GB2458269A

GB2458269A - A monopole antenna having an open circuit element connected to the radiating element

Info

Publication number: GB2458269A
Application number: GB0804400A
Authority: GB
Inventors: Jimmy Ho; Simon Christopher Raymond Munday; Charanjit Sailopal; David Harold Broadman; Barry John Talbot
Original assignee: Jaybeam Ltd
Current assignee: Jaybeam Ltd
Priority date: 2008-03-10
Filing date: 2008-03-10
Publication date: 2009-09-16
Also published as: GB0804400D0

Abstract

A monopole antenna 10, for operating at mobile telecommunications frequencies, comprises a radiating element 26 and a ground plane substantially extending perpendicular to the radiating element 26. The radiating element 26 is short-circuited via a conductive arm 28 to the ground plane such that direct current signals are grounded. The antenna 10 has an open circuit element comprising an electrically conducting wire 70 electrically connected to the radiating element 26. In another embodiment a stub 64 parallel to, spaced from and electromagnetically coupled to the radiating element (26) is provided. The monopole antenna arrangement provides a cheap, compact, broadband antenna without the need for complex impedance matching circuitry.

Description

AN ANTENNA

The present invention relates to an antenna and, in particular, a monopole antenna for operating at mobile telecommunications frequencies, such as a multi-band omni-directional antenna for indoor and outdoor applications, which operate over a wide frequency band.

Background of the Invention

Monopole antennas are widely used in the wireless communication industry, and in particular for indoor applications and at mobile telecommunication frequencies. This is because monopole antennas provide the desirable characteristics of a generally omni-directional radiation pattern as well as the ability to transmit and receive simultaneously in two or more frequency bands.

In these applications, it is desirable that good impedance match and radiation pattern performance is achieved in the desired frequency bands over a very large frequency band. For mobile telecommunication use, the monopole antennas performance should be optimised over the desired band or bands, for example GSM, DCS1800, DCSI900, IMT-2000, UMTS, WLAN and DMB bands at frequency ranges of 870MHz to 960MHz and 1710 to 2500MHz.

Monopole antennas are disclosed in US patent No. US-B-7 027 004, US-B-6809687, US-B-7148848, US-B-7 193978, and US-B-7298346. **S * * S *5 *

* 20 In a typical indoor monopole antenna, an omni-directional radiating element projects outwardly from a metal base plate and the entire arrangement is ::: enclosed by a plastic protective enclosure called a radome. It is generally considered desirable that the radome is compact and with an aesthetically pleasing shape. S...

Furthermore, it is desirable to have a simple design with a small number of components that does not need a complex impedance matching network in order to provide an adequate impedance match. It is also desirable that the antenna has a short labour time (typically, a few minutes) to assemble in order to have low manufacturing costs. Furthermore, it is desirable that the antenna is made from cheap materials again to keep the costs of making the device low.

Examples of the monopole antenna described in this application provide these advantages.

Summary of the Invention

The invention is defined in the independent claims below to which reference should now be made. Advantageous features are set forth in the dependent claims.

A preferred embodiment of the invention is described in more detail below and takes the form of a monopole antenna for operating at mobile telecommunications frequencies. The monopole antenna 10 comprises a radiating element and a ground plane substantially extending perpendicular to the radiating element and electrically conductively connected to the radiating element. The antenna has an open circuit comprising an electrically conducting wire etectrically conductively connected to the radiating element. The antenna is grounded to direct current.

Brief Description of the Drawings

The invention will be described in more detail by way of example with reference to the accompanying drawings, in which: Figure 1 is a perspective view from the side of a monopole antenna as an example of the present invention; Figure 2 is a perspective view from the opposite side of the monopole antenna : *. shown in Figure 1; Figure 3 is a perspective view from above of the monopole antenna of Figures 1 and 2 on a metallic ground plane; Figure 4 is a perspective view of the monopole antenna of Figures ito 3 from underneath; Figure 5 is a perspective view of another example of a monopole antenna of the present invention from the same side as the example of Figure 1; Figure 6 is a perspective view from the opposite side of the monopole antenna shown in Figure 5 (from the same side as the other example antenna of Figure 2); Figure 7 is a perspective view of a further example of a monopole antenna of the present invention from the same side as the view of the example antennas of Figures 2 and 6; Figure 8 is a perspective view from above of a radome for covering the example monopole antennas of Figures 1 to 7; Figure 9 is a graph of return loss versus frequency for an example monopole antenna of the present invention; Figure 10 is a graph return loss versus frequency for the example monopole antennas of Figures 1 to 7; Figure 11 is a measured azimuth radiation pattern at 900MHz for the example monopole antennas of Figures 1 to 7; and Figure 12 is a measured azimuth radiation pattern at 2000MHz for the example monopole antennas of Figures 1 to 7.

