EP1441412B1

EP1441412B1 - Antenna with distributed ground

Info

Publication number: EP1441412B1
Application number: EP03001733.9A
Authority: EP
Inventors: Hiroki c/o Sony Ercisson Mobile Comm. Int.AB Ito; Osamu c/o Sony Ericsson Mobile Comm. Int Matsuta; Akio c/o Sony Ericsson Mobile Comm. Int Ikushima
Original assignee: Sony Ericsson Mobile Communications AB
Current assignee: Sony Mobile Communications AB
Priority date: 2003-01-27
Filing date: 2003-01-27
Publication date: 2015-12-30
Anticipated expiration: 2023-01-27
Also published as: EP1441412A1

Description

The present invention relates to an antenna, in particular to a built-in antenna for use in mobile terminals.
For achieving mobile terminals of very compact dimensions, the antenna is usually integrated into the casing of the mobile terminal.
Various types of built-in antenna concepts are known like e.g. spiral-type antennae, microstrip antennae or others which allow to design an antenna small enough to fit within the casing of a mobile terminal. Technologies allowing to design a thin antenna element, e.g. microstrip, patch, printed dipoles or the like are favoured by many manufacturers of mobile terminals as a respective flat antenna device can be integrated conveniently within the casing of a mobile terminal as for instance indicated with dashed lines in Figure 1. The radiating part of a respective thin antenna element is hereby usually placed just underneath the back cover of a mobile's casing opposite the key and/or display supporting front side.
Most commonly, patch antennae are used which are implemented in microstrip technology. The term 'microstrip' is hereby to be seen in the broadest sense covering a thin conductive strip or layer separated from one or more large-surface counter electrodes by a non-conductive volume. The common concept of a microstrip antenna e.g. provides for a radiating component implemented in form of a thin conductive electrode arranged above a counter electrode which is set to ground potential. As the lateral dimensions of the radiating electrode are far bigger than its thickness, a respective type of electrode is usually referred to as a patch. The patch is separated from the counter or ground electrode by a spacer formed of non-conductive material such serving as a dielectric. A conductive strip extending from the patch forms the feeding line of the antenna which may be connected to a transceiver circuitry. The patch may show a planar as well as a curved surface. The lateral dimensions of the patch correspond to a certain part of the possible radio-frequency (RF) wavelengths to be received or transmitted such, that a standing wave condition for the antenna current distribution on the patch is fulfilled. The shape of the patch may hereto resemble a rectangle or also a more complex geometry.
The radiation efficiency and the bandwidth of a microstrip antenna are strongly affected by the antenna thickness given by the height or thickness, respectively, of the spacer separating the patch from the ground plate. Generally, a use of a thicker spacer results in an improved antenna performance as characterised by a better radiation efficiency combined with a broader bandwidth. Currently a spacer thickness of several millimetres is considered necessary to obtain a sufficient radiation characteristic, which constitutes a requirement that forms a major obstacle in further reducing the thickness of future mobile terminals.
It is therefore an object of the present invention to propose an antenna for use in mobile terminals where the thickness of the spacer can be reduced without degrading the antenna performance.
This object is achieved by an antenna for use in a mobile terminal to transmit and receive radiation of one or more radio-frequency bands as claimed in independent claim 1 enclosed. The antenna has an electrically conductive radiation antenna element which is adapted for being connected to an RF circuitry of the mobile terminal. It further comprises a spacer formed of non-conductive material with a first face supporting the radiation antenna element and a ground electrode, which is adapted to provide a ground potential for the radiation antenna element. The ground electrode is hereby located on the side of the spacer opposite its first face, whereby at least part of the ground electrode is located at a distance from the second face of the spacer opposite its first face.
An antenna according to the present invention allows to reduce the volume occupied by the spacer without worsening the antenna performance by simply shifting the ground electrode away from the spacer.
Additional advantageous features of the present invention are claimed in the respective sub-claims.
The second face of the spacer favourably faces a first surface of a printed wiring board to obtain a compact arrangement of components inside the mobile terminal. Effectively, the spacer is hereby mounted with its second face onto the first surface of the printed wiring board.
For increasing the effective antenna thickness, at least part of the ground electrode is advantageously formed by an electrically conductive patch supported by a second surface of the printed wiring board located opposite the first surface of the printed wiring board.
According to an advantageous development, the effective antenna thickness may also be increased by having at least part of the ground electrode being formed by a shielding component located inside the mobile terminal and by a conductive layer of a display located at the front side of the mobile terminal.
In the following description, the present invention is explained in more detail with respect to special embodiments and in relation to the enclosed drawings, in which

Fig. 1 shows a schematic representation of a mobile terminal with a built-in microstrip patch antenna in a front and in a side view,
Fig. 2 shows a common arrangement of components in a mobile terminal with a state-of-the-art microstrip patch antenna,
Fig. 3 shows a first embodiment of an antenna according to the present invention,
Fig. 4 shows a second embodiment of an antenna according to the present invention, and
Fig. 5 shows a third embodiment of an antenna according to the present invention,

