WO2002082665A2

WO2002082665A2 - Antenna system

Info

Publication number: WO2002082665A2
Application number: PCT/GB2002/001509
Authority: WO
Inventors: Henry O'tani
Original assignee: Dodd, Alexander; Rogers, Howard, Leslie; Rogers, Louise
Priority date: 2001-04-06
Filing date: 2002-04-08
Publication date: 2002-10-17
Also published as: GB2374203A; GB0108692D0; AU2002251211A1; WO2002082665A3

Abstract

The invention relates to antenna systems for wireless communications networks, and in particular concerns antenna systems for low power licence exempt broadband wireless local area networks (WLANs). In one embodiment the antenna system comprises: a first antenna configured for receiving radio signals for a receiver element of a radio transceiver, and a second antenna for transmitting radio signals generated by a transmitter of the transceiver. The gain of the first antenna is greater than the gain of the second antenna. The lower gain of the dedicated transmit antenna enables the radiated power to be minimised without adversely affecting range. Switching means are provided for connecting either the first antenna to the receiver or connecting the second antenna to the transmitter.

Description

ANTENNA SYSTEM

This invention relates to an antenna system for a wireless communications network, and in particular concerns an antenna system for low power licence exempt broadband local area networks.

The maximum Effective Isotropic Radiated Power (EIRP) is one measure that is used by Regulatory Authorities to limit electromagnetic emissions from wireless transmitters. Wireless standards such as Bluetooth and IEEE 802.11 A for Wireless Local Area Networks (WLAN's) limit the effective Isotropic Radiated Power so that devices that conform to these standards are licence exempt. Wireless LANs conforming to the IEEE 802.11A standard are typically limited to 100 mW and this restricts the range of the individual wireless transmitters to a maximum range in the region of 300m. The maximum range of Bluetooth devices is restricted to about 30m.

Wireless LANs are typically installed in buildings to provide fast data connections over a relatively small area. Hitherto, it has been possible to increase the range of such LANs by providing repeater stations which may be in the form of an Ethernet Bridge or an ATM switch which join other sub-networks in the LAN. In this way a wireless LAN may be provided to cover the area of a small building, a research or university campus.

In recent years, the development of products which conform to standards such as IEEE802.11A has led to the development of ad-hoc community based wireless LANs which offer open and sometimes free broadband access to networks such as The Internet. The advantage of such networks is that they offer connectivity at very fast broadband data rates, typically 11 Mbits/sec per second for IEEE802.11A which compares with 2 Mbits/sec or less for ADSL. Typically end users access the wireless LAN through WLAN adaptors which may be implemented as PCIMCIA cards in notebook computers, ISA or PCI adapters in desktop computers or fully integrated devices within hand held computers. Access to fixed networks such as the PSTN is provided by wireless access points (WAP) antennas positioned at strategic geographical locations to provide coverage over the area of the wireless LAN.

As mentioned above one of the major drawbacks with licence exempt wireless LAN such as those conforming to IEEE 802.11. A is that the range is limited to only a few hundred metres. It is possible to overcome this problem in an open network in which users provide stepping stones for re-transmission of wireless data signals between other network nodes. However, this requires significant co-ordination of the network nodes to provide the links necessary for relatively long distance wireless communications.

There is a requirement tp increase the operational range of licence exempt wireless communications in wireless LAN type networks.

Another problem associated with wireless networks is that many kinds of modern wireless systems such as mobile telephones and wireless data networks devices do not possess a detachable external antenna but instead have a built-in or integral antenna. Integral antennas may comprise conventional tuned antenna elements which are plastic moulded or internally constructed, say by using printed circuit track elements.

Although integration improves product reliability and reduces manufacturing costs, integrated antennas do not permit the user of such devices to readily connect to external antenna resources.

The absence of an external antenna connector on most wireless devices such as WLAN cards can constitute a considerable performance limitation for wireless connectivity.

It is generally not possible for a user to alter a wireless appliance's internal circuitry, that is to say by fitting an external antenna connector, without comprising "type approval" requirements of national regularity type approval frameworks.

