EP1389353A1

EP1389353A1 - Transmission line

Info

Publication number: EP1389353A1
Application number: EP02745389A
Authority: EP
Inventors: Francisco Javier Vazquez Sanchez
Original assignee: ERA Patents Ltd
Current assignee: ERA Patents Ltd
Priority date: 2001-05-23
Filing date: 2002-05-22
Publication date: 2004-02-18
Anticipated expiration: 2022-05-22
Also published as: EP1389353B1; EP1263077A1; DE60202061D1; DE60202061T2; WO2002095864A1; ATE283554T1

Abstract

A device for transmitting electromagnetic energy is provided which comprises a conductive sheet (1), the conductive sheet having: a conductor portion (4); at least one ground plane portion; and at least two rows of slots (3) formed in the conductive sheet separating the conductor portion (4) from each ground plane portion, the slots in each row being periodically spaced from one another and adjacent rows of slots being shifted relative to one another. In use the conductor portion transmits electromagnetic waves. The use of a dielectric material is not required, substantially reducing cost and ease of manufacture. Microstrip-like patch radiators can also be implemented from a dielectric-less microstrip by enlarging the central region of the line. <IMAGE> <IMAGE>

Description

TRANSMISSION LINE

The present invention relates to electromagnetic transmission lines. Microstrip, stripline and coplanar transmission lines have been developed as planar structures to guide electromagnetic energy. These planar structures can be used to implement circuits with surface mounted components and beamforming networks for planar antennas . Transmission lines of this type typically require a continuous dielectric substrate or periodic spacers to maintain the correct separation between the transmission line conductors. Most commonly they are fabricated on dielectric substrates using low cost photolithographic techniques.

However, dielectric materials introduce additional losses which degrade the performance of the transmission lines. The choice of suitable dielectric materials is limited, since any material must have low electrical losses and must comply with often stringent mechanical, chemical and thermal requirements. In some applications which involve particularly harsh environments, for example, space, aircraft, military, etc., dielectric materials cannot be used at all . Power handling is also limited due to electrical breakdown of the dielectric materials in the presence of high electromagnetic fields. Mechanically rigid, chemically stable dielectric materials with low losses at microwave and millimetre wave frequencies are expensive, becoming a limiting factor in the reduction of the cost of antenna and microwave technology.

The development of large planar array antennas (eg for Ku-Band TVRO applications) has largely been limited by the losses in low cost dielectric substrates, since the gain of array antennas is physically limited by the insertion losses of the beamforming network. The need to use high quality dielectrics is a major cost for planar antennas, reducing the economic advantage of using simple manufacturing techniques .

The suspended stripline concept was developed to avoid the costs and losses associated with high quality dielectric materials used in large antenna beamformer applications, see for example US-A-4614947 and US-A- 4527165. The suspended stripline concept is based on using a thin sheet, typically less than 0.15mm thick, of dielectric material to support the central conductor of the stripline. As it is very thin, the contribution to losses from the dielectric substrate is minimised. This means that lower cost dielectric materials can be used, producing acceptable line attenuation (typically 2-4dB/m at 10GHz) at an acceptable price.

However, there are a number of problems with the suspended stripline concept. Firstly, it still employs a dielectric substrate, albeit thin. Therefore, dielectric losses are not completely removed, and they can still represent a significant proportion of total losses, typically more than 50%, particularly when low cost dielectric materials are used. Secondly, even if the cost of the dielectric sheet is minimised by selecting low cost materials such as mylar, polystyrene or kapton, it can still represent about 20-30% of the total cost of the transmission line. Thirdly, as the dielectric sheet should be kept thin, it is not rigid and therefore not self- supporting. As a result, the channel width of suspended striplines is limited by mechanical constraints to a few millimetres (typically < 10mm) . Finally, the suspended line concept is very difficult to implement in open structures such as microstrip or coplanar lines.

According to a first aspect of the present invention, a device for transmitting electromagnetic energy comprises a conductive sheet, the conductive sheet having: a conductor portion; at least one ground plane portion; and, two parallel rows of slots formed in the conductive sheet, separating the conductor portion from the or each ground plane portion, the slots in each row being periodically spaced from one another along the row and adjacent rows of slots being shifted relative to one another in the row direction. Preferably, the slots are all of identical length. Preferably, adjacent rows of slots are shifted relative to one another by half the length of the periodic spacing of the slots within each row.

