ANTENNA

FIELD OF THE PRESENT INVENTION

The present invention relates to an antenna and in particular, but not exclusively to an antenna arranged to receive radio frequency signals such as digital audio broadcasting (DAB) signals. The present invention also relates to a device incorporating such an antenna.

BACKGROUND

Mobile telephones, handsets or similar equipment are known. In this document, the term "user equipment" will be used, which is intended to use these devices as well as other items such as PDAs (personal digital assistant), portable computers or the like.

DAB is for example transmitted at a Band III frequency range 174-240MHz. Due to relatively low frequency range, a relatively long antenna length is needed for good reception. In certain applications, there can be difficulty in providing an antenna of a suitable length. Previous proposals have included making use of the headset as an externa! antenna or having a telescopic antenna, when used for applications involving mobile devices. These solutions can be disadvantageous in some circumstances in that the user of the user equipment is required to use a supplied headset or have the telescopic antenna deployed.

The inventors have appreciated that there are no compact antenna designs available suitable for mounting in an electronic device suitable for DAB Band 111 frequency reception.

SUMMARY OF INVENTION

It is an aim of some embodiments of the present invention to address or at least mitigate the problems described above.

According to the first aspect of the invention there is provided a user equipment comprising; a receiver for receiving transmitted data, said receiver comprising an antenna arrangement comprising a ferrite core, said core being surrounded by a sleeve, said antenna arrangement being such that said user equipment is arranged to receive signals in a frequency range of 50 to 400MHz.

According to the second aspect of the invention there is provided an antenna arrangement comprising; an antenna having a core and a sleeve arranged to surround said core, said antenna having a first bandwidth for receiving signals; circuitry coupled to said antenna, said circuitry being arranged so that a second broader bandwidth of signals as compared to the first bandwidth is receivable by said antenna arrangement.

According to the third aspect of the invention there is provided a user equipment comprising; an antenna comprising a ferrite core surrounded by a sleeve, said user equipment having a use orientation in which a plane of said user equipment is perpendicular to ground, and said antenna is arranged in said user equipment such that said antenna has its longitudinal axis parallel to ground when said user equipment is in said use orientation.

According to the fourth aspect of the invention there is provided an antenna comprising: a ferrite core; a sleeve at least partially surrounding the core, wherein said ferrite core comprises a nickel zinc ferrite core.

According to the fifth aspect of the invention there is provided an antenna arrangement comprising; an antenna having a core and a sleeve arranged to surround said core, said antenna having a first resonant frequency range; and circuitry coupled to said antenna, said component being arranged such that said antenna arrangement is arranged to receive signals in a second frequency range, different to first frequency range.

According to the sixth aspect of the invention there is provided an antenna arrangement comprising: a ferrite core; a sleeve at least partially surrounding

the core; a variable capacitance arrangement, wherein the capacitance provided by said variable capacitance arrangement is variable to control the frequency receivable by said antenna.

According to the seventh aspect of the invention there is provided an antenna arrangement comprising; an antenna comprising a ferrite core and a sleeve at least partially surrounding said ferrite core; at least one capacitor coupled to said antenna; and an amplifier coupled to said antenna.

BRIEF DESCRIPTION OF FIGURES

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

Figure 1 shows a ferrite loaded antenna mounted on a printed circuit board

PCB, in accordance with an embodiment of the present invention;

Figure 2 shows the arrangement of Figure 1 from the underside of the PCB;

Figure 3 shows a metal sleeve with micro strip feed line connection of the antenna of Figure 1;

Figure 4 shows the antenna of Figure 1 mounted on the PCB with a cover;

Figure 5 shows an antenna embodying the present invention used with a balanced low noise amplifier;

Figure 6 shows an antenna embodying the present invention used with a low noise amplifier, using a single end receiver application;

Figure 7 shows an antenna embodying the invention in an arrangement with tuning control;

Figure 8 illustrates the antenna slot of Figure 1 ; and

Figure 9 shows an antenna embodying the present invention, when incorporated in user equipment such as a mobile telephone.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference is made to Figures 1 to 4 and 8. This shows an antenna resonant circuit 1. The physical components of the antenna resonant circuit comprise a metal sleeve or coil 2 which is wrapped around a ferrite core 4. This forms an inductive component.