There is a further requirements to enhance the range of wireless network equipment having integrated network antenna's.

According to an aspect of the invention there is provided an antenna system for a wireless transceiver network node; the said system comprising: a first antenna configured for receiving transmitted radio signals for subsequent transmission to a receiver element of a radio transceiver node; a second antenna configured for transmitting radio signals generated by a transmitter of the said transceiver node; the said first antenna having a gain greater than the gain of the said second antenna; and, switching means for connecting either the said first antenna to the said receiver for receiving radio signals or the said second antenna to the said transmitter for transmitting radio signals.

In this aspect of the invention a wireless transceiver has its antenna connection separated between "send" and "receive", and uses a high gain antenna for receiving transmitter signals while ensuring the transmitter output signal is radiated by a second antenna which has a power setting below the maximum allowable for licence exempt operation.

Preferably the said switching means comprises a PIN diode switch, a FET switch or an electromagnetic relay. In this way radio frequency switching can be implemented by PIN diodes or FET's. At lower frequencies and switching rates co-axial relays may be used.

In preferred embodiments, the antenna system further comprises a radio signal detecting means for detecting transmission signals from the said transmitter and for switching the said switching means in response to a transmission signal being detected. A radio frequency detector can be used to detect for the presence of the transmitted signal on say a WLAN card connection port and automatically generate a switching current for the semi-conductor diode or FET or relay. In this way, the switching means can be switched by automatic carrier sensed radio frequency switching in the same way that carrier sensed radio frequency switching is used to switch whole circuit elements such as filters, pre-arnplifiers and power amplifiers etc. In preferred embodiments, the said detecting means comprises a carrier sensing means for sensing radio frequency transmission signals on a Wireless Network Interface Card of the said transceiver, and a switching current generator for switching the said switching means in response to a detected transmission signal from the said transmitter.

Preferably, the Effective Isotropic Radiated Power (EIRP) of the said second antenna is less than 100 mW. In this way the radiated power can be controlled so that it does not exceed the maximum allowed for licence exempt radio operation.

In preferred embodiments, the said first antenna is a high gain antenna selected from the group comprising: an omnidirectional antenna, a sector antenna, a horn antenna, a yagi antenna, or a parabolic antenna. High gain antennas such as omnidirectional, sector, horn, yagi or parabolic can be employed to increase the apparent receiver gain at each end of the communication link and therefore increase the link's effective range without exceeding the maximum transmitter power allowed for licence exempt operation. This increase in system range is equal to that provided by the high gain antenna at the end of the communication link path with the least gain. At UHF and microwave frequencies, domestic size parabolic dish and other high performance antennas may be used to improve the operating range by an order or two of magnitude for example by a factor of 30.

According to another aspect of the invention there is provided an antenna system for a wireless transceiver network node; the said system comprising: a common transmit and receive antenna for a receiver and a transmitter of a wireless transceiver; and, an attenuator connected between the said antenna and the said receiver and transmitter, the said attenuator having asymmetric attenuation properties such that transmission signals from the said transmitter means to the antenna are attenuated greater than received signals from the antenna to the said receiver means.

The common antenna may comprise a high gain antenna of the type mentioned above. In this way the receive and transmit antenna paths can be differentiated so that the increased transmitter output signal is selectively attenuated before being radiated by the high gain antenna so that the radiated power does not exceed the predetermined maximum defined for unlicensed radio operation.

In preferred embodiments the said attenuator comprises a uni-directional circuit which attenuates transmission signals in the transmission direction only. In this way a single path uni-directional circuit can be used which passes signals in the forward (receive) path with little or no signal degradation but reliably attenuate the signal in the return or backwards (transmit) path so as to maintain the radiated power within the licence exempt limit.

Preferably, the said attenuator is selected from the group comprising: a biased switching diode circuit, an unswitched pre-amplifier circuit optionally including variable attenuation, a phase balanced or un-balanced network circuit, a circulator circuit, or an isolator. The attenuator is selected for mW power transmitters. In one embodiment, the attenuator is an un-switched amplifier stage which attenuates the path to the antenna without impairing incoming signals and being damaged by backward transmit signals. In this embodiment, the degree of overall attenuation is not readily controllable but the embodiment provides a very simple and robust solution.

