EP1305845A1

EP1305845A1 - Planar radiating surface antenna and portable telephone comprising same

Info

Publication number: EP1305845A1
Application number: EP01958206A
Authority: EP
Inventors: François BLANCHO
Original assignee: Sagem SA
Current assignee: Vallaroche SAS
Priority date: 2000-08-01
Filing date: 2001-07-31
Publication date: 2003-05-02
Anticipated expiration: 2021-07-31
Also published as: US20030146873A1; DE60137047D1; EP1305845B1; JP2004505581A; WO2002011236A1; FR2812766A1; FR2812766B1; EP1305845B9

Abstract

The invention concerns a transmission/reception antenna comprising a ground plane and at least a planar radiating surface extending plumb over said ground plane and parallel thereto. The invention is characterised in that the radiation section(s) are linked to means defining at least an adaptation circuit which, in transmission as well as in reception is resonant on simultaneously at least two frequency bands.

Description

ANTENNA WITH RADIANT SURFACE (S) PLANE (S) AND PORTABLE TELEPHONE COMPRISING SUCH ANTENNA

GENERAL FIELD OF THE INVENTION AND STATE OF THE ART

The present invention relates to an antenna of the type with planar radiating surface (s).

The invention proposes in particular an antenna of this type able to be used in transmission / reception on at least two frequency bands. It also offers a mobile telephone structure comprising such an antenna.

It has already been proposed to use for mobile telephones flat antennas called PIFA (“Planar Inverted-F Antenna” or inverted F type flat antenna) which comprise, as illustrated in FIG. 1, a ground plane 1 and a planar conductive surface 2 which is superimposed on this ground plane 1, and extends to the right and parallel to it. Such an arrangement has a resonant wavelength which is a function of the dimensions of the planar conductive surface 2 and of the height which separates it from its ground plane 1. These antennas in particular have an important advantage in terms of size.

However, a limitation of these antennas is that their pass bandwidth is all the more limited as the height h which separates their radiating surface 2 from their ground plane 1 is small. This in fact prevents having an optimal gain over all the channels used on the same bandwidth.

In particular, mobile phones are already known which use antennas of the aforementioned type which have two radiating surfaces which are of different dimensions and which are arranged in the same plane, above the same ground plane.

These two radiating surfaces make it possible, as illustrated by the graph in FIG. 2, to have two frequency bands for the antenna, one around 900 MHz, the other around 1.8 GHz. However, the use of a "duplex" separation on transmission / reception, as is usually the case in systems

GSM, leads to use for the transmit and receive frequencies f _T χ, f _R x the edges of the antenna resonance bands, where the adaptations are not very satisfactory.

PRESENTATION OF THE INVENTION

An object of the invention is to overcome the aforementioned drawbacks and to propose an antenna of the aforementioned type with an optimized gain for different channels in the same resonance band of the antenna.

Another object also of the invention is to provide an antenna with at least two transmit / receive bands which is of reduced bulk.

WO90 / 13152 discloses an antenna structure which comprises two radiating elements, one which is used for transmission, the other which is used for reception. It is not intended to use a resonant circuit simultaneously on several frequency bands, the switching to different reception sub-bands being done by switching of capacitive or inductive means. It is also known, for example from EP 687 030, to modify a resonant frequency of an antenna by means of a capacitor switched by a diode.

Furthermore, EP 892 459 discloses circuits making it possible to switch from a transmission band to a reception band. These different structures do not achieve the aims of the invention.

The invention proposes, for its part, a transmit / receive antenna comprising a ground plane and at least one planar radiation surface which extends in line with said ground plane and parallel to it, characterized in that the or the radiation sections are connected to means defining at least one adaptation circuit which, on transmission as on reception, is resonant on simultaneously at least two frequency bands. In particular, the means defining an adaptation circuit are advantageously mounted in parallel with capacitive or inductive means themselves mounted in series with means capable of being selectively switched between a passing state and a blocked state, these capacitive or inductive means shifting the antenna resonance bands, depending on whether the means capable of being switched are in the on state or in the blocked state.