Because the antenna has a ferrite core 4, this means that it is able to respond the magnetic component of an electro-magnetic wave. This makes an antenna embodying the present invention particularly suitable for use in an electrically noisy environment of user equipment such as mobile phone or PDAs. A magnetic antenna is generally less susceptible to electromagnetic interference from the other electronics of the user equipments, The user equipment incorporating an antenna embodying the present invention is preferably arranged so that no other ferromagnetic material is arranged in close proximity to the antenna.

In preferred embodiments of the present invention, the ferrite core 4 is generally cylindrical. However, in alternative embodiments of the present invention, the ferrite core 4 may have other shapes. The metal sleeve or coil 2 is shaped so that it can fit around the core 4 and accordingly in preferred embodiments of the present invention, the sleeve has a cylindrical shape. The shape of the sleeve 2 can be clearly seen from Figure 3.

It should be appreciated that whilst the sleeve 2 has been described in preferred embodiments as being of metal, in alternative embodiments of the present invention, the sleeve may be made of any other suitable conducting material. The sleeve may be made of any suitable metal, preferably one with good conducting properties. A preferred material which gives a good balance, for some implementations, between conductivity and cost is copper.

In preferred embodiments of the present invention, the ferrite material is such that it concentrates the magnetic field and causes a tighter coupling between the radiating element defined by the metal sleeve 2 and the field. The ferrite

material also increases the metal sieeve structure inductance to offer lower resonant frequencies within a smaller physical volume.

In one embodiment of the present invention, the ferrite material comprises nickel-zinc ferrite which has good magnetic permeability and low loss tangent and high Q (Quality factor) at VHF-UHF frequencies. A high Q implies a lower loss and therefore better antenna efficiency. .

Other high frequency ferrite materials can be used in embodiments of the present invention for Band III frequency ranges.

In preferred embodiments of the present invention and as can be seen from Figures 1 and 8, the metal sleeve 2 does not extend to the ends of the ferrite core 4. Instead, a portion 6 of the ferrite core 4 protrudes from each of the opposite ends of the sleeve 2. This means that the magnetic field at the edges of the sleeve 2 is kept within the ferrite core 4.

In preferred embodiments of the present invention, the circumference of the metal sleeve 2 is equal to the length I of the metal sleeve 2. In other words, πd=I where d is the diameter of the sleeve.

In preferred embodiments of the present invention, the sleeve 2 has a slot 15 which runs along the length of the sleeve 2, parallel to the longitudinal axis of the ferrite core. This slot can be clearly seen from Figures 3 and 8.

Discrete capacitors 17 are arranged across the slot 15. These discrete capacitors 17 thus act as distributed capacitance across the slot. These capacitors sum to form the overall capacitive component of the resonant circuit.

The resonant components can be regarded as the sum of the capacitors C 17 resonating with the inductance L produced by the metal sleeve 2 and the ferrite core 4. In some embodiments the resonant frequency is set to be at the

required DAB receive frequency, in other embodiments the components are arranged so that they are inherently non-resonant at the desired operating frequency ω. Thus, jtσL ≠ at the DAB receive frequency operating jmC frequency..

The discrete capacitors have been distributed along the length of the slot 15 to form a resonant circuit with a distributed inductance made up from the metal sleeve and the ferrite core. The number of capacitors and the distance between them will be optimised for a particular application, also taking into account cost. The more capacitors there are, the better the performance, but this needs to be balanced against cost and the practicalities of making the antenna.

Because of the presence of the slot, the antenna behaves as an electrically short magnetic slot dipole. This slot transmission line uses the ferrite as the propagating medium for the quasi-TEM (transverse electromagnetic mode) wave.

As shown in Figures 1 to 3, the antenna is mounted on a printed circuit board 20. The printed circuit board has a slot 22. Slot 22 is aligned with the slot 15 of the sleeve 7. This is to accommodate the capacitors 17.

Reference is made to Figure 4 which shows the antenna arrangement encapsulated in plastic or otherwise mounted in a plastic cover to enhance the durability of the array and also for ease of manufacture. In alternative embodiments of the present invention, the encapsulation or protective housing for the antenna can be of any other suitable material which is non-metallic and robust to provide protection for the antenna. In particular, ferrite materials can be relatively fragile and the housing or encapsulation will give extra robustness to the arrangement.

