WO2007036607A1

WO2007036607A1 - Transmission line structure

Info

Publication number: WO2007036607A1
Application number: PCT/FI2006/050411
Authority: WO
Inventors: Vladimir Stoyanov; Harri Närhi
Original assignee: Filtronic Comtek Oy
Priority date: 2005-09-27
Filing date: 2006-09-26
Publication date: 2007-04-05
Also published as: FI20055511A; FI20055511A0

Abstract

A transmission line structure suited as a transmission path for antenna feed in base stations. The circuit board technique is applied to the manufacture of a transmission line structure (400) for implementing an air-insulation between the centre and ground conductor of the transmission line. The circuit board is a multi- layer board, and a microstrip (440) functions as the line centre conductor. Material has been removed from adjacent layers (432, 433) at the centre conductor and its sides, and a cavity thus formed functions as an air-insulation between the centre conductor (440) and ground conductor (410) of the transmission line. The ground conductor is a part of the metal body of the device containing the transmission line. The structure can comprise several transmission lines, each of which has a cavity of its own in the same multi-layer board. The structure can also be two- sided so that there are similar cavities and a ground conductor on each side of the layer, which comprises the centre conductor(s). The losses of a transmission line are relatively low, and its production costs also are relatively low. Other functional parts, such as directional coupler and power divider, can be integrated to the structure without adding the stage number of the manufacture.

Description

Transmission line structure

The invention relates to a transmission line structure, which is suited to a transmission path for antenna feed in base stations, for example.

The aim of a transmission line is to transfer electromagnetic energy from a site to another site. Therefore, a basic requirement for a transmission line is that the losses taking place in it are relatively low. In addition, a good shielding against external interfering fields is desirable especially in communication lines. Mechanically, a transmission line has to be so firm that a normal treatment and a strain during the use do not cause transformations in it and thus deterioration in its elec- trie characteristics.

The coaxial cables and in microwave range also waveguides are largerly used as antenna feed lines, among other things. Especially the waveguides have low losses and good shielding against interferences. The same holds in the case of coaxial cable, if it is made to have a relatively large diameter and to be almost air- insulated such that the centre conductor has been separated from the outer conductor only by dielectric discs. However, these transmission line types have disadvantages such as high price and relatively laborious mounting. A more inexpensive solution is to implement a transmission line by circuit board technique so that the conductors are microstrips on a surface of a dielectric plate. In a co-planar structure a ground conductor travels on both sides of the centre conductor. When using a multilayer board, ground conductors can be also both above and below the centre conductor. A disadvantage of that kind of transmission lines made by circuit board technique are relatively high losses.

Fig. 1 shows an example of a known transmission line structure, in which the cir- cuit board technique is applied so that the electric characteristics will be better than of usual circuit board lines. The drawing presents a cross-section of the structure. There is a transmission line 100 with a gutter-like metal body 110 and a one- layer circuit board 120. The circuit board comprises a dielectric plate 130 and a microstrip 140 on upper surface of the plate. The circuit board is located below the metal body against the edges of the gutter formed by the body so that a closed inner space, or a transmission line cavity, exists between the circuit board and the gutter. The microstrip 140 is located in that cavity and it functions as the centre conductor of the transmission line. The metal body 110 in turn functions as the ground conductor of the transmission line. The body, or the ground conductor, is preferably made of aluminium, and it is relatively bulky. The losses of the line are low compared with usual circuit board lines, because the field propagating in the line is mostly located in the airspace between the circuit board and the metal body, and only to a small extent in the circuit board material. Therefore the dielectric losses are relatively low. Instead, the production costs are higher compared with usual circuit board lines, and the line takes more space. Certainly the metal body 110 can be a part of the supporting structure of the whole device containing the transmission line, in which case not much additional space is required for the line.

Fig. 2 shows a known version of the transmission line structure in Fig. 1. Now the structure comprises a transmission line 200, which is two-sided so that it has a structure similar to that presented in Fig. 1 and a mirror image structure of it. The symmetry axis is inside the dielectric plate 230, which plate is shared between the halves of the transmission line. Thus the number of the body pieces is two. The first body piece 21 1 is located against the upper surface of the plate 230, and the second body piece 212 is located against the lower surface of that plate. The di- electric plate 230 belongs to the circuit board 220, which further comprises the centre conductor 240 of the transmission line. Also the centre conductor is two- sided; it comprises a first microstrip 241 on the upper surface of the plate 230 and a second microstrip 242 on the lower surface of that plate. These microstrips are connected to each other by vias, such as the via 243 visible in the figure, the vias being located at regular distances from each other. The circuit board may also comprise more than one centre conductor, in which case the whole structure correspondingly comprises more than one transmission line. The body pieces then function as shared ground conductors between all transmission lines.

The losses in the structure according to Fig. 2 are lower than in the structure ac- cording to Fig. 1 , although they would be made to have the same cross-sectional area. This is due to that, compared with the line according to Fig. 1 , a smaller proportion of the electromagnetic field propagating in the line of Fig. 2 is located in the dielectric material of the circuit board. In addition, the shielding against the external interfering fields is better and, in the case of several parallel lines, the coupling between the lines is weaker, i.e., the isolation is better. Also the transmission line structure according to Fig. 2 has the disadvantage that the production costs are relatively high.

