DATATERMINALS

Inventors: Devis lellici and Robert Schlub

BACKGROUND

Virtually all modern cellular radio handset terminal antennas, including dielectric variants, are unbalanced. This means that what is often thought of as the antenna is only half of the radiation mechanism, and the handset chassis acts as the other half. Here, chassis is a general term for the printed circuit board (PCB) together with any conductive components and assemblies connected to it, typically including the battery, keyboard, display housings and any conductive paint applied to the case to enhance electromagnetic compatibility (EMC) performance.

The reason for using the chassis as part of the antenna is that in the low bands (824 to 960MHz), the wavelength is so long that the antenna on its own is inside the Chu- Harrington limit, [L. J. Chu, "Physical Limitations of Omni-Directional Antennas," Journal of Applied Physics, Vol. 19, pp. 1163-1175, 1948 or R.C. Hansen, "Fundamental Limitations in Antennas," Proceedings of the IEEE, Vol. 69, No. 2, pp. 170-182, 1981]. The implication of being inside the Chu-Harrington limit is that the antenna will either be an inefficient radiator or will lack sufficient bandwidth. Both are deficiencies that are not acceptable in a modern handset.

Unfortunately, there is a price to be paid for using the chassis as part of the antenna: if the groundplane size is changed, or other components are moved around on the groundplane, the antenna detunes and has to be redesigned. In simple bar (candy-bar) phones this problem has been studied extensively and it was reported some time ago that the impedance bandwidth of the antenna on a bar phone is particularly sensitive to the length of the chassis of the phone [Kivekis, O.; Ollikainen, J.; Lehtiniemi, T.; Vainikainen, P. Antennas and Propagation, 2003, (ICAP 2003), Twelfth International Conference on (Conf. Publ. No. 491) VoI 2, 31 March-3 April 2003 Page(s):735 - 738 vol.2 or W Dou and M Y W Chia, "Chassis Influence on the Input Impedance and SAR Characteristics of Handset Antennas," IEEE Antennas and Propagation Society International Symposium, 2001].

A typical relationship between dual band antenna bandwidth and chassis length is shown in Figure 1 taken from D. Herberling, "Antennas and technologies for the communication future", IEE lecture, London, 4 May 2004.

The curves relate the bandwidth of an antenna of a particular size to the dimensions of the chassis; it is of the same form as the curves in Kivekas et. al. [Kivekis, O.; Ollikainen, J.; Lehtiniemi, T.; Vainikainen, P. Antennas and Propagation, 2003. (ICAP 2003). Twelfth International Conference on (Conf. Publ. No. 491) VoI 2, 31 March-3 April 2003 Page(s):735 - 738 vol.2].

It is clear from Figure 1 that as the chassis length becomes shorter than about 100mm, it becomes increasingly difficult to design antennas with the bandwidth in the low band that is required for modern cellular radio communication systems. As a consequence, when a short bar-phone design is required, it becomes necessary to provide some kind of chassis extension device, often at the opposite end from the antenna. Existing extension devices include slotting the PCB groundplane, printing a meander line thereon, printing a meander line on a small separate PCB or printing an extension line inside the handset case by means of conductive paint. None of these are particularly effective techniques. Similarly, also shown by Figure 1 , when the chassis gets longer than about 80mm it gets increasingly hard to get good high band performance.

It is known, for example from WO 99/54956, to provide groundplane extensions for use in mobile radio devices. Groundplane extensions are used to lengthen or extend a longest dimension of a groundplane (e.g. a PCB groundplane), and may be configured so as to be variably extensible. These types of groundplane extension are specifically designed to radiate at the same frequency as a main antenna component provided in the mobile radio device, and to couple, e.g. capacitively, with a main ground connection provided on the groundplane. The groundplane is configured with first and second shorter edges, the distance between these edges defining the longest dimension of the groundplane. The main antenna component and the main ground connection are located close to the first shorter edge, and the groundplane extension is located at the second shorter edge, i.e. remote from the main antenna and main ground connection. The groundplane extension serves to improve gain and reduce specific absorption rate (SAR).

