Antenna device and portable radio communication device comprising such an antenna device

(54) Name: Antenna device and portable radio communication device including such antenna device (56) Publications cited: - (47) Summary:

An antenna device for a portable radio communication device adapted to operate in at least a first and a second frequency band and comprising a first electrically conductive plane radiating element (10) with a supply member (12) connectable to a supply device (RF) of the radio communication device and a second electrically conductive device; flat, radiant element (20) with a grounding element connectable to ground. A controllable switch (30) is arranged between the first and second radiating elements for selectively interconnecting and disassembling the radiating elements, the state of the switch being controlled by a control voltage input (V_ltoU). A first filter (16) is arranged between the supply portion and the control voltage input, the first filter being arranged to block radio frequency signals. By providing a high-pass filter (32) between the first and second substantially planar radiating elements disposed over a ground plane, four-band operation with high efficiency is achieved in a physically small antenna device.

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Summary

An antenna device for a portable radio communication device adapted to operate in at least a first and a second frequency band and comprising a first electrically conductive plane radiating element (10) with a supply member (12) connectable to a supply device (RF) of the radio communication device and a second electrically conductive device; flat, radiant element (20) with a grounding element connectable to ground. A controllable switch (30) is provided between the first and second radiating elements for selectively interconnecting and disassembling the radiating elements, the state of the switch being controlled by a control voltage input (V_switoh). A first filter (16) is arranged between the supply portion and the control voltage input, the first filter being arranged to block radio frequency signals. By providing a high-pass filter (32) between the first and second substantially planar radiating elements disposed over a ground plane, four-band operation with high efficiency is achieved in a physically small antenna device.

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Field of the Invention

The present invention relates generally to antenna devices and, more specifically, to a controllable internal, multi-band antenna device for use in portable radio communication devices, such as in mobile phones. The invention also relates to a portable radio communication device comprising such an antenna device.

Background

Internal antennas have been used for some time in portable radio communication devices. There are a number of advantages associated with the use of internal antennas, of which one can mention that they are small and lightweight, making them suitable for applications where size and weight are of importance, such as in mobile phones. One type of internal antenna that is often used in portable radio communication devices is the so-called flat inverted F antenna (PIFA).

However, the use of internal antennas in a mobile phone places certain limitations on the antenna's design, such as the dimensions of the radiating or radiating elements, the exact location of the feed and grounding parts, etc. These limitations make it difficult to find a design of the antenna which gives a wide working hand. This is especially important for antennas intended for multi-band operation, in which the antenna is arranged to operate in two or more spaced apart frequency bands. In a typical two-band telephone, the lower frequency band is centered around 900 MHz, the so-called GSM 900 band, while the Upper frequency band is centered around 1800 or 1900 MHz, the DCS and PCS band respectively. If the top frequency band of the antenna device is made sufficiently wide, covering both the 1800 and 1900 MHz bands, a telephone will be obtained which operates in three different standard bands. In the near future, it is possible to think of antenna devices that operate in four or even more different frequency bands.

The number of frequency bands in passive antennas is limited by the size of the antenna. To further increase the number of frequency bands and / or reduce the antenna size, active frequency control can be used. An example of active frequency control is described in Patent Abstracts of Japan 10190347, which discloses a patenting device capable of handling multiple frequencies. For this, there is a base patch portion and a further patch portion which are interconnected by PlN diodes arranged to selectively couple and disassemble the patch portions. Although this provides a frequency control, the antenna device is still large in size and is not well adapted for switching between two or more relatively separated frequency bands, such as between the GSM and DCS / PCS bands.

Instead, this example of known devices is typical in that the switching on and off of additional patches has been used for tuning rather than creating additional frequency bands at a distance from a first frequency band.

Patent abstracts of Japan with publication number JP2000-236209 disclose a monopole antenna comprising a linear conductor on a dielectric substrate, see Figure 1. Radiant portions of the antenna consist of at least two metal portions connected by diode switching circuits. The radiating elements have feed points connected to one end of a filter circuit which blocks a high frequency signal. A signal V_swltch is used to control the diode switch. The configuration described is limited to monopoly or dipole antennas. Furthermore, the purpose of the antenna according to the aforementioned Japanese documents is not to provide a small size antenna.

