GB2157131A - Cordless telephone - Google Patents

Abstract

A cordless telephone comprises a base set 1 and a handset 2. Radio transmissions from base set 1 to handset 2 are made directly between aerials 9,12 mounted on the base set 1 and handset 2 respectively. For transmission in the other direction, however, a leaky feeder 7 is provided, coupled to the base set receiver 4. Radio transmissions emitted by a second aerial 10 of the handset 2 in the vicinity of the leaky feeder 7, are picked up and carried to the base set 1 by the leaky feeder 7. Embodiments of the invention find particular application in crowded office environments. <IMAGE>

Description

SPECIFICATION Cordless telephone The present invention relates to mobile communication apparatus and in particular to cordless telephones.

Cordless telephones may comprise a base set and a handset. The base set may be connected, for instance by means of a wire pair, into the public telephone network. It is therefore fixed in one location. The handset however is an independently portable unit, communicating with the base set by means of air transmitted energy such as radio waves.

If a telephone call is directed to, or to be made from, the cordless telephone, the base set cooperates with the public telephone network in a conventional manner. Communication between the base set and the handset however may be carried out by means of modulation, for instance fre quency modulation, of a radio carrier wave. Known cordless telephones operate using two different frequencies of carrier wave: a first in transmitting from base set to handset, and a second in trans mitting from handset to base set. The particular frequencies of carrier wave used are constrained by law in Great Britain to avoid interference with, for example, other users such as the emergency services. Carrier frequencies which comply with the legal requirements lie within the ranges 1.5 to 2.0 MHz and 45.0 to 50.0 MHz.The two frequencies used in a cordless telephone are selected as a pair, one frequency lying in each of those ranges.

In order for communication to take place between the handset and base set, each is equipped with a receiver and a transmitter. Each may also be further equipped with two separate aerials: one for use in transmission of radio waves from the base set to the handset, and the other for use in trans mission of radio waves from the handset to the base set.

it has been found, using the above equipment in a building, that the range between base set and hand set over which communication is acceptable can be extremely limited. In some environments it may be less than 20m. The range of communication cannot however simply be increased by in creasing transmitter power since that is limited by the Radio Regulatory Department of the Depart ment of Trade and Industry in order to avoid interference with other equipment.

It is an object of the present invention to provide a cordless telephone which can be used over an increased range between handset and base set.

The present invention provides a cordless telephone having a base set and a handset, the receiver of the base set having a leaky feeder coupled thereto such that communication energy emitted by the handset may be transmitted to the receiver via the leaky feeder.

Various types of leaky feeder are known and a number are described in the following publication: "Radio and Electronic Engineer; volume 45, num ber 5, May 1975". They can generally be described as imperfect transmission lines which set up electro-magnetic fields in their vicinity, along their length. They have the characteristic, in air , of being able to receive and transmit radio frequency electromagnetic energy from and to the surrounding air at all points along their length, that energy being able to propagate along the leaky feeder.

The leaky feeder employed with the present invention may be optimised for reception at the shorter wavelength used between handset and base set where transmission in the other direction is by way of direct broadcast.

Embodiments of the present invention have the advantage that signal amplification can be conveniently provided between the handset and the base set by the connection of one or more amplifiers to the leaky feeder. This allows enhancement of the communication power received at the base set from the handset, again without there being an increase in transmitter power.

A cordless telephone according to an embodiment of the invention will now be described, with reference to the accompanying drawings in which: Figure 1 represents the cordless telephone in block form.

Referring to Figure 1, the cordless telephone comprises a base set 1 and a handset 2. The base set 1 is connected to the public telephone network via an interface unit 8 and is adapted to communicate with the handset 2 by means of radio waves.

The base set 1 also includes a control unit 11 for the control of signalling and the like.

Cordless telephones of this general type are known and details of the interface unit 8 and control unit 11 are therefore not described.

The base set 1 is equipped with a transmitter 3 and a receiver 4. The transmitter 3 has an associated aerial 9 by means of which radio transmissions based on carrier frequency 1.7 MHz may be radiated, in known manner into the surrounding air. The receiver 4 has coupled to it a leaky feeder 7 such that electromagnetic energy carried by the feeder 7 is fed to the receiver 4.

The handset 2 is also equipped with a transmitter 5 and a receiver 6. The handset transmitter 5 is adapted to emit radio signals based on carrier frequency 47.5 MHz, in known manner, into the surrounding air. The handset receiver 6 is adapted to receive radio signals based on carrier frequency 1.7 MHz. The transmitter 5 has an associated aerial 10, external to the handset 2, which may be for instance of telescopic or helical type. The receiver 6 has an associated aerial 12 which may be internal to the handset 2 amd may comprise for instance a ferrite rod. (The frequencies quoted above are nominal not actual, being given to an accuracy of within 0.1 MHz.) For use, the base set 1 is installed in a building, in connection with the public telephone network, and the feeder 7 is distributed around the building in a convenient manner.The feeder 7 is arranged so that the handset 2 can be brought within for example 5m of the cable at any location where it is likely to be necessary to transmit signals from the handset 2 to the base set 1.

