Some telecommunications systems operate at different frequencies. A telecommunications device for use with such systems therefore needs be to able to receive and transmit signals at those different frequencies. However, an antenna is normally only active, i. e. resonates, at a particular frequency thereby limiting a device including such an antenna to operation at that single frequency.

One solution to this problem would be to provide a communications device with multiple antennae, each of which is active at a particular frequency. However, this approach involves increased cost and complexity. A conventional single antenna may have a number of resonant modes at different frequencies, but. only a one of those modes may generate a beam shape suitable for use in a telecommunications system. Therefore there is a need for a simple antenna which produces a suitable beam shape for telecommunications at the difference frequencies utilized by the telecommunications system.

According to a first aspect of the present invention, there is provided an antenna for a telecommunications device, comprising at least three helical radiating elements substantially equally spaced about, and concentric to, a longitudinal axis of the antenna, in which the antenna has a different physical property along the radiating elements such that when an electrical signal is supplied to each

radiating element, with the phase relationship between each electrical signal being 360 degrees divided by the number of radiating elements, the antenna resonates at at least two different frequencies.

The use of at least three radiating elements having a helical form and'equiangularly displaced about a common central axis and fed with separate suitably phased electrical signals allows a beam shape suitable for a telecommunications applications to be generated. By configuring the multi-filar antenna with a physical property that alters whilst traversing the path of the radiating elements, the antenna can be made to resonate at at least two different frequencies with a beam shape suitable for telecommunications application. This provides a single physical antenna which is operable at at least two different frequencies thereby providing the multi-band performance required in a telecommunications system.

Preferably the antenna includes a feed for supplying the electrical signals to the radiating elements.

The radiating elements may extend along the longitudinal axis of the antenna. This helps to improve the radiation beam shape generated by the antenna such that it is suitable for a particular application.

The antenna can have more than three radiating elements.

Preferably the antenna has four radiating elements. In the latter case, the feed is arranged to provide electrical signals with a relative phase difference of 90° between adjacent radiating elements in sequence.

Preferably, the feed is configured to provide electrical signals in which the phase relationship between the signals applied to adjacent radiating elements in sequence corresponds to the sense of the helical shape of the radiating elements. For a right handed helical shape, the phase of the electrical signals can increase in a clockwise sense when viewed from the fed end toward the opposite, free end. For a left hand helical shape, the phase of electrical signals increases in an anti-clockwise sense when viewed from the fed end toward the free end. Matching the phase relationship to the sense of helix ensures an optimum beam shape is generated.

Preferably, the antenna is configured such that the resonant modes resonate at gigahertz frequencies.

Preferably the ends of the radiating elements at a free end of the antenna are connected together. Connecting the free ends of the radiating elements together improves the polarisation of the radiation. It also improves the mechanical strength of the antenna.

The diameter of the helix can be the physical property that varies.

Preferably, the pitch of the helix is the physical property of the antenna that varies. The antenna can have a pitch that varies along the entire length of the antenna. The antenna can have sections of substantially constant pitch.

The length of radiating element in each section can vary.

Preferably, the antenna includes a section of substantially constant pitch helix corresponding to each resonant mode of the antenna.

The antenna can include tap elements which connect the radiating elements to a ground plane. Such tap elements improve the voltage standing wave ratio (VSWR) for the feed to the antenna at the resonant frequencies thereby improving the performance of the antenna. The antenna can have tap elements interior to the body of the antenna. The tap elements can be fed and the radiating elements can be connected directly to a ground plane.

Preferably, the antenna includes a dielectric medium between the radiating elements. The dielectric medium can provide a support structure for the antenna. The antenna can be, encapsulated in a dielectric medium. The dielectric medium can be a foam. A cover can be provided for the antenna.

Preferably, the dielectric medium has a relative permittivity in the range of substantially 2 to 10. The dielectric medium can be a plastics material. The dielectric medium can be resilient. The dielectric medium can be a ceramic material. The dielectric material can be loaded with high relative permittivity particles.

Preferably, the radiating elements have a section with a length capable of sustaining the resonant mode for each resonant mode supported by the antenna. The length of the section can be an integer fraction of the wavelength of the resonant mode generated by that section. Most preferably, each section has a length of substantially one half the wavelength of the resonant mode. The length of section can depend on the dielectric medium in which the radiating element is located.

According to a second aspect of the invention there is provided a telecommunications device including an antenna

according to the first aspect of the invention. Preferably, the device is a satellite communications device. This allows the telecommunications device to be used in a telecommunications system in which messages are transferred via satellites. The device can be a mobile telephone.

