GB2396967A

GB2396967A - Strip feed arrangement for a compact internal planar antenna element

Info

Publication number: GB2396967A
Application number: GB0230266A
Authority: GB
Inventors: Ming Zheng; Hanyang Wang; Jason Goward
Original assignee: Nokia Oyj; Nokia Inc
Current assignee: Nokia Oyj; Nokia Inc
Priority date: 2002-12-30
Filing date: 2002-12-30
Publication date: 2004-07-07
Also published as: AU2003298454A8; GB0230266D0; WO2004059785A2; WO2004059785A3; AU2003298454A1

Abstract

An antenna element 112, for a radio transceiver, comprises a planar portion 114 with first and second strips 118, 120 extending from the said portion and a third strip 122 connecting between the first and second strips. The first strip 118 may be a feed line and the second strip 120 a ground line whilst the third strip 122 is a bridge connecting the feed and ground lines. The planar portion 114 and the said strips 118, 120, 122 may be integrally formed from a single piece of metal sheet. The feed and ground line strips 118, 120 may be parallel and extend from the edge of the planar portion 114 within the same plane as the said portion until they reach a knee area where they are bent to go in a direction orthogonal to the said plane. The distal ends of the feed and ground lines 118, 120 connect to a printed circuit board 108. There may be one or more bridging strips 122 which may vary in shape and dimensions and may also be located in the same plane as the planar portion 114 on the section of the feed and ground lines located between the planar portion and the said knee area. The antenna element 112 provides a compact planar inverted F antenna (PIFA) arrangement 110 which can be used in a mobile radio transceiver.

Description

1 2396967

AN INTERNAL ANTENNA ELEMENT

Embodiments of the present invention relate to an internal antenna element.

Fig. 1 illustrates a prior art internal antenna 10 suitable for use in a mobile telephone.

The antenna 10 is a planar inverted-F antenna (PIFA). The antenna 10 comprises an antenna element 12 and a parallel ground plate 8, which is generally a printed circuit board (PCB). The antenna element 12 has a planar plate portion 14 and an antenna feeding structure 16. The antenna feeding structure 16 has a feed line strip 18 and an earth point strip 20. The feed line strip 18 extends from an edge of the planar portion 14 and in the plane of the planar portion 14, before bending at a 'knee' portion and terminating at a feed line contact point 19 that contacts the PCB 8. The earth point strip extends parallel to the feed line 18, from an edge of the plate portion 14 and in the plane of the planar portion 14, before bending at a 'knee' portion and terminating at an earth contact point 21 that contacts the PCB 8.

Slots 22 between the feed line 18 and the earth pin 20 provide adequate mechanical spring force for the feed line 18 and the earth pin 20 where they contact the PCB 8.

One problem with this type of internal antenna is that the slots 22 reduce the closed À À effective area of the antenna plate portion. This reduces the efficiency of the internal À..

À..... antenna. À ee.

À Another problem with this type of internal antenna is that as the size of a device housing Àe À À the antenna decreases, the PCB size must also decrease. A decrease in the PCB size À À decreases the effectiveness of the antenna. À À À Àe Àe À .

À It would be desirable to provide an efficient internal antenna that is suitable for small devices.

According to one aspect there is provided an internal antenna element, for a radio transceiver, comprising: a planar portion; a first strip extending from the planar portion; a second strip, adjacent the first strip, extending from the planar portion; and a connecting strip joining the first strip and the second strip.

The connecting strip provides sufficient spring force between the first and second strips without the presence of slots between the first and second strips. The perimeter of the closed antenna area passes through the connecting strip. Consequently, the inventive antenna element has an increased effective area and has improved efficiency.

The configuration of the connecting strip can be varied to vary the electrical characteristics of the internal antenna element. The size, position, number or shape of the connecting strip may be varied. This provides an extra degree of design freedom for the antenna element which is not constrained by the PCB to which it will be attached.

For a better understanding of the present invention and to understand how the same may be brought into effect reference will now be made by way of example only to the accompanying drawings in which:

Fig. 1 illustrates a prior art internal antenna;

Fig. 2 illustrates an internal antenna according to one embodiment of the present invention; Fig. 3 illustrates how the electrical characteristics of an internal antenna may vary with the position of the bridge; Figs. 4A, 4B and 4C illustrate alternative configurations for the bridge; À À À Fig 5 illustrates a template for producing an internal antenna element; and À.. Fig. 6 illustrates a radio transceiver device, such as a mobile telephone, having an internal antenna. À: Àe

À À Fig. 2 illustrates an unbalanced internal antenna 110 suitable for use in a mobile a... . telephone. The antenna 110 is a planar inverted-F antenna (PIFA). The antenna 110 À Àe comprises an antenna element 112 and a ground plate 108, which is generally a printed À circuit board (PCB). The antenna element 112 has a planar plate portion 114 that is parallel to the ground plate 108 and an antenna feeding structure 116, connecting the ground plate 108 and the planar portion 114. The antenna feeding structure 116 has a feed line strip 118, an adjacent earth point strip120 and a bridge 122 joining the feed line strip 1 18 and the earth point strip 120.

