WO2005029634A2

WO2005029634A2 - Dielectric loading of distributed printed circuits

Info

Publication number: WO2005029634A2
Application number: PCT/IL2004/000876
Authority: WO
Inventors: Meir Gazit; Danny Alon
Original assignee: Vishay Advanced Technologies Ltd.
Priority date: 2003-09-22
Filing date: 2004-09-21
Publication date: 2005-03-31
Also published as: TW200524491A; WO2005029634A3

Abstract

A method for making a conductive pattern (104) with distributed printed circuit (DPC) properties on a substrate (102) and dielectrically loading it by applying a high dielectric constant low loss layer (106) that substantially encapsulates the conductive pattern (104), the conductive pattern is a small antenna (100) formed on a printed circuit board (PCB) and covered by a high dielectric constant (104), low loss plate pressed against the antenna and glued to the PCB.

Description

DIELECTRIC LOADING OF DISTRIBUTED PRINTED CIRCUITS

FIELD AND BACKGROUND OF THE INVENTION The present invention relates to dielectric loading of distributed printed circuits (DPC), and, more particularly, to a process of covering distributed printed circuits with materials having a high dielectric constant, in order to achieve reduced size, enhance performance and reduce cost. The dielectric constant is the ratio of the permittivity of a substance to the permittivity of free space. It is an expression of the extent to which a material concentrates electric flux. Distributed circuits are circuits in which physical dimensions influence performance. In other words, distributed circuits are circuits with dimensions of the same order of magnitude as the wavelength that passes through them. Examples for such circuits include antennas, filters, couplers (including directional couplers), matching circuits and baluns. The performance, cost and size of distributed circuits designed for the consumer market are strongly linked; the physical dimensions determine the performance of the distributed circuit. The size of the distributed circuit greatly influences its cost. In prior-art designs, dielectric loading is often achieved by printing the distributed circuits directly onto a ceramic substrate that has a high dielectric constant. These devices are later assembled on the printed circuit board and are known as Surface Mounted Technology (SMT). These same devices are sometimes also called "chip" devices (e.g. chip antenna, chip balun etc.). While this method significantly reduces the size of distributed circuits, it also increases their cost. SMT production requires specialized experience with ceramic material production techniques and distributed circuits design. Thus, production of miniaturized SMTs is generally limited to companies that specialize in this field. All SMTs on the market today are designed to specific parameters. In many cases engineers must compromise their demands in order to fit the specifications available in the market Standard commercial loaded circuits in SMT form require, in addition to the distributed circuit itself, conducting surfaces that enable the devices to be electrically connected to the printed board on which they are mounted. These surfaces might include soldering pads and waveguides, and add attenuation and area to the device. In order to avoid these complications, many designers turn to printed distributed circuits. While these circuits can be easily designed to match any given application while maintaining low cost, they suffer from a great disadvantage in that they require large areas on the printed boards. There is thus a widely recognized need for, and it would be highly advantageous to have, a technique in which dielectric loading can be added to printed distributed circuits without the manufacturing complications involved in SMT production.