In preferred embodiments, the said attenuator is positioned in series in an antenna feed line for connecting the said antenna to the said transceiver. In this way the antenna system may be fitted to an existing antenna feed cable of an existing radio frequency transceiver or incorporated into the transmitter stages of a new design.

According to another aspect of the present invention there is provided an antenna system for a wireless transceiver network node; the said system comprising: a common transmit and receive antenna for a receiver and a transmitter of a wireless transceiver; the said antenna being connectable to the said receiver by a first connection path and to the said transmitter by a second connection path; the second path having an attenuator for attenuating transmission signals from the said transmitter to the antenna; and, switching means for connecting either the said receiver to the said antenna by the said first path or the or the said transmitter to the said antenna by the said second path.

In this way, high speed switching can be used to automatically attenuate the transmitted power by an amount approximately equal to the gain of the high gain antenna without causing the receive signal to be attenuated. The amount of attenuation may be predetermined or determined automatically by radio frequency sensing and measurement of the transmitter wireless carrier. In this aspect of the invention the switching means separates the single antenna path used for transmitting and receiving radio frequency signals and recombines these paths to feed the common antenna.

Preferably, the said switching means comprises a PIN diode switch, a FET switch or an electromagnetic relay. In one embodiment the switching means may be a PIN diode in a through channel normally biased ON and turned OFF by a current of a few mW. This embodiment could be implemented in a phase cancelling strip-line circuit in a similar way to the hybrid mixer circuits used for microwaves.

Preferably, the said first path comprises an amplifier. In this way it is possible to insert a low noise amplifier in the receivers antenna path.

According to another aspect of the invention there is provided apparatus for coupling two or more antennas comprising: a first antenna having a feed line connected to an antenna circuit; the said circuit being capable of electromagnetically coupling the said antenna to an integrated antenna electrically connected to and integrated with a radio transceiver.

This aspect of the invention enables an existing wireless appliance fitted with an integrated antenna to be coupled to an external antenna connection. The antenna connection may be made without physically altering the existing wireless appliance In this way, the advantages obtained with external antenna resources such as the use of a high gain antenna and appropriate transmitter and/or receiver amplifiers maybe obtained by coupling an external antenna to the internal antenna of a wireless appliance.

In one embodiment, the antenna circuit is passively coupled to the integrated antenna by simply positioning the antenna circuit in the radiated electromagnetic field of the integrated antenna such that the antenna circuit picks up radiation from the integrated antenna in one sense and transmits electromagnetic radiation to the integrated antenna. In this way, a single connection path is provided between the antenna circuit and the first antenna by the an uninterrupted feed line.

In another embodiment, the said feed line comprises first and second connection paths for connecting the said circuit to the said antenna; the second path having an attenuator for attenuating transmit signals from the said circuit to the antenna; and, switching means for connecting the said circuit to the said antenna by the said first path for signal reception or the or the said circuit to the said common antenna by the said second path for signal transmission. In this embodiment, an external amplifier may be used for simplex transceivers to compensate for the high coupling losses associated with the wireless coupling of the antennas. In particular, by positioning the antenna circuit after the pre-amplifier, the noise-figure for the system can be maintained and/or improved. In the above mentioned coupling system the antenna circuit can be attached to the existing wireless appliance preferably in the region of the integrated antenna and may be included as a pickup loop within a mobile telephone cradle or within a large plastic clip designed to fit on the wireless appliance's outer casing.

According to a further aspect of the invention there is provided an apparatus for coupling two or more antennas comprising: a first antenna and a second antenna; the said first antenna having a higher gain than the said second antenna; the said second antenna being electrically connected to and integrated with a radio transceiver and positioned with respect to the said first antenna such that electromagnetic coupling of the electromagnetic fields of said first and second antenna is optimised to increase the range of the said integrated antenna.

In this way it is possible to obtain the same advantages as would be obtained with an external antenna connection, and a length of radio frequency co-axial cable to a high gain antenna, simply by positioning the transceiver within the prime signal field of the high gain antenna, normally occupied by the antenna's active element.