However, in dual- or multiband devices, simple groundplane extensions serve to improve low band performance at the expense of high band performance. This is because, in GSM applications, the optimum groundplane length for the high band is around 80mm and for the low band is around 120-130mm. Adding a groundplane extension to an e.g. 90mm PCB will thus reduce high band performance. Furthermore, adding a groundplane extension will tend to lower the overall resonance frequency of the antenna system for the simple reason that half the antenna (the groundplane) is made longer. In other words, given that in the low cellular radio bands (850-900MHz) the groundplane is the effective radiator, extending the groundplane by increasing its effective length will lower the resonant frequency of the antenna as a whole.

Moreover, simple groundplane extensions do not cause any change in the vertical polarisation created by, for example, a planar inverted-F antenna (PIFA) mounted on a vertical groundplane.

BRIEF SUMMARY OF THE DISCLOSURE

Embodiments of the present invention seek to augment the antenna performance on all electrically small devices in all radiating bands without resorting to chassis or groundplane extension techniques. The invention is particularly useful for realizing efficient antenna performance in the cellular radio low bands when the antennas are mounted on short groundplanes. Other situations where the invention may be useful are when the antenna is low in height, has a small volume, is crowded by other components or is installed on a difficult platform such as a two-part slider phone where the effective groundplane length is not constant. Different numbers of bands may be jointly and severally addressed by this technique.

According to a first aspect of the present invention, there is provided an antenna system for a mobile radio platform, the antenna system comprising at least first and second antenna components, the second of which is configured as a chassis formed from or including at least one conductive component, wherein at least one conductive resonator is connected to the at least one conductive component in such a way that an effective length of the conductive component is not extended by the conductive resonator.

According to a second aspect of the present invention, there is provided a method of improving performance of an antenna system comprising at least first and second antenna components, the second of which is configured as a chassis formed from or including at least one conductive component, wherein at least one conductive resonator is connected to the at least one conductive component in such a way that an effective length of the conductive component is not extended by the conductive resonator.

The conductive component includes at least a groundplane of the antenna system, together with other conductive parts of the chassis electrically connected to the groundplane. The effective length of the conductive component may be considered as the major length dimension of the smallest imaginary rectangle that can be drawn around a plan projection of the conductive component onto a plane substantially parallel therewith.

The conductive resonator is then mounted on or connected to the conductive component so as not to project beyond the major length dimension of the imaginary rectangle. In other words, the conductive resonator is wholly contained within the footprint (that is, the plan projection) of the chassis and/or the conductive component.

The conductive resonator will generally project in a direction substantially normal to, or have a resolved component of projection normal to, a major plane of the conductive component or chassis, while still being contained within the footprint thereof.

Alternatively or in addition, the conductive resonator may be mounted within a cut-out formed in an edge of the conductive component in such a way that the conductive resonator does not project beyond the edge (although it may project above and/or below the major plane of the conductive component).

The conductive resonator preferably takes the form of a helical or spiral conductive element.

The first antenna component may be any type of antenna component, including dielectric antenna components (e.g. dielectric resonator antenna, broadband dielectric antenna, dielectrically loaded antenna, dielectrically excited antenna, hybrid dielectric antenna) and traditional conductive antenna components. The first antenna component

is generally also mounted on the chassis, but may alternatively just be electrically connected thereto.

The conductive resonator may be provided at an end of the chassis remote from a location of the first antenna component, but in other embodiments may be provided on any part of the chassis other than the end remote from the location of the first antenna component.

It has been found that the addition of small resonant components to the handset chassis form a small and simple way of significantly increasing the bandwidth of both low and high band antennas in mobile terminals with small chassis dimensions. The resonant component may be of a generally helical or spiral form and may be tuned to enhance antenna performance in the lower band or the high band, or both, or other bands of interest such as GPS, UMTS, WLAN, etc.

