WO1999033139A2

WO1999033139A2 - Directional coupler for high power rf signals

Info

Publication number: WO1999033139A2
Application number: PCT/SE1998/002410
Authority: WO
Inventors: Gunnar Persson
Original assignee: Allgon Ab
Priority date: 1997-12-19
Filing date: 1998-12-21
Publication date: 1999-07-01
Also published as: AU2193999A; SE9704795D0; WO1999033139A3

Abstract

A directional coupler for coupling an RF signal from a first conduit (30) to an adjacent second conduit (20), said first and second conduits having parallel conduit portions located side by side, each of said conduit portions comprising a sandwich structure with an upper conductive strip (20; 30), an intermediate layer (1) constituted by said dielectric substrate material and a lower conductive strip connected to said upper strip, so as to eliminate any electrical field in said intermediate layer. The upper and lower conductive strip (20; 30) cooperates with upper and lower ground plane layers, respectively located at a distance therefrom, the two parallel conduit portions being surrounded sideways by air gaps (40) therebetween, whereby the RF signals will propagate with the electric fields being located substantially in air gaps surrounding the upper and lower conductive strips.

Description

DIRECTIONAL COUPLER FOR HIGH POWER RF SIGNALS

Tne present invention relates to a direction.*--.- coupler for coupling a RF signal from a first conduit to a" adjacent seconc conduit, said first and second conduits having oarallel ccnαuit portions located side by side, each of said conαu t portions comprising a strip of an electrically conducting material supported by a dielectric substrate material ana cooperating with a ground plane layer of an electrically conductive material, each of said conduit portions comprising a sandwich structure with an upper conductive strip, an intermediate layer constituted by said dielectric substrate material and a lower conductive strip, said upper conductive strip cooperating with an upper ground plane layer located at a distance therefrom and said lower conductive strip cooperating with a lower ground plane layer located at a distance therefrom, the two parallel conduit portions being separated sideways by an air gap therebetween, whereby the RF signals will propagate with the electric fields being located substantially in air gaps above the upper conductive strip and below the lower conductive strip.

Directional couplers are used generally for measuring in a given direction characteristics of radio frequency signals, e.g. to enable measurement of the return loss (or the corresponding standing wave ratio) in the feed line to an RF antenna, especially a microwave antenna, in order to monitor the proper operation thereof. In such applications, m particular m cellular telephone systems, it is of course essential that the measurements are reasonably accurate and that the coupler has a high degree of directivity with low energy losses in the measurement device.

Prior art couplers of the kind stated in the first paragraph normally comprise microstrip or striplme embodiments, where the high power RF signal propagates in various electromagnetic modes within the dielectric substrate material wnich supports the electrically conducting strip. When using standard substrate board materials, such as glass fiber reinforced epoxi resin laminate materials, it is inevitable that the dielectric properties, such as the ε value, and the thickness of the la i- nate material vary considerably. Such variations will cause an unbalance between different modes, e.g. between even and uneven modes, in terms of propagation velocity. In turn, an unbalance of this kind will cause a substantial loss in the directivity of the coupled signal and an accompanying error in the measure- ment value.

Accordingly, the object of the present invention is to provide a directional coupler which is less sensitive to variations in the substrate material and which is apt to give accurate measurement values in a wide measurement range.

This object is achieved by a directional coupler having at least one further longitudinal air gap, viz. a second air gap along the side of said second conduit portion facing away from said first conduit portion and/or a third air gap along the side of said first conduit portion facing away from said second conduit portion, the respective upper and lower strips being mutually connected for substantially eliminating any electrical field in the substrate material between them. With such a structure, the electric field component of the electromagnetic signal, which propagates along the conductive strips, will thus be located substantially in air in gaps surrounding the first and second conduits. This improves accuracy in the measurements and the ability to discriminate signals propagating in a given direction. In the respective substrate material, on the other hand, there will hardly be any signal propagation at all. Therefore, the dimensions and properties of the substrate material do not influence the characteristics of the directional coupler.

In a preferred embodiment, the substrate material is a part of a printed circuit board which carries the two conduits, associ- ated terminals and possible other components, the printed circuit board being mounted m a casing having top and bottom wall portions serving as the upper and lower ground plane layers. The strip interconnections are spaced m order to attain a suitable combination of electrical, mechanical, and cost parameters. In an intended use, it appears as though measurement performance is improved by providing a smaller width of the second conduit compared to that of the first conduit .

These and other advantageous features are stated m the _claιms and will appear from the detailed description below. Alternative forms of defining the inventive concept will be apparent from the present specification and claims.

The invention will now be explained further with reference to the appended drawings illustrating a preferred embodiment.

