EP1366538A1

EP1366538A1 - Microstrip transition

Info

Publication number: EP1366538A1
Application number: EP02701844A
Authority: EP
Inventors: Jan Grabs
Original assignee: Saab AB
Current assignee: Saab AB
Priority date: 2001-03-05
Filing date: 2002-03-04
Publication date: 2003-12-03
Anticipated expiration: 2022-03-04
Also published as: WO2002071533A1; DE60224012T2; SE0100725D0; DE60224012D1; EP1366538B1; WO2002071533A8; SE518679C2; ES2298344T3; ATE381118T1; SE0100725L

Abstract

A device for transferring microwaves between a mechanical waveguide (115) and a microstrip line, which microstrip line comprises a conductor (140) and an earth plane (150), which are arranged on each side of a dielectric substrate (130). The microstrip line is plane-parallel to the waveguide and the waveguide has a cavity (124) adjacent to that end of the waveguide into which the microstrip line is partly inserted. The cavity is formed, for example, in the bottom (120) of the waveguide and extends perpendicular to the direction of the microstrip line. The distance (D2) between the plane in which the centre axis of the microstrip line is located and the bottom of the cavity is between lambda /8 and 3 lambda /8.

Description

MICROSTRIP TRANSITION

Technical Field

The present invention relates to the field of microwave technology and, in particular, a device for trans- ferring microwaves between a mechanical waveguide and a microstrip line, which microstrip line comprises a conductor and an earth plane, which are arranged on each side of a dielectric substrate. This type of transition for transferring microwaves will in this document also be referred to as microstrip transition.

Background Art

Microwaves are used, among other things, in radar technology, in radio links, in satellite communication, in microwave ovens, in mobile telephony and in microwave measurement technology. At the high frequencies of microwaves, special components in the form of mechanical cavity waveguides are used. When miniaturising microwave circuits, a technology called microstrip technology is used for manufacturing transmission lines etc. on printed circuit cards and in integrated microwave circuits. The microstrip technology means that conductors are applied to one side of a dielectric substrate, the other side of which consists of an earth plane. Within the field of mi- crowave technology, transitions thus have to be designed for transferring microwaves between mechanical waveguides and printed circuit cards, or circuits, which use the microstrip technology.

For instance, when power is received from a waveguide, it is desirable that the power which is obtained in the load, i.e. in the microstrip conductor, be essentially as large as the power that is supplied to the waveguide . A known way to form a transition between a mechanical waveguide and a microstrip conductor applied to a printed circuit card by using microstrip technology is to make contact between the top of the waveguide and the mi- crostrip conductor at the end of the waveguide. The contact is made, for example, by means of a metal sheet or a soldering point between the wall of the waveguide and the microstrip conductor. However, applying this metal sheet or providing this soldering point is a difficult and la- borious step and, thus, causes problem in automated manufacturing of the transition. Furthermore, there is a considerable risk of the metal sheet or soldering point not giving the desirable contact or of a breakage arising at the location of the earlier contact after some time due to, for example, temperature variations. A further disadvantage of these types of transitions is that the microstrip line becomes earthed in terms of direct current.

Summary of the Invention An object of the invention is to provide an improved structure for transferring microwaves between a mechanical waveguide and a microstrip line. Another object is that this structure should be easy to implement and suitable for automated manufacturing. According to the invention, these objects are achieved by means of a device for transferring microwaves between a mechanical waveguide and a microstrip line which has a constructive design in accordance with the features defined in the appended claims. According to the present invention, the waveguide has at one end a cavity which extends perpendicular to the longitudinal direction of the waveguide. The microstrip line is arranged plane-parallel to the longitudinal direction of the waveguide and is inserted into the waveguide end having said cavity.

In this context, the cavity constitutes a microwave resonator and is provided, for example, by a demarcated waveguide section. The cavity can preferably be formed by letting a part of the waveguide, adjacent to its end into which the microstrip line is inserted, deflect perpendicular to the longitudinal direction of the rest of the waveguide .

