WO2008010149A1

WO2008010149A1 - Antenna with reduced sensitivity to user finger position

Info

Publication number: WO2008010149A1
Application number: PCT/IB2007/052730
Authority: WO
Inventors: Zidong Liu
Original assignee: Nxp B.V.
Priority date: 2006-07-17
Filing date: 2007-07-10
Publication date: 2008-01-24
Also published as: TW200814427A

Abstract

An antenna has a ground plane (30) and a radiating element (15), and a shorting path (10, 110, 160, 170), coupling the ground plane (30) and the radiating element (15), the shorting path (10, 110, 160, 170) having an inductive element (10). The inductive element (10) can be a variable inductor. The antenna can have a short circuit path (160) across the inductive element (10), with a switch (170) to switch the short circuit path (160) in place of the inductive element (10).

Description

DESCRIPTION

ANTENNA WITH REDUCED SENSITIVITY TO USER FINGER

POSITION

TECHNICAL FIELD

This invention relates to antennas, and to portable or handheld devices having an antenna.

BACKGROUND ART Modern mobile phone handsets and other portable devices typically incorporate an internal antenna, such as a Planar Inverted-F Antenna (PIFA) or similar. Antennas in mobile terminals are required to cover an increasing number of communications bands, for systems such as CDMA850, GSM900, GSM1800, PCS1900, and UMTS2000. At the same time, the size of mobile terminals has been reduced dramatically. The miniaturisation of mobile terminals leaves ever less space for the antenna. However, there are fundamental limits on bandwidth as a function of antenna volume. Generally speaking, the smaller the antenna size, the narrower the bandwidth.

PIFAs are popular in mobile phone handsets because they exhibit low SAR (Specific Adsorption Ratio) which means that less transmitted energy is lost to the user's head and they are compact. They are installed above the phone circuitry and, therefore, make fuller use of the space within the phone casing. Such antennas are small (relative to a wavelength) and therefore, owing to the fundamental limits of small antennas, narrowband. However, cellular radio communication systems typically have a fractional bandwidth of 10% or more. To achieve such a bandwidth from a PIFA for example requires a considerable volume, there being a direct relationship between the bandwidth of a patch antenna and its volume, but such a volume is not readily available with the current trends towards small handsets. Further, PIFAs become reactive at resonance as the patch height is increased, which is necessary to improve bandwidth. Our co-pending PCT Patent Application 02/071535 discloses a dual- band Planar Inverted F-Antenna arrangement comprising a relatively small patch conductor, acting as a radiating element, supported substantially parallel to a ground plane. The patch conductor includes first and second connection points, for connection to radio circuitry and a ground plane, and further incorporates a slot between the first and second points. The radiating element is typically located near an edge of the ground plane. The short circuit is implemented typically in the form of a pin or stub or plate, and typically at the edge of the radiating element, and providing a pure short circuit. The antenna can be operated in a plurality of modes. For example, if signals are fed to the first point then a high frequency antenna is obtained by connecting the second point to ground and a low frequency antenna by leaving the second point open circuit. This can enable shifting the resonant frequency of the antenna to compensate for impedance changes caused by, for example, the presence of a user's hand. This shifting is provided by electrically shorting the antenna at different positions.

A further problem occurs when a dual band antenna is required. In this case two resonances are required from a single structure, which usually requires a compromise to be made between the two bands. Furthermore, in order to maximise power transfer between an antenna and circuitry connected thereto, it is important to ensure that the impedance of the antenna is matched to that of the circuitry. However, the impedance of the antenna will also vary with operating conditions, for example when a finger is on the top of the antenna whilst the mobile terminal is in use. A perceived drawback of mounting PIFAs inside the housings of portable telephones and locating them just under the outer cover is that they are very susceptible to detuning by a person holding the telephone.

A traditional dual-band planar inverted F-antenna operates in two frequency bands. However, when the antenna is covered by the user's hand, the resonant frequencies are shifted downwards. Therefore, the user's hand causes significant impedance change and gain reduction under usual talk position. In order to overcome this problem, the antenna needs to be retuned when the user's finger is on the top of the antenna. It is known that the resonant frequencies of a dual-band PIFA can be varied if it is electrically shorted at different positions. However the antenna input impedance cannot be well matched to 50Ω at all shorted positions, particularly for the lower frequency band. SUMMARY OF THE INVENTION

It is an object of the invention to provide improved apparatus or methods. According to a first aspect of the invention, there is provided an antenna comprising a ground plane and a radiating element, and a shorting path coupling the ground plane and the radiating element, the shorting path having an inductive element.

Compared to the above referenced existing methods, by replacing the electrical short by an inductive element, the antenna can be well-matched at various positions of a user's hand.