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Thus, a problem with known antenna devices is to provide a single-band PIFA-type multi-band antenna with small size and volume and wide frequency bands that maintain good performance.

Summary of the Invention

It is an object of the present invention to provide an antenna device of the type mentioned in the introduction, at which the frequency characteristic gives four comparatively wide frequency bands while the overall size of the antenna device is small.

Another object is to provide an antenna device with better multiband performance than known devices.

The invention is based on the understanding that multiple frequency bands can be provided in a physically very small antenna by arranging the antenna such that first portions of two radiating elements are interconnected for radio frequency signals and second portions of the radiating elements are selectively interconnected by a switch controlled by a DC. voltage. This DC voltage is applied to a control input, whereby a filter arrangement arranged between the RF supply part and the DC control input blocks RF signals.

According to a first aspect of the present invention there is provided an antenna device as defined in claim 1.

According to a second aspect of the present invention there is provided a portable radio communication device, as defined in claim 10.

Further preferred embodiments are defined in the dependent claims.

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The invention provides an antenna device and a portable radio communication device in which the problems of known devices are avoided or at least reduced. Thus, a multi-band antenna device is provided having an antenna volume as small as about cm³ which means that the size of the antenna is reduced compared to conventional multi-band patch antennas but still retains RF performance. Furthermore, the bandwidths of the antenna device according to the invention can be improved compared to corresponding known devices but without any increase in the physical size, which is believed to be the result of the use of the two-band antenna structure.

The switch is preferably a PlN diode which has good properties as it functions as an electrically controlled RF switch.

Brief description of the drawings

The invention will now be described by way of example with reference to the accompanying drawings, in which:

Figure 1 is a description of a known monopoly antenna;

Figure 2 shows an overview diagram of a PIFA antenna device according to the invention;

Figures 2a and 2b show the PIFA antenna of Figure 2 in a first and second working mode, respectively;

Figure 2c is a frequency diagram of the working modes of the antenna shown in Figure 2;

Figure 3 is an overview of a printed circuit board arranged for mounting in a portable communication device and having an antenna device according to the invention;

528 Figure 4 shows an embodiment of the antenna device, wherein capacitive coupling between radiating elements is provided by a conductive plate;

Figure 5 shows a further embodiment of the antenna device, wherein capacitive coupling between radiating elements is provided by means of a meandering interface between the radiating elements;

Figure 6 shows yet another alternative design of radiant elements;

Figure 7 shows an alternative embodiment of an antenna device according to the invention, in which there are three radiating elements and Figures 7a-d show different operating modes for the antenna device shown in Figure 7.

Detailed description of the invention

In the following, a detailed description of preferred embodiments of an antenna device according to the invention will be given. In the specification, for explanatory non-limiting purposes, specific details will be described, such as specific hardware, applications, techniques, etc. to provide a good understanding of the present invention. However, those skilled in the art will recognize that this invention can be used in other embodiments which deviate from these specific details. In other cases, detailed descriptions of well-known methods, devices and circuits are omitted so as not to obscure the description of the present invention with unnecessary details.

Figure 1 has been described in the background section and will not be included any more.

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Figure 2 shows an antenna device, generally designated 1. The antenna device comprises a first substantially flat, rectangular radiating element 10 made of an electrically conductive material, such as a sheet or flex film, as is conventional. A source RF for radio frequency signals, such as electronic circuits of a portable radio communication device, is connected to a supply portion 12 of the first radiating element.

The antenna device also comprises a second substantially flat rectangular radiating element 20. A switching element 30 is arranged between the two radiating elements 10, 20. This switching element is preferably a PIN diode, i.e. a silicon diode having a slightly doped intrinsic layer which acts as a dielectric. barrier between p- and n-layers. Ideally, a PIN diode switch is characterized as an open circuit with infinite isolation in open mode and as a short circuit without resistive losses in closed mode, making it suitable as an electronic switch. In practice, the PIN diode switch is not ideal. In open mode, the PIN diode switch has capacitive characteristics (0.1-0.4 pF) resulting in finite isolation (15-25 dB @ 1 GHz) and in closed mode the switch has resistive characteristics (0.5-3 ohms) resulting in resistive losses (0.05-0.2 dB).

