GB2401249A

GB2401249A - Attaching dielectric antenna structures to microstrip transmission line feed structures formed on dielectric substrates

Info

Publication number: GB2401249A
Application number: GB0415804A
Authority: GB
Inventors: Rebecca Thomas; Susan Williams; James William Kingsley
Original assignee: Antenova Ltd
Current assignee: Antenova Ltd
Priority date: 2002-05-15
Filing date: 2003-05-15
Publication date: 2004-11-03
Anticipated expiration: 2022-05-15
Also published as: GB0415798D0; GB2400498A; GB2401249B; GB0415804D0; GB2400498B

Abstract

A dielectric antenna comprises a dielectric resonator 1 mounted in direct contact with a microstrip transmission line 2 formed on one side of a printed circuit board 3. The dielectric antenna 1 may be a dielectric resonator antenna (DRA), a high dielectric antenna (HDA) or a dielectrically-loaded antenna. Electrically conductive pads 4 are provided between the dielectric resonator 1 and the printed circuit board 3 to provide good mechanical stability. The simple construction of the antenna leads to improved manufacturing reliability and efficiency, and allows all functional features of the antenna to be located on one side of a printed circuit board (PCB) substrate. Particular areas of the substrate and/or resonator 1 may be metallised. One or more dielectric resonators 1 may be glued by conductive epoxy or metallised and soldered on to the transmission line 2. The said resonators 1 may be centred 115 or off-centre 116 with regard to the said line 114.

Description

2401 249

IMPROVEMENTS RELATING TO ATTACHING DIELECTRIC ANTENNA

STRUCTURES TO MICROSTRIP TRANSMISSION LINE FEED

STRUCTURES FORMED ON DIELECTRIC SUBSTRATES

The present invention relates to techniques for attaching antenna structures, such as dielectric resonators or pellets, to microstrip transmission line electrical feed structures formed on dielectric substrates so as to fomm antennas, for example dielectric resonator antennas (DRAs), high dielectric antennas (HDAs) and dielectricallyloaded antennas (DLAs).

The present application is divided out of UK patent application no 0311181.2 (GB2388964).

Dielectric resonator antennas are resonant antenna devices that radiate or receive radio waves at a chosen frequency of transmission and reception, as used in for example in mobile telecommunications. In general, a DRA consists of a volume of a dielectric material (the dielectric resonator or pellet) disposed on or close to a grounded substrate, with energy being transferred to and from the dielectric material by way of monopole probes inserted into the dielectric material or by way of monopole aperture feeds provided in the grounded substrate (an aperture feed is a discontinuity, generally rectangular in shape, although oval, oblong, trapezoidal or butterfly/bow tie shapes and combinations of these shapes may also be appropriate, provided in the grounded substrate where this is covered by the dielectric material.

The aperture feed may be excited by a strip feed in the form of a microstrip transmission line, coplanar waveguide, slotline or the like which is located on a side of the grounded substrate remote from the dielectric material). Direct connection to and excitation by a microstrip transmission line is also possible. Alternatively, dipole probes may be inserted into the dielectric material, in which case a grounded substrate is not required. By providing multiple feeds and exciting these sequentially or in various combinations, a continuously or incrementally steerable beam or beams may be fommed, as discussed for example in the present applicant's US patent number US 6,452,565 and the publication by KINGSLEY, S.P. and O'KEEFE, S.G., "Beam steering and monopulse processing of probe-fed dielectric resonator antennas", FEE Proceedings - Radar Sonar and Navigation, 146, 3, 121 - 125, 1999, the full contents of which are hereby incorporated into the present application by reference.

The resonant characteristics of a DRA depend, ironer alla, upon the shape and size of the volume of dielectric material and also on the shape, size and position of the feeds thereto. It is to be appreciated that in a DRA, it is the dielectric material that resonates when excited by the feed. This is to be contrasted with a dielectrically loaded antenna, in which a traditional conductive radiating element is encased in a dielectric material that modifies the resonance characteristics of the radiating element.

DRAs may take various forms, a common form having a cylindrical shape dielectric pellet which may be fed by a metallic probe within the cylinder. Such a cylindrical resonating medium can be made from several candidate materials including ceramic dielectrics.