In a preferred embodiment, the antenna comprises at least two radiation surfaces connected by means forming at least one adaptation circuit.

The two radiation surfaces are advantageously in the same plane.

In addition, the antenna is connected to the ground plane and to an electronics supply of RF signals and of reception by two points on either side of a radiating section.

The means forming an adaptation circuit are then split on one side and on the other of the radiating sections.

The invention also relates to a portable telephone which includes such an antenna. In particular, it offers a mobile telephone structure comprising a box in the bottom of which is a ground plane and speaker electronics, a structure in which said ground plane is extended by a shielding part which has an upward recess speaker and which constitutes the ground plane of an antenna of the aforementioned type, the radiating section or sections of which are arranged in the housing on the side of the shielding part opposite the loudspeaker.

Other characteristics and advantages of the invention will also emerge from the description which follows, which is purely illustrative and not limiting and should be read with reference to the appended drawings in which: - Figure 1, already discussed, schematically represents an antenna of PIFA type conforming to a known state of the art; FIG. 2, also already discussed, is an adaptation / frequency graph illustrating the distribution of the transmit / receive frequencies relative to the frequency bands of a double-band PIFA type antenna conforming to a known state of the art;

- Figure 3 is a schematic representation from above of an antenna according to a possible embodiment of the invention; - Figure 4 is a schematic representation from above of an antenna according to a preferred embodiment of the invention;

FIG. 5 is a graph similar to that of FIG. 2 illustrating the operation of an antenna of the type of that of FIG. 4.

The antenna shown in Figure 3 has a ground plane 10 and two planar radiating elements 11 and 12 which extend superimposed on said ground plane 10, being parallel to it and arranged in the same plane.

These two elements 11 and 12 are both of rectangular sections. They are in this case aligned with respect to each other and have the same width I. Their respective lengths, referenced by L1 and L2, may be equal or different.

More precisely, these two radiating sections 11 and 12 substantially verify the following equation:

^> h + L + L ₂ + U2> ^ - 4 ' ² 4

with A, = -.

J n

At its opposite end to the radiating section 12, the radiating section 11 is connected on one side to the ground plane (connection point 13) and on the other to an electronic power supply / reception of RF signals (connection point 14). The two sections 11 and 12 are connected to each other by at least one circuit which allows the antenna to resonate on two frequency bands centered on the frequencies F1 and F2.

More specifically, two identical LC plug circuits are provided on either side of the median length of the two sections 11 and 12, which can be understood in this case, taking account of the arrangement of points 13 and 14, as a line of symmetry for mounting. The values of the components L and C of these plug circuits are chosen so that these circuits have a resonant frequency substantially in the middle of F1 and F2.

In a simplified manner, the operation of an assembly of the type which has just been described can be understood as follows.

It is known that a plug circuit behaves substantially as a purely inductive element before its resonant frequency and as a purely capacitive element after its resonant frequency.

The introduction of a plug circuit between the two sections will complement these by - depending on whether there is before or after said resonant frequency - a capacitor or an adaptation inductor. However, adding an inductance to a planar antenna has the same effect as if it were lengthened: we move the resonance band towards the low frequencies; adding a capacitor to a planar antenna has the same effect as if we shortened it: we move the resonance band towards the high frequencies.

An assembly of the type represented in FIG. 1 therefore works with two resonance bands, which by varying the dimensions of the radiating sections and the height h, as well as the values of the components L and C, can be centered on the two resonant frequencies F1 and F2.

The antenna which is illustrated in FIG. 4 (apart from differences in representation scales) takes up all of the various elements of the assembly of FIG. 3. It furthermore, mounted in parallel to the plug circuit 17, a branch which comprises a capacitor C1 mounted in series with an element D forming a controlled switch, which in this case is a PIN diode. A similar assembly is mounted in parallel with the plug circuit 18. The two diodes D of these assemblies are controlled by a voltage signal CT transmitted through shock inductors Le.

A protection capacitor C2 is also provided between point 214 and the power / reception electronics associated with the antenna. Depending on whether or not a voltage signal CT is injected on the anode of the diodes D, the latter are blocked or on the contrary pass-through.

