GB2282028A

GB2282028A - Cordless telephone with loop antenna

Info

Publication number: GB2282028A
Application number: GB9317811A
Authority: GB
Inventors: Kenneth Ashcroft; David William Knight
Original assignee: BETACOM PLC
Current assignee: BETACOM PLC
Priority date: 1993-08-26
Filing date: 1993-08-26
Publication date: 1995-03-22
Also published as: GB9317811D0

Abstract

In a cordless telephone system, the transmitting antenna (1) of a base unit and the transmitting antenna (3) of a handset are each in the form of a loop antenna. The loop may be formed from a number of turns of wire and tuned by a capacitor 5 via an externally mounted knob. An LED 7 is provided to indicate when the circuit is tuned. <IMAGE>

Description

CORDLESS TELEPHONE SYSTEMS The present invention relates to cordless telephone systems.

Cordless telephone systems are commonly used to allow the user to communicate with the local telephone system by means of two-way radio communication units. Such systems typically consist of a base unit directly connected to a telephone line and having a transmitter-receiver unit therein configured for two-channel duplex operation. A battery-powered portable handset is also provided, similarly having a duplex transmitter-receiver therein.

Each unit is provided with an associated transmitting/receiving antenna or transmitting and receiving antennae.

One form of cordless telephone system uses an MF radio link from the base unit to the handset, operated on one of a group of 8 channels near 1.7MHz, and a VHF link from the handset to the base unit operated on one of a group of 8 channels near 47MHz. Narrow band frequency or phase modulation is used, with the audio signal transmitted in simple analogue form. Data exchanges between the handset and base unit normally make use of the audio sub-band.

Both transmitters are limited to a maximum effective carrier power of 10mW, while the receivers are limited to a maximum sensitivity of lpV for a signal to noise ratio of 12dB. The maximum range of such a link, seldom achieved in practice, is about 200m.

Normally, short whip antennae are used for the 47MHz transmitter and receiver. The 1.7MHz handset receiving antenna is a ferrite rod and the 1.7MHz transmitter antenna is a length of wire hanging out of the back of the base unit. The overall arrangement is untidy and, particularly in the case of the 1.7MHz transmitter, susceptible to environmental factors which lead to a high rate of customer complaints. The 47MHz whip antennae are inconvenient, occasionally cause physical injury and are frequently broken.

US-A-4 876 709 discloses a cordless telephone system in which a transmitting/receiving antenna in the handset is in the form of a plate-form antenna, in which system the base unit may also include such a plate form transmitting/receiving antenna.

According to the present invention, there is provided a cordless telephone system comprising a base unit and a handset, in which the transmitting antenna of the base unit comprises a loop antenna and/or the transmitting antenna of the handset comprises a loop antenna.

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Fig. 1 is a block diagram of the circuitry of the base unit of one example of a cordless telephone system according to the present invention; Fig. 2 is a block diagram of the circuitry of the handset of the system; Fig. 3 is a view of the underneath of the base unit; Fig. 4 is a view of the underneath of the handset; and Fig. 5 is a side view of the handset.

The example of the present invention comprises a modified form of a cordless telephone system called Venture XT of Betacom plc. In such a system, a base unit connected to a telephone line transmits to a handset via an MF radio link on one of a group of 8 channels near 1.7MHz, the transmitting antenna of the base unit being a length of wire hanging out of the back of the base unit and the receiving antenna of the handset being a ferrite rod; and the handset communicates with the base unit via a VHF radio link on one of a group of 8 channels near 47MHz, the transmitting antenna of the handset and the receiving antenna of the base unit each being a respective whip antenna. According to the example of the present invention, each of the transmitting antenna of the base unit and the transmitting antenna of handset is a respective loop antenna.

Referring first to Fig. 1 of the accompanying drawings, reference numeral 1 designates a wire loop (frame) transmitting antenna of the base unit and reference numeral 2 designates a whip receiving antenna of the base unit. The remaining circuitry is essentially standard circuitry of a Venture XT cordless telephone system base unit and will not be described in detail. The loop antenna 1 is contained within the casing of the base unit.

Referring to Fig. 2, reference numeral 3 designates a wire loop (frame) transmitting antenna of the handset and reference numeral 3 designates a ferrite rod receiving antenna of the handset. The remaining circuitry is essentially standard circuitry of a Venture XT cordless telephone system handset and will not be described in detail.

In the base unit, the loop antenna 1 is in the form of a transformer with, by way of example, the following parameters: Dimensions (cm): 11 x 9 Area (m2): 0.010 Tuned winding turns: 10 Tuning capacitor (pF) 316 Link turns: 1 Input impedance at resonance (Q): 100 The above parameters may be different and the antenna 1 may be configured differently from a transformer, for example the loop of the antenna could be a series-tuned loop, a parallel-tuned loop, in the form of an autotransformer or in a w-network.

The base unit is provided by modifying a standard Venture XT base unit by complete removal of the mains transformer and by bringing the main (nominally llV) supply rail out to a 2.lmm power jack at the rear of the case. The loop antenna secondary winding has no earth connection. The loop tuning capacitor is brought out to a knob on the side of the case, with an LED next to it. The LED is at its brightest when the loop is at resonance. A 100pF silvered mica fixed capacitor is placed across the tuning capacitor in order to prevent the loop from being tuned to the second harmonic of the transmitter output.

The 47MHz receiving whip antenna may be removed along with isolating capacitor and loading coil, the received signal being obtained by sensing between the line cord and the DC input lead. The loop is positioned around the area vacated by the main transformer, overlapping the main circuit board in the area of the power supply and the VHF receiver RF and IF signal chain.

In the handset, by way of example only, the antenna 3 is a 50 x 47mm loop of 18swg wire, mounted directly behind the earpiece transducer and spaced about 10mum away from the main circuit board. This is tuned by means of a 20pF preset capacitor. Output from the 47MHz final amplifier is injected via a tap at 1/2 turn from the earthed end of the loop. The 1.7MHz receiver ferrite rod antenna 4 may be mounted vertically, in order to make its sensitivity omnidirectional.

In Fig. 3 reference numeral 5 designates the tuning capacitor; reference numeral 6 designates tuning indication circuitry; and reference numeral 7 designates the tuning indication LED.

In Figs. 4 and 5, reference numeral 8 designates the battery of the handset; reference numeral 9 designates the microphone of the handset; reference numeral 10 designates the earpiece of the handset; and reference numeral 11 designates the tuning capacitor for the loop antenna 3.

Loop antennae are, of themselves, known (see, for example, R.E. Schemel "The loop aerial revived", Wireless World, July 1979, pages 48-52) but their major use has been as receiving antennae. Also, although it is well known that the voltage available at the terminals of a receiving loop antenna is magnified in proportion to the Q of loop as a tuned circuit, what appears not to have been noted in the past in that the Q of the loop makes a contribution to the usefulness of the loop as a transmitting antenna. The present invention utilises this to advantage by using a loop antenna as a transmitting antenna in a cordless telephone system.

Claims

1. A cordless telephone system comprising a base unit and a handset, in which the transmitting antenna of the base unit comprises a loop antenna and/or the transmitting antenna of the handset comprises a loop antenna.

2. A system according to claim 1, wherein each of said transmitting antennae comprises a loop antenna.

3. A system according to claim 1 in which the transmitting antenna of the base unit comprises a loop antenna, or a system according to claim 2, wherein the transmitting antenna of the base unit is a loop antenna in the form of a transformer.

4. A cordless telephone system, substantially as herein described with reference to the accompanying drawings.