Detailed Description of the Preferred Embodiment S... * S * S. *

* *** A monopole antenna 10 for operating at mobile telecommunications frequencies will now be described with reference to Figures 1 to 4. The monopole : *. 20 antenna 10 has a radiating element 12 and a ground plane 14 extending * perpendicular to the radiating element and electrically conductively connected to the radiating element. The monopole antenna 10 is 80mm high. The radiating element 12 and the ground plane 14 are made of copper on a printed circuit board (PCB) S...

substrate 16 of epoxy resin reinforced by woven glass fibre called FR4 (flame *..*S* retardant 4) also known by trade marks including GlO, Micarta and Garolite.

FR4, although inexpensive, does not hold its tolerance very well and is very lossy. The dielectric constant can vary from supplier to supplier and the PCB board thickness tolerance for a 0.8mm substrate (such as the present substrate) is �0.1mm.

The example antenna described herein reduces the impact of the material's drawbacks. In particular, it provides an adequate impedance match without requiring a complex (and expensive) impedance matching network.

As shown in Figure 1, the radiating element substrate 16 is a sheet or plate and planar. It is 0.8mm thick (although thickness could vary, for example, between 0.7mm and 0.9mm). The sides 18 of the radiating element substrate project from the ground plane 14 to form a body 15 with parallel sides. The sides narrow to form shoulders 20. A neck 22, with parallel sides, narrower than the shoulders, project outwardly and co-planar from the shoulders. The neck narrows to form a head 24 which terminates the substrate. That is to say, the radiating element narrows in part at the end spaced from the ground plane.

The main radiating element 26 is located on one face of the substrate 16 and is made from an electrical conductor, for example, a metal such as copper. It is largely the same shape as the substrate although it terminates slightly short of the edges. An arm 28 extends down one side of the radiating element from one shoulder 30 to form an electrically conducting connection with the ground plane 14. In other words, the main radiating element 12 is short circuited to a ground plane 14 at a point 29. The point should be as small as possible to provide a direct current short circuit to ground. At the junction between the main radiator 12 and the PCB ground 14, solder is applied to provide a short circuit. The short circuit expands the operating bandwidth to the low end of the frequency band of interest without having to increase the height or width of the radiating element. The bottom or lower edge 30 of the radiating element 26 at the base of body 32 is spaced from the base of the body 15 S...

: of the substrate 16 so that there is no electrical contact with the ground plane 14. * S S...

The ground plane substrate 40 is also a sheet or plate and planar and is :.:::. 25 made from FR4. It is rectangular. It has a thickness of 8mm. The ground plane 14 on the upper surface of the substrate is formed from copper 42 or other electrical conductor or metal on the ground plane substrate 40. The copper 42 terminates a S...

short space from the edges of the substrate 40. At two diagonally opposing corners * :. * 44, there is a rectangular notch 46 in the copper exposing a portion of electrical insulating substrate. The notch 46 has a small electrically conducting or copper portion or mount 48 on it. The mount 48 is spaced from the ground plane and electrically insulated from the ground plane.

The underside of the ground plane substrate does not have any copper and only electrically insulating substrate is exposed. This provides capacitive grounding.

This PCB ground plane can either be DC grounded or not.

As an alternative, the radiating element is soldered to a common ground plane for direct current grounding.

In the example of Figure 3, the monopole antenna 10 is located on a metallic base 50, which forms a ground plane. The base is circular and has mounting holes 52 located around its circumference. There are three holes all in recesses 54. The ground plane 14 is capacitively coupled to the metallic ground plane 50.

There is a hole extending through the centre of the ground plane, ground plane substrate and the metallic base 50. The hole houses the electrical connector 36 for transmitting and receiving electrical signals to the antenna. The electrical connector is a coaxial cable, and in particular an n-type connector 36. The connector has an electrically conducting overhang around its circumference forming a collar 47 with a flat face, which rests on the upper surface of the ground plane 14. As shown best in Figure 4, the connector has a threaded stem 49, which projects from the underside of the ground plane. A nut 51 is located around the stem. The nut of the n-type connector is tightened to the ground plane. The antenna is grounded to direct current (DC), which is achieved through metallic contact between the flat face of the collar of the n-type connector with the metal track or copper surface of the PCB ground 14. * * .