According to a second aspect of the invention there is provided a handheld device having an antenna in accordance with the first aspect of the invention. Additional features and advantages will be described below. Any of the additional features can be combined together or with any of the aspects of the invention, as would be apparent to those skilled in the art. Other advantages may be apparent to those skilled in the art, especially over other prior art not known to the inventors. Embodiments of this invention can include additional features, as described below, some of which are summarised as follows. The inductive element can comprise a variable inductor or a switched array of inductors. A short circuit path can be provided across the inductive element, with a switch to switch the short circuit path in place of the inductive element. A switch controller can be provided, arranged to control the switch such that for retuning to a lower frequency band, the inductive element is switched in, and for retuning to a higher frequency band the short circuit path is switched in. The switch controller can be arranged to detect an input impedance mismatch of the antenna and control the switch according to the detected input impedance mismatch. The radiating element can have a slot to divide the element into more than one region. The radiating element can be substantially rectangular with the shorting path coupled to a corner. The shorting path can comprise a number of paths to different locations on the radiating element, and a switching arrangement to control which of the paths is in use. The radiating element can comprise a conductive layer on a circuit board mounted parallel to the ground plane, the circuit board also being used for mounting circuit components. The antenna can be arranged as a dual or multi-band PIFA. The bands can comprise any one or more of those used for CDMA850, GSM900, GSM1800, PCS1900, UMTS2000, Bluetooth or IEEE 802.11 b at 2.4 to 2.5GHz, TD-SDCMA at 2.3 to 2.4GHz, or UMTS future expansion at 2.5 to 2.7GHz. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of example, and with reference to the accompanying drawings, in which: Figure 1 is a schematic view of an inductor-grounded planar inverted F- antenna, according to an embodiment of the invention;

Figure 2 is a schematic view of a traditional PIFA with a finger on the top;

Figures 3a, 3b, 3c and 3d are graphs of simulated input return loss (S11 ) against frequency for the antenna of Figure 2;

Figure 4 is a schematic view of an inductor-grounded planar inverted F- antenna that can be tuned to overcome the proximity effect due to the user's finger;

Figures 5a and 5b are graphs of simulated input return loss (S11 ) against frequency for the antenna of Figure 4;

Figure 6 shows an embodiment of an antenna; and Figure 7 shows an embodiment of a device having an antenna. MODES FOR CARRYING OUT THE INVENTION

The invention will be described with reference to an Inverted F Antenna (IFA), although the invention is not limited to such antennas. An Inverted F antenna (IFA) can be regarded as a classic monopole having a radiating element extending normal to a ground plane, with its top section folded to be parallel with the ground plane. The parallel section introduces capacitance to the input impedance of the antenna, which is compensated by implementing a shorting path to the ground plane away from the feed point. The term shorting path will be used in this document even when the path is not a pure short circuit and has some inductance. In operation, currents in the radiating element excite currents in the ground plane, such that the resulting electromagnetic field is formed by the interaction of the IFA and an image of itself below the ground plane. In practice the ground plane is not infinite and the radiating element/ground plane combination will behave as an asymmetric dipole. The ground plane length is preferably around one half of the operating wavelength, to provide an omni-directional far-field pattern, for a sufficiently wide bandwidth, and a high impedance seen at the feed point of PIFA so that the coupling between the radiating element and ground plane is strong. The location of the shorting path relative to the feed point can be significant, particularly the distance separating them.

Figure 1 shows an embodiment of an antenna according to the invention, in the form of a perspective view of an inductor-grounded planar inverted F- antenna (IGPIFA) mounted on a PCB. The antenna comprises a ground plane 30 and a radiating element 15, separated from the ground plane by a dielectric which in the illustrated embodiment is air. The ground plane 30 is shown in the form of a conductive layer on a bottom PCB. The ground plane 30 need not be planar. The radiating element 15 can be in the form of a rectangular radiating plate as shown as a layer on part of top PCB 40, or can be of other configuration or shape. The radiating element 15 need not be planar, need not be parallel to the ground plane nor be the same size or shape as the ground plane 30, nor completely overlapping the ground plane. It is fed via a feed pin 20 extending between the two PCBs at a corner of the radiating plate. A shorting path from the radiating element 15 to the ground plane 30 is located spaced apart from the feed pin, and provided in the form of an inductor 10. The inductor can be implemented in the form of discrete or distributed components. Although applicable to single band or multiple band antennas, in the example shown two resonant frequencies are created by cutting a slot 12 in a meandering shape as shown in Figure 1 , or other shape of slot, into the radiating plate 15, following established practice. Embodiments in the form of dual band antennas can also have the slot 12 replaced by a resonator, or in the form of single band antennas can have the slot 12 replaced by a simple inductance, as is explained further in our patent application WO2005011055, and so this will not be explained further here.

