WO2007023442A2

WO2007023442A2 - Device comprising an antenna for exchanging radio frequency signals

Info

Publication number: WO2007023442A2
Application number: PCT/IB2006/052880
Authority: WO
Inventors: Anthony Kerselaers
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2005-08-24
Filing date: 2006-08-21
Publication date: 2007-03-01
Also published as: WO2007023442A3; RU2008110925A; CN101243583A; JP2009506616A

Abstract

Devices (1) comprising antennas (2) with conductive planes (201-202) with slots (205) comprising open ends and closed ends for exchanging radio frequency signals are provided with reactance circuits (203) coupled to the conductive planes (201-202) in parallel to the slots (205) to be able to tune the antenna (2) to a predefined operating frequency (band) by adjusting frequency dependent characteristics of the reactance circuits (203). The reactance circuits (203) are tunable reactance circuits (203) to be tuned via base band circuits (502). The slots (205) divide the conductive planes (201-202) into mainly exchanging planes (201) and mainly coupling planes (202). Feeding points (204) are located in the coupling planes (202) between the closed ends of the slots (205) and the reactance circuits (203). The exchanging planes (201) form part of printed circuit boards or other laminate material comprising radio frequency circuits (501) coupled to the base band circuits (502). The reactance circuits (203) comprise variable capacitors such as switchable capacitors and/or fixed capacitors (221) coupled to variable capacitance diodes (222).

Description

Device comprising an antenna for exchanging radio frequency signals

The invention relates to a device comprising an antenna for exchanging radio frequency signals with an other device, and also relates to an antenna, and to an exchanging method, and to a tuning method.

Examples of such a device are home theatre devices, surround sound devices, wireless headphone devices, second room wireless audio devices, bio-sensing devices, positioning tracking devices, mobile terminals and wireless interfaces.

A prior art antenna is known from US 2002/0135525 Al, which discloses in its Figures an antenna comprising a conductive flat plate with a slit comprising an open end and a closed end. The slit divides the conductive flat plate into a ground portion and a radiating portion. The length of the radiating portion is substantially a multiple of a quarter of a wavelength attained at the operating frequency of the antenna by an odd-numbered value. So, the operating frequency is defined by selecting the length of the radiating portion. Most of the time, different regions in the world require different operating frequencies. For example for wireless headphones, Europe requires an operating frequency of 864 MHz, The United States requires an operating frequency of 914 MHz, Hong Kong requires an operating frequency of 820 MHz, Korea requires an operating frequency of 930 MHz and some other countries even require an operating frequency of 434 MHz. The known antenna is disadvantageous, inter alia, owing to the fact that its size depends on the region in the world where the device comprising the antenna is going to be sold.

It is an object of the invention, inter alia, to provide a device comprising an antenna having a size that does not depend on the region in the world where the device comprising the antenna is going to be sold.

Further objects of the invention are, inter alia, to provide an antenna having a size that does not depend on the region in the world where a device comprising the antenna is going to be sold, and an exchanging method and a tuning method both for use in combination with an antenna having a size that does not depend on the region in the world where the device comprising the antenna is going to be sold.

The device according to the invention comprises an antenna for exchanging radio frequency signals with an other device, which antenna comprises

- a conductive plane with a slot comprising an open end and a closed end; and

- a reactance circuit coupled to the conductive plane in parallel to the slot.

By coupling a reactance circuit to the conductive plane in parallel to the slot, which reactance circuit comprises - by definition - a frequency dependent characteristic, it is no longer necessary to tune the antenna to a predefined operating frequency (band) by selecting a size for this antenna. The antenna can be tuned to a predefined operating frequency (band) by selecting (adjusting) the frequency dependent characteristic of the reactance circuit.