Since the waveguide is formed with a cavity at its end, the electromagnetic field in that part of the waveguide which precedes the cavity will interact with the electromagnetic fields in the cavity. Thus, the elec- tromagnetic field will be at its strongest at the location where the microstrip line is inserted into the waveguide. If the waveguide is terminated with a perpendicular bend, interaction between electromagnetic fields will occur correspondingly and the electromagnetic field will be at its strongest immediately before the perpendicular bend, i.e. at the location where the microstrip line is inserted.

The construction according to the present invention thus satisfies the need for a microwave transition be- tween a waveguide and a microstrip line, where the microstrip line is positioned in the same plane as the waveguide. It is advantageous to construct a microwave transition in one plane, which, among other things, allows a common cover to be positioned on the waveguide, to form its top, and on the printed circuit card on which the microstrip line is mounted. By forming the bottom and walls of the waveguide as a bottom plate and the top of the waveguide as a cover, the bottom plate and the cover being formed in blocks, for example, by casting or by some other mechanical process, the cover can easily be placed on the bottom plate and on the applied microstrip line as well as on part of, or the whole, associated printed circuit card during an automated manufacturing process. Consequently, an assembly which includes the printed circuit card, the waveguide and the transition therebetween can be manufactured in a considerably easier way. The construction also allows the microstrip line to be plane-parallel to the waveguide at the same time as the waveguide has a cavity in the end which communicates with the microstrip line. The construction thus provides advantages in manufacturing as regards, among other things, complexity and price, at the same time as it provides a high degree of efficiency.

The present construction has no direct wiring between the top of the waveguide and the conductor of the microstrip line. Preferably, there is an air gap between the top of the waveguide and the conductor of the microstrip line. This results in the above-mentioned cover being very easy to mount. Thus, the absence of metal sheets, soldering points and the like between the waveguide top and the conductor of the microstrip line further contributes to a simpler automated manuf cturing of the construction which includes the microstrip transition. The construction requires only accurate positioning of the microstrip line, but not necessarily of the top of the waveguide, any metal sheet or any soldering point. Moreover, the construction then avoids possible breaks which otherwise may arise at the contact which is provided by such metal sheets and soldering points.

Preferably, the inner dimensions of the mechanical waveguide are such that the mechanical waveguide is lim- ited to form at its end a narrow section into which the microstrip line is inserted. This section is narrower than 1/2 wavelength in free space in order to prevent the waveguide mode from leaking out of the waveguide .

In addition, it is preferred that the top of that part of the waveguide which constitutes the wall of the cavity nearest said end of the waveguide into which the microstrip line is inserted has a bevel of the edge facing the cavity. Surprisingly enough, it has been found that this bevel results in the degree of efficiency of the transition of microwaves between the waveguide and the microstrip line being reinforced. The frequency range for which the cavity gives resonance can advantageously be made trimmable after the manufacturing of said cavity. One way is to make at least one of the walls of the cavity movable, whereby displace- ment of this wall, by e.g. a screw means, affects the resonance frequency of the cavity. Alternatively, it is possible to screw in a screw directly into said cavity, the length of that part of the screw which is screwed into the cavity affecting the resonance frequency of the cavity. Those skilled in the art realise that also other shapes of the cavity are possible.

The microstrip line which is inserted into the waveguide end has no earth plane in that portion of the microstrip line which is positioned in the cavity, while the line on each side of the cavity comprises an earth plane. According to one embodiment of the invention, the conductor of the microstrip line is connected to the earth plane of the microstrip line near to that end of the line which is inserted into the mechanical waveguide and which is positioned just outside the cavity. The cavity will then reinforce the coupling between the magnetic field of the waveguide and the current loop which is thus formed at the end of the microstrip conductor, the current loop consisting of the conductor, the earth plane and part of the waveguide which defines the cavity. According to one embodiment, the conductor of the microstrip line is connected to the earth plane over the end of the substrate and, according to another embodiment, via a lead-through in the substrate. Both embodiments make it easy to manufacture a microstrip transition in a plane where the conductor of a printed circuit card is inductively coupled to the magnetic field of the waveguide .