Since the first systematic study of dielectric resonator antennas (DRAB) in 1983 [LONG, S.A., McALLISTER, M.W., and SHEN, L.C.: "The Resonant Cylindrical Dielectric Cavity Antenna", IEEE Transactions on Antennas and Propagation, AP-31, 1983, pp 406-412], interest has grown in their radiation patterns because of their high radiation efficiency, good match to most commonly used transmission lines and small physical size [MONGIA, R.K. and BHARTIA, P.: "Dielectric Resonator Antennas - A Review and General Design Relations for Resonant Frequency and Bandwidth", International Journal of Microwave and Millimetre-Wave Computer Aided Engineering, 1994, 4, (3), pp 230-247]. A summary of some more recent developments can be found in PETOSA, A., ITTIPIBOON, A., ANTAR, Y.M.M., ROSCOE, D., and CUHACI, M.: "Recent advances in Dielectric-Resonator Antenna Technology", IEEE Antennas and Propagation Magazine, 1998, 40, (3), pp 35 - 48. s t

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A variety of basic shapes have been found to act as good DRA resonator structures when mounted on or close to a ground plane (grounded substrate) and excited by an appropriate method. Perhaps the best known of these geometries are: Rectangle [McALLISTER, M.W., LONG, S.A. and CONWAY G.L.: "Rectangular Dielectric Resonator Antenna", Electronics Letters, 1983, 19, (6), pp 218-219].

Triangle [ITTIPIBOON, A., MONGIA, R.K., ANTAR, Y.M.M., BHARTL\, P. and CUHACI, M.: "Aperture Fed Rectangular and Triangular Dielectric Resonators for use as Magnetic Dipole Antennas", Electronics Letters, 1993, 29, (23), pp 2001- 2002].

Hemisphere [LEUNG, K.W.: "Simple results for conformal-strip excited hemispherical dielectric resonator antenna", Electronics Letters, 2000, 36, (11)].

Cylinder [LONG, S.A., McALLISTER, M.W., and SHEN, L.C.: "The Resonant Cylindrical Dielectric Cavity Antenna", EKE Transactions on Antennas and Propagation, AP-31, 1983, pp 406-412].

Half-split cylinder (half a cylinder mounted vertically on a ground plane) [MONGIA, R.K., ITTIPIBOON, A., ANTAR, Y.M.M., BHARTIA, P. and CUHACI, M: "A Half-Split Cylindrical Dielectric Resonator Antenna Using SlotCoupling", EKE Microwave and guided Wave Letters, 1993, Vol. 3, No. 2, pp 38-39].

Some of these antenna designs have also been divided into sectors. For example, a cylindrical DRA can be halved [TAM, M.T.K. and MURCH, R.D.: "Half volume dielectric resonator antenna designs", Electronics Letters, 1997, 33, (23) , pp 1914 - 1916]. However, dividing an antenna in half, or sectorising it further, does not change the basic geometry from cylindrical, rectangular, etc. High dielectric antennas (HDAs) are similar to DRAB, but instead of having a full ground plane located under the dielectric pellet, HDAs have a smaller ground plane or no ground plane at all. Removal of the ground plane underneath gives a less well- defined resonance and consequently a very much broader bandwidth. HDAs generally radiate as much power in a backward direction as they do in a forward direction.

In both DRAs and HDAs, the primary radiator is the dielectric pellet. In DLAs, the primary radiator is a conductive component (e.g. a metal wire or printed strip or the like), and a dielectric component then just modifies the medium in which the DLA operates and generally allows the antenna as a whole to be made smaller or more compact.

A DLA may also be excited or formed by a direct microstrip feedline. In particular, the present applicant has found that a pellet of dielectric material may be placed on or otherwise associated with a microstrip feedline or the like so as to modify radiation properties of the feedline when operating as an antenna.

The present application is particularly but not exclusively directed towards techniques for constructing DRAB, HDAs and DLAs by way of assembly-line processes in a large-scale industrial context. Furthermore, the present application is particularly but not exclusively concerned with DRAs or HDAs comprised as a piece of high dielectric constant ceramic material excited by some form of feed structure on a printed circuit board (PCB), and also with DLAs comprising a conductive radiator provided with a pellet of dielectric material.

For the purposes of the present application, the expression "dielectric antenna" is hereby defined as encompassing DRAB, HDAs and DLAs.