Figure 1 is a representation of a typical state-of-the-art mobile terminal 1 with a built-in microstrip patch antenna 2 indicated by dashed lines for establishing a wireless link to a telecommunication network. The mobile terminal may be optimised for telecommunication purposes as shown, with a display 13, an earpiece 15, and a keypad 14 having functional and numerical keys which may further be used for an input of alphabetic characters. The mobile terminal 1 may also be optimised for other purposes like e.g. with organiser functions and an alpha-numerical keypad to serve as a Personal Digital Assistant (PDA) or for instance with a bigger display and a joystick-like input facility to serve as a game console optimised for wirelessly playing interactive games or the like. To obtain a compact design, all these different types of mobile terminals are preferably equipped with a built-in microstrip patch antenna such as the one indicated with dashed lines in the example of Figure 1.
Prior art documents EP0982798A , EP1020947A and EP1209760A2 show a mobile terminal with a buitl-in antenna according to the state-of-the-art.
As the lateral dimensions of a patch antenna are of the order of a quarter wavelength of an RF-signal to be received or transmitted, respectively, and as its thickness is typically below 1 cm, a patch antenna is flat enough for being placed inside a casing of a conventional mobile terminal 1. The flat design furthermore allows to reduce the portion of the radiation emitted from the antenna into the direction of the front side of the casing by means of very simple shielding mechanisms. But for a sufficient radiation performance, the patch antenna still requires a thickness of typically around 7 mm. This forms a major problem for further reducing the weight and overall size of a mobile terminal 1 as is the common trend, since the common microstrip patch antennae occupies a space within the mobile terminal 1, that cannot be shared with other components therein.
Figure 2 shows an exemplary arrangement of components inside a conventional mobile terminal 1 in a schematic representation. A conventional microstrip patch antenna 2 is typically composed of a radiating thin antenna element 3 implemented in form of a conductive material like e.g. a metal-plating or a conductive epoxy thick film printed on top of a dielectric 5 or the like. The radiating thin antenna element 3 will subsequently be referred to as patch 3. The indication of directions 'top' and 'bottom' used in the ongoing of the description refer to the corresponding directions of the enclosed figures. As the radiating component, the patch 3 is connected to an RF-circuit 4 of the mobile terminal 1. A counter electrode 6 is arranged opposite the patch 3 at the bottom face of the dielectric 5. This counter electrode 6 provides a ground potential for the radiating patch antenna element 3 and is therefore also referred to as ground electrode. The ground electrode 6 may be formed on the bottom face of the dielectric 5 by any known kind of plating, deposition, laminating or coating, in a comparable or in a completely different way than the patch antenna element 3. In this case, the respective patch antenna 2 is a single piece device with its thickness substantially defined by the thickness of the dielectric 5 used. Since the dielectric 5 separates the patch 3 from the ground electrode 6 by a certain distance defined by its thickness, it will also be referred to as spacer 5.
With regard to a precise indication of faces and surfaces, the face of the dielectric supporting the patch 3 of the antenna 2 will be referred to as the first face of the spacer 5, while the spacer's face opposite said first face will be identified as the second face of the spacer. Likewise, the surface of the printed wiring board (PWB) 8 facing the spacer will be denoted as the first surface of the PWB, and the surface opposite it on the PWB as the respective second surface.
For a good mechanical support and to ensure a short feeding line, a patch antenna 2 is preferably mounted on a main PWB 8 like shown in Figure 2. Usually in this case, the ground electrode 6 is formed by a conductive patch-like layer located on top of the PWB with the dielectric 5 placed with its second face upon it. The total height of the resulting antenna structure is thus given by the thickness of the spacer 5. Due to their film-like character, the contribution of patch 3 and ground electrode 6 to the overall structure height of the antenna can be neglected hereby.
As mentioned above, a spacer thickness of about 7 mm or more is commonly required for a sufficient antenna performance. This implies, that a respective microstrip patch antenna occupies a considerable volume of a mobile terminal's interior, a characteristic being opposed to the requirement of further reducing the size of future mobile terminals 1. The present invention is based on the assumption, that a microstrip patch antenna doesn't have to be implemented in form of a discreet device, but may be built-up with components arranged distributed within the mobile terminal 1. The antenna component which can most easily be placed apart from the rest of the antenna structure has been identified as the ground electrode 6.
Shifting the ground electrode 6 away from the second face of the spacer 5 increases the distance between the patch 3 and the ground electrode 6 thus improving the antenna performance. Seen the other way round, without worsening the antenna performance, the thickness of the spacer 5 can be reduced when the ground electrode is at the same time shifted away from the second face of the spacer.
A first embodiment implementing this insight is illustrated in Figure 3. Part of the ground electrode area is located on the second surface 10 of the PWB 8. The part of the ground electrode 6 on this second surface of the PWB is preferably implemented as a conductive patch-like structure 9. To form a continuous ground plane, the bottom-side ground electrode element 9 is electrically connected to the parts of the ground electrode remaining on the upper first surface 7 of the PWB 8, e.g. by means of one or more feedthroughs 16 as indicated in Figure 3.
The ground electrode arrangement shown in Figure 3 is particularly useful when components of the mobile terminal 1 are sensitive to an electromagnetic field which can build-up between the patch antenna element 3 and the corresponding ground electrode 6. With part of the ground electrode still formed on top of the PWB 8, those components mounted at the bottom face of the PWB or below are protected from the electromagnetic field. But it is clear from the description that alternatively to the representation in Figure 3, the ground electrode may also be formed exclusively on the second surface 10 of the PWB 8.
In a respective design, the antenna thickness is substantially defined by the sum of the thickness of the spacer 5 with the thickness of the PWB 8. For the design shown in Figure 3 the said applies only to the region with the ground electrode 9 built on the bottom side of the PWB 8. For the remaining regions with the ground electrode 6 formed atop the PWB 8, the antenna thickness is given by the spacer 5 thickness. Nevertheless a performance improvement is obtained as the effective height of the antenna structure is given by an average according to the relative contribution of each component.
A second embodiment increasing the effective height of the antenna arrangement according to the present invention is illustrated in Figure 4. Usually some components of a mobile terminal 1 which have a low level electromagnetic compatibility are located inside a shielding case 17 or are covered by a shield 17. At least an area of one outer surface of such a shield is set to ground potential for the shielding purpose. Sometimes shields 17 are used to block a passage for radiation instead of protecting components.
As at least part of an outer surfaces of a shielding or shielding case is set to ground potential, the respective surface or surfaces can be utilised to further dislodge the ground electrode 6 from the second face of the spacer 5. A respective example is shown in Figure 4 in a schematic representation. A conductive surface of a shield 17 serves as an extension 11 for the ground electrode 6. The antenna thickness at the extension part of the ground electrode is then substantially given by the sum of the thickness of the spacer 5 and PWB together with the distance of the extension 11 from the bottom surface of the PWB. Although some part of the ground electrode 6 may still remain on the first and/or second surface of the PWB, the effective thickness of the antenna structure is considerably improved thus allowing a further reduction of the spacer thickness.
Finally, conductive surfaces at or close to the front side of a mobile terminal may be integrated in a ground electrode 6. Usually a display 13 of a mobile terminal has a backside shield 12 which is suited to serve as a ground electrode. The backside shield may be formed by a metal cover, a metallisation or any other known type of electrically conductive layer or foil. As a display 13 is usually integrated in the front of a mobile terminal while the radiating patch 3 of a microstrip antenna 7 is preferably placed close to the back cover, the backside shield 12 is generally the most distant contiguous conductive layer usable as a part for a ground electrode 6. Using the backside shield 12 as a major extension of the ground electrode 6 allows to increase the effective antenna thickness further than with the examples given above. Like for that examples, the ground electrode 6 may be formed by the backside shield 12 only or by a combination of the shield 12 together with other ground potential layers like e.g. those formed on a shield 11, on the second surface and/or first surface of the PWB 8.
As the ground electrode is relocated from the second face of the spacer 5 at least in parts, the necessary thickness of the spacer 5 can be reduced thus enabling a thinner design of mobile terminals than presently possible.