SUMMARY OF THE INVENTION According to the present invention there is provided a method for providing a low cost, high performance distributed printed circuit comprising the steps of forming a conductive pattern with distributed circuit properties on a substrate, and dielectrically loading the conductive pattern by applying a high dielectric constant, low loss layer that substantially encapsulates the conductive pattern, thereby providing a smaller effective wavelength for the conductive pattern. As used herein, "effective wavelength" refers to the length of a wave on a conductive surface. According to one feature in the method for providing a low cost, high performance distributed printed circuit of the present invention, the step of forming a conductive pattern with distributed circuit properties on a substrate includes forming the conductive pattern on a printed circuit board (PCB), thereby obtaining a distributed printed circuit (DPC). According to another feature in the method for providing a low cost, high performance distributed printed circuit of the present invention, the step of dielectrically loading the DPC includes applying a high dielectric constant plate on top of the DPC. According to yet another feature in the method for providing a low cost, high performance distributed printed circuit of the present invention, the step of applying a high dielectric constant plate includes pressing the plate to the DPC to form a minimal gap between the plate and the DPC, and fixedly attaching the plate to the PCB. According to yet another feature in the method for providing a low cost, high performance distributed printed circuit of the present invention, the substep of fixedly attaching the plate to the DPC includes gluing the plate to the PCB, wherein the glue fills the gap. According to yet another feature in the method for providing a low cost, high performance distributed printed circuit of the present invention, the gluing of the plate to PCB includes using glue with a high dielectric constant. According to yet another feature in the method for providing a low cost, high performance distributed printed circuit of the present invention, the PCB is made of FR4, and wherein the pressing a high dielectric constant plate includes pressing a Rogers 6010 plate to the FR4 PCB. According to yet another feature in the method for providing a low cost, high performance distributed printed circuit of the present invention, the step of forming a conductive pattern with distributed circuit properties on a PCB includes forming an antenna. According to yet another feature in the method for providing a low cost, high performance distributed printed circuit of the present invention, the step of foraiing a conductive pattern with distributed circuit properties on a PCB includes forming a conductive pattern selected from the group consisting of an antenna, a filter, a coupler, a directional coupler, a matching circuit, a balun and a combination thereof. Preferably, in the method, the forming is effected by a process selected from the group consisting of printing, etching, painting, spraying, plating, gluing, sputtering, and metal vapor deposition. Preferably, the dielectric loading material is assembled using standard placement and gluing techniques. The proposed method provides an additional benefit in the ability to correct deviations caused by manufacturing tolerances. Since the circuit's dimensions influence its performance, distributed circuits are vulnerable to deviations caused by production tolerances. By measuring the circuit's electric properties before assembling the dielectric loading layer, it is possible to correct deviations by controlling the thickness of the added dielectric material. This process offers control of the effective dielectric constant, and therefore allows high accuracy to be achieved. According to the present invention there is provided a DPC with reduced size comprising a conductive pattern with distributed circuit properties formed on a non- conductive substrate that has a first dielectric constant, and a dielectric loading element having a second dielectric constant, the loading element disposed so as to substantially cover the conductive pattern. According to one feature in the DPC of the present invention, the non- conductive substrate is a PCB and the conductive pattern is selected from the group consisting of an antenna, a filter, a coupler, a directional coupler, a matching circuit, a balun and a combination thereof. According to another feature in the DPC of the present invention, the dielectric loading element is a layer applied by a method consisting of printing, etching, painting, spraying, plating, gluing, sputtering and metal vapor deposition. According to yet another feature in the DPC of the present invention, the PCB is made of FR4, and wherein the dielectric loading element is a Rogers 6010 layer. The present invention advantageously solves the problems presented by standard commercial loaded circuits in SMT form technologies, mcluding the added attenuation and area required by solder pads and waveguides.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: FIG. 1 shows in schematic cross section: a) a conductive pattern formed on a substrate; b) a dielectric loading layer applied on top; c) a dielectric loading layer applied conformally; FIG. 2 shows: (a) a printed antenna A on a FR4 substrate without dielectric loading; a reduced size antenna B designed for dielectric loading; antenna B with dielectric loading; (b) dimensions of antennas B and C. Antennas A and B are designed to radiate RF energy at the same frequency;; FIG. 3 shows a S 11 plot measured for antenna C in Fig. 2; FIG. 4 shows a radiation pattern measured for for antenna C in Fig. 