If a telephone call is directed to, or made from, the cordless telephone, communication between the base set 1 and the hand set 2 is made in two different ways. From the base set 1 to the handset 2, signals are transmitted as modulation of an airborne radio carrier wave at frequencyl.7MHz, from the base set aerial 9 to the handset receiver aerial 12. In the opposing direction, from the handset 2 to the base set 1, signals are transmitted as modulation of a radio carrier wave at frequency 47.5 MHz, from the handset transmitter aerial 10, into the leaky feeder 7, and along the feeder 7 to the base set receiver 4.

The following rough comparison can be made regarding signal losses between a handset and a base set in a building, at 47.5 MHz, using firstly a coaxial cable leaky feeder as above and secondly direct aerial to aerial transmission. If the handset transmitter 5 emits a 10 mW signal, an acceptable maximum power loss between it and the base set receiver 4 is 120 dB. Where the leaky feeder 7 is used, and the handset 2 is positioned 5m from the feeder 7, a power loss in the signal path from the transmitter 5 and into the feeder 7 of about 70 dB can occur.

Once in the leaky feeder 7, however, the signal suffers a power loss of only about 5 dB per 100m.

This gives a range between the handset 2 and base set 1, for a total power loss of 120 dB, of up to 1 km. Where direct aerial to aerial transmission is employed, a power loss of about 20 dB occurs at the aerial to air interfaces. The remaining 100 dB loss of the acceptable total has been found to occur, in a building, over a distance of about 20m.

The range between the handset and the base set in this case is therefore only 20m, compared with the 1 km range where the leaky feeder 7 is employed.

It should be noted that the range of 20m stated is dependent on the structure and design of the building concerned and may vary considerably.

However,even line of sight transmission out of doors only gives a range of about 300m between the handset and the base set which is still significantly low when compared with the 1 km quoted above.

If one or more amplifiers are connected into the leaky feeder then the range can be increased still further, to an extent determined by the arrangement of amplifiers selected.

It will be clear from the reference quoted above that a leaky feeder suitable for use in embodiments of the invention may vary considerably in structure. For instance a coaxial cable with a perforated screen, such as either a loosely woven braided screen, or a screen comprising a continuous metal tape with holes punched through it, may be used.

Another suitable form of leaky feeder is a twin pair of wires, embedded in a dielectric.

Although in the embodiment described the feeder is arranged so that the handset 2 can be brought within 5m of it in order to transmit to the base set 1, this distance may be greater, or may even be less. It will be determined, as will be clear to one skilled in the art, by a number of factors such as the power of the handset transmitter, or whether or not there are amplifiers connected into the leaky feeder 7.

Although only one base set is mentioned in the embodiment described above, it may be convenient to connect more than one base set to a common leaky feeder.

Signals from the base set to the handset of a cordless telephone may also be transmitted via the leaky feeder to a handset receiver aerial. Due to the characteristics of leaky feeders, however, it is likely that the radio wave carrier frequency used in such transmissions would have to be increased from the 1.7 MHz quoted in the above embodiment. An optimal frequency in the case of coaxial cable leaky feeders has been found to be about 30 MHz.

A further advantage of embodiments such as the one described with reference to Figure 1, using aerial to aerial transmission from the base set ot the handset, is that in practice the presence of the leaky feeder 7 tends to lead to an improvement in the quality of signal transmission from the base set aerial 9 to the handset 2.

Claims (8)

1. A cordless telephone having a base set and a handset, the handset having an aerial and the receiver of the base set having a leaky feeder coupled thereto such that signals emitted by the handset aerial may be transmitted to said receiver via the leaky feeder.

2. A cordless telephone according to claim 1 wherein the leaky feeder is a coaxial cable, having a perforated screen.

3. A cordless telepone according to claim 1 wherein the leaky feeder is a twin pair of wires, embedded in a dielectric.

4. A cordless telephone according to any preceding claim wherein the base set is provided with an aerial and the handset is provided with two aerials, signals emitted by the base set aerial being received at the handset via one of the handset aerials.

5. A cordless telephone according to any preceding claim wherein the signals comprise modulated radio carrier waves.

6. A cordless telephone according to claim 5 wherein signals emitted by the handset have a carrier frequency lying in the range 45 MHz to 50 MHz.

7. A cordless telephone according to claims 5 or 6 wherein signals emitted via the base set aerial have a carrier frequency lying in the range 1.5 MHz to 2.0 MHz.

8. A cordless telephone substantially as described herein with reference to the accompanying Figure.