GB2442032A - Earphone cable antenna arrangements and circuitry - Google Patents

Abstract

An antenna system for a portable device comprises first and second respective earphone cable pairs 20, 21 which are mutually connected to circuitry means to provide polarisation diversity. The system may include spatial, frequency or antenna element pattern diversity arrangements or a 4-port 180� hybrid junction. The system may be switched between a horizontally polarized balanced dipole mode and a vertically polarized unbalanced monopole mode. The circuitry may include amplifier, impedance matching and filter arrangements. The system may provide multiple independent RF output signals with low polarisation and spatial correlation. The antenna system makes use of polarisation diversity in an earphone cable pair to enhance signal reception and, optionally, transmission.

Description

I

ANTENNA ARRANGEMENTS AND CIRCUITS

BACKGROUND

This invention relates to antenna systems for portable devices such as mobile phones, personal digital assistants (PDA5), laptop computers and purpose designed portable entertainment equipment.

The broadcast digital transmission of radio, television and other data streams in a form suitable for use on mobile terminals is generically referred to as datacasting. The frequencies to be used for datacasting are likely to extend over a wide spectrum, subject to the forthcoming agreements on standards and on their adoption by regulatory authorities and potential broadcasters. It is already apparent that many transmissions are likely to take place on frequencies lower than those in current use for mobile telephony.

The use of frequencies in the range of a few tens or hundreds of megahertz raises important issues in relation to antenna design. Antennas for portable devices are necessarily small, and at the frequencies to be used by many datacasting services the dimensions of the device will be only a very small fraction of the wavelength of the received signal. The performance of electrically small antennas is subject to well-known constraints [Chu, U, "Physical Limitations on Omni-Directional Antennas," Journal of Applied Physics, Vol 19, pp 1163-1175, Dec 1948; and Hansen, RC, "Fundamental limitations in antennas" Proc IEEE, Vol 69, No 2, pp 170-182, Jan 1981].

It is well-known that the operation of an electrically small receiving antenna can be enhanced by the connection of a radio-frequency amplifying circuit directly to the terminals of the antenna. This amplifier serves both to increase the level of the received signal, to match the impedance of the antenna to that of the input to the receiver and to ensure that the whole receiver (including the amplifier) has low internal noise (a low noise figure). Such antenna/amplifier configurations are sometimes known as active antennas'.

Experience with receivers fitted with small antennas shows that they are sensitive to positioning, both in respect of the polarisation of the received signal and in respect of local maxima and minima of the wanted signal, especially in indoor locations.

In order to obtain more satisfactory reception it is known that use can be made of a cable which may be used to connect the portable device to earphones or headphones which may be worn by the user, especially when the device is used in a public place or a vehicle.

In all forms of mobile communication, the signal which is being received is subject to multiple reflections by objects surrounding the receiver. This is particularly so in the case of indoor reception, or reception in vehicles such as cars or trains. This is recognised in the design of antennas for base stations for mobile communications systems and is combated by the use of diversity in the receive path. If signals are received by two (or more) independent antennas and the resulting signals are appropriately combined, then the worst effects of spatial fading are overcome and the user observes less frequent interruptions to the signal. Receiver systems commonly use spatial diversity in which separate samples of the signal are received in two physically separate locations, or polarisation diversity in which the samples of the signal are differentiated by their polarisation. The relative merits of these systems are discussed in Collins B S, Polarization Diversity Antennas for Compact Base Stations," Microwave Journal, Vol. 43, No. 1, Jan 2000, pp. 76-88 in the context of a mobile radio base station. The frequencies in use by some datacasting services (perhaps operating at frequencies between 1 MHz and 800MHz) may differ from those in use by existing mobile networks (typically around 850, 900, 1800, 1900 or 2100MHz according to country and mobile standard), but the principal of diversity reception is still valid.

Use of earphone or headphone cables as a balanced dipole-like external antenna is known. However, existing arrangements, as shown for example in Figure 1, provide essentially horizontal polarisation. Even when the cables are not truly horizontal, vertical polansation components cancel each other out while horizontal polarisation components add.