Referring back to Figure 1, the main radiating element 26 is electrically conductively connected by solder to the inner conductor 34 of the n-type connector * *S * * S *5S* Figure 2 shows the reverse or opposite side 60 of the radiating element **** substrate shown best in Figure 1. The reverse of the substrate has two stubs 62, 64 formed of metal, and in particular copper, on it. The stubs 62,64 are * 1 electromagnetically coupled to the main radiator and form part of the radiating element 12. They are for improving the impedance match of the antenna. One of the stubs 62 is formed in the head and neck of the substrate and has the same shape, which narrows towards the top. The edges of the stub 62 are spaced slightly from the edges of the substrate so that electrically insulating substrate is exposed. The other stub 64 is located in the body of the substrate. It is rectangular in shape. It is spaced from the edges of the substrate so that electrically insulating substrate is exposed.

An open circuit is provided by an electrically conducting wire or cable 70 electrically conductively connected to the radiating element. One and only one open circuit wire or cable is required. In the embodiment of Figure 2, the wire is electrically conductively connected, at one end 71, to one of the stubs, and in particular the rectangular stub 64 on the reverse of the substrate. The other end 72 of the wire 70 is electrically insulated from the ground plane. It is conveniently connected to the mount 48 (which is spaced from the ground plane and electrically insulated from the ground plane) on the same side of the antenna as the open circuit wire.

In the alternative example of Figures 5 and 6, in which like features have been given like reference numerals to those of Figures 1 to 4, the open circuit is provided by an electrically conducting wire 70 electrically conductively connected to main radiating element 26. The open circuit wire is shown electrically conductively connected, at one end 71, to the shoulder 30 of the main radiating element from which the arm 28 extends down one side of the radiating element. The other, open end 72 of the wire is connected to the mount 48 on the same side of the antenna as the open circuit wire. In this embodiment, one and only one open circuit wire is provided. As shown in Figure 6, there is no open circuit wire on the reverse side of the antenna.

In an alternative example shown in Figure 7, in which like features have been : given like reference numerals to those of Figures 1 to 4, the open circuit is provided by an electrically conducting wire 70 electrically conductively connected, at one end 71, to the stub 62 on the head portion of the radiator substrate, which is :.: : * 25 electromagnetically coupled to the main radiating element. The open circuit wire is connected, at the other open end 72, to the mount 48 on the same side of the antenna as the open circuit wire. In this embodiment, one and only one open circuit S...

* , * * wire is provided. The reverse side of the antenna element is the same as that shown in Figure 1 and there is no open circuit wire on the reverse side of the antenna A radome 100 for covering and protecting the monopole antenna is shown in Figure 8. It is attached to holes 52 in the metallic base 50. It is axially symmetrical with a corresponding shape in cross section to the antenna. It has a flat rim 80 that lies over the metallic base 50. Then, in cross section, it has a body with sides 82, which narrow slightly. The sides then narrow more sharply to form shoulders 84. A head 86 projects outwardly from the shoulders with narrowing sides.

The return loss performance of the antenna of Figures 1 to 8 without an open circuit wire is shown in Figure 9. That is, an example, with one face of Figure 1 and the other face as Figure 6. The performance is very good from 1710 to 2500MHz. This antenna can also extend beyond its required frequency band as evident in the return loss plot. Although the return loss meets the requirement for a return loss of -10dB between 870 to 960MHz, this specification is only marginally met. This electrical performance can be improved with a more complex matching network. This would change the width of the element which depletes the purpose of a small unobtrusive indoor antenna design. A more complex matching network is also not desirable because it is expensive and it reduces the advantage of using cheap FR4 as a PCB substrate.

A much better alternative is to apply an open circuit stub or wire 70 that is anchored on the PCB ground as shown in the embodiments of Figures 1 to 7.

This allows the size of the radiating element to remain the same but with significantly improved return as shown in the return loss performance graph of Figure 10.

