WO2002084799A1

WO2002084799A1 - Antenna

Info

Publication number: WO2002084799A1
Application number: PCT/GB2002/001714
Authority: WO
Inventors: Peter Stuart Excell; Raed Abd-Alhameed
Original assignee: University of Bradford
Current assignee: University of Bradford
Priority date: 2001-04-17
Filing date: 2002-04-17
Publication date: 2002-10-24
Anticipated expiration: 2003-10-17
Also published as: GB0109329D0

Abstract

A multi-filar spiral antenna for a telecommunications device. The antenna comprises at least two spiral arm radiating elements each having an outer end and an inner end. A feed is connected to each outer end. When an electrical signal is supplied to each respective feed with a respective phase step of substantially 360° divided by the number of spiral arms an electromagnetic radiation field pattern for a telecommunications system is generated.

Description

Antenna

The present invention relates to an antenna, and in particular to a multi-filar spiral antenna for a telecommunications device.

Difficulties arise in providing a reduced size antenna that will fit in a mobile telecommunications device whilst still delivering acceptable electrical performance. In particular conventional spiral antennae have a feed at the antenna centre. The size of the spiral therefore needs to be sufficiently large to accommodate central feeding ports . Further, the feeding ports need to be sufficiently spaced to prevent mutual couplings affecting the antenna performance.

The present invention therefore provides a feeding arrangement allowing a reduced size antenna suitable for a telecommunications device to be provided and also simplifies the provision of ancillary circuitry for the antenna.

According to a first aspect of the present invention, there is provided a multi-filar spiral antenna for a telecommunications device, comprising at least two spiral arm radiating elements, each having an outer end and an inner end, and a feed connected to each outer end in which when an electrical signal is supplied to each respective feed with a respective phase difference of substantially 360^c divided by the number of spiral arms, an electromagnetic radiation field for a telecommunications system is generated.

The feed for the spiral arms of a multi-filar spiral antenna is provided at the exterior, rather than the centre, of the antenna. This means that no space is required at the centre of the antenna to accommodate the feeds and so the antenna can be made smaller. Further, the feed arrangement is simplified as the individual feeds are spread out and so mutual coupling between the feeds is rendered negligible.

The spiral arm radiating elements can be coplanar. This provides an optimally compact antenna in which the antenna radiating elements lie in a common flat plane.

The antenna structure can be provided as a part of a printed circuit. In this way, ancillary circuitry for the antenna can be provided in the printed circuit in the same plane as the antenna. This provides an improved and simplified manufacturing route.

A phasing circuit can be provided to provide the required phase relationship between the electrical signals supplied to the feeds. The phasing circuit can be provided as a part of the printed circuit.

An impedance matching circuit can be provided to help match the impedance of the antenna to the supply of electrical signals. Preferably, the impedance matching circuit is provided as a part of a printed circuit.

Preferably, the antenna includes a ground plane and the feeds extend in a direction away from the ground plane. The spiral arms of the antenna can lie in a plane substantially parallel to the ground plane.

The feeds can extend in a direction at an angle to the direction perpendicular to the ground plane. This helps to improve the circular polarisation of the electromagnetic radiation field pattern generated by the antenna. Preferably, the feeds extend at an angle of between substantially 0° and 60°. More preferably, the feeds extend at an angle of between substantially 30° and 40°, and most preferably substantially 33°, away from perpendicular to the ground plane .

The inner ends of the spiral arms of the antenna can be free. The inner ends of the spiral arms of the antenna can be connected together. This minimises the size of the antenna .

Most preferably, the antenna has four spiral arm radiating elements . Four spiral arm radiating elements have been found to provide an optimal electrical performance with regard to the complexity of the antenna structure. At least two spiral arm radiating elements can be used to provide a polarisation purity suitable for use in a telecommunications system.

Preferably, the antenna is suitable for use in a satellite telecommunications system. Preferably, the antenna is for use in a personal communications mobile terminal for a low earth orbit satellite based telecommunications system. The antenna can be a dual band antenna operating at substantially 1.6 Ghz and 2.5 Ghz . The antenna can be a dual -band antenna operating at two frequencies having a ratio in the range between substantially unity and two.