In the first case, the assembly is equivalent to that of FIG. 3. In the second case, the plug circuits 17, 18 are modified by the presence of the capacitors C1 mounted in parallel to it, so that the frequencies of the two resonance bands then created are offset from those obtained when the diodes D are blocked. This is illustrated by the two gain curves which are represented in FIG. 5.

As will be understood, the values of the various components of the assembly which has just been described and in particular the inductive and capacitive values are chosen so that the resonance frequencies F1 and F2 correspond in one case to the emission frequencies "duplex "fγx (curve in solid lines in FIG. 5) and in the other case at the" duplex "reception frequencies f _R χ (curve in dotted lines in FIG. 5).

With such an arrangement, the antenna is perfectly resonant for one and the other of the two bands and this both at the emission and at the reception. Such an antenna is advantageously integrated into a mobile telephone structure, as illustrated in FIG. 6.

This portable telephone structure comprises in a box B a card 19 which is arranged in the vicinity of the front face of the telephone box and which carries the various keys 20 of the keyboard, means for controlling the screen 21 of the telephone, as well as means 22 forming a loudspeaker.

In the bottom of the housing B is arranged a metallization 23 which defines the neutral (or mass) of the portable telephone and to which the card 19 and any other cards of the telephone (card 24 in FIG. 6) are connected.

This ground plane 23 is extended by a metal shield 25 which extends with a recess to the right of the means 22 forming a loudspeaker. This metal shield 25 is used to make the ground plane of the antenna, the sections 11 and 12 of the latter being arranged on the bottom of the housing B, in line with the shield 25.

With such a configuration, the volume occupied by the antenna mounting is minimal, the height h between the shield 25 and the radiating sections 11 and 12 can be 8 mm or less.

In addition, an important advantage of this configuration is that the shielding effectively protects the user against electromagnetic radiation emitted by the antenna. Of course, the invention has been described here in the case of dual-band transmission / reception but could be applied more generally for any transmission / reception around n frequencies.

In addition, the invention has been described here in the case of radiating surfaces of rectangular sections, but could of course be applied with radiating sections having other shapes.

Claims

1. Transmitting / receiving antenna comprising a ground plane and at least one planar surface of radiation which extends in line with said ground plane and parallel to it, characterized in that the radiation section or sections are connected to means defining at least one adaptation circuit which, on transmission as on reception, resonates simultaneously on at least two frequency bands.

2. Antenna according to claim 1, characterized in that the means defining an adaptation circuit are mounted in parallel with capacitive or inductive means themselves mounted in series with means capable of being selectively switched between a passing state and a blocked state, these capacitive or inductive means shifting the resonance bands of the antenna, depending on the on or blocked state of the means capable of being switched.

3. Antenna according to one of the preceding claims, characterized in that it comprises at least two radiation surfaces connected by means forming at least one adaptation circuit.

4. Antenna according to claim 3, characterized in that the two radiation surfaces are in the same plane.

5. Antenna according to one of claims 3 or 4, characterized in that it is connected to the ground plane and to an RF power / reception electronics by two points on either side of a radiating section.

6. Antenna according to claim 5, characterized in that the means forming an adaptation circuit are split on one side and on the other of the radiating sections.

7. Antenna according to one of claims 5 or 6, characterized in that the electronics for supplying RF signals and for receiving are connected to the radiating section by a capacitor.

8. Antenna according to claim 2 taken alone or in combination with one of claims 3 to 6, characterized in that the means capable of being selectively switched between a passing state and a blocked state comprise at least one PIN diode.

9. Mobile phone, characterized in that it comprises an antenna according to one of the preceding claims.

10. Mobile telephone comprising a box in the bottom of which is disposed a ground plane and a loudspeaker electronics, characterized in that said ground plane is extended by a shielding part which has a step towards the loudspeaker and which constitutes the ground plane of an antenna according to one of claims 1 to 8, the radiating section or sections of which are arranged in the housing on the side of the shielding part opposite the loudspeaker.