In summary, examples of the antenna described herein have a simple structure that can achieve good impedance match over a wide frequency band by utilising an open circuit stub soldered to a small stub in the board to prevent the cable from moving under vibration, a short circuit to provide both impedance bandwidth extension and DC grounding and also good omni-directional pattern * *4 over a wide frequency band as shown in Figures 11 and 12. Figure 11 shows the measured azimuth radiation pattern at 900MHz. Figure 12 shows the measured azimuth radiation pattern at 2000MHz. S... * S *4*S

* : *.: The wideband characteristic of the antenna is achieved by shorting one side of the radiating patch and EM (electromagnetically) coupling to a stub on the reverse side of the FR4 PCB. These two features open up or increase the bandwidth. However, the voltage standing wave ratio (VSWR) will meet the required return loss specification marginally at the bottom of the band unless a matching network is employed. However, by using an open circuit stub in the form of a cable or a wire anchored in place by soldering to an isolated stub on the FR4 ground plane, the impedance match is improved substantially as shown in Figure 10 compared to Figure 9. This is a surprising and unexpected result. One and only one open circuit stub is required. The length of the open circuit wire or cable is dependent on the intended frequencies of operation. It can be soldered to either the main radiator or either of the coupled stubs. If the cable or wire is applied to the main radiator, then one end of the cable or wire is soldered to point and the other end is soldered to an isolated stub at point 48. Similarly, if the cable or wire is soldered to one of the coupled stubs, then one end of the cable/wire is soldered to point 62 0164 and the other end is soldered to an isolated stub on the PCB ground at point 48.

To achieve good omni-directional patterns, many designs opt for generally a 3D (three dimensional) axially symmetric element. Compared to a known Jaybeam disc cone, examples of the antenna described herein achieve the same or better omni-directional patterns. Although the invention described is two dimensional (2D), the element can be revolved and assume a 3D or axially symmetric element.

Although many different configurations are possible, the preferred example of the invention comprises an inexpensive FR4 PCB multi-band monopole shorted to an FR4 ground plane to provide both low frequency band extension and DC grounding and also an open circuit wire or cable to provide ., substantial improvement in VSWR and return loss to a certain part of the band * .** * ** (dependent on the length of the cable). To achieve a good or improved * * impedance match within a restrictive space, the low frequency performance is critically dependent on the element being grounded and improved with the open circuit wire or cable.

The dielectric constant (OK) and thickness of the FR4 material are not held to the same stringent tolerance as PTFE (polytetrafluoroethylene) substrates.

Therefore the simpler the structure, the less reliant it is on the material properties.

The example antennas provide a reduction of 14% in height over a known A/4 monopole (where A is the mean or average operating wavelength of the antenna). Furthermore, the ability to shape the top or bottom of the antenna with more flexibility allows for more choice of shape or a more aesthetically pleasing shape. These embodiments are inexpensive, both because of low labour costs because they are quick and simple to make and because they are made from cheap material. They are also tolerant to variation in material properties. Below is a comparison of material properties between FR4 and PTFE.

Material Properties of FR4 material from Isola Laminates Systems Corp. (http:I/www.isola.de/eI), Reference Name: Duraver -E -Cu: DK � 1MHz = 4.7 (5.4 max), Loss tangent � 1MHz = 0.035, and Material thickness = 0.8mm � 0.1mm.

Material properties of PTFE laminate from laconic (http:/Iwww.taconic-add.com/en....index.php): DK = 2.55 � 0.04, Loss tangent @1MHz = 0.0009, Material thickness = 0.8mm.

The antenna described herein can be used either indoors or outdoors.

Example antennas are shown in Figures 1 to 7 with the radome removed. They show various views of the broadband element connected to an n-type connector and the FR4 PCB ground. The radiating element is etched on an inexpensive 0.8mm thick FR4 PCB substrate. On the reverse side of the FR4 PCB, there are two stubs which are coupled electromagnetically from the main radiating element and help improve the impedance match. These stubs have minor influence on a...

expanding the frequency bandwidth but help improve the match in the frequency * a..

*** * band of interest by optimising the position and size of the stubs. Although only two stubs are shown, more stubs could be utilized. * .. * . . *..*

The antenna arrangements described herein ensure that the height of the element is low meeting the requirements of indoor and outdoor antenna * a..

manufacturers. Shorting the radiating element extends the bandwidth to the low end of the frequency band as shown in Figure 9 without increasing the height or width of the radiating element. Shorting the radiating element to the PCB ground also allows the antenna to achieve DC grounding for lightning protection in outdoor applications. The DC grounding is achieved whilst still maintaining good IMP performance.