Fig. 1 is a top perspective view of a directional coupler loca- ted m a casing, an upper lid being taken off to make the inside visible;

Fig. 2 shows the casing of fig. 1, a printed circuit board carrying the directional coupler being taken away;

Fig. 3 shows a cross-section along the line III-III m fig. 1; and

Fig. 3a shows a modified embodiment of the conductive strips on the printed circuit board (m a detailed sectional view) .

Referring to figs. 1 and 3, there is shown a directional coupler arranged on a printed circuit board 1 located m a box-like casing 2 having a bottom wall 3, side walls 4, 5 and a top wall in the form of a lid 6. The casing 2 and the lid 6 are made of an electrically conductive material, preferably an aluminum alloy. The casing 2 with its bottom and top walls 3, 6, side walls 4, 5 and end walls 7, 8 (fig. 1) defines a rather deep (m relation to the printed circuit board 1) chamber 9 which is empty and only contains air except for the relatively thin printed circuit board 1. The latter is firmly positioned, by means of screw fasteners 11 onto a peripheral bottom shoulder 10, which extends around the contour of the bottom wall inside the side and end walls 4, 5, 7, 8. Thus, the board 1 is held at a well- defined distance from the bottom wall 3 (and also from the upper lid 6) . As appears from fig. 2, the shoulder 10 is partially continuous and partially formed as discrete projections 10a, there being threaded holes 12 distributed around the periphery of the chamber 9.

The lid 6 is fixed onto the upper edge surface of the walls 4, 5, 7 and 8 by means of screws 13 (fig. 3) fitted into threaded holes 15 (figs. 1 and 2) .

The directional coupler as such comprises electrically conductive first and second conduit portions, each m the form of a pair of oppositely located strips 30, 31 and 20, 21 on the upper and lower sides of the board 1. The strips consist of a thin metal material, such as Al or Cu, and are mutually connected by means of small holes 25, 35 extending through the board 1 and being plated with the same metal material. As an alternative the side edges of the strips may be connected by side edge metal coatings 26 and 36, as illustrated m fig. 3a. In either case, because the upper and lower strips 20, 21 and 30, 31, respectively, have the same electric potential value, there will be no electric fields within the substrate dielectric material of the board 1.

Instead, the electric field components of the electromagnetic signal propagating along the strips will be confined to the air gaps between the upper strips 20, 30 and the lid 6 and between the lower strips 21, 31 and the bottom wall 3, the lid 6 and the bottom wall 3 serving as ground plane layers.

Also, m accordance with the present invention, there is an air gap in the form of a longitudinal slot 40 between the mutually parallel pairs of strips 20, 21 and 30, 31. So, even m this region, the electric fields will be located in air.

In the region, where the first and second conduits 30, 31 and 20, 21 extend rectilmearly m parallel to each other, there are longitudinal slots 41 and 42 on the other side of each conduit portion as well, so as to keep the major part of the electric field m air.

As is known per se, the second conduit portion 20, 21 forms a part of a secondary or coupling branch for measuring the return loss or VSWR between associated terminals 22, 23 (at the outside of the casing) , whereas the first conduit portion 30, 31 forms a part of a primary or main branch of the coupler having an antenna terminal 32 and terminals 37, 38 to be connected to transmitter and receiver devices, respectively.

The sandwich structure according to the invention has proven to give a very small power loss while providing a high degree of directivity. Moreover, the production is relatively simple and cost-effective, since standard printed circuit boards can be used, the dielectric properties and dimensions (thickness) being of minor importance.

The plated holes 25 and 35 also serve to prevent the formation of any propagating or standing electromagnetic waves within the sandwich structure, between the opposite strips 20, 21 or 30, 31. Such plated holes may be replaced by other kinds of electrically conductive interconnections or discontinuities along the respective conduits. The ground plane means would be generally a box-like structure having planar surfaces but a more complex geometry could prove useful, for instance, providing different spacing to the first and second conduits

Claims

1. Directional coupler for measuring in a given direction characteristics of radio frequency signals, comprising: - a first conduit providing a path for said radio frequency signals, a second conduit adjacent to said first conduit for enabling said measuring, said first conduit having a first conduit portion and said second conduit having a second conduit portion, said first and second conduit portions being parallel and located side by side, ground plane means of an electrically conductive material, said first conduit portion comprising a sandwich structure with a first upper conductive strip, a first intermediate supporting layer constituted by a first dielectric substrate material and a first lower conductive strip, said second conduit portion comprising a sandwich structure with a second upper conductive strip, a second intermediate supporting layer constituted by a second dielectric substrate material and a second lower conductive strip, each of said conduit portions interacting with said ground plane means, a first air gap separating said first and second conduit portions, and at least one of: a second air gap along a side of said second conduit portion facing away from said first conduit portion and a third air gap along a side of said first conduit portion facing away from said second conduit portion, said air gaps being formed in said dielectric substrate materials so that electric fields will propagate substantially in air surrounding said parallel conduit portions, said first upper and lower conductive strips and said second upper and lower conductive strips, respectively, having conductive interconnections for substantially eliminating any electrical field in the dielectric substrate material between them.