The first and second radiating elements 10, 20 are also capacitively coupled by means of a high-pass filter, shown as a capacitor 32 in the figures. The high pass filter allows RF signals to pass and this means that the two radiating elements from an RF perspective are a single element, as will be further described with reference to Figures 2a-c.

The first and second radiating elements 10, 20 are arranged at a predetermined distance over an earth plane, such as a

528 017 printed circuit board described below with reference to FIG

3rd ------- ---- A DC control input, designated V_switch in the figures, for controlling the operation of the PIN diode, is connected to the first radiating element 10 via a filter block 16 so as not to affect the RF characteristic of the antenna device. This means that the filter characteristics of the filter block 16 are designed to block RF signals. In the preferred embodiment, filter block 16 includes a low pass filter.

Finally, the second radiating element is connected directly to ground at an earthing portion 22. This earthing portion works for both RF signals emanating from the RF input and DC signals emanating from the control input.

The antenna is preferably designed for 50 ohms.

The switching of the antenna device works as follows. The RF source and other electronic circuits of the communication device operate at a given voltage level, such as 1.5 volts. The condition is that the voltage level is high enough to create the necessary voltage drop across the PlN diode, ie about 1 volt. This means that the control voltage V_BWitch switched between the two voltages high and low, such as 1.5 and 0 volts respectively. Then V_switoh is high, there is a voltage drop across the PIN diode 30 and a corresponding current thereby of about 5-15 mA. This voltage drop makes the diode conductive, effectively connecting the two radiating elements 10, 20 to the diode 30.

With the control voltage V_switoh low, the voltage drop across the pin diode 30 is not sufficient to make it conductive, i.e. it is open. The other radiant element is then effective

528 017 connected to the first radiating element only through capacitor 32.

The size and design of the two radiating elements are selected to obtain the desired resonant frequencies, such as the 850 and 1800 MHz bands with the switch open and the 900 and 1900 MHz bands with the switch closed.

Referring now to Figure 2a, therein is shown how the two radiating elements 10, 20 from an RF perspective function as a single radiating element of general C-shape. The reason for this is that the capacitor 32, which operates as a high-pass filter, acts as an RF bridge between the two radiating elements. The switch 30 in the form of a PIN diode is open, i.e. non-conducting in Figure 2a due to the control voltage V_Awitch is low. The C-shape of the combined radiating elements in combination with the position of the feed portion 12 causes the arrangement to pivot at two frequencies, effectively making it suitable for two-band operation.

In Figure 2b, the switch 30 is closed, i.e. the diode is conductive. This effect is then achieved by a high control voltage V._wltch The control input is applied, see Figure 2. This voltage creates a DC current flowing through the LP filter 16, over the first radiating element 10, through the diode 30, over the second radiating element 20 and to ground via the grounding part 22. With the switch 30 closed , i.e. with the diode conducting, the RF bridge is widened between the two radiating elements. This is clearly seen in Figure 2b when compared to Figure 2a.

This change in the geometry of the effective radiating elements adjusts the resonant frequencies of the antenna device. This is evident from Figure 2c, where the dashed curves correspond to the operating mode shown in Figure 2a and the solid curves corresponding to the operating mode shown in Figure 2b. This means an antenna device

528 017 which can operate in four different frequency bands is obtained, such as in the aforementioned 850/900/1800/1900 MHz bands.

The adjustment of the resonant frequencies shown in Figure 2c can be used to an advantage in so-called weight telephones. In this type of communication device, the resonant frequency of an internal antenna element tends to move downward in frequency as the phone's position changes from folded to folded mode. With the antenna device according to the invention, when the telephone is folded out, the movement of the resonant frequencies can be counteracted by closing of the switch 30. Thus, with the telephone weight, the control voltage V_switoh2 low and with the phone folded, the control voltage is high. The antenna device then functions as a two-band antenna with substantially constant resonant frequency regardless of the mode of operation of the communication device (folding weight / weight).