Electrical signals can be fed to the antenna by either coaxial cables or strip lines. Embodiments 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 a first antenna according to the present invention; Figure 2 shows a schematic illustration of a second antenna according to the present invention; Figure 3 shows a graph illustrating the VSWR performance of a prototype antenna;

Figure 4 shows a graph illustrating the return loss of a prototype antenna;

Figure 5 shows a graph illustrating the VSWR performance of a further prototype antenna; Figure 6 shows a graph illustrating the return loss performance of a further prototype antenna; and Figure 7 shows a schematic diagram of a phasing circuit for use with the antenna of the invention.

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

With reference to Figure 1, there is shown a schematic illustration of a quadrifilar spiral antenna, generally designated by reference numeral 100. The antenna 100 includes four spiral arm radiating elements 102, 104, 106, 108. The antenna spiral arms are disposed above and substantially parallel to a ground plane 110. Four feed connectors 112 are provided by which an electrical signal can be connected to feeds 114 which extend away from the ground plane 110 and connect each feed 114 to an outer end 116, 117, 118, 119 of respective spiral arms. The spiral arm radiating elements 102, 104, 106, 108 traverse a spiral path within a substantially flat plane substantially parallel to the ground plane and each terminate at a respective free inner end 120, 121, 122, 123 at the centre of the spiral .

Each feed 114 is inclined at an angle 126 of substantially 33° away from an axis 128 substantially perpendicular to the ground plane and plane of the spiral arms of the antenna.

There is an angular displacement of substantially 90° between neighbouring spiral arms. A substantially equi-magnitude voltage signal from a telecommunications device is fed to each of the helical arms with a progressive 90° relative phase between neighbouring arms . For the sense of winding of the spiral antenna shown in Figure 1, and with the voltage fed to spiral arm 102 as the reference, the voltage applied to arm 104 leads by 90°, the voltage applied to spiral arm 106 by 180°, and the voltage applied to arm 108 by 270°. By applying the substantially same magnitude but phase stepped signal to the radiating elements, a circularly polarised electromagnetic radiation field pattern suitable for telecommunication applications is generated. The sense of the polarisation generated by antenna 100 with the afore described voltage phasing would be left handed.

A suitable phasing circuit 300 is shown in Figure 7. The phasing 300 provides four output voltage signals 301, 302, 303, 304 to the radiating elements with the necessary phase relationship. The four voltage signals are then supplied to the fed ends of the radiating elements in the appropriate sense. The triple hybrid phasing circuitry 300 consists of transmission lines deposited on a substrate with a high relative permittivity. An input line 306 is provided by which input signal 305 is supplied to the feed. The phasing circuitry includes a rat race 308. A first transmission line 310 taps the rat race at a λ/4 section of transmission line from the signal feed point 312. A hybrid provides a phase- shifting device 320 and is connected to a 50Ω matched load 322 and provides substantially 270° and 180° phase outputs 301, 302. A further 50Ω matched load 324 connects a further λ/4 section of the rat race and a second transmission line 314 further taps rat race 308 at a further λ/4 section of transmission line. Transmission line 314 connects to a second hybrid 330 connected to 50Ω matched load 326 and providing outputs 303 and 304 at phases of substantially 0° and 90°.

In the case of three spiral arms, the arms would be equi- angularly displaced with an angle of 120° between them and a phase step of 120° in the applied voltage signal. In the case of five spiral arms, the arms would be displaced by 72° and the voltage signals phased with 72° steps. In general for n spiral arms the arms are displaced by 360°/n and the voltages are phased with 360°/n steps.

Each spiral arm traverses an Archimedean spiral path. The Archimedean spiral antenna function can be expressed as r = r₀ + aφ, where a is the spiral constant in metre/radian, r₀ is the start radius of the spiral arm and φ is the winding angle. The pitch distance between the turns of the spiral is given by a/2π. The parameters of the spiral, such as the number of turns and pitch distance between turns can be derived as follows : « <_. BS & 0 W (_. V I f

7

Number of spiral turns = total length of one spiral arm space between turns of a spiral arm

Pitch distance = spiral radius at φ = cp-_t spiral radius at φ = φ_end

The start and end angles can be stated as φ_st = φ₀ and φ_end = cp_c + 2π x number of turns .