An embodiment of the invention will now be described in detail, by way of example only, and with reference to the accompanying drawings, in which: Figure 1 shows a schematic illustration of an antenna according to the present invention; Figure 2 shows a schematic diagram of the radiating elements of an antenna including tap elements; Figure 3 shows a schematic illustrations of suitable feed means for use with the antenna; Figures 4a and b show graphs illustrating the beam shape generated by an antenna according to the invention operating at 1.62GHz and 2.48GHz respectively; and Figure 5 illustrates a prototype antenna according to the present invention.

Similar items in different figures share common reference numerals, unless indicated otherwise.

With reference to figure 1, there is shown an antenna 100 including a radiating part 110 and a feed part 150. The radiating antenna part 110 consists of four coaxial, helical, conducting, wires which provide radiating elements 112,113,114,115. The four radiating elements are each angularly displaced by 90° around a common longitudinal axis extending along the length of the antenna. The four

radiating elements are fed at a fed end with identical voltage signals differing only by a sequential 90° phase shift.

The sense of the phase shift of the electrical signals matches the sense of rotation of the helical shape of the radiating elements. In figure 1, the antenna has a left handed helical shape and so the radiating elements are fed with signals having relative phase shifts of 0,-90,-180 and-270° respectively.

The antenna element 110 has a first section 120 and a second section 122. The radiating elements have a constant pitch over the first section and a smaller constant pitch over the second section 122. The radiating elements in the first and second sections each rotate through approximately one and one quarter revolutions. The free ends of the radiating elements at the distal free end 124 of the antenna element are connected together by short terminating sections of wire. The length of the radiating elements in the first and second sections are each substantially one half the wavelength of the resonance sustained by the respective sections.

Figure 2 shows the antenna element 110 of figure 1 in more detail and includes tap members 126 by which the antenna element is connected to a ground plane 128. Each radiating element 112,113,114,115 is provided with a conducting tap element 126 by which the radiating element is connected to electrical ground. The radiating elements themselves are fed from below as indicated in figure 1. The tap elements follow the path of the helix of the radiating elements to

which they are connected but with an increasing diameter in the direction toward the ground plane.

As an alternative to the tap element configuration shown in Figure 2, the tap elements can be located within the antenna body defined by the radiating elements and connected to the electrical feed to the antenna. The parts of the radiating elements'below the tap point are connected directly to the ground plane instead of. being fed. The tap elements then provide the feed to the antenna, rather than the bottom parts of the radiating elements.

Suitable dimensions for a dual-band, end-fire quadrifilar helix antenna with an air dielectric and resonating at 1.62GHz and 2.48GHz are as follows. The radiating elements comprise copper wire with a radius of 0.00089m and are formed into a helix with a radius of 0.007m. The first section has 1.25 turns over a length of 0.085m along the longitudinal axis and the second section 122 has the same number of turns over a length of 0.035m along the longitudinal axis.

In the first section, the length of radiating element is 0.1012m corresponding to a frequency of 1.48GHz, which is substantially equivalent to 1.62GHz, and the radiating elements in the second section 122 have a length of 0.0652m corresponding to 2.3GHz which is substantially equivalent to 2.48 gigahertz. The length of radiating elements is slightly longer than the length exactly equivalent to the resonant mode for separate resonators. The spacing between turns in the first section is 0.068m and the spacing between turns in the second section is 0.028m.

The tap elements are made from the same wire and are formed into 0.25 of a turn of a helix with a diameter of 0.007m where they tap into the radiating elements and a radius of 0.0105m where they connect to the ground plane. Each tap element extends over 0.017m along the longitudinal axis of the antenna element and each has a length of 0.0217m. The spacing between the turns of the tap elements is the same as for the first section.

The feed circuitry 150 provides four voltage signals to the radiating elements with the necessary phase relationship.

Figure 1 illustrates a schematic block diagram illustrating the phasing characteristic required. An input signal 155 is split into two signals by a hybrid power divider introducing a phase shift of 180° and the split signals are passed through further hybrid power dividers to provide further split signals each with a relative phase shift of 90°. The four voltage signals 156,157,158 and 159 are then supplied to the fed ends of the radiating elements in the appropriate sense.

For a left handed helix, the phase of the signals increases by 90° in an anti-clockwise direction when viewed in the direction from the fed end to the free end of the antenna.

For a right handed helix the phase increases by 90° in a clockwise direction when viewed in the same manner. This provides a forward fire (end fire) radiation mode for the antenna generating a circularly polarised radiation beam extending over substantially +/-60° with respect to the longitudinal axis of the antenna. The radiation pattern has a single lobe over its forward direction therefore providing substantially uniform coverage over the entire solid angle of the forward radiation beam pattern.