According to another aspect of the invention there is provided a method of coupling at least two antennas including a first antenna having a feed line connected to an antenna circuit capable of electromagnetically coupling the said first antenna to an integrated second antenna electrically connected to and integrated with a radio transceiver; the said method comprising the step of: positioning the said antenna circuit in the electromagnetic field of the said second antenna.

The embodiments of the invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of an antenna path switch for a dual antenna system for a radio frequency transceiver;

Figure 2 is a schematic representation of a network link comprising a pair of radio frequency transceivers incorporating switches of Figure 1;

Figure 3 is a schematic representation of a single path uni-directional attenuator for a radio frequency transceiver having a single antenna system;

Figure 4 is a schematic representation of a dual path active attenuator for a radio frequency transceiver having a single antenna system;

Figure 5 is a schematic representation of a network link comprising a pair of radio frequency transceivers incorporating the attenuator of Figure 3 or Figure 4; Figure 6 is a schematic representation of a passive antenna coupling system;

Figure 7 is a schematic representation of an active antenna coupling system;

Figure 8 is a schematic representation of an embodiment of a direct antenna coupling system;

Figure 9 is a schematic representation of another embodiment of a direct antenna coupling system;

Figure 10 is a schematic representation of a further embodiment of a direct antenna coupling system.

Referring to Figure 1, a transceiver 10 comprises independent transmitter and receiver elements 11 and 12 which are connected on a common antenna path to a switch 13 which defines an antenna re-separator. The switch may comprise a semi-conductor PIN diode or a FET for low power automatic carrier sensed switching driven by a radio frequency detector (18) positioned between the switch and the transceiver elements. The switch operates to connect the transceiver to either a first antenna 14 at the end of a first antenna path 15 or a second antenna 16 at the end of a second antenna path 17. The switch is normally biased to connect the transceiver to the second or receive path 17 and is switched by the carrier sense radio frequency detector 18 to connect the transceiver to the first or transmit path 15. The first antenna is a low power low gain antenna which has an EIRP less than say lOOmW. The second antenna has a much higher gain to increase the sensitivity or operating range of the receiver, the gain may be 25dB for example.

Referring to Figure 2, a network link 20 comprises a pair of radio frequency transceivers incorporating switches 13 to provide two-way radio communication between a first transceiver 21 and a second transceiver 22. The link 20 comprises an uplink 23 between the low gain transmitter antenna 14 of the first transceiver 21 and the high gain antenna of the second transceiver 22, and an downlink 24 between the low gain transmitter antenna 14 of the second transceiver 22 and the high gain antenna 16 of the first transceiver 21.

The high gain antennas 16 may comprise an omnidirectional antenna, a sector antenna, a horn antenna, a yagi antenna, a parabolic antenna, for example, to increase the effective range of the radio links without increasing the transmitter power or gain of the transmitter antenna 14 beyond a pre-determined limit.

Referring to Figure 3, in an alternative arrangement a single path uni-directional attenuator 30 for a radio frequency transceiver 10 is provided between a single common receive/transmit antenna 31 and the transceiver 10. In the arrangement of Figure 3, the attenuator 30 can be a passive diode circuit, an un-switched low-noise pre-amplifier with adjustable backward attentuation, a phase balanced (an unbalanced) network, a circulator, a hybrid ring stripline, or a microwave ferrite isolator, for example. The attenuator 30 constitutes a uni-directional passive circuit in that it passes signals in the forward (receive) path with little or no degradation but reliably attenuates signals in the backward (transmit) path.