The top PCB 40 can be used for structural support of the radiating element 15 and can simulate the effect of a casing of a mobile device for testing purposes. The dielectric constant and the thickness of both PCB boards in this example are 3.38 and 0.813mm. The PCB dimensions are 40mmx100mm. The bottom PCB provides a feed signal to the antenna and can also support the conductive ground plane at its back. Electronic components in RF shields (otherwise called RF cans) can be mounted on both sides of the bottom PCB and the electrically conductive ground plane 30 surrounds these components and covers the remaining area of the PCB. The ground plane 30 and/or the radiating element 15 can be curved, e.g. to match the contour of a phone case. The impedance and peak resonance frequency of the antenna can be set by various dimensions of the radiating antenna and its slot 12, and by the location of the feed pin 20 and shorting path. The provision of an inductive element in the shorting path means that the impedance of the antenna can be better matched even if there is still sensitivity to the user's hand position. The inductor 10 can have a fixed value or a variable value, or can be switched to any of a number of discrete values. The inductor value can be selected, even if it cannot be varied or switched dynamically, such that even if the mismatch varies with hand position, an average mismatch, or a range of mismatches for various hand positions, can be optimised to improve an overall matching of the antenna impedance to the RF circuitry coupled to the antenna. This can enable RF power to be transmitted more efficiently and battery life and/or range for portable devices can be increased.

The inductor 10 can also have some effect on making the peak resonance frequency less sensitive to user's hand position. If the inductor 10 has a variable or switchable value, the value can be controlled to maintain the antenna impedance and/or peak resonance frequency more stable, to reduce sensitivity to users hand position, although variable devices tend to be more lossy and more expensive. The inductive element can have a network of individual inductors coupled in series or parallel. Other components such as resistive elements can be incorporated in the shorting path, to provide a wider bandwidth.

Figure 2 is a schematic view of a traditional PIFA having a ground plane 30 with a users finger 70 shown as a cuboid on the top of the upper PCB 40. Usually a PIFA is designed and optimised without the user's hand on. A shorting pin 60 is located at the position A near the feed pin 20. Figure 3a and Figure 3b show the simulated input-return loss (S11 ) as a function of frequency when the finger isn't on the top of the antenna. Two resonances are clearly observed and centred on 0.87GHz, and 1.78GHz. Figure 3c and Figure 3d show the simulated input-return loss (S11 ) as a function of frequency when the user's finger is on the top of the antenna. They show that both the resonant frequencies have been shifted downwards by about 138MHz for the low band and by about 370MHz for the high band, compared to those without the finger on.

Figure 4 shows another embodiment of the invention, similar to that of Figure 1 , and similar reference signs are used as appropriate. Circuitry 110 is provided at location B, in this example at one corner of the radiating element 15. It includes a switch 170 provided to connect either the inductor 10 or a short circuit in the form of a shorting pin 160 to the ground plane 30, as shown in Figure 4. An advantage of this is that some of the flexibility of a variable inductor can be achieved with less complexity, cost, and loss. This flexibility means the mismatch can be controlled more accurately for a range of different hand positions. The short circuit can be provided by a conductive tab or plate or strip or other arrangement. Other switch layouts can be envisaged to achieve a similar effect, for example the inductor 10 could be coupled permanently and the switch arranged to open or shut only the branch having the shorting pin, to short circuit the inductor 10 without making the inductor open circuit. Any such switched arrangement can be combined with a variable inductance or switched array of inductors. Both the shorting pin 160 and the inductor 10 are located at the position B as shown.

The antenna's resonant frequencies are retuned upwards by switching in the inductor 10 for the low band and switching in the shorting pin 160 for the high band. The switch 170 can be controlled to achieve this by control circuitry arranged to receive a signal indicating which band is to be used. This can enable better mismatch control for two band operation. Such control circuitry can be mounted on either PCB. Alternatively or as well, the control circuitry can be arranged to control the switch according to a level of antenna input impedance mismatch which can be detected by suitable circuitry. Such detection can be implemented by conventional circuitry, based on detecting reflected power for example. Use of such a direct measurement can enable better control of mismatch than estimates or indirect measures for example.