This antenna is further advantageous in that it is based on planar slot technology which makes it small sized. The antenna can be used for transmitting as well as for receiving radio frequency signals. The antenna has a sufficient bandwidth, is low cost and can easily be manufactured. Finally, the antenna can be used for example for wireless headphones between 350 MHz and more than 1 GHz just by adapting the reactance circuit. It should be noted that WO 02/27865 Al discloses a tunable antenna based on inverted F technology. This is a different technology (no slot technology) which results in a relatively large antenna that cannot be tuned over a relatively large frequency range. WO 2004/109850 Al discloses a frequency variable antenna with a radiating electrode comprising a frequency variable circuit in series with this radiating element. This is a different technology (no slot technology) which results in a relatively inefficient antenna owing to the fact that such an antenna has a small antenna radiating resistance. Finally, JP

2003298341 discloses a tunable antenna comprising microstrips on both sides of a conductive film. This is an antenna based on a different technology (no single layer planar technology).

An embodiment of the device according to the invention is defined by the reactance circuit being a tunable reactance circuit. By making the reactance circuit a tunable reactance circuit, a same reactance circuit can be used for an antenna for a device to be sold in different regions in the world requiring different operating frequencies by just tuning this same reactance circuit per region.

An embodiment of the device according to the invention is defined by further comprising a base band circuit coupled to the reactance circuit for tuning this reactance circuit. Such a base band circuit is usually already present for other purposes such as signal processing purposes and can therefore be used efficiently.

An embodiment of the device according to the invention is defined by the slot dividing the conductive plane into a mainly exchanging plane and a mainly coupling plane, the mainly exchanging plane comprising 90% or more of a surface of the conductive plane and the mainly coupling plane comprising 10% or less of this surface. A mainly coupling plane is a mainly non-radiating plane. The word "mainly" here means that the coupling plane of the antenna of the device according to the invention has a coupling function of more than 50%, and a radiating function of less than 50%. An embodiment of the device according to the invention is defined by the reactance circuit being coupled to the exchanging plane and to the coupling plane at both sides of the slot. Such a reactance circuit bridges the slot.

An embodiment of the device according to the invention is defined by the antenna further comprising - a feeding point located in the coupling plane between the closed end of the slot and the reactance circuit.

The antenna input impedance depends for example on the size and/or the shape of the slot and on a location for the feeding point. The location of the feeding point in the coupling plane between the closed end of the slot and the reactance circuit has found to be advantageous.

An embodiment of the device according to the invention is defined by the exchanging plane forming part of a printed circuit board or other laminate material comprising a radio frequency circuit coupled to a base band circuit. Such an exchanging plane is more efficient in that it is used for two purposes, an antenna purpose and a carrying purpose. Preferably, empty parts between the radio frequency circuit and/or the base band circuit are to be filled with copper.

An embodiment of the device according to the invention is defined by the reactance circuit comprising a variable capacitor. Variable capacitors can be easily tuned. An embodiment of the device according to the invention is defined by the variable capacitor comprising a set of switchable capacitors and/or comprising a fixed capacitor coupled in series to a variable capacitance diode. These are well performing embodiments of a variable capacitor. Other serial constructions and/or parallel constructions are not to be excluded. An embodiment of the device according to the invention is defined by the reactance circuit further comprising a coil coupled in parallel to the variable capacitor. This parallel circuit has found to perform well. Other parallel constructions and/or serial constructions are not to be excluded. Preferably, a length of the slot is between 10% and 50% of a wavelength of the radio frequency signals. The length of the slot is a matching parameter, in an optimum case this length will be about 25% of the wavelength of the radio frequency signals. In case of the slot being a bended or folded slot, the length of the slot is the sum of the lengths of the parts of the slot. Embodiments of the antenna according to the invention and of the exchanging method according to the invention and of the tuning method according to the invention correspond with the embodiments of the device according to the invention.

The invention is based upon an insight, inter alia, that in an ideal situation a size of an antenna should be independent of a region of the world in which a device comprising the antenna is going to be sold, and is based upon a basic idea, inter alia, that the antenna based on a conductive plane with a slot should be provided with a frequency dependent circuit coupled to the conductive plane in parallel to the slot.