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is of a method for providing a low cost, high performance distributed printed circuit, specifically an antenna on a printed circuit board, by adding a dielectric loading material on top of the printed circuit after the circuit formation. By dielectrically loading the circuit, the effective wavelength is decreased. This fact allows a shorter loop to carry the same frequency as a longer loop that does not use the loading process. The discussion below, by way of example only, shows how the method of the present invention may be applied to provide a compact, low-cost, printed antenna for transmission and reception of^' radio signals. The antenna is formed on a PCB (exemplarily made of FR4) and dielectrically loaded by a high dielectric constant material (exemplarily Rogers 6010, Rogers Corp., 100 S. Roosevelt Ave., Chandler, AZ 85226, USA), in a process that may include printing, etching, painting, spraying, plating, gluing, sputtering, and metal vapor deposition. The exemplary use of the Rogers 6010 dielectric loading layer on a FR4 PCB allows formation of a reduced size antenna with a conductive path having a length of approximately half the effective wavelength (half the length of an antenna that does not use the dielectric loading). A "high dielectric constant" according to the present invention is a constant higher than that of air or of the substrate on which the distributed circuit is formed. Most commercial low loss materials can reach constants up to 100. The method starts with the step of forming a conductive pattern of distributed circuit (exemplarily an antenna) on a substrate, preferably a PCB substrate, using any of the forming techniques mentioned. Preferably, the conductive pattern is placed on an exterior layer of the PCB in order to maximize the loading effect of the dielectric material that will be placed upon it. A cross section of an antenna 100 formed on a PCB substrate 102 is shown in FIG. la. The figure shows a conductive pattern 104 on top of substrate 102. The conductive pattern forming step is followed by a dielectric loading step in which a high dielectric constant, low loss layer is applied on top of, or essentially on top and around of the conductive pattern. The dielectric loading layer substantially encapsulates the conductive pattern, thereby providing a smaller effective wavelength for the conductive pattern. FIG. lb shows a dielectric layer 106 applied on top of conductive pattern 104, while FIG lc shows a dielectric layer 106 conformally coating conductive pattern 104 and encapsulating the PCB as well. Layer 106 may be in the form of a flat flexible or inflexible plate (e.g. used in FIG. lb), a material compliant enough to be pressed against the conductive layer plane and on top of it. or any solid or liquid dielectric material that can be applied on and around the conductive pattern and the substrate is it formed on. Preferably, the dielectric material physical properties should allow it to withstand the mechanical demands of the entire circuit (such as bending, dropping, shaking etc.). However, the dielectric material does not require any other chemical or physical properties that might enhance its cost such as good adhesion to printed metallic layers, matching thermal expansion at high temperatures and so on. Preferably, the dielectric material is placed in a fashion that covers all of the relevant parts in the circuit, while maintaining minimal dimensions, as the cost is directly linked to the dielectric material area. Preferably, the dielectric material is pressed against the printed circuit in a way that insures a minimal gap between the two materials. As the effective dielectric constant is a combination of the dielectric constants of the printed board, the loading material and the substance between the two, it is preferable to fill the gap with a substance of high dielectric constant. The method described above was used to form a reduced size antenna on a FR4 PCB substrate. The antenna was exemplarily made using the process described in US Patent Application No. 20040135726 by A. Shamir and M. Gazit, "Method for designing a small antenna matched to an input impedance, and small antennas designed according to the method", which is incorporated by reference for all purposes set forth herein. The dielectric material (Rogers 6010 with a dielectric constant of 10.2) was attached to the FR4 (dielectric constant of about 4.2) using a commercial epoxy glue (Quick Setting Epoxy manufactured by Pacer Technology, Ca, USA). This type of glue has typically a dielectric a constant of ~4. Other glues of the same family are used in standard assembly processes. Care was taken to insure that no air remained in the gap between the FR4 and Rogers 6010. The dielectric constant of the glue also has an effect on the final product. It is therefore preferable to use a glue with a high dielectric constant. The epoxy glue also provided physical support to the structure. Thus, the present invention provides for the creation of a small, distributed circuit for the cost of a simple printed-circuit board and a dielectric die. Table 1 shows the effective wavelengths for the various materials discussed herein.