According to the present invention, there is provided a dielectric antenna comprising a dielectric pellet mounted in direct contact with a microstrip transmission line formed on one side of a dielectric substrate, wherein at least one electrically conductive pad is formed or provided between the substrate and the pellet so as to provide structural stability.

The dielectric substrate may be in the form of a printed circuit board (PCB) and may have optional metallisation on at least part of one or other of its major surfaces.

In preferred embodiments, the dielectric pellet is made of a ceramic material, preferably with a high dielectric constant.

The dielectric antenna may be a DRA, an HDA or a DLA.

This has the advantage of making an antenna with good gain and bandwidth and a very simple method of assembly because everything is on one side of the dielectric substrate or PCB (with slot feeding, for example, the microstrip is on one side of the board and the ceramic pellet is on the other). On a production line, a pick-and-place machine can take ceramic pellets supplied on a reel and place these directly onto the dielectric substrates or PCBs.

Several methods of attachment can be used such as gluing or gluing with conducting epoxy. The present applicant has discovered that it is possible to solder the ceramic pellets into place, and that this can give a very strong joint with good electrical and radio-frequency properties. In production, the microstrip will have been already screen-printed with solder paste before the pick-and-place machine positions the ceramic pellet onto the dielectric substrate or PCB. The substrate or PCB with ceramic pellet attached is then passed into a reflow oven that melts the solder, thereby soldering the ceramic resonator in place. This is a procedure ideally suited to modern automated electronic assembly production lines.

Solder will not generally adhere directly to ceramic materials, so the ceramic pellets are advantageously first metallised. Several metals can be used for this and can be deposited in different ways, but the present applicant has found that conductive silver paint is a particularly efficient and cost effective solution for preferred dielectric antenna products. A screen-printing process can easily apply the paint. In some cases (i.e. for some types of paint and for some ceramics) the paint can be allowed to dry, but usually it is preferable for the painted ceramic to be fired in an oven or on a hot plate to ensure good adhesion and a surface that has a low loss at radio frequencies.

With direct microstrip feeding it is often advantageous to have the ceramic pellet substantially offset from the microstrip, as this gives improved gain, bandwidth and match to 50 ohms (an industry standard impedance in antenna design). However, with such an offset the joint is not strong mechanically because the ceramic pellet is balanced on the microstrip line (see Figure 1). The present invention improves the mechanical strength of the joint by the insertion or formation of electrically conductive (e.g. metal or metallic) pads, preferably by way of soldering, under corner or edge portions of the ceramic pellet (see Figure 2). It has been found that the pads may be extended to form a continuous support (see Figure 3) without impairing the performance of the dielectric antenna formed thereby. Indeed, in many cases this technique may advantageously be used to improve the performance of the antenna.

In general, metallisation of parts of the lower surface of a dielectric pellet (e.g. a ceramic pellet) and/or the substrate or PCB surface beneath the resonator will cause a concentrating effect on the electric field inside the dielectric, thereby changing the electrical performance of the antenna. The effect of metallisation can even cause the antenna to resonate in a different mode with a consequently larger change in the electrical performance. The shape and extent of the microstrip line feeding the dielectric antenna also affects the overall performance. With careful design, these changes can be used to improve the antenna performance. Whilst it is usual for the metallisation on the two surfaces (underside of dielectric/pellet and substrate/PCB) to be matched with each other, the present applicant has found a few cases where improved antenna performance can be obtained with the metallisations being non- matching.

The present applicant has successfully created DRAs and HDAs with rectangular ceramic pellets acting as dielectric resonators and also with half-split cylindrical ceramic pellets in this way. By extension, all or most other shapes of dielectric pellet (such as those mentioned in the introductory part of the present application) may therefore be attached to a dielectric substrate/micros/rip transmission line assembly in this manner.

When using a direct connection (e.g. a direct microstrip connection) to feed a DRA or HDA, the present applicant has found that the position of the dielectric material (the dielectric pellet) relative to the direct connection (e.g. a microstrip) influences the direction of a resultant radiation beam. Where a dielectric material of appropriate shape is placed centrally on top of a microstrip transmission line, the dielectric material will tend to generate a beam in a vertical direction. When the dielectric material is placed on top of the microstrip line with a greater volume of the material to the right or left of the microstrip line, a beam having respectively a rightward or leftward component is generated. This technique may be used to help aim a radiation beam in a desired direction andlor to broaden a radiation beam by using a plurality of dielectric resonators positioned in different ways on the microstrip transmission line.