Referring to Figure 4, in an alternative arrangement a dual path attenuation circuit 40 for a radio frequency transceiver 10 is provided between a single common receive/transmit antenna 31 and the transceiver 10. The transceiver 10 is connected on a common antenna path 41 to a switch 42 which defines an antenna path re- separator switch which switches between a first position connecting the transceiver to a receiver amplifier 43 on a first receive antenna connection path 44, and a second position connecting the transceiver to a transmitter attenuator 45 on a second transmit antenna path 46. The paths 44 and 46 are again switched at a second switch 47 to a common antenna path 48. The switches 42 and 47 may comprise a semi-conductor PIN diode or a FET for low power automatic carrier sensed switching driven by a radio frequency detector (not shown) positioned between the switch 42 and the transceiver elements. The switches 42 and 47 operates to connect the transceiver to either the first antenna path or to the second antenna path. The switches are normally biased to connect the transceiver to the first antenna path and are switched by the carrier sense radio frequency detector to connect the transceiver to the second antenna path comprising the attenuator 45.

The antenna in the arrangements of Figures 3 and 4 is a high gain antenna, the gain may be 25dB for example.

Referring to Figure 5, a network link 50 comprises a pair of radio frequency transceivers 10 incorporating a dual path attenuator circuit 40 to provide two-way radio communication between a first transceiver 51 and a second transceiver 52. The link 50 comprises an uplink 53 between the high gain antenna 53 of the first transceiver 51 and the high gain antenna 54 of the second transceiver 52, and an downlink 55 between the antenna 54 of the second transceiver 52 and the high gain antenna 53 of the first transceiver 51.

Referring to Figure 6, a transceiver 10 comprises an internal integrated antenna 60. In the arrangement of Figure 6 the advantages associated with an external antenna 62, including for example, the use of a high gain antenna and appropriate transmitter and/or receiver amplifiers, can be realised by coupling the internal antenna 60 by a radio frequency integral-antenna coupling system in the form of a tuned antenna circuit 61 positioned close to the integral antenna.

In one embodiment, the coupling system 61 may be "passive" as shown in Figure 6, or it may be "active" as shown in Figure 7. Although, passive coupling is recognized as a fundamentally inefficient and lossy process, the practical convenience of very high gain antenna in the order of 25dB or more, at microwave frequencies, can compensate for this and passive coupling is an increasingly attractive and practical option at frequencies above say 1GHz. The arrangement of Figure 7 is similar to that of Figure 6 but further comprises an attenuation circuit 70 similar to the circuit 40 of Figure 4. The amplifier of the circuit 70 can be adjusted to fully compensate for the high coupling losses associated with non connective wireless coupling. In Figure 7 the coupling circuit is positioned after the external pre-amplifϊer so that the system noise-figure can be maintained or improved.

The two coupling systems described with reference to Figures 6 and 7 can be used by simply attaching the coupling circuits to the wireless device, for example by positioning a pick-up loop within a mobile telephone cradle or providing the loop within a large clip which fits on the device's outer casing, such as the casing of a laptop wireless enabled computer.

In further arrangements shown in Figures 8, 9 and 10, the wireless device is positioned such that the integral antenna is positioned within and orientated with respect to the prime signal field of the antenna, that is to say in a position used normally to locate the active component or components of the antenna.

In the arrangement of Figure 8, direct antenna coupling is shown for a Yagi directional field array antenna. In the arrangement of Figure 8, direct antenna coupling is shown for a parabolic antenna.

In the arrangement of Figure 8, direct antenna coupling is shown for a parabolic antenna.

Although aspects of the invention have been described with reference to the embodiments shown in the accompanying drawings it is to be understood that the invention is not limited to those precise embodiments and various changes and modifications may be affected without exercise of further inventive skill and effort.

Claims

1. An antenna system for a wireless transceiver network node; the said system comprising: a first antenna configured for receiving transmitted radio signals for subsequent transmission to a receiver element of a radio transceiver node; a second antenna configured for transmitting radio signals generated by a transmitter of the said transceiver node; the said first antenna having a gain greater than the gain of the said second antenna; and, switching means for connecting either the said first antenna to the said receiver for receiving radio signals or the said second antenna to the said transmitter for transmitting radio signals.

2. An antenna system as claimed in Claim 1 wherein the said switching means comprises a PIN diode switch, a FET switch or an electromagnetic relay.

3. An antenna system as claimed in Claim 1 or Claim 2 further comprising a radio signal detecting means for detecting transmission signals from the said transmitter and for switching the said switching means in response to a transmission signal being detected.