In one embodiment of the present invention, there is provided two helical or spiral coils, one end of each coil being connected to the conductive component of the chassis, and the other end of each coil being open-circuited.

It is not necessary to have two such coils, although it has been found advantageous in some circumstances to have more than one. Each additional resonator adds an extra resonance to the return loss (and may thereby improve bandwidth), and further improves efficiency. The resonators may be configured with various different orientations. The resonators can be simply manufactured as stand alone components or incorporated in a plastic or other dielectric structure. Using a pair of helical coils, it has been found that the antenna efficiency in the low band of a 70mm board (PCB chassis) is approximately the same as the efficiency of the same antenna on an 85mm board (PCB chassis) without resonators. Resonators operating in different bands may be incorporated in a single structure, e.g. one inside another.

The technique may be used to improve the antenna performance of electrically small devices in several different ways:

1. Improving the absolute antenna efficiency and return loss on a radio device of fixed form factor and population.

2. Improving the relative antenna efficiency and return loss on a short board compared with that of the same antenna on a longer board.

3. Improving the relative antenna efficiency and return loss of an antenna that is lower in height, or smaller in some other respect, or cheaper to manufacture compared with a larger or more expensive antenna mounted on the same board.

It is to be appreciated that the resonant devices are not simple groundplane extensions because:

a) They can be attached anywhere on the groundplane or conductive component (with varying effect) as opposed to being limited to changing the longest dimension of a PCB; and

b) They do not lengthen the groundplane and do not lower the resonant frequency of the antenna as a whole. Instead, they introduce a new resonance into the return loss plot of the antenna which may be used to widen the bandwidth. Simple groundplane extensions do not introduce or exhibit resonances in the return loss plots in the manner of embodiments of the present invention.

c) They do not impair high band performance by increasing the length of the groundplane. Accordingly, extra resonances can be introduced into the low band return loss so as to improve antenna efficiency without significant effect on high band performance.

d) They may be engineered to give horizontal polarisation so that orthogonal currents are created in the groundplane that do not interfere with vertically polarised currents due to the main antenna.

e) When a helical coil is mounted close to a bottom edge of a groundplane in accordance with embodiments of the present invention, straightening out the coil so that it extends beyond the effective length of the groundplane results in a cessation of the advantages provided by embodiments of the invention, and the coil reverts to behaviour typical of a simple groundplane extension, lowering the main resonant frequency.

Embodiments of the present invention may be used with various types of mobile telephone handset (including bar phones, clamshell phones, slider phones, swing

phones, flip phones etc). Embodiments of the present invention may also be used with various other small radio platforms, including PDAs (personal digital assistants), PCMCIA cards, laptop and palmtop computers and other small data devices and terminals.

At least one resonator is provided, although it is often advantageous to provide two or more resonators for each groundplane. Where more than one resonator is provided, the resonators may be the same or different.

Where more than one resonator is provided, each resonator may be tuned or configured to a different frequency band. In particular, dual- or multi-band resonators may be used to augment antenna performance in dual- or multi-frequency bands.

The at least one resonator may be located anywhere on the groundplane or the conductive component, and may be configured so as to be generally orthogonal thereto, although this is not always required. Where first and second groundplanes are provided (for example on opposed surfaces of a PCB, or where more than one PCB is provided such as in a clamshell phone), the at least one resonator may be provided on either one or both of the groundplanes or conductive components.

In one possible arrangement, there is provided at least one slot in the PCB and associated groundplane, for example at an edge thereof, and a generally helical resonator is located within the slot such that a longitudinal axis of the resonator is generally parallel to the groundplane.

Different resonators adapted for different frequencies (e.g. low band, high band and intermediate band resonators) may be located at different places on the groundplane or conductive component. For example, low band and high band resonators may be located at opposite ends of the groundplane.

In some embodiments, the resonators may be connected directly to the first antenna component, or may be mounted on the groundplane at a location directly beneath or adjacent to the first antenna component.

The at least one resonator may take various shapes and configurations, and is not limited to a regular helix or spiral.