Preferably, the conductor portion is formed centrally on the conductive sheet, extends from one edge of the conductive sheet to another and is separated from the ground plane portions by two rows of slots formed on both sides of the conductor portion. More than two rows of slots may be used to separate the conductor from the ground plane to implement a multiconductor transmission line.

In an alternative embodiment, the conductor portion does not extend from one edge of the conductive sheet to another and comprises a widened end section, which in use forms a radiating element. The present invention provides a low cost alternative to existing transmission lines and radiating elements, as there is no need to use expensive dielectric material in a device according to present invention. Furthermore, transmission lines and radiating elements according to the present invention can be more finely tuned than conventional transmission lines and radiating elements and present minimal scattering of incident electromagnetic waves .

In one embodiment of the invention an additional conductive sheet is placed in a plane parallel to the slotted conductive sheet. In an alternative embodiment two additional conductive sheets are placed parallel to the slotted conductive sheet, one above and one below the slotted conductive sheet . In further embodiments a conductive channel may back or completely surround the slotted conductive sheet.

In a second aspect of the invention a filter includes a device according to the first aspect of the invention, wherein the frequency response includes a passband and selective attenuation at frequencies outside the passband. Examples of the present invention will now be described with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram of a transmission line according to present invention;

Figure 2 is a graph showing the transmission characteristic of the transmission line shown in Figure 1; Figure 3 is a schematic showing alternative embodiments of the present invention; and,

Figure 4 is a schematic diagram of a further embodiment of the present invention. The dielectric-less transmission line described with reference to Figure 1 is coplanar (one metal plane) but the present invention can be embodied as a microstrip transmission line (two metal planes) or in stripline form

(three metal planes) . The transmission line of Figure 1 consists of a metallic sheet 1, which is perforated by two rows of slots 3, periodically distributed in a given direction. Adjacent rows of slots are displaced relative to one another by half the periodic inter-row spacing. The slots define a central metallic region 4 which acts as the conductor of the transmission line itself, two inter-slot metallic regions between adjacent rows 5, and ground planes 2 on both sides. In the example shown, the centres of slots in each row are separated by about half a wavelength at the nominal operating frequency, and slots in adjacent rows are offset by about a quarter wavelength.

A TEM type wave, periodically loaded, is supported by the structure and it can be used to guide electromagnetic energy between the central conductor 4 and the two outer ground planes 2. In an alternative embodiment the conductor can be formed at the edge of a conductive sheet, with two rows of slots separating the conductor from the rest of the sheet. The structure can be seen as a multi-conductor transmission line in which the conductors are short- circuited at alternated periodic intervals. In a simple coplanar line configuration, two possible symmetrical TEM waves are supported by the structure . One TEM mode (Ml "resonating mode") is defined between the outer ground plane 2 and the central conductor 4, with the intermediate conductors at the same potential as the ground plane . A second TEM mode (M2 "propagating mode") is defined when the intermediate conductors are at the mean potential between the central conductor 4 and the ground plane 2. The structure can be seen as a transmission line supporting symmetrical mode M2, loaded periodically with short circuited stubs of line supporting the mode Ml. The length of those stubs is half of the slot length and its period is the same as the slots 3. The separation of the slots within each row is less than a tenth of the wavelength of the highest frequency of operation. The separation between adjacent rows is about half the pitch of the slots. The frequency response of the dielectric-less line is similar to some types of bandpass filter based on periodic loading of transmission lines with λ resonating stubs . The transmission line propagates energy in the frequency bands where the length of the slots 3 is close to nλ/2 (n is an integer) . The line provides selective filtering of signals outside the primary operating band of the transmission line. A typical frequency response is shown in Figure 2. The width of the transmission band can be controlled using the slot width and separation, as these parameters define the characteristic impedance of the resonating mode Ml. The width of the central region can be used to control the impedance of the line since it defines characteristic impedance of the propagating mode M2. Very large transmission bands, typically up to 80%, can be achieved through proper design of the line. The line characteristic impedance and propagation constant change with frequency, especially at the edges of the transmission band. In the central part of the band the line exhibits very low dispersion.

Multiple transmission lines can be formed on a single conductive sheet, with adjacent conductor portions separated by a common ground plane portion.

This coplanar dielectric-less line concept can be directly extended to different configurations, simply by placing additional ground planes above and/or below the slotted one. The different configurations are shown in Figure 3; coplanar as described above, in Figure 3a, microstrip in Figure 3b, and stripline in Figure 3c. The additional ground planes of Figure 3b should be at the same potential as the coplanar ground planes. Closed lines

(suspended stripline, shown in Figure 3c, or coaxial type, not shown) , are those in which a metallic channel encloses the central metallic region. The walls of the metallic channel are connected to the outer coplanar ground plane.