In an alternative embodiment, the conductor of the microstrip line is unearthed at the mechanical waveguide and, thus, has the function of a capacitively operative aerial . Consequently, the conductor of the microstrip line has no connection with the earth plane of the microstrip line, and the cavity will then reinforce the coupling between the electric field of the waveguide and the microstrip conductor. By means of this embodiment, it is easy to manufacture a microstrip transition in a plane where a printed circuit card has a conductor which is ca- pacitively coupled to the electric field of the waveguide in the same way as an aerial .

Apart from the fact that the present invention im- plies an easier way to provide the printed circuit card with a cover, the invention also results in simplifications as regards the construction of the actual printed circuit card whether or not the microstrip conductor is inductively or capacitively coupled to the electromag- netic field of the microwave guide.

The cavity described above allows a high power drain in the microstrip conductor, even though this is plane- parallel to the longitudinal direction of the waveguide. Since it is desirable that the power which is supplied to the microstrip conductor be essentially as large as the power that is supplied to the waveguide, there is, however, a need of attempting to further increase the efficiency, i.e. further increasing the field strength in the waveguide. Usually, the width of the waveguide is double the size of its height. An optimal power drain is not obtained with these dimensions in a plane-parallel microstrip conductor arranged according to the invention. One way of further increasing the field strength is to reduce the height of the waveguide. Since the power in the waveguide is transmitted in the form of electric and magnetic fields, the flow area will then decrease, whereby the field strengths will increase in order to maintain the power level .

It is thus preferred that the perpendicular distance between the inner bottom of the waveguide and the inner top of the waveguide decreases gradually as regards a portion of the waveguide in the direction of, and in con- nection with, that part of the end of the waveguide which communicates with the microstrip line. In two alternative embodiments this can take place either by discrete steps or continuously. Apart from the fact that the decreasing distance between bottom and top allows a higher power drain in a plane-parallel microstrip conductor, this also leads to adaptation of the impedance of the waveguide to the impedance of the microstrip line by the impedance of the waveguide decreasing in the direction of the micro- strip line. When the size is changed by discrete steps, the steps are adapted in such a manner that the desirable impedance is obtained as regards the end of the waveguide. When the size is changed continuously, the change in size is made over a longer portion of the waveguide than in the case of discrete steps with the purpose of obtaining the desirable impedance.

It is further realised that the conductor of the microstrip line cannot only be earthed to the earth plane belonging to the microstrip technology, or unearthed with respect to this earth plane, but also connected to a matching network made by microstrip technology. The conductor can be designed in a shape that is straight, zigzag-shaped for a longer conductor length with maintained plug-in depth, or in some other shape. In the above-mentioned embodiments of the present invention, there are different shapes of the mechanical waveguide and its cavity, as well as variants of designs of the conductor of the microstrip line in order to provide a capacitive/inductive coupling between the conduc- tor and the electric/magnetic field of the cavity. Combining these shapes, alternatives and variants in order to provide an embodiment which suits the application in question, is considered to be within the scope of invention. For example, the transition can be formed by a printed board, in which the conductor of the microstrip line has not been pulled down to the earth plane of the microstrip line, being inserted into the end of a waveguide, which end comprises a cavity, the resonance frequency of which can be adjusted by means of a screw, and one wall of which has a bevelled edge, and where the waveguide for a portion beyond the cavity has an inner size that increases continuously along the waveguide in the direction away from the microstrip line of the printed board.

The following embodiments which will be shown by way of example constitute only a selection of all the combi- nations of features which can be made within the scope of invention with the purpose of obtaining an embodiment of the invention suitable for an application of immediate interest .