Accordingly, there may be provided one or more dielectric resonators mounted on a microstrip transmission line, wherein at least one of the dielectric resonators is positioned off-centre on the microstrip transmission line.

There may also be provided a method of feeding a DRA or HDA or an array thereof, wherein at least one dielectric resonator is positioned offcentre on the microstrip transmission line in a predetermined direction so as to generate a beam having a directional component in the predetermined direction.

For a better understanding of the present invention and to show how it may be carried into effect, reference shall now be made by way of example to the accompanying drawings, in which: FIGURE 1 shows side and plan views of a rectangular ceramic pellet mounted on a direct microstrip transmission line on one side of a PCB; FIGURE 2 shows side and plan views of a rectangular ceramic pellet mounted on a direct microstrip transmission line on one side of a PCB with additional support pads printed on the PCB; FIGURE 3 shows side and plan views of a rectangular ceramic pellet mounted on a direct microstrip transmission line on one side of a PCB with a continuous support strip printed on the PCB; FIGURE 4 shows various metallisation patterns on an underside of a dielectric pellet; and FIGURE 5 shows a direct microstrip feed network with an array of dielectric resonators located thereon.

Figure 1 shows side and plan views of a rectangular metallised ceramic resonator pellet 1 soldered onto a direct microstrip transmission line 2 formed on one side of a PCB 3. A conductive ground plane (not shown) may be formed on an opposed side of the PCB 3. The pellet 1 is mounted offcentre, and the soldered joint has good electrical contact but poor mechanical strength.

Figure 2 shows side and plan views of a rectangular metallised ceramic resonator pellet 1 soldered onto a direct microstrip transmission line 2 formed on one side of a PCB 3 as in Figure 1. Additional conductive pads 4 are printed on the PCB 3 so as to support corner portions 5 of the pellet 1, thereby increasing the mechanical strength of the assembly.

Figure 3 shows side and plan views of a rectangular metallised ceramic resonator pellet 1 soldered onto a direct microstrip transmission line 2 formed on one side of a PCB 3 as in Figures 1 and 2. An additional conductive strip 6 is printed on the PCB 3 so as to support an edge portion 7 of the pellet 1, thereby forming a single continuous support that increases the mechanical strength of the assembly.

Ceramic materials with relative permittivities ranging from 37 to 134 have been successfully used as resonator pellets 1 fed directly by microstrip transmission lines 2. Specific paints suitable for metallisation of the pellets 1 vary according to the type of ceramic material. Examples of suitable metallic paints include DuPont 8032 and 54341, which may be used with Solderplus 42NCLR-A solder paste.

Generally the benefits that can be obtained by metallising parts of the undersurface of the pellets are improved bandwidth and lower resonant frequency (resulting in a smaller antenna for a given operating frequency) .

The return loss bandwidth of an antenna is dependent upon: The resonant mode of the antenna À The characteristic impedance of the antenna À The feed impedance À The matching circuit À The return loss at which the match is measured.

In effect, metallisation used to improve the soldered joint can affect the first three items on the list above. Examples where metallisation of a rectangular pellet for solder purposes have resulted in an increase in bandwidth and reduced frequency without adversely affecting the other properties of the antenna are shown in Figure 4.

The shaded areas indicate the metallised areas.

Specifically, Figure 4(i) shows an underside of a rectangular dielectric pellet 1 in which large corner portions 10 are metallised, leaving a rhombus of unmetallised surface in a central part of the underside of the pellet 1.

Figure 4(ii) shows an underside of a rectangular dielectric pellet 1 in which small corner portions 11 are metallised, as is a central strip 12 along a central longitudinal axis of the underside of the pellet 1.

Figure 4(iii) shows an underside of a rectangular dielectric pellet 1 in which two small corner portions 11 are metallised on a right hand side of the underside, as is a strip 13 along a left hand side of the underside.

Figure 4(iv) shows an underside of a rectangular dielectric pellet 1 on which two metallised strips 14 and 15 are provided, one along each of the left and right hand longitudinal sides of the underside.

Figure 5 shows a direct microstrip feed network comprising a microstrip transmission line 114 with three dielectric resonators 115, 116 and 117 mounted thereon.