The adjustment of resonant frequencies shown in Figure 2c can also be used to advantage in two-band, non-foldable telephones. In the frequency bands used for mobile communication, the transmission and reception frequencies (TX, RX) are separated by approximately 45-90 MHz. By using frequency adjustment, close to optimal efficiency can be obtained by adjusting the frequencies to the TX and RX frequencies instead of the wider frequency band comprising the TX and RX frequencies.

In Figure 3, the two radiating elements 10, 20 are arranged substantially parallel to and spaced from a printed circuit board (PCB) 70 arranged for mounting in a portable communication device 80, such as a mobile telephone. The PCB acts as a ground plane for the antenna device. The general contours of the communication device are shown by dashed lines in Figure 3. Typical dimensions of the antenna device 1 are a height of about 4 millimeters and a total volume of about 3 cm³.

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It will be appreciated that all components except the two radiating elements 10, 20, switching element 30 and capacitor 32 can be arranged on the PCB, thereby enabling easy mounting of the antenna device. This is further facilitated by the fact that there is no separate supply of the switching element.

A conventional method of production of antenna devices is to provide an electrically conductive layer which forms the radiant portions of the antenna on a carrier made of a non-conductive material, such as a polymer or other plastic material. The carrier is thus made of a heat sensitive material, and a small heating area is desired to keep the temperature as low as possible when components are soldered to the antenna device.

Figure 4 shows how the capacitive bridge can be provided by a conductive layer 34 arranged under a portion of the two radiating elements 10, 20 at the RF bridge. If a multilayer flex film is used to provide the radiant elements, the radiating elements 10, 20 may be arranged on one side of the flex film and the conductive layer 34 on the other. In this way, discrete components are avoided to provide the capacitive coupling between the radiating elements.

Figure 5 shows how the capacitive bridge can be provided by means of a meandering interface between the two radiating elements 10, 20. In this way, discrete components are also avoided to provide the capacitive coupling between the radiating elements.

Figure 6 shows an alternative design of the radiant elements. In all aspects, this antenna device functions as that described above with reference to Figures 2 and 2a-c.

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In an alternative embodiment shown in Figure 7, generally designated 100, a further third radiating element 140 is provided together with a second control input, designated V_awitch2connected to the third radiating element via a low pass filter 142. This third radiating element is connected to the second radiating element 120 by a second switch 144 in the form of a PIN diode.

Also in the embodiment shown in Figure 7, the first radiating element 110 is connected to ground at an earthing portion 114 via a high pass filter 118 which blocks DC signals. Finally, the second radiating element 120 is connected to ground at an earthing portion 122 via a low pass filter 124 which blocks RF signals. Thus, in this embodiment, there are separate grounding parts for RF signals and DC signals (i.e., control signals).

The antenna device of Figure 7 operates as follows. The first control voltage V_switch functions as in the first embodiment shown in Figure 2. Thus, high voltage creates a current flowing through the first switch 130 and to ground via the low-pass filter 124. With the second control voltage V_switch2 low does not conduct the second switch 144. This means that the third radiating element 140 is effectively disconnected from the second radiating element, see Figures 7a and 7b.

With the position of the supply portion 112 and the first switch 130 open, as in Figure 7a, the first and second radiating elements 110, 120 coupled by the capacitor 132 at a first frequency oscillate. With this first switch closed, as in Figure 7b, the combination of the first and second radiating elements oscillates at a second frequency.

With the second switch 144 closed, as in Figures 7c, 7d, ie with the second control voltage high, the combination oscillates

528 017 of the first, second and third radiating elements 110, 120, 140 at a third or fourth frequency, depending on whether the first switch 130 is open or closed. Thus, four-band operation is achieved with this design.

Preferred embodiments of an antenna device according to the invention have been described. However, it will be appreciated that these may be varied within the scope of the appended claims. Thus, a PIN diode has been described as a switch element. It will be appreciated that other types of switching elements can also be used.

The radiant elements shown in Figures 2, 3 and 7 have been described as substantially planar and generally rectangular. It will be appreciated that the radiating elements may take any suitable shape, such as bent to conform to the housing of the portable radio communication device in which the antenna device is mounted.

A switch has been shown to connect two radiant elements. It will be appreciated that more than one switch, such as multiple parallel PiN diodes, can be used without departing from the scope of the invention.

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