Brief Description of the Drawings

In the following, embodiments of the invention will be described by way of example and with reference to the accompanying drawings, in which

Fig. la is a top plan view of a transition between a mechanical waveguide and a microstrip line according to one embodiment of the invention,

Fig. lb is a cross-section along the line I-I in Fig. la,

Fig. 2a is a top plan view of a transition between a mechanical waveguide and a microstrip line according to another embodiment of the invention,

Fig. 2b is a cross-section along the line II-II in Fig. 2a,

Fig. 3a is a top plan view of a transition between a mechanical waveguide and a microstrip line according to yet another embodiment of the invention, and

Fig. 3b is a cross-section along the line III-III in Fig. 3a.

Description of Preferred Embodiments

An exemplifying embodiment will now be described with reference to Figs la and lb which show a transition, also referred to as microstrip transition, between a mechanical waveguide and a microstrip line. Fig. la is a top plan view of the microstrip transition and Fig. lb is a cross-section through the microstrip transition along the line I -I in Fig. la. With the aim of simplifying the description, the plane shown in Fig. la will in the following be referred to as the horizontal plane.

The conductive walls of the mechanical waveguide 115 are formed from a bottom plate 120 and a cover 110. The part of the cover that is placed over the waveguide part is in the shown embodiment completely plane. The bottom plate forms the bottom and walls of the waveguide while the cover only forms the top of the waveguide . The bottom plate and the cover can be formed in blocks, for example by casting or by some other mechanical working. It should here be pointed out that the cover does not need to be plane with grooves formed in the bottom plate, but the grooves can, wholly or partly, also be formed in the cover which thus is not plane. As indicated in Fig. lb, the cover does not only constitute a top of the waveguide but also a cover of the printed circuit card, on which the microstrip line is positioned.

The microstrip line comprises a conductor 140 which is also referred to as a microstrip and is arranged on one side of a dielectric substrate 130, and a conductive earth plane 150, 151 arranged on the other side of the substrate. The microstrip line is via its earth plane attached directly to the bottom plate 120 by means of layers of adhesive 160, 161 which have electrically conduc- tive properties. Alternatively, the direct contact is provided by soldering. The microstrip line is attached in such a manner that the dielectric substrate is plane- parallel to the mechanical waveguide 115, i.e. so that the extension of the microstrip line at least adjacent to the waveguide is horizontal.

As shown in Fig. lb, there is an intervening space, or an air gap, 145 between the top 110 of the waveguide and the conductor 140 of the microstrip line. Consequently, the conductor 140 has no contact with the cover 110. The conductor will have the function of an aerial which provides a capacitive coupling to the electric field in the waveguide.

At the bottom, the walls of the groove have a pair of protruding portions 128 extending perpendicular to the longitudinal direction of the waveguide at the end of the mechanical waveguide into which the microstrip line is inserted. The microstrip line is inserted into the section that is formed between these protruding portions. At said end, a portion of the waveguide forms a cavity 124 which communicates with the remaining part of the waveguide . The cavity 124 has been formed in the bottom of the waveguide by a perpendicular bend of the waveguide relative to the overall longitudinal direction of the waveguide. The cavity constitutes a microwave resonator which reinforces the electromagnetic field of the micro- waves within a frequency range that is desirable for the application. The bottom of the cavity 124 is positioned at a distance D2 from the conductor 140 of the microstrip line, preferably corresponding to 1/4 of a waveguide wavelength. The bottom of the cavity constitutes a short- circuit plane, a maximum for the electric field of the microwaves arising 1/4 wavelength from the bottom, i.e. at the location of the conductor 140.