The dielectric-less lines can be used to implement power dividers, couplers and other passive devices typically used in microwave networks. The lines may be curved by bending the slots accordingly whilst maintaining the slot period and length as constant as possible.

Power dividers, for example T-splitters, can also be implemented by bending the slots of the input line at the junction of the lines. The central conductor width is used to control the impedance of the lines. Impedance transformers can also be implemented by stepping the width of the central conductor at the mid-point of adjacent slots. The width of the slot should be also properly modified.

Other passive circuit devices such as branch couplers, hybrids, etc. can also be implemented using the dielectric- less line concept .

Radiating elements can also be directly implemented using dielectric-less lines. In the dielectric-less transmission line, the slots are excited by the TEM mode M2 as it propagates in the structure. In open structures (i.e. coplanar and microstrip) the line radiates energy which introduces additional line losses. This radiation is minimised when the width of the transmission line is much smaller than the wavelength. However, the slots can be separated and enlarged to radiate energy. A radiating slot can be directly fed from a dielectric-less line just by bending the adjacent slots 90° and widening them to obtain adequate impedance level and bandwidth.

Microstrip-like patch radiators can also be implemented from a dielectric-less microstrip by enlarging the central region of the line 20, creating a resonating cavity, as shown in Figure 4. The slots surrounding the metallic central patch 21 form an array of radiators that drive the fields inside the cavity into the open space. A key benefit of the present invention is the use of low cost manufacturing techniques for the implementation of the dielectric-less transmission line structures. Possible techniques for the manufacture of the central conductor include: sheet metal stamping or pressing, chemical etching, laser cutting, aluminium casting, plastic or metal injection moulding, and metallisation.

For approaches using additional ground planes, the following fabrication methods could be used to form channelised microstrip or suspended stripline: multilayer sheet metal structures, in which the intermediate layers are profiled to form channels, sheet metal pressing, aluminium casting, plastic or metal injection moulding, and metallisation.

In some circumstances it may be desirable to include a layer of dielectric material in a transmission line or radiating element according to the present invention, in order to improve the mechanical characteristics of the structure. For example, an airborne antenna element without a layer of dielectric material may be too flexible for a particular application. Insertion of a layer of dielectric material provides additional rigidity. Although this increases the expense of the antenna to that of a conventional antenna, it has benefits over a conventional antenna in that it is more highly tuned and hence is less susceptible to electromagnetic interference from outside the operating band of the antenna. Furthermore, an antenna according to the present invention presents minimal scattering of incident electromagnetic waves.

Claims

1. A device for transmitting electromagnetic energy comprising a slotted conductive sheet (1) , the slotted conductive sheet (1) having: a conductor portion (4) extending from one edge of the conductive sheet; at least one ground plane portion (2) ; and, two rows of parallel slots (3) formed in the conductive sheet separating the conductor portion (4) from the or each ground plane portion, the slots in each row being periodically spaced from one another along the row and adjacent rows of slots being shifted relative to one another in the row direction.

2. A device according to claim 1, wherein the slots are all of identical length in the row direction.

3. A device according to claim 1 or 2 , wherein adjacent rows of slots (3) are shifted relative to one another by half the length of the periodic spacing of the slots within each row.

4. A device according to any one of the preceding claims 1, wherein the conductor portion (4) is formed centrally on the conductive sheet and extends from one edge of the conductive sheet to another, and ground plane portions (2) are formed on both sides of the conductor portion (4) .

5. A device according to any one of claims 1 to 4 , wherein the frequency response is adjustable by varying the width of the slots and/or the gap between adjacent slots in the row direction.

6. A device according to any one of claims 1 to 3 , wherein the conductor portion does not extend from one edge of the conductive sheet to another, and comprises a widened end section (21), which in use forms a radiating element.

7. A device according to any one of the claims 1 to 4, wherein an additional conductive sheet is placed in a plane parallel to the slotted conductive sheet .

8. A device according to any one of claims 1 to 4 , wherein two additional conductive sheets are placed parallel to the slotted conductive sheet, one above and one below the slotted conductive sheet .

9. A device according to any one of claims 1 to 4, wherein a conductive channel backs or completely surrounds the slotted conductive sheet.

10. A filter including a device according to any one of claims 1 to 5, wherein the frequency response includes a passband and selective attenuation at frequencies outside the passband.