In this description the transmission line is also called line, for the sake of briefness. "Line" then means the same as "transmission line". An object of the invention is to alleviate the disadvantages associated with the prior art. The transmission line structure according to the invention is characterized in that which is specified in the independent claim 1. Some advantageous embodiments of the invention are specified in the other claims.

The basic idea of the invention is as follows: The circuit board technique is applied to the manufacture of a transmission line structure for implementing an air- insulation between the centre and ground conductors of the transmission line. The circuit board is a multi-layer board, and a microstrip being located on the surface of a layer functions as the line centre conductor. Material has been removed from adjacent layers at the centre conductor and its sides, empty space thus being formed in the structure. The empty space, or the cavity, functions as an air- insulation and it ends in a conductor plane, which functions as a ground conductor of the transmission line. That conductor plane is preferably a part of the metallic body of a device containing the transmission line. The structure can comprise sev- eral transmission lines, each of which has a cavity of its own in the same multilayer board. The structure can also be two-sided so that there are similar cavities and a ground conductor on each side of the layer, which comprises the centre conductor(s).

The invention has the advantage that the losses of a transmission line according to it are relatively low. This is due to that most of the field propagating in the line is located in said cavity and not in the dielectric material. This matter is contributed, if the layer comprising the line centre conductor is made as thin as possible. Another advantage of the invention is that the production costs of a transmission line according to it are relatively low. This is due to that relatively cheap materials can be used in the structure and its manufacture can be automated almost completely. A further advantage of the invention is that the structure according to it is firm and can be easily mounted. A further advantage of the invention is that also other functional parts, such as directional coupler and power divider, can be integrated into the structure without adding the stage number of the manufacture.

The invention is described in detail below. The description refers to the enclosed drawings, in which

Fig. 1 presents an example of a known transmission line structure, in which the circuit board technique is applied,

Fig. 2 presents a known version of transmission line structure in Fig. 1 , Fig. 3 presents another example of a known transmission line structure, in which the circuit board technique is applied,

Fig. 4 presents an example of a transmission line structure according to the invention, Fig. 5 presents an example of the structure according to Fig. 4 seen from a- bove,

Fig. 6 presents a version of transmission line structure in Fig. 4, and

Fig. 7 presents another example of a transmission line structure according to the invention.

Figs. 1 and 2 were described already in connection with the description of prior art.

Fig. 3 shows an example of a transmission line structure, the main principle of which is known of the publications US 6,266,016 and Fl 1 13580. The drawing presents a cross-section of the structure. There is a transmission line 300 comprising a centre and ground conductor, which line is wholly implemented by a multi-layer circuit board. The layers are, from bottom to top, a basic layer 331 , a first raising layer 332, a second raising layer 333 and a covering layer 334. The centre conductor of the transmission line is a microstrip 340 on the upper surface of the basic layer. So the number of the raising layers is two in this example. By means of them the transmission line is made to be air-insulated, i.e. an air-filled cavity 360 is formed between the centre and ground conductor. For that purpose a portion is removed from the raising layers, the portions being located at the centre conductor 340, as seen from above. The width of the removed portions is greater than the width of the centre conductor so that the formed cavity extends to both sides of the centre conductor. The covering layer 334 closes the cavity 360. The ground con- ductor 310 of the transmission line is a conductive coating of the covering layer on its lower surface at the cavity.

Fig. 4 shows another example of a transmission line structure according to the invention. The drawing presents a cross-section of the structure. There are a transmission line 400 comprising a centre and ground conductor and a multi-layer cir- cuit board with a basic layer 431 , a first raising layer 432 and a second raising layer 433, like in Fig. 3. Unlike the structure in Fig. 3, the multi-layer circuit board now does not have a covering layer with the ground conductor. Instead of them there is a metal body 410, which closes the cavity 460 and functions as the ground conductor of the transmission line. Preferably the metal body is a part of the me- chanical structure of a device, which contains the transmission line, such as a base station.

The impedance of the transmission line is mostly determined by the widths of the centre and ground conductor and by the distance therebetween. This distance in turn is determined by the number and thickness of the raising layers, in compliance with what is described above. The raising layers are arranged so that the eventual line impedance has the desired value. The width of the centre conductor is e.g. 6.5 mm and the width of the ground conductor and cavity is e.g. 15 mm. The distance between the centre and ground conductors, or the cavity height, is e.g. 1.7 mm.

The losses in the transmission line according to Fig. 4 are relatively low for the same reason as in the structures according to Figs. 1 and 3: The field propagating in the line is mostly located in the line cavity and only to a small extent in the circuit board material. Here that advantage is obtained using only circuit board technique, which is advantageous manufacture-wise. Naturally the dielectric losses are the lower the less there is circuit board material in the places where the field strength is significant. For this reason especially the basic layer 431 is made as thin as possible. In practice its thickness is e.g. 0.4 mm.