BRIEF SUMMARY OF THE DISCLOSURE

According to a first aspect of the present invention, there is provided an antenna system for a portable device, the antenna system comprising first and second respective earphone cable pairs, the earphone cable pairs being mutually connected by way of circuitry means so as to provide polarisation diversity.

Advantageously, there may further be provided at least one additional output configured to provide spatial diversity.

Preferably, the antenna system further includes a 4-port 1800 hybrid junction.

The system may be operable to function in an unbalanced monopole-like mode.

Alternatively or in addition, the system may be operable to function in a balanced dipole-like mode.

The system may be operable to provide polarisation diversity by using both a horizontal mode and a vertical mode.

Alternatively or in addition the system may be operable to provide polansation diversity by switching between a horizontal mode and a vertical mode.

The system may be operable to function in an independent mode in which dominantly horizontally and dominantly vertically polarised signals are simultaneously and independently available at different output terminals.

The outputs may be used in various combinations so as to provide additional diversity bra nches.

In this way, embodiments of the present invention seek to provide an enhanced reliability of reception by exploiting an earphone or headphone cable and/or one or more antennas internal to the portable device to provide diversity (polarisation diversity and optionally spatial diversity) reception.

In some embodiments, there may be provided an unbalanced monopole-like mode where horizontal polarisation components cancel and vertical polarisation components add. This may be achieved by making use of coaxial cable connectors to each earpiece of a pair of ear-or headphones, and connecting an inner conductor of each coaxial cable connector to respectively left and right outputs by way of inductors.

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 a typical portable user device with connected earphones; FIGURES 2(a) to 2(i) show various antenna arrangements demonstrating different radio frequency current modes; FIGURE 3 shows a 4-port 180-degree hybrid junction; FIGURE 4 shows a first embodiment of the present invention; FIGURE 5 shows a second embodiment of the present invention; FIGURE 6 shows a third embodiment of the present invention; and FIGURE 7 shows a fourth embodiment of the present invention.

FIGURE 8 shows computer simulation results for the performance of the antenna arrangements shown in Figures 2a and 2b.

FIGURE 9 shows computer simulation results for the performance of the antenna arrangements shown in Figures 2e and 2f.

DETAILED DESCRIPTION

Fig. 1 provides a diagrammatic view of a typical portable device I with a branched earphone cable 2 attached thereto by means of a stereophonic audio jack-plug 3. Such a plug has a grounded projecting connection with two insulated regions near the end engaged with the connected device; these regions are conventionally used in conjunction with the grounded body to provide connections for the right-hand and left-hand earphone.

Fig. 2 indicates a number of different current modes which can be excited in the branched earphone cable. The balanced mode of Fig. 2(a) excites the two branches of the cable, creating a dipole antenna which responds to a signal polarised in the direction of the dotted arrow. The unbalanced mode of Fig. 2(b) excites the two branching cables substantially in phase and may be regarded as a monopole mode, responding to signals polansed in the direction of the dotted arrow.

Fig. 2(c) shows an independent current mode in which the antenna system is arranged to drive a pair of cophased outputs.

Fig. 2(d) is similar to Fig. 2(b), but the antenna terminals are at the lower end of the cable; in this configuration the counterpoise is provided by the connected user device.

Fig. 2(e) shows a dipole configuration suited to internal mounting in the user device, in which the currents in the outer linear conductors flow in phase with one another. An alternative configuration is shown in Fig. 2(f) in which the currents in the outer linear conductors flow in opposite directions. The arrangement in Fig. 2(g) provides switches allowing the configuration to be changed between those shown in Figs. 2(e) and 2(f) by operating both switches together.

Fig. 2(h) shows the usual balanced mode in a loop antenna, while Fig. 2(i) shows the unbalanced mode in which the user device acts as a counterpoise.

In practice the cables connecting the right and left earphones will not be physically in line as shown in Fig. 2, but will assume positions more like those shown in Fig. 1. This difference in disposition has only a small effect on the polarisations of the modes, which remain substantially orthogonal. Even if the branching cables are significantly asymmetric and the vertical' cable is inclined, the resulting coupling between the planes of polarisation is still likely to provide signal samples which are sufficiently de-correlated to provide effective polarisation diversity [Collins B S, "The effect of imperfect antenna cross-polar performance on the diversity gain of a polarization-diversity receiving system", Microwave Journal April 2000, Volume 43, No4, pp. 84-94].