2. Directional coupler of claim 1, wherein said conductive interconnections include spaced electrically conductive discontinuities .

3. Directional coupler of claim 2, wherein said conductive discontinuities have an intermediate spacing substantially less than half a wavelength of said radio frequency signals.

4. Directional coupler of claim 2, wherein said conductive discontinuities include at least one conductive layer between edges of said conductive strips.

5. Directional coupler of claim 2, wherein said conductive discontinuities are constituted by plated holes through said sandwich structure.

6. Directional coupler of claim 1, or any preceding claim, wherein said conductive interconnections include at least one continuous conductive layer between edges of said upper and lower conductive strips.

7. Directional coupler of claim 1, or any preceding claim, wherein a second width of second conduit portion is smaller than a first width of said first conduit portion to enhance directivity performance of said directional coupler.

8. Directional coupler of claim 6, or any preceding claim, wherein said second conduit portion exhibits a characteristic impedance smaller than 50 ohms and said second conduit is provided with impedance transformation means for transforming its characteristic impedance to 50 ohms.

9. Directional coupler of claim 1, or any preceding claim, wherein said dielectric substrate material is a part of a printed circuit board carrying said first and second conduits, associated terminals and possible other components, and wherein the printed circuit board is mounted in a casing having top and bottom wall portions serving as said upper and lower ground plane layers.

10. Directional coupler of claim 9, wherein at least one of said air gaps is constituted by a longitudinal slot in said printed circuit board.

11. Directional coupler of claim 1, or any preceding claim, wherein said ground plane means includes an upper ground plane layer located at a first distance from said upper conductive strips and interacting therewith.

12. Directional coupler of claim 1, or any preceding claim, wherein said ground plane means includes a lower ground plane layer located at a second distance from said lower conductive strips and interacting therewith.

13. Directional coupler of claim 11 and 12, wherein said first distance being substantially greater than said second distance.

14. A directional coupler for coupling a RF signal from a first conduit (30, 31) to an adjacent second conduit (20, 21), said first and second conduits having parallel conduit portions located side by side, each of said conduit portions comprising a strip (20, 21; 30, 31) of an electrically conducting material supported by a dielectric substrate material (1) and cooperating with a ground plane layer of an electrically conductive material, each of said conduit portions (20, 21; 30, 31) comprising a sandwich structure with an upper conductive strip (20; 30), an intermediate layer (1) constituted by said dielectric substrate material and a lower conductive strip (21; 31), said upper conductive strip (20; 30) cooperating with an upper ground plane layer (6) located at a distance therefrom and said lower conductive strip (21; 31) cooperating with a lower ground plane layer (3) located at a distance therefrom, the two parallel conduit portions being separated sideways by an air gap (40) therebetween, whereby the RF signals will propagate with the electric fields being located substantially m air gaps above the upper conductive strip and below the lower conductive strip, c h a r a c t e r i z e d i n that two further longitudinal air gaps (41, 42) are located adjacent to said conduit portions, a first one (41) along the side of said second conduit portion (20, 21) facing away from said first conduit portion (30, 31) and a second one (42) along the side of said first conduit portion (30, 31) facing away from said second conduit portion (20, 21), and m that the respective upper and lower strips are mutually connected for substantially eliminating any electrical field m the dielectric substrate material between them.

15. A directional coupler as defined m claim 14, wherein electrically conductive discontinuities (25; 35) are located along said first and second conduit portions (20, 21; 30, 31).

16. A directional coupler as defined m claim 15, wherein said discontinuities are constituted by plated holes (25, 35) through said sandwich structure.

17. A directional coupler as defined m any one of claims 14- 16, wherein side edges of the respective upper and lower strips are connected by side edge metal coatings (26; 36) .

18. A directional coupler as defined in any of claims 14-17, wherein said dielectric substrate material is a part of a printed circuit board (1) carrying said first and second conduits, (30, 31; 20, 21), associated terminals and possible other components, and wherein the printed circuit board is mounted m a casing (2) having top and bottom wall portions (6, 3) serving as said upper and lower ground plane layers.

19. A directional coupler as defined m claim 18, wherein each of said air gaps is constituted by a longitudinal slot (40) m said printed circuit board (1) .