4. A antenna system as claimed in Claim 3 wherein the said detecting means comprises a carrier sensing means for sensing radio frequency transmission signals on a Wireless Network Interface Card of the said transceiver, and a switching current generator for switching the said switching means in response to a detected transmission signal from the said transmitter.

5. An antenna system as claimed in any preceding Claim wherein the Effective Isotropic Radiated Power (EIRP) of the said second antenna is less than 100 mW.

6. An antenna system as claimed in any preceding Claim wherein the said first antenna is a high gain antenna selected from the group comprising: an omnidirectional antenna, a sector antenna, a horn antenna, a yagi antenna, or a parabolic antenna.

7. An antenna system for a wireless transceiver network node; the said system comprising: a common transmit and receive antenna for a receiver and a transmitter of a wireless transceiver; and, an attenuator connected between the said antenna and the said receiver and transmitter, the said attenuator having asymmetric attenuation properties such that transmission signals from the said transmitter means to the antenna are attenuated greater than received signals from the antenna to the said receiver means.

8. A system as claimed in Claim 7 wherein the said attenuator comprises a unidirectional circuit which attenuates transmission signals in the transmission direction only.

9. A system as claimed in Claim 8 wherein the said attenuator is selected from the group comprising: a biased switching diode circuit, an unswitched pre-amplifier circuit optionally including variable attenuation, a phase balanced or un-balanced network circuit, a circulator circuit, or an isolator.

10. A system as claimed in any one of claims 7 to 9 wherein the said attenuator is positioned in series in an antenna feed line for connecting the said antenna to the said transceiver.

11. An antenna system for a wireless transceiver network node; the said system comprising: a common transmit and receive antenna for a receiver and a transmitter of a wireless transceiver; the said antenna being connectable to the said receiver by a first connection path and to the said transmitter by a second connection path; the second path having an attenuator for attenuating transmission signals from the said transmitter to the antenna; switching means for connecting either the said receiver to the said antenna by the said first path or the or the said transmitter to the said antenna by the said second path; and radio signal detecting means for detecting transmission signals from the said transmitter and for switching the said switching means in response to a transmission signal being detected.

12. An antenna system as claimed in Claim 11 wherein the said switching means comprises a PIN diode switch, a FET switch or an electromagnetic relay.

13. A antenna system as claimed in Claim 11 wherein the said detecting means comprises a carrier sensing means for sensing radio frequency transmission signals on a Wireless Network Interface Card of the said transceiver, and a switching current generator for switching the said switching means in response to a detected transmission signal from the said transmitter.

14. An antenna system as claimed in any one of Claims 11 to 13 wherein the Effective Isotropic Radiated Power (EIRP) of the said second antenna is less than 100 mW.

15. An antenna system as claimed in any one of Claims 11 to 14 wherein the said first antenna is a high gain antenna selected from the group comprising: an omnidirectional antenna, a sector antenna, a horn antenna, a yagi antenna, or a parabolic antenna.

16. An antenna system as claimed in any one of Claims 11 to 15 wherein the said first path comprises an amplifier.

17. Apparatus for coupling two or more antennas comprising: a first antenna and a second antenna; the said first antenna having a higher gain than the said second antenna; the said second antenna being electrically connected to and integrated with a radio transceiver and positioned with respect to the said first antenna such that electromagnetic coupling of the electromagnetic fields of said first and second antenna is optimised to increase the range of the said integrated antenna.

18. An antenna system as claimed in Claim 17 wherein the Effective Isotropic Radiated Power (EIRP) of the said second antenna is less than 100 mW.

19. An antenna system as claimed in Claim 18 wherein the said first antenna is a high gain antenna selected from the group comprising: an omnidirectional antenna, a sector antenna, a horn antenna, a yagi antenna, or a parabolic antenna.

20. A method of coupling at least two antennas including a first antenna having a feed line connected to an antenna circuit capable of electromagnetically coupling the said first antenna to an integrated second antenna electrically connected to and integrated with a radio transceiver; the said method comprising the step of: positioning the said antenna circuit in the electromagnetic field of the said second antenna.