Reference is now made to Figure 5 which shows the antenna 1 connected to a low noise amplifier 30. The arrangement of Figure 5 is such that the antenna

has a balanced feed to the low noise amplifier 30. In other words, the low noise amplifier 30 has a first input from one end of the antenna and a second input 34 from the other end of the antenna. The low noise amplifier 30 provides two outputs which are received by a balanced receiver input circuit 36.

The antenna design leads itself to a "balanced" receiver input design. DAB receivers tend to have balanced RF inputs as this may improve sensitivity. Using a balanced antenna coupled to a balanced low noise amplifier which in turn is fed to a balanced receiver may give an improved performance over a single ended design. The balanced nature of the antenna means that it is easy to use in conjunction with the balanced low noise amplifier and receiver.

Alternatively, embodiments of the present invention may have the configuration shown in Figure 6 where one end 38 of the antenna is connected to ground whilst the other end 40 is connected to a low noise amplifier 42. The output of the low noise amplifier 42 is input to a receiver input circuit 44.

In the embodiment shown in Figure 6, the low noise amplifier is arranged so that there is a single ended connection between the antenna and the input to the low noise amplifier 42.

In one embodiment of the present invention, the low noise amplifier is a discrete component common emitter type amplifier, in one embodiment of the present invention, the low noise amplifier employs negative feedback. This assists the amplifier in providing a flat broadband gain response and helps define the input and output impedance. It also helps control the performance of the amplifier from variations in the parameters of individual transistors.

For the LNA to have good noise performance, i.e. it adds little noise when amplifying the received signal, it is important to set the input impedance of the amplifier so that it is operating at its minimum noise point. This input impedance tends to be around 50ohms.

in the above arrangements, the LNA is connected to the antenna by a micro strip or a strip line of desired characteristic impedance. This technology is known and provides tracks on printed circuit boards which work well with radio frequencies. In preferred embodiments of the invention, the tracks are provided on the upper side of the printed circuit board, that is the side of the printed circuit board to which the antenna is mounted. If appropriate, a micro strip can be used to connect the low noise amplifier to the receiver front end circuitry. In alternative embodiments, the feed line could be of any other suitable transmission line technology, such as CPW (coplanar waveguide), grounded CPW or slot line.

The loss caused by the micro strip is dependent on its length, width and materials used. In preferred embodiments, the micro strip is made as short as possible, that is the distance between the antenna and the LNA is made as small as possible. This is to minimise RF power losses between the antenna and the LNA thus improving system sensitivity.

In a further modification to the above arrangements and which is shown in Figure 7, the capacitors are replaced by variable capacitors 46 or varactors. A tuning control circuit 48 is provided to control the value of the varactors 46 in order to allow the antenna to be wide-band tuneable. Again, one end of the antenna is connected to a low noise amplifier 42, the output of which is input to a receiver input circuit 44. This is similar to the arrangement shown in Figure 6

The varactor capacitors, in series with DC blocking capacitors, are placed across the slot instead of the fixed capacitors previously defined. The control circuit allows the antenna to be tuned over an even broader range of frequencies. In this way, the wide bandwidth performance of the structure can also be enhanced.

In preferred embodiments of the present invention, the low noise amplifier is incorporated with the ferrite antenna and included in the housing 24. Thus, the

antenna component can be defined by the ferrite core, the metal sleeve together with the mounting PCB, discrete capacitors, feed iine between the antenna and the low noise amplifier and the low noise amplifier.

Using an impedance transformation network between the antenna and LNA allows the low noise amplifier input impedance to be matched to the antenna output impedance. This means that there is an efficient power transfer from the antenna to the LNA giving rise to low ioss, tow Q and wide bandwidth match. The output impedance of the low noise amplifier is also matched to the receiver front end circuitry 44 again improving power transfer to the load provided by the receiver front end.

In some embodiments of the present invention, the resonating structure is detuned away from the desired resonant frequency to allow a simple low loss single series element discrete capacitor match to the LNA input frequencySO Ohms. The antenna is coupled to the low noise amplifier using a semi- balanced line and matching.