Table 1. FIG. 2 shows a printed antenna A designed to radiate at 2.44 GHz on a FR4 substrate, formed using the process described in detail in US Patent Application No.

20040135726. The figure also shows an antenna B with the same design as A but with reduced dimensions, prior to dielectric loading. The figure also shows antenna B with an added Rogers 6010 dielectric loading layer on top. The B and C antenna dimensions are length 12.3 mm, width 12 mm ard gap 0.65 mm, and are shown in more detail in FIG. 2b. All of the other parameters (PCB thickness, RF feed dimensions, strip width and thickness etc.) remain unchanged from A to B, C. Since the effective wavelength is decreased by half when Rogers 6010 replaces air, the method of the present invention allows formation of a significantly smaller antenna that will radiate in the same frequency. Figures 3 and 4 present measured values of standard parameters for antenna C used for antenna analysis, see for example C.A. Balanis, "Antenna Theory Analysis and Design", John Wiley & Sons inc., Chapter 2, 1997 (2^nd edition), indicating that the electric properties of the antenna have not suffered from the dielectric loading of the circuit. Although the preceding discussion has dealt with a distributed circuit formed on a circuit board, it will be apparent to those skilled in the art that, if the conductive parts of the circuit are made of material sufficiently rigid so as to be self-supporting, or, if the conductive parts are provided with appropriate supports, a distributed circuit substantially as described herein can be constructed without the use of a circuit board.

In such a circuit, the spaces between, above and below the conductive parts serve as dielectric loading. Such a circuit can be formed using standard techniques, such as the typical metalworking processes of bending, casting and stamping. Such a circuit can economically be formed from sheet metal. The scope of the present invention includes such circuits formed without a circuit board. All publications, patents and patent applications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention. While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.

Claims

WHAT IS CLAIMED IS

1. A method for providing a low cost, high performance distributed printed circuit comprising the steps of: a. forming a conductive pattern with distributed circuit properties on a substrate; and b. dielectrically loading said conductive pattern by applying a high dielectric constant, low loss layer to cover said conductive pattern, thereby providing a smaller effective wavelength for said conductive pattern.

2. The method of claim 1 , wherein said step of forming a conductive pattern with distributed circuit properties on a substrate includes forming said conductive pattern on a printed circuit board (PCB), thereby obtaining a distributed printed circuit (DPC).

3. The method of claim 2, wherein said step of dielectrically loading said DPC includes applying a high dielectric constant plate on top of said DPC.

4. The method of claim 3, wherein said applying of a high dielectric constant plate includes pressing said plate to said DPC to form a minimal gap between said plate and said DPC, and fixedly attaching said plate to said PCB.

5. The method of claim 4, wherein said pressing and fixedly attaching said plate to said DPC includes gluing said plate to said PCB, wherein said glue fills said gap.

6. The method of claim 5, wherein said gluing said plate to said PCB includes using a glue with a high dielectric constant.

7. The method of claim 3, wherein said PCB is made of FR4, and wherein said pressing a high dielectric constant plate includes pressing a Rogers 6010 plate to said FR4 PCB.

8. The method of claim 2, wherein said step of forming a conductive pattern with distributed circuit properties on a PCB includes forming an antenna.

9. The method of claim 2, wherein said step of forming a conductive pattern with distributed circuit properties on a PCB includes forming a conductive pattern selected from the group consisting of an antenna, a filter, a coupler, a directional coupler, a matching circuit, a balun and a combination thereof.

10. The method of claim 1, wherein said step of forming includes using a process selected from the group consisting of printing, etching, painting, spraying, plating, gluing, sputtering and metal vapor deposition.

11. A distributed printed circuit (DPC) with reduced size comprising: a. a conductive pattern with distributed circuit properties formed on a non-conductive substrate that has a first dielectric constant; and b. a dielectric loading element having a second dielectric constant higher than said first dielectric constant, said loading element disposed so as to substantially cover said conductive pattern.

12. The DPC of claim 11, wherein said non-conductive substrate is a printed circuit board (PCB), and wherein said conductive pattern is selected from the group consisting of an antenna, a filter, a coupler, a directional coupler, a matching circuit, a balun and a combination thereof.

13. The DPC of claim 12, wherein said dielectric loading element is a layer applied by a method consisting of printing, etching, painting, spraying, plating, gluing, sputtering and metal vapor deposition.

14. The DPC of claim 13, wherein said PCB is made of FR4, and wherein said dielectric loading element is a Rogers 6010 layer.