BACKGROUND OF THE INVENTION Mobile communications are carried out using portable or mobile radio communications units known in the art as mobile stations' (e. g. mobile or portable telephones, radios and the like) which include (amongst other things) a transmitter to convert messages or information of a user input in the form of speech, text data or <BR> <BR> visual images etc. , into radio frequency (r. f. ) signals for transmission to a distant receiver, and a receiver to convert received RF signals from a distant transmitter back into information which can be understood by the user. Many components of the transmitter and receiver are common components usually forming a single transceiver unit.

In a mobile station, the function of sending and receiving an r. f. signal via an air interface to and from a distant transceiver is carried out by a component referred to in the art as an antenna or aerial. In general, an antenna is a device which converts an electrical signal oscillating at r. f. frequency into a radiated electromagnetic energy signal and vice versa.

In modern mobile communications, such as using digital technology, the r. f. signals generally have a high frequency, e. g. above 10MHz. For example, for systems operating according to TETRA standard procedures, the operating frequency is in one of a series of the bands in the range from 380 MHz to 470 MHz. TETRA (Terrestrial Trunked Radio) is a set of standards defined by the European Telecommunications Standards Institute (ETSI).

It is known to use a GPS (global positioning system) receiver in conjunction with a mobile station to provide information for use by the station relating to the current location of the station. Such receivers operate at a frequency of 1575.42MHz, which corresponds to a wavelength of about 19cm.

Currently, the GPS antennae employed for use with mobile stations are internal low gain antennae which are inferior in performance. Dual band antenna solutions are known but these do not provide a performance comparable with that of a separate dedicated GPS antenna at GPS frequencies. Furthermore the dual band solutions which have been proposed are complex and costly to implement.

Thus there is a need for an improved antenna construction for use in a mobile station which can provide operation at both UHF, e. g. at TETRA, and GPS frequencies.

SUMMARY OF THE PRESENT INVENTION According to the present invention in a first aspect there is provided an antenna structure for use in at least two different r. f. operational frequency bands in a radio communication device, the antenna comprising (i)

an elongate conducting radiator; (ii) a feedline including a signal conductor electrically connected to the elongate conducting radiator and a screening conductor; and (iii) a conducting member adjacent to a portion of the elongate conducting radiator and a portion of the feedline connected thereto, the conducting member providing a required r. f. impedance between the feedline and the elongate conducting member; wherein the elongate conducting radiator provides a monopole antenna having an electrical length of about a quarter wave or less in a first r. f. operational frequency band and a half wave dipole antenna in a second r. f. operational frequency band.

In the antenna structure according to the first aspect the feedline preferably comprises a coaxial connector.

The screening conductor may in use be connected to a ground plane conductor of a mobile station. The signal conductor may be coupled to a (signal r. f. processing section of a transceiver of the mobile station.

In the antenna according to the first aspect of the invention, preferably the conducting member partly or fully encloses adjacent portions of the feedline and the elongate conducting radiator. The conducting member may comprise a curved plate or sleeve (hollow cylinder). The conducting member may contribute capacitively to an r. f. impedance to r. f. signals transmitted between the feedline and the elongate conducting radiator. The r. f. impedance may at the frequencies of the second frequency band be a standard impedance of approximately 50 ohms.

In a first alternative form of the first aspect of the invention, the conducting member is electrically isolated from the screening conductor. This form is

suitable for use where the antenna structure is to be connected to a mobile station having a body of short length, e. g. not greater than about 8cm.

In a second alternative form of the first aspect of the invention, the conducting member is electrically connected to the screening conductor to provide a balun.

The antenna according to the first aspect of the invention may operate in a first frequency band which is a frequency band in the range 380 MHz to 420 MHz, in particular a band suitable for TETRA operation, and in a second frequency band which includes the GPS frequency 1575 MHz, whereby a mobile station including the antenna structure is suitable for receiving GPS signals. The antenna may act as a radiator or receptor of r. f. signals in one or more frequency bands in the selected range 380 MHz to 470 MHz.

According to the present invention in a second aspect there is provided a mobile station for use in mobile radio communications which includes an antenna according to the first aspect.