In this configuration, the antenna 110 defines an aperture bounded by the planar plate portion 114, the feed line strip 118, the bridge 122, and the earth point strip 120.

The feed line strip 118 extends from an edge of the planar portion 114 and in the plane of the planar portion 114, until it bends at a 'knee' portion and terminates at a feed line contact point 119 that contacts the PCB 108. The earth point strip 120 extends parallel to the feed line 118 from an edge of the planar portion 114 and in the plane of the planar portion 114, until it bends at a 'knee' portion and terminates at an earth contact point 121 that contacts the PCB 108. The spacing between the feed line contact point 119 and the earth contact point 121 is referred to as the feeding gap.

The bridge 122 is a connecting strip that connects the feed line strip 118 and the earth point strip 120. The connecting strip 122 joins the feed line strip 118 at a first connection 124 and joins the earth point strip 120 at a second connection 126.

In the illustrated example, the connecting strip 122 lies in the plane of the planar portion 114 and joins the feed line strip 118 and the earth point strip 120 where they extend parallel to the plane of the planar portion 114. The feed line strip 118 and the earth point strip 120 are parallel to each other and the connecting strip is perpendicular to both. The connecting strip 122 is positioned closer to the knees of the feed line strip 118 and the earth point strip 120 than to the points where the feed line strip 118 and the earth point À strip 120 join the planar portion 114. The length of the feed line strip 118 between the first me.

À. connection 124 and the feed line contact point 119 is approximately twice the length of the feed line strip 118 between the first connection 124 and where the feed line strip 118 Àaa.e joins the planar portion 114. The length of the earth point strip 120 between the second Àee.

À connection 126 and the earth contact point 121 is approximately twice the length of the À . . feed line strip 118 between the second connection 126 and where the earth point strip À Àe joins the planar portion 114. À Àe

The configuration of the antenna feeding structure 116 may be varied to vary the electrical characteristics of the antenna 110. For example the distance between the feed line strip 118 and the earth point strip 120 may be varied and the width of the feed line strip 118 and the width of the earth point strip may be varied.

The configuration of the bridge may be varied to change the electrical characteristics of the antenna 110. The configuration of the bridge may be varied by changing the position of the connecting strip 122. Fig. 3 illustrates how the parameter S11 varies with frequency for each one of several different positions of the connecting strip 122. The parameter S11 is the return loss of the antenna.

The trace A is for an antenna element for which the distance between first connection 124 and the feed line contact point 119 is zero mm and the distance between the second connection 126 and the earth contact point 121 is zero mm. The trace B is for an antenna element for which the distance between first connection 124 and the feed line contact point 119 is 1 mm and the distance between the second connection 126 and the earth contact point 121 is1mm. The trace C is for an antenna element for which the distance between first connection 124 and the feed line contact point 119 is 4mm and the distance between the second connection 126 and the earth contact point 121 is 4 mm. The trace D is for an antenna element for which the distance between first connection 124 and the feed line contact point 119 is 8mm and the distance between the second connection 126 and the earth contact point 121 is 8 mm.

The preferred position, in this example of an antenna for an EGSM mobile telephone, is when the distance between first connection 124 and the feed line contact point 119 and the distance between the second connection 126 and the earth contact point 121 are both 8-10mm. The distance between first connection 124 and the feed line contact point 119 and the distance between the second connection 126 and the earth contact point . . should be a minimum of 4mm. bare

The width of the connecting strip can also be varied. One example of an internal antenna 110, for an EGSM mobile telephone, has a connecting strip of width 0.5mm and length of e I,.e À À approximately 3.5 mm. The feed line strip 118 has a length of approximately 13mm and a . , width of 2.12mm. The portion of the feed line strip 118 parallel to the planar portion, ad before the knee bend, has a length of approximately 5 mm. The portion of the feed line strip 118 perpendicular to the planar portion, after the knee bend, has a length of approximately 8 mm. The portion of the earth point strip 120 parallel to the planar portion, before the knee bend, has a length of approximately 5 mm. The portion of the earth point strip 120 perpendicular to the planar portion, after the knee bend, has a length of approximately 8 mm. The connecting strip 122 is positioned approximately 4.3mm from where the feed line strip 118 and the planar portion meet and approximately 4.3mm from where the earth point strip 120 and the planar portion 114 meet. The distance between first connection 124 and the feed line contact point 119 is approximately 8.5mm. The distance between second connection 126 and the earth contact point 121 is approximately 8.5mm.