The inductor has the inductance of 8.5nH in this example. Other embodiments can have inductive elements having values in a range of 2nH to 20 nH for example. An appropriate value can be determined by testing the antenna in its product casing and measuring the antenna impedance or the impedance mismatch for various values of inductance, until the impedance is close to 50 Ohms or other desired impedance to give good matching with the associated RF circuitry. The embodiment of Figure 4 has been simulated in software. Graphs showing input-return losses for both bands based on simulations are shown in Figures 5a and 5b. The relative dielectric constant and conductivity of the finger material are 41.5 and 0.97 S/m for the low band and 40.0 and 1.4 S/m for the high band, respectively for this simulation. The switch used in embodiments of this invention can be a PIN-diode or FET or MEMS or any other type. In the above simulations, a small piece of rectangular copper (1 mmxO.88mm) is used to represent the switch in the ON state.

Figure 6 shows another embodiment which is frequency- reconfigurable, as well as being well matched when the hand is on or off, by using two shorting paths at different locations. In this example the shorting paths are illustrated at positions A and B, though other positions are possible. At position A, a switchable shorting pin 320 is provided. At position B an inductive element such as an inductor 10 is provided in parallel with a shorting pin 31. The circuitry at position A is used when the hand isn't on, in which case the results of Figures 3a and 3b apply. The circuitry at position B is used when the hand is on, the shorting pin 310 being switched in for high band and the inductor 10 being switched in for low band; the results of Figures 5a and 5b apply. Control circuitry for the switches in Figure 6 involves detecting the impedance mismatch.

Figure 7 shows a handheld battery powered device 290 in schematic form according to an embodiment of the invention. This shows device circuitry 280 leading to RF amplifier and matching circuitry 240. This feeds an antenna 260. The antenna has a ground plane, a radiating element 15 and shorting path circuitry 250. The shorting path circuitry has a shorting path 230, a controller 200, and a mismatch detector 210. The controller has an input 205 indicating whether a high or low frequency band is used, and an input from the mismatch detector. The controller 200 controls controllable elements in the shorting path, which can include a switch 170 and/or a variable inductor 180 and/or a switchable array inductor 190, and/or a shorting path location switching arrangement 195. The shorting path 230 is coupled between the ground plane and the radiating element.

In summary, an inductor-grounded antenna has been described, which can provide improved tolerance to handling in applications such as mobile phones. The shorting pin of a radiating element can be replaced by an inductor, so that the antenna can be well matched when either the user's finger is on the top of the antenna or the antenna needs to be tuned to operate in a different frequency band.

In the present specification and claims the word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

Further, the word "comprising" does not exclude the presence of other elements or steps than those listed. The inclusion of reference numerals in the claims is not intended to be limiting.

From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the design, manufacture and use of planar antennas and component parts therefor and which may be used instead of or in addition to features already described herein.

INDUSTRIAL APPLICABILITY Wireless communication equipment, particularly hand-held devices.

Claims

1. An antenna comprising a ground plane (30) and a radiating element (15), and a shorting path (10, 160, 170, 180, 190, 195, 230, 310, 320) coupling the ground plane (30) and the radiating element (15), the shorting path having an inductive element (10, 180, 190).

2. The antenna of claim 1 , the inductive element (10, 180, 190) comprising any or both of a variable inductor (180), or a switched array (190) of two or more inductors.

3. The antenna of claim 1 or 2, comprising a switch (170) arranged for switching a short circuit path (160, 310) in place of the inductive element (10, 180, 190).

4. The antenna of claim 3 being a dual band antenna and comprising a controller (200), arranged to control the switch (170) such that for retuning to a lower of the frequency bands, the inductive element (10, 180, 190) is switched in, and for retuning to a higher of the frequency bands the short circuit path (160, 310) is instead switched in.

5. The antenna of claim 2, 3 or 4, comprising a detector (210) arranged to detect an input impedance mismatch of the antenna and a controller (200) arranged to control, according to the detected input impedance mismatch, the variable inductor (180) or switched array (190) when dependent on claim 2, or the switch (170) when dependent on claim 3 or 4.

6. The antenna of any preceding claim, the shorting path (10, 160, 170, 180, 190, 195, 230, 310, 320) comprising a number of paths to different locations on the radiating element (15), and a switching arrangement (195) to control which of the paths is in use.

7. The antenna of any preceding claim, the radiating element (15) comprising a conductive layer on a circuit board (40) mounted parallel to the ground plane (30), the circuit board (40) also being used for mounting circuit components.

8. The antenna of any preceding claim, arranged as a dual or multi-band PIFA.

9. The antenna of any preceding claim, arranged to transmit or receive any one or more bands used for any one or more of CDMA850, GSM900, GSM1800, PCS1900, UMTS2000, Bluetooth or IEEE 802.11 b at 2.4 to 2.5GHz, TD-SDCMA at 2.3 to 2.4GHz, or UMTS future expansion at 2.5 to 2.7GHz.

10. A handheld device (290) having the antenna of any preceding claim.