Fig. 3 shows a known radio frequency circuit device capable of providing the required characteristics, the device being known as a 4port 180-degree hybrid junction. A signal input at Port I emerges as two equal co-phased signals from Ports 3 and 4; a signal input at Port 2 emerges as two anti-phase signals from Ports 3 and 4.

Correspondingly the vector sum of two signals input at Ports 3 and 4 appear at Port 1 while their vector difference appears at Port 2. The required functionality can be realised using vanous circuit technologies according to the operating frequency band including ferrite-cored transformers and various types of transmission line circuits; miniature hybrid junctions constructed using low-temperature co-fired ceramic techniques are commercially available.

Antennas as described above are likely to have input impedances which are unsuitable for matching to a conventional receiver or radio frequency amplifier. The use of a low-noise amplifier connected directly to the antenna terminals is a known method whereby the antenna can be better matched to the receiver to which it is connected, and a lower noise figure provided for the whole receiver system [Meinke, H., "Aktive Antennen," Nachnchtentechnische Zeitschrift (NTZ), Vol. 19, No. 12, Dec 1966, pp. 697-705; Lindenmeier, H.K., "The Transistorized Receiving Antenna with a Capacitive High Impedance Amplifier for an Optimum Solution for Receiving at Low Frequencies," Nachrichtentechnische Zeitschrift (NTZ), Nov 1974, pp. 411-418; and Landstorfer, F.M., "New Developments in VHF/UHF Antennas," Proceedings of the International Conference on Antennas and Propagation, 1978, pp. 132-141]. For receiving applications, all the configurations described in this document may be connected to an amplifier through some form of radio frequency transmission line or through the amplifier directly connected to the antenna terminals.

There is a variety of configurations in which the hybrid antenna arrangement described above can be implemented, and some of these are now described by reference to Figs. 4 to 7.

Each configuration provides a balanced and unbalanced RF output covering the frequency band defined by the characteristics of the amplifiers. The two amplifiers may cover the same frequency band (providing frequency diversity) or different frequency bands (allowing different services to be provided).

For each arrangement, the RF outputs may be carried on the same wires as the left and right hand audio channels, using simple and well-known filtering circuits comprising capacitors and inductors. The DC supply to the amplifier may optionally be combined with the audio and RF signals using well-known blocking and filtering techniques. This arrangement allows the use of only three conductors in the connection between the earphones and the mobile device and permits the use of a standard stereophonic jack plug as currently used for headphones, but a system using a connector with an additional contact may be preferred because the system will not operate correctly if a standard headphone (with no amplifiers or filter circuit) is plugged into the socket. A system using a fourth contact for DC power allows the device to be used with an internal antenna for some services, perhaps telephone use or use as a DVD player, while requiring a purpose-designed antenna-headphone' to provide full performance and functionality. The use of a fourth contact removes the need for circuits to detect the presence of a standard headphone to avoid a loud click in the ear when connecting to the energised contact and possible damage to a standard headphone.

Fig. 4 shows a circuit arrangement providing the balanced mode RE output corresponding to Fig 2(a). The earphones 10, 11 are connected by cables 8, 9 each of coaxial form with an inner conductor 21 and an outer screening conductor 20. The outer conductors 20 form the RF conductors of the antenna and are connected via capacitors 13, 15 to the inner conductors of the connecting cables 16, 17 which carry audio signals to the two earphones. The inner conductors of the cables 21 are connected via RF chokes 12, 14 to the inner conductors of the connecting cables. The function of the capacitors 13, 15 is to block the audio frequency currents and that of the chokes 12, 14 is to prevent RF currents flowing in the headphones. The inductors 18 and 19 provide an audio-frequency return path. By these means the two connecting cables carry both the left/right audio signals and the balanced RE currents. A similar pair of filtering circuits is provided at the user device to separate the combined RF and the audio signals which are connected to the RF and audio amplifiers respectively.