The reinforcement of the electromagnetic field depends on the Q value of the load and is proportional to *^» . The Q value of the load indicates the ratio of the reactive power spinning in the cavity, or the microwave resonator, to the power which is taken out. A high Q value gives high fields but at the same time the resonator serves as a bandpass filter having a relative band- width which is 1/Q. It is desirable that as low Q value of the resonator as possible be chosen. However, it should be taken into consideration that high Q values also make greater demands on manufacturing tolerances. The resonance frequency should be accurate so that the transferred frequency does not fall outside the desired frequency band. In the embodiment which is shown in Figs la and lb, transition of power takes place between the waveguide and the conductor of the microstrip line by a capacitive coupling. In the embodiments which are referred to by Figs 2a and 2b, and 3a and 3b, respectively, the conductor of the microstrip line forms a current loop and transition of power between the waveguide and the conductor of the microstrip line through an inductive coupling.

In Fig. lb, on the other side of the cavity, seen from the microstrip line, the mechanical waveguide 115 has an inner size which vertically, i.e. perpendicular relative to the horizontal plane, gradually and continuously increases along the waveguide in a direction away from the microstrip line as regards a portion of the waveguide. This is brought about by the groove in the up- per side of the bottom plate gradually and continuously becoming deeper in a direction away from the microstrip line, a sloping bottom 126 being formed as regards a portion of the waveguide. This sloping bottom will cause an adaptation of the impedance of the waveguide to the im- pedance of the microstrip line by the impedance of the waveguide being reduced in the direction of the microstrip line.

A second embodiment will now be described with reference to Figs 2a and 2b which show a microstrip transi- tion between a mechanical waveguide and a microstrip line. Fig. 2a is a top plan view of the microstrip transition and Fig. 2b is a cross-section of the microstrip transition along the line II-II in Fig. 2a. On the analogy of the first embodiment, the plane shown in Fig. 2a will in the following be referred to as the horizontal plane. The designation of the reference numerals in Figs 2a and 2b has been made by analogy with the designation in Figs la and lb. However, it should be noted that in Figs la and lb, the reference numerals begin with number 1, and in Figs 2a and 2b with number 2. In the description of the second embodiment, only features distinguish- ing it from the first embodiment are stated.

On the other side of the cavity 224, seen from the microstrip line, the mechanical waveguide 215 has an inner size which vertically, i.e. perpendicular relative to the horizontal plane, gradually increases by discrete steps along the waveguide in the direction away from the microstrip line as regards a portion of the waveguide. This is achieved by the groove in the upper side of the bottom gradually and by discrete steps becoming deeper in the direction away from the microstrip line and, thus, forming a step-shaped bottom 226 as regards a portion of the waveguide. This step-shaped bottom implies an adaptation of the impedance of the waveguide to the impedance of the microstrip line.

The conductor 240 of the microstrip line is in Figs 2a and 2b connected to the conductive earth plane 251 of the microstrip line via a metallisation 242 which extends from the conductor to the earth plane over the end of the dielectric substrate 230 that is positioned beyond the cavity in the waveguide 215. Thus, an electric loop is formed by the conductor 240, earth planes 250, 251 and the part of the waveguide bottom that defines the cavity. This loop provides an inductive coupling to the magnetic field in the cavity.

Fig. 3a shows a microstrip transition between a me- chanical waveguide and a microstrip line according to a third embodiment of the invention. Fig. 3a is a top plan view of the microstrip transition and Fig. 3b is a cross- section of the microstrip transition along the line III- III in Fig. 3a. On the analogy of the previous embodi- ments, the plane which is shown in Fig. 3a will be referred to below as the horizontal plane. The designation of the reference numerals in Figs 3a and 3b have been made by analogy with the designation in Figs la and lb, and 2a and 2b. However, it should be noted that in Figs 3a and 3b, the reference numerals begin with number 3. In the description of this third embodiment, only features which distinguish it from the first and the second embodiments are indicated.

In this embodiment, the cavity 324 exhibits a bevel 325 of the edge nearest the waveguide end into which the microstrip line is inserted. Surprisingly enough, it has been found that this bevel results in the degree of efficiency of the transfer of microwaves between the waveguide and the microstrip line being reinforced.