An antenna construction for use in a mobile station for operation at UHF (e. g. 380 MHz to 470 MHz) and at the GPS frequency (1575 MHz) has to be large enough to receive the required GPS signals, yet small enough to be integrated in the mobile station. These two requirements are conflicting and have not been satisfactorily met in the prior art. The present invention unexpectedly and beneficially satisfies these requirements by converting an existing elongate conducting radiator known for use in a mobile station (and often referred to as a whip' antenna') to a novel structure which will radiate properly in both required frequency bands. The

construction can also provide excellent efficiency and gain properties in both required frequency bands, which in the case of the GPS band is in contrast to the relatively poor antenna performance obtained with prior art antennae. Furthermore, a specific connection of the antenna may be provided to a second antenna adapted specifically to act together with the first mentioned antenna as a receptor for circularly polarized radiation to further enhance performance at the GPS frequency.

This is achieved by arranging the two antennae to have mutually orthogonal operational axes.

Although the electrical length of the elongate conducting radiator in the antenna according to the invention is desirably a quarter wavelength at the mean frequency of the band of lower frequency, i. e. the first band (e. g. mean frequency 425 MHz), the antenna will still work well as a so called quarter wave antenna if the electrical length is less than a quarter wavelength, e. g. up to 50% less in electrical length than a quarter wavelength. A preferred length for the elongate conducting radiator is from 10cm to 14cm and a preferred length of the conducting member is from 2cm to 4cm.

According to the present invention in a second aspect there is provided a mobile station which includes the novel antenna according to the first aspect.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

FIG. 1 is a front elevation partly in schematic form of a mobile station incorporating an antenna structure embodying the invention.

FIG. 2 is an equivalent circuit diagram of the antenna structure shown in FIG. 1.

FIG. 3 is a graph of radiation intensity versus direction angle showing a radiation pattern obtained from the antenna structure shown in FIG. 1.

FIG. 4 is a front elevation partly in schematic form of a mobile station incorporating an alternative antenna structure embodying the invention.

FIG. 5 is a graph of radiation intensity versus direction angle showing a radiation pattern obtained from the antenna structure shown in FIG. 4.

FIG. 6 is a front elevation partly in schematic form of a mobile station incorporating an alternative antenna structure embodying the invention.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION As shown in Figure 1, an antenna construction 1 embodying the invention is operably connected to a body 3 of a mobile station (e. g. a mobile radiotelephone) comprising a transceiver for use in wireless communications. A coaxial feedline 5 comprising an inner conductor 5a, an outer conductor 5b and an insulating sleeve 5c separating the two connects between the body 3 and the antenna construction 1. In particular, the inner conductor 5a acts as a signal conductor and is connected to a r. f. processing section 3a of the transceiver of the mobile station. The outer conductor 5b acts as a screening conductor and is connected to a conducting

sheet 3c inside the mobile station body 3 which acts as a ground plane. The r. f. processing section 3a of the transceiver of the mobile station is connected to a baseband processing section 3b of the transceiver operating together in a generally known manner.

In the antenna structure 1, an elongate linear conductor 7 is electrically connected to the inner conductor 5a of the feedline 5 and extends along the axis of the feedline 5. A conducting sleeve 9 also having the same axis as the feedline 5 is fitted to cover an end portion 5d of the feedline 5. The sleeve 9 also covers an end portion 7a of the conductor 7 which is connected to the feedline 5. The sleeve is tapered so that its diameter at its end covering the portion 5d of the feedline 5 is greater than its diameter at its other end. The feedline 5 (outside the body 3), the conductor 7 and the sleeve 9 are encapsulated within a moulded insulating plastics case 11 which is tapered so that its diameter is reduced as it extends away from the body 3.

The sleeve 9 has an axial length (approx. 3cm) which is approximately one quarter that of the conductor 7 (approx. 12cm). During manufacture, the relative linear position of the sleeve 9 to the end portion 5d of the feedline 5 and the end portion 7a of the conductor 7 is finely adjusted (by theory and/or practice) to give the following required electrical properties.

The combination of the end portion 5d of the feedline 5 and the sleeve 9 covering it as indicated by a region of length A forms an equivalent electrical inductor and the combination of the end portion 7a of the conductor 7 and the sleeve 9 covering it as indicated by a region of length B forms an equivalent electrical capacitor.

The antenna construction 1 is adapted to provide a simple quarter wave antenna or radiator across the TETRA UHF bands (380-470 MHz, mean frequency 425MHz) ('TETRA frequencies') and a half wave antenna at the GPS frequency, namely 1575.42 MHz. The same antenna structure 1 is used for both of these different frequency applications, by adaptation of the electrical properties of the antenna structure 1 so that the correct feed impedance is respectively presented, especially at the GPS frequency.