The invention solves the problem, inter alia, to provide an antenna having a size that does not depend on the region in the world where a device comprising the antenna is going to be sold, and is further advantageous, inter alia, in that it is based on planar slot technology which makes it small sized. The antenna can be used for transmitting as well as for receiving radio frequency signals. The antenna has a sufficient bandwidth, is low cost and can easily be manufactured. Finally, the antenna can be used for example for wireless headphones between 350 MHz and more than 1 GHz by just adapting the reactance circuit. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments(s) described hereinafter.

In the drawings: Fig. 1 shows diagrammatically a device according to the invention comprising an antenna according to the invention;

Fig. 2 shows diagrammatically an antenna according to the invention in greater detail; Fig. 3 shows diagrammatically an antenna according to the invention in greater detail;

Fig. 4 shows diagrammatically a reactance circuit for use in an antenna according to the invention in greater detail; Fig. 5 shows a top view (upper Figure) and a bottom view (lower Figure) of an antenna according to the invention;

Fig. 6 shows a measured return loss of an implementation of an antenna according to the invention;

Fig. 7 shows a relation between an operating frequency of an antenna according to the invention and a capacitance of a reactance circuit, and

Fig. 8 shows a measured return loss for different settings of a capacitance of a reactance circuit of an antenna according to the invention.

The device 1 according to the invention as shown in Fig. 1 such as for example a home theatre device, a surround sound device, a wireless headphone device, a second room wireless audio device, a bio-sensing device, a positioning tracking device, a mobile terminal or a wireless interface comprises an antenna 2 according to the invention coupled to a radio frequency circuit 501. The radio frequency circuit 501 is further coupled to a base band circuit 502. The base band circuit 502 for example comprises a processor and an amplifier and is further coupled to loudspeakers 504.

The antenna 2 according to the invention shown in Fig. 2 and 3 comprises a conductive plane 201-202 with a slot 205 comprising an open end and a closed end. The slot 205 divides the conductive plane 201-202 into a mainly exchanging plane 201 and a mainly coupling plane 202. The mainly exchanging plane 201 mainly exchanges (receives and/or transmits) radio frequency signals with (from and/or to) an other device not shown. The mainly coupling plane 202 mainly couples the mainly exchanging plane and the circuitry 300. The antenna 2 further comprises a reactance circuit 203 coupled to the conductive plane 201-202 in parallel to the slot 205. The circuitry 300 for example comprises the radio frequency circuit 501 and the base band circuit 502 shown in Fig. 1. The antenna 2 further comprises a feeding point 204 located in the coupling plane 202 between the closed end of the slot 205 and the reactance circuit 203. The slot 205 has a width 401 and the mainly coupling plane has a width 402. These widths 401 and 402 usually have a same order of magnitude. The planes 201 and 202 form for example part of a printed circuit board or other laminate material.

The reactance circuit 203 shown in Fig. 4 is a tunable reactance circuit 203 comprising a variable capacitor for example in the form of a serial circuit of a fixed capacitor 221 coupled to a variable capacitance diode 222. Alternatively the variable capacitor may comprise a set of switchable capacitors. The reactance circuit 203 further comprises a coil 223 coupled in parallel to the variable capacitor. The main terminals 210 and 214 of the reactance circuit 203 are coupled to the planes 201 and 202, and a control terminal 212 of the reactance circuit 203 is for example coupled via a resistor 224 to a common point of the serial circuit and is further to be coupled to the base band circuit 502 for tuning the reactance circuit 203.

The circuits 501 and 502 for example perform filtering, amplifying, mixing, detecting etc. The width 401 of the slot 205 between the planes 201 and 202 is relatively small compared to a wavelength of interest. The variable reactance circuit 203 allows the tuning of the antenna 2 to a wanted frequency (band). This can be done in a continuous way or a discrete way by means of a Digital Signal Processor forming part of the base band circuit 502 or can be a selected preset frequency. The reactance circuit 203 can be tuned with a voltage at the control terminal 212. The performance of the antenna 2 further depends on the dimensions and the shape of the mainly exchanging plane 201 which is usually defined by the outer dimensions of the device 1 (the final product).