For example, the resonator may be:

i) helical or spiral with non-circular cross-section (e.g. oval, rectangular etc)

ii) helical or spiral with non-regular pitch

iii) helical or spiral with non-constant diameter (e.g. increasing or decreasing)

iii) helical or spiral with non-regular pitch and diameter

Where the resonator is generally helical, an end of the helix closest to the conductive component or groundplane may be directly connected thereto. Alternatively, an end of the helix furthest from the conductive component or groundplane may be connected thereto, for example by way of a conductive connection through the loops of the helix, leaving the closest end free.

Where the resonator is generally spiral, especially a planar spiral, it may be elevated over the conductive component or groundplane and disposed generally parallel thereto. This allows integrated circuits or other components to be mounted on the conductive component or groundplane underneath the spiral, thereby saving space on the PCB.

A generally spiral, especially planar spiral, resonator may also be disposed generally perpendicular to the conductive component or groundplane.

Helical resonators may also be elevated over the conductive component or groundplane, including any integrated circuits etc mounted on the groundplane. For example, a longitudinal axis of a helical resonator may be disposed generally parallel to the groundplane, with one end of the helical resonator comprising a conductive connection thereto.

The or each resonator may have two or more resonances in any given frequency band, or may be configured as dual- or multi-band resonators having resonances in different frequency bands. This may be achieved by using changes in resonator coil diameter, and/or by locating one resonator inside another, and/or by incorporating changes of pitch, among other techniques.

The resonators may be mounted inside non-conductive containers, or embedded in dielectric blocks, so as to provide robust units for "snap-on" mounting. Two or more resonators may be provided inside a given container or dielectric block.

Both left-handed and right-handed spirals or helices may be used. Where the resonators are provided pair-wise, the members of each pair may have the same or opposing handedness.

Helical resonators may be short circuited to themselves at either end or both ends.

A compound resonator may be constructed by connecting two or more helical resonators to each other at their free ends. One of the helices may be shorted than the other, or may have a different pitch and/or diameter. This can provide multiple resonances in one band, or multi-band resonance.

The resonators can be tuned to a desired frequency or frequencies by way of various techniques. For example, the resonators may be air-spaced, may have a low dielectric insert such as a plastics former, or a high dielectric insert such as a high dielectric constant ceramic material. In other embodiments, the resonators may be provided with a high permittivity insert such as a ferrite core.

Where an insert is provided, the resonator may in some embodiments be tuneable by varying the degree of insertion of the insert into the coil of the resonator.

Alternatively or in addition, the resonators may be electronically tuneable by changing predetermined properties of the coils of the resonators. For example, the resonators may be made of a ferroelectric material having a permittivity that can be changed by varying an applied electric field. It is also possible to tune some materials by varying an applied magnetic field.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", means "including but not limited to", and is not intended to (and does not) exclude other moieties, additives, components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention and to show how it may be carried into effect, reference shall now be made by way of example to the accompanying drawings, in which:

FIGURE 1 shows RF currents flowing in a prior art mobile telephone PCB;

FIGURE 2 shows RF currents flowing in a mobile telephone PCB provided with a helical resonant component embodying the present invention;

FIGURE 3 is a plot showing terminal efficiency in the low band around 900MHz for a normal PIFA on a prior art mobile telephone PCB;

FIGURE 4 is a plot showing terminal efficiency in the low band around 900MHz for a PIFA on a mobile telephone PCB provided with a helical resonant component as in Figure 2;

FIGURE 5 is a plot showing terminal efficiency in the low band around 900MHz for a PIFA on a mobile telephone PCB provided with a shorter helical resonant component than that producing the plot of Figure 4;

FIGURE 6 is a return loss plot for a PIFA on a PCB without a resonant component (the arrangement of Figure 1);

FIGURE 7 is a return loss plot for a PIFA on a PCB with a resonant component (the arrangement of Figure 2);

FIGURE 8 shows a typical relationship between dual band antenna bandwidth and chassis length;

FIGURES 9 and 10 show an embodiment of the present invention;

FIGURE 11 shows an Sn return loss for the embodiment of Figures 9 and 10 compared with a prior art antenna;

FIGURE 12 shows measured terminal efficiency for the embodiment of Figures 9 and 10 compared with a prior art antenna;

FIGURE 13 shows an alternative embodiment of the present invention; and

FIGURES 14 to 19 show various alternative resonator configurations.