Claims

Mobile terminal (1) comprising
- a printed wiring board (8);

- a display (13) located at the front side of the mobile terminal (1); and

- an antenna (2) to transmit and receive radiation of one or more radio-frequency bands, the antenna (2) comprising:
-- an electrically conductive patch antenna element (3) adapted for being connected to an RF circuitry (4) of the mobile terminal (1),

- a spacer (5) formed of non-conductive material having a first face supporting the patch antenna element (3) and a second face opposite the first face, and

- a ground electrode (6, 9, 11, 12) adapted to provide a ground potential for the patch antenna element (3) and being located on the side of the spacer (5) opposite the first face, whereby the second face of the spacer (5) faces a first surface (7) of the printed wiring board (8), and

whereby at least part of the ground electrode (6, 9, 11, 12) is located at a distance from the second face of the spacer (5), and
whereby at least part of the ground electrode (6, 9, 11, 12) is formed by a shielding component (11) located inside the mobile terminal (1) on the opposite side of the printed wiring board (8) in relation to the patch antenna (3),
characterised in that
at least part of the ground electrode (6, 9, 11, 12) is formed by a conductive layer (12) of the display (13).
Mobile terminal (1) according to claim 1,
characterised in
that the spacer (5) is mounted with its second face onto the first surface (7) of the printed wiring board (8).
Mobile terminal (1) according to claims 1 or 2,
characterised in
that an electrically conductive patch (9) supported by a second surface (10) of the printed wiring board (8) located opposite the first surface (7) of the printed wiring board (8) forms at least one part of the ground electrode (6, 9, 11, 12).