Fig. 5 shows a top view (upper Figure) and a bottom view (lower Figure) of an antenna according to the invention as a part of an implementation of a wireless digital headphone board HDlOOO that is currently under development. The mainly exchanging plane 201 contains at least most of the electronics. The planes 201 and 202 are implemented as copper sheet on FR4, 1.6mm printed board material. The overall dimensions of the board are 58 x 60 x 1.6 mm.

Fig. 6 shows a measured return loss of an implementation of an antenna according to the invention of a wireless digital audio headphone (return loss in dB versus frequency in MHz). The headphone should for example be operational from 800 MHz to 930 MHz and the antenna should be adjustable for the detuning effects of the human head. Obviously, the antenna has a very good return loss over a wide frequency range.

Fig. 7 shows a relation between an operation frequency of an antenna according to the invention and a capacitance of a reactance circuit (capacitance in pF versus frequency in MHz). The capacitance is for example realized via the series circuit of the varicap 222 with capacitor 221.

Fig. 8 shows a measured return loss for different settings of a capacitance of a reactance circuit of an antenna according to the invention (magnitude in dB versus frequency in MHz). Again, the capacitance is for example realized via the series circuit of the varicap 222 with capacitor 221.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "to comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. Device (1) comprising an antenna (2) for exchanging radio frequency signals with an other device, which antenna (2) comprises

- a conductive plane (201-202) with a slot (205) comprising an open end and a closed end; and - a reactance circuit (203) coupled to the conductive plane (201-202) in parallel to the slot (205).

2. Device (1) according to claim 1, the reactance circuit (203) being a tunable reactance circuit (203).

3. Device (1) according to claim 2, further comprising a base band circuit (502) coupled to the reactance circuit (203) for tuning this reactance circuit (203).

4. Device (1) according to claim 1, the slot (205) dividing the conductive plane (201-202) into a mainly exchanging plane (201) and a mainly coupling plane (202), the mainly exchanging plane (201) comprising 90% or more of a surface of the conductive plane (201-202) and the mainly coupling plane (202) comprising 10% or less of this surface.

5. Device (1) according to claim 4, the reactance circuit (203) being coupled to the exchanging plane (201) and to the coupling plane (202) at both sides of the slot (205).

6. Device (1) according to claim 4, the antenna (2) further comprising

- a feeding point (204) located in the coupling plane (202) between the closed end of the slot (205) and the reactance circuit (203).

7. Device (1) according to claim 4, the exchanging plane (201) forming part of a printed circuit board or other laminate material comprising a radio frequency circuit (501) coupled to a base band circuit (502).

8. Device (1) according to claim 1, the reactance circuit (203) comprising a variable capacitor.

9. Device (1) according to claim 8, the variable capacitor comprising a set of switchable capacitors and/or comprising a fixed capacitor (221) coupled in series to a variable capacitance diode (222).

10. Device (1) according to claim 8, the reactance circuit (203) further comprising a coil (223) coupled in parallel to the variable capacitor.

11. Antenna (2) for use in a device (1) as defined in any one of the claims 1-10.

12. Exchanging method for exchanging radio frequency signals, which exchanging method comprises a step of using an antenna (2), which antenna (2) comprises - a conductive plane (201-202) with a slot (205) comprising an open end and a closed end; and

- a reactance circuit (203) coupled to the conductive plane (201-202) in parallel to the slot (205).

13. Tuning method for tuning an antenna (2), which antenna (2) comprises

- a conductive plane (201-202) with a slot (205) comprising an open end and a closed end; and

- a tunable reactance circuit (203) coupled to the conductive plane (201-202) in parallel to the slot (205), the tuning method comprising a step of tuning the tunable reactance circuit (203).