The conductor 340 of the microstrip line is here extended to the conductive earth plane 351 of the micro- strip line via a lead-through 342 in that part of the dielectric substrate 330 which is located beyond the cavity in the waveguide 315.

In the embodiments mentioned above by way of example, different shapes of the mechanical waveguide beyond the cavity seen from the microstrip line have been described; different variants of the design of the conductor of the microstrip line for providing a capacitive/an inductive coupling between the conductor and the electric/magnetic field of the cavity have also been de- scribed. In one embodiment also a bevel of that edge of the cavity which is located at that end of the waveguide into which the micostrip line is inserted has been described. In order to limit the number of embodiments to a manageable number, all the combinations of these shapes, alternatives and variants have not been described. However, within the scope of invention it is possible to combine these shapes, alternatives and variants in order to provide an embodiment that is suitable for an application of immediate interest. Thus, the transition can, for example, be constructed with a bevel of the edge of the cavity, wherein no extension of the conductor of the microstrip line has been made to its earth plane. This can then be combined with a waveguide which has a sloping bottom or a step-shaped bottom on the other side of the cavity seen from the microstrip line.

Claims

1. A device for transferring microwaves between a mechanical waveguide (115) and a microstrip line, which microstrip line comprises a conductor (140) and an earth plane (150) arranged on each side of a dielectric substrate (130) , c h a r a c t e r i s e d in that the microstrip line is plane-parallel to the waveguide and partly inserted into one end of the waveguide, and that the waveguide has a cavity (124) adjacent to said end, which cavity extends in a direction perpendicular to the direction of the microstrip line, the distance (D2) between the plane in which the centre axis of the icro- strip line is located and the bottom of the cavity being between 1/8 and 3/8 of a waveguide wavelength.

2. A device as claimed in claim 1, wherein said distance between the centre axis of the microstrip line and the bottom of the cavity essentially is 1/4 of a waveguide wavelength.

3. A device as claimed in claim 1 or 2 , wherein the waveguide and the conductor (140) of the microstrip line are arranged in such a manner that an airgap (145) is formed between the top (110) of the waveguide and the conductor of the microstrip line.

4. A device as claimed in any one of the preceding claims, wherein the inner dimensions of the mechanical waveguide are such that the mechanical waveguide is limited to form a narrow section (Dl) into which the micro- strip line is inserted, the section not being wider than

1/2 wavelength in free space.

5. A device as claimed in any one of the preceding claims, wherein the top of that part of the waveguide which constitutes the wall of the cavity (324) nearest said end of the waveguide has a bevel (325) of the edge facing the cavity.

6. A device as claimed in any one of the preceding claims, wherein the conductor (140) of the microstrip line is unearthed.

7. A device as claimed in any one of claims 1-5, wherein the conductor (140) of the microstrip line is connected to the earth plane (150) of the microstrip line adjacent to that end of the microstrip line which is partly inserted into the mechanical waveguide.

8. A device as claimed in claim 7, wherein the con- ductor (240) of the microstrip line is connected to the earth plane (250) of the microstrip line by the conductor being pulled down (242) over the substrate (230) of the microstrip line and over that end of the microstrip line which is partly inserted into the mechanical waveguide.

9. A device as claimed in claim 7, wherein the conductor (350) of the microstrip line is connected to the earth plane (350) of the microstrip line via a lead- through (342) in the substrate (330) of the microstrip line .

10. A device as claimed in any one of the preceding claims, wherein the distance between the inner bottom of the waveguide and the inner top of the waveguide is gradually decreasing in the direction of that end of the waveguide into which the microstrip line is inserted and for a portion of the waveguide adjacent to said end.

11. A device as claimed in claim 10, wherein said gradual decreasing takes place by discrete steps (226) .

12. A device as claimed in claim 10, wherein said gradual decreasing takes place continuously (126) .