The equivalent electrical circuit of the antenna construction 1 is shown in FIG. 2. The inner conductor 5a is connected to one end of an inductive portion L (formed in the region A of FIG. 1) which is connected at its other end to one end of the elongate conducting radiator 7 and to one end of a capacitive portion C (formed in the region A of FIG. 1). The combination of L and C transforms the a. c. impedance of the end fed dipole at 1575 MHz to about 50 ohm as required in industrial standards (the same input impedance is required at the lower frequency range 380 MHz to 480 MHz). The inductance and capacitance values of the equivalent components L and C are small enough at 1575 MHz so as not to affect the antenna monopole performance at the TETRA frequencies (380-470 MHz). In practice we have found that the actual effect on performance is negligible, with a resulting frequency shift less than 2% which is well within the bandwidth available.

FIG 3 shows the radiation pattern obtained with the antenna structure in so called elevation cut'at GPS frequency. The axis z corresponds to the axis of the body 3 of the mobile station shown in FIG. 1. The axis x

corresponds to an axis at 90 degrees to the z axis in the plane of the drawing in FIG. 1. The radiation pattern shows good all round coverage although there is some degradation in directions which are at an angle A from the z axis of 0,90, 180 and 270 degrees. In particular, the degradation at angles A of 90 and 270 degrees increases as the size of the mobile station body 5 increases (because the size of conducting components in the mobile station then increases). This problem can be avoided by use of the alternative construction shown in FIG. 4.

FIG. 4 shows an alternative form of antenna structure.

Components which are similar to those shown in FIG. 1 are given like reference numerals. In FIG. 4, the conducting sleeve 9 has an annular conducting flange 13 at its end which is nearer the body 3 which makes electrical contact with the outer conductor 5b of the feedline 5. The sleeve is thereby grounded to the conducting sheet 5c acting as ground plane of the mobile station via the outer screening conductor 5b. This arrangement forms a bazooka balun (quarter wave balun). A balun is a balanced-unbalanced transformer for a transmission line. The purpose of such a transformer is to convert a balanced signal (two conductors carrying opposite currents/voltages) to an unbalanced signal (current/voltage on a signal conductor with a separate ground conductor}, and vice versa. In the present case, the balun prevents significant current flow in the metal parts of the mobile station, e. g. on a printed circuit board or metal enclosure used in the construction of the mobile station or in the sheet 3c. The frequency of resonance of the balun arrangement shown in Fig. 4

depends on the relative permittivity of the plastics material between the sleeve 9 and the feedline 5.

Operation at GPS frequency may conveniently be obtained by selection and use of a plastics material having a relative permittivity of about 5.

FIG. 5 shows the radiation pattern obtained using the construction shown in FIG. 4. Beneficially, no degradation is seen at the direction angles A from the z axis of 90 degrees and 270 degrees.

In a further embodiment shown in FIG. 6, the antenna structure of FIG. 1 is modified by connection to a further antenna structure 15. In particular, the inner conductor 5 is connected at its inner end inside the mobile station body 3 to the further antenna structure 15. The further structure 15 may be one specifically for use with the antenna structure 1 to emit or receive radiation in circularly polarized form. This helps to enhance the reception performance at GPS frequency. The further antenna structure 15 is connected to the r. f. signal processing section 3a of the mobile station.

The further antenna structure and antenna 1 may together form an antenna arrangement as described in Applicant's copending International Application PCT/EP 03/50735 which may be summarized as follows. An antenna arrangement for radiating and/or receiving electromagnetic signals is operably coupled to a r. f. transmitter and/or a receiver, for transmitting/receiving a radio communication signal.

The antenna arrangement comprises an internal antenna (in this case the further antenna structure 15) located within the mobile station and an external antenna (in this case the antenna structure 1) located substantially

outside of the mobile station, such that both the internal antenna and the external antenna co-operate on substantially the same electromagnetic signal.

In this manner, by provision of both an internal and an external antenna the mobile station is able to function adequately should an antenna become disconnected, malfunction, or its performance suffer from impedance mismatching. Preferably, the internal and external antennae can be configured to be orthogonal to one another (i. e. having mutually orthogonal constructional axes), thereby providing the mobile station with the ability to operate with a substantially circular or elliptical polarization.