Figure 3 shows an illustration of a suitable embodiment of a part of the feed 150. The circuitry 150 consists of transmission lines deposited on a substrate with a high relative permittivity, e. g. Cr = 10.2. A connector 156 is provided by which signal 155 is supplied to the feed. The feed circuitry includes a rat race 157. A first transmission line 158 taps the rat race at a A/4 section of transmission line from the signal feed point 159. A hybrid provides a phase shifting device 160 and is connected to ground by resistor 161 and provides substantially 0° and 90° phase outputs 162, 163. A further resistor 164 connects a further A/4 section of the rat race to ground and a second transmission line 165 further taps rat race 157 at a further X/4 section of transmission line. A second transmission line 165 is connected to a second hybrid 166 connected to ground by resistor 167 and providing outputs 168 and 169 at phases of substantially 180° and 270° relative to output 162.

It will be appreciated that the embodiment of the feed circuit shown in figure 3 will require a separate feed circuit for each resonant mode of the antenna as the feed circuit is frequency dependent. This can easily be achieved by a simple switching mechanism. Alternative suitable feed circuits to provide the required voltage signal splitting and phasing for multiple frequencies are considered to be within the ability of a man of ordinary skill in this art and so have not been described further.

With regard to figures 4 and 5, there is shown a prototype telecommunications device handset 500 including an antenna 100 according to the present invention. Figures 4a and 4b respectively illustrate the radiation beam pattern measured for the antenna at frequencies of 1.62GHz and 2.48GHz

respectively. The ordinate indicates the axial ratio of the radiation beam with a value of 1 indicating a perfectly circularly polarised radiation beam. The abscissa indicates the angle away from the longitudinal axis of the antenna.

As can be seen a substantially uniform radiation beam is generated by the antenna over an angular spread suitable for telecommunications application, in particular for satellite telecommunications systems.

The prototype antenna 100 includes a central acrylic rod with radial struts distributed along its length at each turn providing a support for the antenna elements. The acrylic rod provides a minimally distorting dielectric support for the antenna.

A structural support can instead, or also, be provided by a structural foam. This provides resilience to the antenna.

A protective cap could also be provided with or without an internal supporting structure. In figure 5, the radiating elements are in an air dielectric. It will be understood by a man of skill in the art that by locating the radiating elements in a dielectric environment with a high dielectric constant, the physical dimensions of the radiating elements will need to be reduced accordingly to provide the correct resonant frequencies required of the antenna.

A particularly advantageous form of the antenna would have the radiating elements encapsulated in a homogeneous plastics dielectric with a high relative permittivity. A plastics material can be loaded with high relative permittivity particles to increase its effective overall relative permittivity.

The tap elements 126 connecting the antenna elements to the ground plane 128 help minimise the VSWR at the feeds to the radiating elements at both 1.6 and 2.48 gigahertz. The function of the tap elements can be understood by analogy with a split gamma-match feed for a half wave dipole grounded antenna. However, in this case, the radiating antenna elements are fed at a point not connected to ground and the tap elements tap into the radiating elements above their feed point.

In the alternative tap element construction described previously, the tap elements would be fed and provide the feed to the radiating elements, while the radiating elements below the tap point are connected directly to the ground plane.

The effect of the tap elements can be understood in terms of the tap elements and radiating elements above them acting as the radiating antenna while the parts of the radiating elements below the tap elements help improve the impedance match between the rest of the radiating elements and the feed to them. The tap elements alter the frequency of both the resonant modes of the antenna, compared to the case in which they are not present. However, by adjusting the parameters of the antenna suitable performance can be established and the tap elements improve the VSWR for the radiating elements thereby improving the overall performance of the antenna.

The radiation beam pattern generated by the helical antenna is dependent on a number of factors including the number of turns of radiating element in each section. A number of turns between 1 and 1.5 have been found to provide a

suitable radiation beam pattern and substantially 1.25 turns have been found to provide an improved radiation beam pattern. In the absence of any turns, the antenna would be a simple dipole with its characteristic radiation pattern, while with too many turns, the antenna is essentially a coil which radiates in a direction substantially perpendicular to the longitudinal axis of the antenna and so does not provide a suitable radiation pattern for some telecommunications applications.

Providing a second section of helical antenna with an increased pitch helps to cause the second resonant mode of the antenna to also be a substantially single lobe forward radiation pattern suitable for telecommunications purposes.

Although the antenna has been described in the case of a quadrifilar helix, it will be appreciated that a trifilar helix, supplied with voltages at relative phases of 0,120 and 240°, would also be suitable. Antenna with higher order multifilar helices, e. g. 5 and upwards, and with a corresponding change to the relative phasing of the voltage signals for each radiating element, area also considered to fall within the ambit of this invention.

Although the embodiment described relates to a dual band antenna, the invention is also considered to encompass triband, and higher, antennas in which three or more physically distinct sections of the radiating elements of the antenna are provided so as to provide three resonant modes of the antenna at three separate frequencies. Again, this principle can be extended to cover higher numbers of frequency bands.