be seen inFIG. 6that the two center conductors5are connected with a conducting element9. Further, in order to maintain the same potential of the walls in both transmission lines, a conductor13is provided connecting both (outer) walls3.

A system10as has been described above can, as an example, be put to use as an antenna feed, feeding antenna elements with signals. Thus, such a system could replace the usual coaxial cables in such an application. This could simply be accomplished by having a system of at least two transmission lines according to the invention connected to a corresponding at least two antenna elements11. (Of course, the use of a singular transmission line of the invention to feed an antenna element is also possible, however that can be thought of as just a singular transmission line rather than a system of transmission lines.) In comparison to a system using coaxial cables, this solution has the prospect of being both inexpensive and allowing for improved production. Such a system in accordance with the invention is depicted inFIGS. 7aand7b.FIG. 7ashows a system of transmission lines1connected to the back of an antenna reflector (no antenna radiating element is shown inFIG. 7a). The different transmission lines1inFIG. 7aare connected to each other in a way shown schematically inFIG. 7b. An antenna feed signal can be supplied at the tail12inFIG. 7b. This signal is then distributed to the different antenna element pairs11by the successive forking of a transmission line into new transmission lines. This can be seen inFIG. 7b; each new fork level (or T-branch level) results in a doubling of the number of branches of the transmission lines. Each level is depicted with a roman numeral (i, ii, iii, iv, respectively) inFIG. 7band each such corresponding level is indicated in the schematicFIG. 7a. In the end, the transmission lines are connected to an antenna element pair11, as seen inFIG. 7b.

The invention also comprises a method for production of a transmission line comprising the steps:

a) manufacturing a groove2defined by two parallel conducting walls3and a conducting floor4all electrically connected to each other, together forming a peripheral conductor of the transmission line, and

b) submersing the center conductor5, at least partly in the groove2, such that the center conductor5is isolated from the conducting walls3and the conducting floor4of the groove2, distinguished in that in step b: submersing a center conductor5that comprises at least one conductor on a side of a printed circuit board6.

The manufacture of a groove2in step a) could be accomplished in many ways. For instance by milling in a solid body such as a metal body. Another alternative would be to create a metal profile by extrusion or folding of a metal sheet. A third alternative would be to connect different separate parts that together form the groove. It should be emphasised that any embodiment of the transmission line and the system described above, could be produced in a corresponding step in the method of the invention.