Resonator 115 is mounted centrally on the microstrip 114 and radiates vertically (out of the plane of the drawing towards the viewer). Resonator 116 is mounted to the left of the microstrip 114 and radiates out of the drawing with a leftward component.

Resonator 1 17 is mounted to the right of the microstrip 1 14 and radiates out of the drawing with a rightward component.

The preferred features of the invention are applicable to all aspects of the invention and may be used in any possible combination within the scope of the appended claims Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", mean "including but not limited to", and are not intended to (and do not) exclude other components, integers, moieties, additives or steps.

Claims

CLAIMS: 1. A dielectric antenna comprising a dielectric pellet mounted in

direct contact with a microstrip transmission line formed on one side of a dielectric substrate, wherein at least one electrically conductive pad is formed or provided between the substrate and the pellet so as to provide structural stability.
2. An antenna as claimed in claim 1, wherein the at least one pad is formed or provided at edge or comer portions of a surface of the pellet facing the substrate.
3. An antenna as claimed in claim 1 or 2, wherein the at least one pad is soldered to the substrate and/or the pellet.
4. An antenna as claimed in any preceding claim, wherein the dielectric substrate is a printed circuit board.
5. An antenna as claimed in any preceding claim, wherein the dielectric pellet is made of a ceramics material.
6. An antenna as claimed in any preceding claim, wherein the dielectric pellet is glued to the transmission line.
7. An antenna as claimed in claim 6, wherein the dielectric pellet is glued to the transmission line with a conducting epoxy.
8. An antenna as claimed in any one of claims 1 to 5, wherein the pellet is soldered to the transmission line.
9. An antenna as claimed in any preceding claim, wherein at least a part of the pellet that contacts the transmission line is metallised.
10. An antenna as claimed in claim 9, wherein the part of the pellet is coated with a conductive silver paint.
11. An antenna as claimed in any preceding claim, wherein the pellet is mounted substantially centrally on the transmission line with reference to a longitudinal extent of the transmission line.
12. An antenna as claimed in any one of claims 1 to 10, wherein the pellet is mounted in an offset position on the transmission line with reference to a longitudinal extent of the transmission line.
13. An antenna as claimed in any one of claims 1 to 10, wherein there is provided a plurality of pellets mounted on the transmission line, and wherein at least one of the pellets is mounted in an offset position of the transmission line with reference to a longitudinal extent of the transmission line.
14. An antenna as claimed in any preceding claim, wherein at least part of a side of the substrate, opposed to that on which the pellet is mounted, is metallised.
15. An antenna as claimed in any preceding claim, wherein the antenna is a dielectric resonator antenna.
16. An antenna as claimed in any one of claims 1 to 14, wherein the antenna is a high dielectric antenna.
17. An antenna as claimed in any one of claims 1 to 14, wherein the antenna is a dielectrically-loaded antenna.
18. An antenna as claimed in claim 17, wherein a side of the substrate opposed to that on which the pellet is mounted is metallised, except for an area corresponding to a location of an end of the transmission line on the said one side of the substrate, and wherein the pellet is mounted so as to contact the end of the transmission line.
19. An antenna as claimed in claim 18, wherein the end of the transmission line contacts an underside surface of the pellet.
20. An antenna as claimed in claim 18, wherein the end of the transmission line contacts a side or top surface of the pellet.
21. An antenna as claimed in claim 20, wherein the side or top surface of the pellet is metallised.
22. A dielectric antenna substantially as hereinbefore described with reference to or as shown in the accompanying drawings. >

KEY TO DRAWINGS: Figure 1: Side and plan views showing a pellet mounted on a direct microstrip feed line creating a joint with good electrical contact but with poor mechanical strength. The pellet has been drawn as transparent to reveal the microstrip underneath.

Figure 2: Side and plan views showing a pellet mounted on a direct microstrip feed line with pads printed onto the PCB to support the pellet and increase the mechanical strength of the assembly.

Figure 3: As figure 2, but with the pads extended to form a single continuous support.

Figure 5: Placing pellets either side of direct microstrip feed causes the beam to move left or right of vertical. In this context, 'vertical' means offthe page towards the reader.

114: Microstrip feed line.

115: Pellet radiating vertically.

116: Pellet rests on microstrip. Pellet radiating to right of vertical.

117: Pellet radiating to left of vertical.