DETAILED DESCRIPTION

The resonant components (generally coils and helices) attached to a groundplane in accordance with embodiments of the present invention are believed to introduce a novel variation of conventional groundplane radiating structures. Helical antennas and radiating structures can operate in one of two principal modes: normal (broadside) mode or axial (endfire) mode. In the normal mode, the dimensions of the helix are small compared with the wavelength of operation and the far field radiation pattern is similar to that of an electrically short dipole. In the axial mode, the helix dimensions are comparable with the wavelength of operation and the antenna produces circular polarisation. Embodiments of the present invention relate to helices that are electrically very short and so produce linear monopole or di pole-like radiation. However, conventionally a helical antenna is fed like a monopole using a groundplane as a counterpoise but in this case the helix is connected to the groundplane and has no separate feed. The way the helix works must thus be studied carefully.

Figure 1 shows the RF currents flowing in the surface of mobile phone PCB 100 at a frequency of about 1.1GHz. No resonant component is present. The antenna (not

shown) is at the top of the PCB 100 and the current vectors are substantially aligned along the length of the PCB 100 giving rise to vertical polarisation.

Figure 2 shows the same situation as Figure 1, but with a resonant spring or coil 101 located near the bottom of the PCB 100. If this were simply a groundplane extension device, it would cause an elongation of the vertically polarised currents resulting in a lower frequency of operation. Maximum radiation would occur from near the geometrical centre of the PCB 100 plus its extension. However, as Figure 2 shows, the effect of the resonant component is to cause horizontal currents to flow across the width of the PCB 100 instead of along its length. This gives rise to horizontally polarised radiation from near the bottom of the PCB 100 as well as the original vertically polarised radiation from the centre of the PCB 100. According to investigations conducted by the present applicant, this effect does not occur with a simple groundplane extension device.

The impact of this effect can be seen in the performance of the antenna. Figure 3 shows the terminal efficiency (radiation efficiency of a mobile phone handset measured in free space) for a normal PIFA in the low band around 900MHz. It behaves in a similar way to a dipole with most of its radiation in the Phi (vertical) polarisation.

Figure 4 shows the same situation as Figure 3 but with the presence of the resonant spring or coil 101. In the low part of the band it still behaves in a similar way to a dipole with most of its radiation in the Phi (vertical) polarisation. However, near the resonant frequency of the resonant spring or coil, at around 936MHz, a significant portion of the radiation occurs in the Theta (horizontal) polarisation.

When the length of the spring or coil 101 is reduced, the resonant frequency rises, as can be seen in Figure 5. This shows that the resonance effect is caused by the spring or coil 101 , and not by the length of the PCB groundplane 100, which has not changed.

The impact of the resonant spring or coil 101 can also be seen in the return loss plots. Figure 6 shows the return loss of the PIFA and PCB 100 used for the experiments above. The performance of the PIFA is very good in the high band covering the GSM, DCS and PCS bands as well as the 3G UMTS band, all with an excellent return loss. However, in the low band there is a single resonance at around 900MHz that is not broad enough to cover the GSM band at a good return loss. The size of the PCB 100 is

40 mm wide by 65 mm long and poor low band performance is to be expected on a PCB groundplane 100 as short as this.

The low band return loss of Figure 6 can be improved through the introduction of a resonant spring or coil 101 , as shown in Figure 7. A double resonance now occurs giving rise to an improved low return loss. It must be stressed that a groundplane extension would not have this effect; it would simply lower the frequency of the single resonance.