In other embodiments, the number and shape of the bridge 122 may be changed as illustrated in Figs 4A, 4B and 4C. Fig. 4A illustrates a bridge 122 comprising multiple parallel connection strips 122a and 122b extending between and perpendicular to the feed line strip 118 and the earth point strip 120. Fig. 4B illustrates a bridge 122 comprising a single meandering connection strip extending between the feed line strip 118 and the earth point strip 120. Fig. 4C illustrates a bridge 122 that has a narrowed portion 122c.

The bridge 122 of the internal antenna 110 acts as a distributed shunt inductor, which compensates the capacitance characteristics presented by the antenna. The bridge is not only part of the antenna feeding structure 116 but also part of the antenna 110. The perimeter of the antenna closed area passes through the connecting bridge 122. This provides a large radiation area and results in a large bandwidth. A larger bandwidth gives a better antenna return loss and better efficiency The antenna element 112 illustrated in Fig.2 is made from the planar antenna element template illustrated in Fig 5. The template is one-piece of sheet metal. It is placed on a mould that supports the planar portion while the feed line strip 118 and earth point strip it. 120 are bent to create the antenna feeding structure 116 illustrated in Fig 2. À À< . À

The antenna element 112 may be used in a mobile radio transceiver device, such as the *.: mobile phone 1 50 illustrated in Fig 6. The mobile phone comprises a PCB 108 to which is À attached an internal antenna element 112 and radio transceiver circuitry 152. The radio À transceiver circuitry is connected to the feed line point 119 of the antenna element. The À *. internal antenna structure has particular application for mobile telephones operating in Hi: the 900MHz EGSM band. À Àe

An antenna element 112 is generally designed before the components to be attached to the PCB 108 are fully specified. Thus, when the final components are added to the PCB 108, the PCB will not necessarily have the same characteristics as that for which the antenna element 112 was designed. The operation of the antenna 110 will be sub-optimal and, if the feeding gap is now fixed on the PCB 108, it may not be possible to optimise it.

The presence of a bridge 122 allows the performance of the antenna 110 to be redesigned by changing the configuration of the bridge 122. The bridge 122 therefore provides an extra degree of design freedom, which is not constrained by the PCB layout.

Although embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the invention as claimed.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon. À. .. À À cee. . me He À

A À s À À . À. Àe À C À

Claims

Claims 1. An internal antenna element, for a radio transceiver,

comprising: a planar portion; a first strip extending from the planar portion a second strip, adjacent the first strip, extending from the planar portion; and a connecting strip joining the first strip and the second strip.
2. An internal antenna element as claimed in claim 1, wherein the first strip, the second strip, the connecting strip, and the planar portion are integral.
3. An internal antenna element as claimed in claim 2, wherein the first strip, the second strip, the connecting strip, and the planar portion are formed from one-piece of metal sheet material.
4. An internal antenna element as claimed in any preceding claim, wherein the connecting strip extends, between the first strip and the second strip, in the plane of the planar portion.
5. An internal antenna element as claimed in any preceding claim, wherein the first and second strips are bent at knee portions and the connecting strip is positioned closer to the knee portions than to where the first and second strips and the planar portion join. À. À. À .
6. An internal antenna element as claimed in any preceding claim wherein the connecting À strip is positioned so that the length of the first strip between the connecting strip and an a'.. extremity of the first strip is greater than the length of the first strip between the connecting strip and where the first strip joins the planar portion. À .
7. An internal antenna element as claimed in any preceding claim wherein the connecting . strip is positioned so that the length of the first strip between the connecting strip and an extremity of the first strip is approximately twice the length of the first strip between the connecting strip and where the first strip joins the planar portion.
8. An internal antenna element as claimed in any preceding claim wherein the connecting strip is positioned so that the length of the first strip between the connecting strip and where the first strip joins the planar portion is no more than twice the length of the first strip between the connecting strip and an extremity of the first strip.
9. An internal antenna element as claimed in any preceding claim wherein the connecting strip is positioned so that the length of the first strip between the connecting strip and an extremity of the first strip is greater than 4mm.
10. An internal antenna element as claimed in any preceding claim wherein the first strip and the second strip are substantially parallel.
11. An internal antenna element as claimed in any preceding claim wherein the connecting strip is substantially perpendicular to the first strip.
12. An internal antenna element as claimed in any preceding claim, wherein the first strip and the second strip extend substantially parallel to the planar portion until knee portions from which they extend substantially perpendicular to the planar portion.
13. An internal antenna element as claimed in any preceding claim, comprising a plurality of connecting strips joining the first strip and the second strip.
14. An internal antenna element substantially as hereinbefore described with reference to and/or as shown in Figs. 2 to 5. e.
15. An internal antenna comprising an internal antenna element as claimed in any preceding claim. . :
16. A mobile radio transceiver comprising an internal antenna as claimed in claim 15. À
17. Any novel subject matter or combination including novel subject matter disclosed, : whether or not within the scope of or relating to the same invention as the preceding claims