Fig. 5 shows an example of the structure according to Fig. 4, as seen from above and the topmost part removed so that the cavity is open and the centre conductor is visible. The transmission line of the example is straight, and its one end is seen.

The reference numerals of Fig. 4 are used in the drawing. The centre conductor

440 is located on the basic layer 431 in an elongated space confined by the raising layer 433. The portions of the raising layer travelling on different sides of the cen- tre conductor unite at the line end. The centre conductor continues (not visible) through a via to an antenna radiator, for example. Naturally also the line ground conductor is in that case connected to the antenna structure.

Fig. 6 shows a version of the transmission line structure in Fig. 4. The structure comprises a transmission line 600, which is now two-sided so that it has a struc- ture similar to that presented in Fig. 4 and a mirror image structure of it. The symmetry axis is inside the basic layer 631 , which layer is shared between the halves of the transmission line. Above the basic layer there are the first 632 and second 633 raising layer, and below the basic layer there are similar third 635 and fourth 636 raising layer. Above the second raising layer there is a first ground conductor 61 1 as a "roof" of the first cavity 661 , and correspondingly below the fourth raising layer 636 there is a second ground conductor 612 as a "floor" of the second cavity 662. At least one of the ground conductors belongs to the metallic body of a greater device. Also the centre conductor 640 is two-sided; it comprises a first mi- crostrip 641 on the upper surface of the basic layer 631 and a second microstrip 642 on the lower surface of the basic layer. These microstrips are connected to each other by vias, such as the via 643 visible in the figure, the vias being located at regular distances from each other.

Compared with the one-sided structure in Fig. 4, the losses in the structure according to Fig. 6 are even lower, although the structures would be made to have the same cross-sectional area. In addition, the shielding against the external interfering fields is better. The reasons are the same as in the case of Fig. 2.

Fig. 7 shows a third example of a transmission line structure according to the invention. The structure 700 comprises in this example two transmission lines and power dividers. In the drawing it is seen from above, and the topmost part re- moved so that the cavities are open and the centre conductors are visible. The first transmission line consists of the first centre conductor 740, the shared ground conductor and their intermediate materials. These materials comprise substantially air. Correspondingly the second transmission line consists of the second centre conductor 750, the shared ground conductor and their intermediate materials. The parallel first and second centre conductor are located on the basic layer 731 , each in an elongated space of its own, confined by the raising layer(s) 732. The first transmission line leads to a first divider 771 , in which the line branches into two parts. Correspondingly, the second transmission line leads to a second divider 772, in which that line branches into two parts. The number of the outputs is then four. The structure can be used for feeding of an array antenna, which comprises four radiators. Each output of each divider will be connected to one radiator. The conductor widths in the dividers are dimensioned so that the incoming energy is divided half-and-half and the impedance matching is maintained.

The directional epithets "above", "below", "upper" and "lower" refer in this descrip- tion and claims to the position of a transmission line structure, where its multi-layer board is horizontal so that at least above the basic layer there are parts belonging to the transmission line. The use position of the structure can naturally be whatever. "Multi-layer circuit board" means in this description and claims a circuit board, which comprises at least two dielectric layers one on the other.

Claims

1. A transmission line structure (400; 600; 700) comprising a ground conductor (410; 611 ) and at least one centre conductor (440; 640; 740, 750), which is a mi- crostrip on upper surface of a dielectric basic layer (431 ; 631 ; 731 ) of a multi-layer circuit board, and the multi-layer circuit board comprises above the basic layer at least one dielectric raising layer (432, 433; 632, 633), which lacks material, seen from above, at the centre conductor (440; 641 ) in an area having greater width than the centre conductor, to form an air-filled transmission line cavity (460; 661 ) between the centre conductor and the ground conductor, characterized in that it is combined with a device having a metallic body so that the ground conductor closing the transmission line cavity belongs to said metallic body (410; 61 1 ).

2. A structure according to claim 1 , characterized in that a thickness of said basic layer is not more than 0.5 mm to reduce losses, which take place in the basic layer.

3. A structure according to claim 1 , characterized in that it is two-sided so that below the basic layer (631 ) there are, symmetrically with said transmission line cavity (661 ) and the ground conductor (61 1 ), a second transmission line cavity (662) and a second ground conductor (612), and the centre conductor (640) further comprises a second microstrip (642) being located on the lower surface of the basic layer and vias (643) being located at regular distances from each other, by which vias said microstrips are connected to each other.

4. A structure according to claim 1 , characterized in that, to form more than one transmission line, the number of centre conductors (740, 750) being supported to the basic layer (731 ) is at least two, and the multi-layer circuit board comprises a separate cavity, closed by the ground conductor, for each centre conductor.

5. A structure according to claim 4, characterized in that it is two-sided so that below the basic layer there are, symmetrically with said transmission line cavities and ground conductor, other transmission line cavities and a second ground con- ductor, and each centre conductor further comprises another microstrip being located on the lower surface of the basic layer and vias being located at regular distances from each other, by which vias the two microstrips of a centre conductor are connected to each other.