Figure 8 is discussed in the Background section of the present application, and shows a typical relationship between dual band antenna bandwidth and chassis length.

Figures 9 and 10 show a mobile telephone handset chassis 1 , which includes a PCB 2 with a conductive groundplane (not shown) on its underside. A first antenna component 3 in the form of a high dielectric ceramic pellet dielectrically exciting a planar inverted-L antenna (PILA) (described in more detail in GB 2 409 345) is mounted on the PCB 2. The ceramic pellet is fed by way of a microstrip transmission line 4, and the PILA is connected to the groundplane. However, most other types of antenna component may be used. The groundplane (and any other components electrically connected thereto) serves as the second antenna component. A pair of conductive helical resonators 5 is mounted at an end of the PCB 2 remote from the location of the first antenna component 3, each of the resonators 5 having one end connected to the groundplane and the other end open-circuited.

Results are presented for a small dual band (880-960MHz and 1710-1990MHz) antenna mounted on a chassis measuring 38mm wide by 70mm long. The first antenna component 3 is 7mm high and has a volume of approximately 3.5cm ³ . A matching circuit (not shown) comprising a series 1.8nH inductor, a shunt 5.6nH inductor and a shunt 1.5pF capacitor was used.

Figure 11 shows the Sn return loss plot for the case where no resonators have been used (trace 6) and where two resonators 5 have been used (trace 7). It is clear from Figure 11 that the required bandwidth (880-960MHz) cannot be achieved at the - 6dB return loss level without the resonators 5. With the resonators 5 there is plenty of bandwidth available. The high band is substantially unaffected by the presence or

absence of these resonators 5 which are tuned to the low band. Resonators tuned to the high band can similarly improve that band without affecting the low band.

The efficiency of the antenna is significantly improved by the addition of the resonators 5, as shown in Table 1 , where an almost 20% improvement can be seen. The same information is plotted in graphical form in Figure 12. Trace (a) shows the measured terminal efficiency in % for two resonators 5 on a 70mm long PCB 2, and trace (b) shows the measured terminal efficiency in % for the same PCB 2 with no resonators.

Table 1

Measured terminal efficiency in % for a handset antenna on a 70 mm board with, and without, helical resonators.

Where a dielectric insert is provided for the resonators 5, the quality of the dielectric material used is important. Low loss materials permit better antenna efficiency than high loss materials.

Many of the effects demonstrated above have been confirmed by simulation using the Ansoft® electromagnetic simulation package HFSS (High Frequency Structure Simulator) and are thus not an artefact of the measurement system.

In addition to helical resonators 5 projecting generally normal to the groundplane as shown in Figures 9 and 10, various other resonator configurations may be used.

For example, Figure 13 shows a configuration in which the PCB 2 has a notch 8 cut out on one edge, and a helical resonator 5 is mounted in the notch 8 with its axis generally parallel to the plane of the PCB 2, but not extending beyond the footprint of the PCB 2.

Figure 14 shows a configuration in which a helical resonator 5 is connected to the groundplane of the PCB 2 by way of a central conductor 9 that feeds the resonator 5 from the top.

Figure 15 shows a configuration in which spiral helical resonator 5 with increasing diameter is connected to the groundplane of the PCB 2. The helical spiral resonator may be inverted and fed with a central conductor as for the Figure 14 embodiment.

Figure 16 shows an embodiment in which an elevated, flat, spiral resonator 5 is fed by a central conductor 9. In this embodiment, the resonator 5 is elevated above various integrated circuit or other components 10 mounted on the PCB 2, thus saving space on the PCB 2.

Figure 17 shows a similar space-saving arrangement, this time with an elevated helical resonator 5 with its axis generally parallel to the PCB 2.

Figure 18 shows an embodiment in which a helical resonator 5 is short circuited to itself at either or both ends.

Figure 19 shows an embodiment in which two helical resonators 5, 5' are connected to each other at their upper ends, with the lower end of the longer resonator 5 being for connection to the groundplane. This arrangement can provide multi-band operation.