Fig. 5 shows a circuit arrangement providing the unbalanced mode RF output corresponding to Fig. 2(b). The connection of the earphones 10, 11 is made by the coaxial cables 8, 9 as before. The inner conductors of the cables 21 are connected to the right and left connecting cables via the inductors 30, 31. The capacitor 32 provides an RF path for the output signal via the inner conductor of the connecting cable 16, while inductor 33 provides a return path for the audio signals and blocks the flow of RF currents.

Fig. 6 illustrates a circuit providing the unbalanced output with an effective RF feed location close to the user device. In this circuit the outer conductors of the earphone cables 20 are connected directly to the inner conductors of the connecting cables 16, 17 and the inner conductors 21 are connected to the outer conductors of 16, 17. At the user device the inner conductors of the connecting cables 16, 17 are connected via RE chokes 52, 53 to the right and left audio circuits. The outer conductor is connected to ground via an RE choke 51 and via a capacitor 50 to the input of the receiver.

Fig. 7 shows a circuit providing a dual-mode RF output and provided with an RF amplifier for each mode. The filtering components and audio circuits are omitted for clarity. The outer conductors of the earphone cables 40, 41 are connected via a transformer with a centre-tapped primary winding 42 and a single secondary winding 44. The ends of the secondary winding 44 of the transformer are connected to the balanced inputs of amplifier 45 and the balanced mode output (Fig 2(a)) is obtained from the output of this amplifier. The second amplifier 46 is connected between the centre tap 43 of the primary winding 42 and ground, providing the unbalanced mode output (Fig 2(b)). By the use of appropriate simple filtering circuits the balanced and unbalanced RF signals and the DC supply for the amplifiers can be combined into the two connecting cables.

The circuit of Fig. 7 may also be applied to a loop antenna when it will couple to the current modes shown in Fig. 2(h) and Fig. 2(i). In this case the balanced and unbalanced current modes provide different radiation patterns but with the same polarisation. The two outputs can provide pattern diversity, responding to direct and reflected signals arising from the wanted incoming signal.

Some of the antenna circuits and arrangements have been simulated using the Ansoft High Frequency Structure Simulator (HESS) package. Figures 8a and 8b show the radiation patterns corresponding to the arrangements of Figures 2a and 2b. It can be seen that the two different ways of driving the antenna give rise to substantially orthogonal radiation patterns. Similarly, Figures 9a and 9b shows the radiation patterns corresponding to the arrangements of Figures 2e and 2f. It can be seen that once again the two different antenna arrangements give rise to substantially orthogonal radiation patterns. A reasonable impedance match across the band 480-700MHz was obtained with the antenna arrangements shown in Figures 2e and 2f.

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.

Throughout the description and claims of this specification in all references to the sense of received polarisations it is to be understood that the polarisations referred to will typically be dominantly but not exclusively received when worn by a user having an upright or seated posture.

Claims (8)

1. CLAIMS: 1. An antenna system for a portable device, the antenna system

   comprising first and second respective earphone cable pairs, the earphone cable pairs being mutually connected by way of circuitry means so as to provide polarisation diversity.
2. 2. An antenna system as claimed in claim 1, further comprising at least one additional output configured to provide spatial diversity.
3. 3. An antenna system as claimed in any preceding claim, including a 4-port 180 hybrid junction.
4. 4. An antenna system as claimed in any preceding claim wherein the system is operable to function in a balanced dipole-like mode.
5. 5. An antenna system as claimed in any one of claims I to 3, wherein the system is operable to function in an unbalanced monopole-like mode.
6. 6. An antenna system as claimed in any preceding claim wherein the system is operable to provide polarisation diversity by using both a horizontal mode and a vertical mode.

   7. An antenna system as claimed in any preceding claim wherein the system is operable to provide polarisation diversity by switching between a honzontally-polansed mode and a vertically-polansed mode.
7. 7. An antenna system as claimed in any preceding claim wherein the system is operable to provide multiple independent output RF signals having independent polarisation or low spatial correlation..
8. 8. An antenna system substantially as hereinbefore described with reference to or as shown in the accompanying drawings.