The example antenna arrangements described herein achieve wideband performance but with significantly improved return performance over the required wireless service bands.

The example antennas are multi-band indoor or outdoor omni-directional antennas offering good impedance match and good radiation patterns suitable for various mobile communication applications. The antennas are capable of providing IMP friendly performance, are DC grounded and can cover the existing GSM,DCSI800,DCS1900 IMT-2000,UMTS,WLAN and DMB bands with good VSWR and radiation paftern performance in those bands. The antennas are easy to assemble giving low labour costs. The example antennas achieve good electrical performance and are aesthetically pleasing but with low component count and only a few minutes of assembly time. The example antennas are a cost effective product.

As an alternative to the n-type connector described, a DIN 7/16 connector could be used, which guarantees good IMP performance.

As an alternative to connecting the open end of the open circuit wire to mount 48, it could be held in place by a cable tie or other suitable device.

The example monopole antennas operate at mobile telecommunications frequencies of 800 MHz to 3000MHz. Alternatively, they operate at 870 MHz to 960MHz. Alternatively, they operate at 1710 MHz to 2500MHz.

: *** Examples of the present invention have been described. However, it will S...

be appreciated that variations and modifications may be made to the examples described within the scope of the present invention. * S *5SS

S *

S

Claims

CLAIMS1. A monopole antenna for operating at mobile telecommunications frequencies, the monopole antenna comprising: a radiating element; a ground plane substantially extending perpendicular to the radiating element and electrically conductively connected to the radiating element; and an open circuit comprising an electrically conducting wire electrically conductively connected to the radiating element; wherein the antenna is grounded to direct current.
2. A monopole antenna according to claim 1, wherein the radiating element comprises a planar electrical insulator with an electrical conductor on one face of it.
3. A monopole antenna according to claim 2, wherein the planar electrical insulator comprises epoxy resin reinforced by woven glass fibre.
4. A monopole antenna according to claim 2 or 3, further comprises at least one stub electro-magnetically coupled to the radiating element.
5. A monopole antenna according to claim 4, wherein at least one of the at least one stub is located on the planar electrical insulator. *..S
6. A monopole antenna according to claim 5, wherein the at least one of the at : * 20 least one stub is on a face of the planar electrical insulator.
7. A monopole antenna according to claim 6, wherein the at least one of the at least one stub is on the opposite face of the planar electrical insulator to the electrical *...conductor.S
8. A monopole antenna according to any of claims 2 to 7, wherein the radiating element is electrically conductively connected to the ground plane by an electrically conducting track.
9. A monopole antenna according to claim 8, wherein the electrically conducting track is on the face of the electrical insulator.
10. A monopole antenna according to claim 9, wherein the electrically conducting track is electrically connected by solder to the ground plane.
11. A monopole antenna according to any preceding claim, wherein the ground plane comprises a planar electrical insulator with an electrical conductor on one face ofit.
12. A monopole antenna according to claim 12, wherein the planar electrical insulator comprises epoxy resin reinforced by woven glass fibre.
13. A monopole antenna according to claim 11 or claim 12, wherein the electrically conducting wire is connected to the planar electrical insulator of the ground plane.
14. A monopole antenna according to any preceding claim for operating at mobile telecommunications frequencies of substantially 800 MHz to substantially 3000MHz.
15. A monopole antenna according to any preceding claim for operating at mobile telecommunications frequencies of substantially 870 MHz to substantially 960MHz.
16. A monopole antenna according to any preceding claim for operating at mobile telecommunications frequencies of substantially 1710 MHz to substantially 2500MHz.
17. A monopole antenna according to any preceding claim, wherein the radiating .. : element narrows at least in part at the end spaced from the ground plane. * * **.*
18. A monopole antenna for operating at mobile telecommunications frequencies, the monopole antenna comprising: a planar radiating element; a ground plane substantially extending perpendicular to the radiating element *** ** and electrically conductively connected to the radiating element; and at least one stub for improving the impedance match of the antenna, the at least one stub being substantially parallel to, spaced from and electromagnetically coupled to the radiating element; wherein the antenna is grounded to direct current.
19. A monopole antenna according to claim 18, further comprising an open circuit compnsing an electrically conducting wire electrically conductively connected to the radiating element.
20. A monopole antenna as substantially hereinbefore described with reference to and as illustrated by the accompanying drawings. * S * ** * S... * * S... * *5 * . . S... S... * SS*S.*..I I