In more detail, in these embodiments the resonating structure which is defined by the ferrite core, the sleeve and the capacitors across the slot of the sleeve are arranged to naturally resonate at a first frequency, This frequency is relatively high, that is higher than the DAB receive frequency. The frequency band the combined antenna LNA combination efficiently resonates over is relatively narrow compared to the DAB frequency band. By using a single matching capacitor connecting the low noise amplifier to the resonating structure, the frequency of resonance is both lowered and the bandwidth over which the antenna combined with LNA efficiently resonates over is increased such that the antenna is abie now to receive well all DAB frequency signals.

The small physical size of the antenna structure helps with matching to the low noise amplifier as this allows the antenna impedance to be ciose to the input impedance of the LNA. In some embodiments the impedances of the antenna and LNA are arranged to be very similar or the same so no matching components are required between the LNA and antenna.

The SWR (standing wave ratio) of the antenna component is not adversely affected by the environmental loading, in preferred embodiments of the present invention. The antenna is arranged to be physically electrically small, The radiation pattern is omni directional in one plane and has a figure of eight pattern in the other two planes. As will be described in more detail, the radiation pattern is omni-directional in the piane perpendicular to the axis of the antenna.

Embodiments of the present invention are less sensitive to near field electronic interference. There is a smaller proximity effect as the only possible interference is another ferromagnetic body.

Embodiments of the present invention are such, that the arrangement is electrically isolated from the phone electronics and other RF systems. This means that a degradation of performance and the lost system sensitivity due to close physical proximity with the phone electronics the can be avoided or at the very least mitigated. This is for several reasons described below. .

Unlike many other current proposals where the DAB antenna in a mobile device have relied on using the ground plane of the host user equipment as the counterpoise for the antenna system, the antenna in this invention is not part of the ground plane of the host device and so circulating currents (noise) on the host ground plane are not transferred to the antenna "ground",.

Embodiments of the present invention are less sensitive to near field electronic interference from either the eiectrical circuits in the host device or other nearby devices. There is a smaller proximity effect as the only possible interference is another ferromagnetic body.

In some embodiments the antenna's radiation pattern is optimised to reject noise from the host devices electronics provided that the antenna is

positioned in its correct mounting so that the interferer from the host electronics is in a null point of the antenna's radiation pattern.

Reference is made to Figure 9 which shows schematically, user equipment 54 embodying the present invention. In this example, the user equipment is a telephone. The user equipment comprises the printed circuit board 20 on which there is the housing 24 in which the antenna component is housed. The user equipment also comprises phone electronics and display 52. The user equipment 54 may often be positioned as shown relative to the externa! ground plane 50. In this orientation the antenna has its longitudinal axis arranged perpendicular to the ground plane. The user equipment itself is thus also positioned perpendicular to the ground.

Schematically shown in figure 9 is the transmitter 56 of the DAB signals. The signal transmitted by the transmitter 56 is vertically polarised. The vertically polarised transmitter 56 has a magnetic component running parallel to the ground. The ferrite antenna is mounted vertically within the mobile device which means that under normal use, (where the device is handheld) the ferrite antenna is in the correct plane for the magnetic component of the electromagnetic wave. It should however be appreciated that the user equipment is generally used in a muiti-path environment so the actual orientation of the user equipment is not critical.

Embodiments of the present invention are such that the antenna performance is not altered significantly by close proximity to a body which makes the arrangement particularly appropriate for use in a handheld device.

Because the antenna structure is magnetic, the performance is less affected by close proximity to metal objects.

Embodiments of the present invention may be omni-directional in a multipath environment.

No production trimming is needed of the resonant frequency of the combined antenna and LNA as working together they are relatively wideband, lowering manufacturing costs.

Preferred embodiments of the invention have used a single capacitor to match the antenna to the LNA. However, it should be appreciated that this function can be achieved by any other suitable element or combination of elements such as capacitive elements, inductive elements or resistive elements.

By providing a compact antenna suitable for mounting within user equipment, this would allow receivers and network operators to better support recording and caching services, as it is not required to use the supplied headset antenna or have an telescopic antenna deployed as it is in previous proposals. Recording and caching is where information is downloaded to the user equipment when the user equipment is not being used, for later use by the user.

The invention is not limited to use for receiving DAB Band IM signals. It is equally suitable for receiving other, non DAB, radio signals transmitted in the VHF-UHF frequency spectrum. Embodiments of the invention, may for example be used to receive any suitable digitally encoded data.