The above pitch distance equation should satisfy the inequality: pitch distance is greater than or equal to 16 times radius of wire, as in each pitch turn there are four wires. Additionally, a space between wires greater than the wire diameter should be allowed in order to prevent degradation by proximity effects between closely parallel wires .

A first prototype antenna has the antenna structure shown in Figure 1 with an 8cm x 8cm ground plane. A spiral support is provided made of polystyrene and the spiral arm radiating elements are made of copper wire in an Archimedean spiral shape within a 3cm x 3cm planar area. An air dielectric is provided between the antenna spiral arms and the ground plane .

The radius of the wire used for the spiral arm radiating elements is .025cm and the spiral height above the ground plane is 1.75cm. Each spiral arm has approximately 1.4 turns with a spacing of 1.3cm between turns. The outer end of each spiral arm is rotated through 13 radians with respect of the inner end of the spiral arm. The inner start radius of the spiral is 1.7cm, the end radius of the spiral is 2.78cm and the spiral constant 0.248 m/radians . Figures 3 and 4 illustrate the measured performance of the first prototype antenna at the L- and S-band uplink and downlink bands of the big LEO satellite system (having frequency ranges 1.61-1.625GHz and 2.4835-2.5GHz respectively). The measured VSWR values are 1.1 and 1.9 at 1.6GHz and 2.48GHz respectively. The return loss is -27.9dB at 1.6GHz and -9.7dB at 2.5GHz providing a suitable performance. The imaginary part of the input impedance is close to zero at the working frequencies of 1.6GHz and 2.48GHz, whereas the real parts of the input impedance are 49.6Ω and 32.9Ω respectively.

The phasing circuit is shown in Figure 7. In practice an Impedance matching circuitry will also be required in order to match the impedance of the path by which signals are fed to the antenna with the input impedance of the antenna itself. Standard techniques can be used to provide the matching circuitry, eg quarter wave section or tapered transmission line.

The first prototype antenna has an air dielectric. In practice, a reduced dimension antenna can be provided by using a dielectric with an increased relative permittivity (e.g. between 2 and 10) and the dimensions of the spiral antenna structure would need to be reduced accordingly, as would be understood by a man of ordinary skill in this art. The antenna can be fabricated as a part of a printed circuit, in which a hybrid phasing network and matching network are integrated in the same plane as the antenna. The substrate of the printed circuit would provide the dielectric medium with a ground plane on reverse side. The external feeds to the antenna would then be provided by a microstrip line, or alternatively by coaxial cables. A further embodiment of the antenna 200 of the present invention is shown in Figure 2 which is substantially similar to the antenna 100 shown in Figure 1 but in which the inner ends of the spiral arm radiating elements are connected at the centre of the spiral at point 210. The connected inner end, externally fed, quadrifilar spiral antenna 200 operates in substantially the same way as the free inner end antenna 100.

A further prototype antenna has the same antenna structure as that shown in Figure 2. The prototype antenna has a ground plane and spiral arm radiating elements 202, 204, 206, 208 and feed elements 210, 211, 212, 213 made from copper wire, supported on a thin polystyrene structure. Again, the feed elements are at an angle of substantially 33° to the normal to the ground plane. The radius of the spiral arm radiating element wire is .025cm and the spiral height above the ground plane is 1.8cm. The number of turns of the spiral arms is 1.3 and the spacing between turns is 1.59cm. The outer end of each spiral arm is angularly displaced with respect to the inner end by approximately 11 radians . The inner radius of the spiral is zero and the end radius of the outer ends of the spiral arms is 2.8cm, with a spiral constant of 0.253m/radians .

The measured performance of the further prototype antenna is illustrated in Figures 5 and 6. The VSWR values at 1.6 GHz and 2.48GHz are 1.56 and 3.97 respectively and the return losses are -13.03dB at 1.6GHz and -4.47dB at 2.5GHz . The imaginary part of the input impedance is small compared to the real parts at the working frequencies, and the real parts are approximately 65Ω and 134Ω at 1.6GHz and 2.4GHz respectively, which values can easily be matched to a 50Ω transmission line.

Again, in practice, a reduced area antenna is provided as a printed element on a high relative permittivity dielectric substrate and the voltage phasing circuitry and any required impedance matching circuitry are provided as integrated elements of the printed circuit .

As the antennae 100, 200 are externally fed, the size of the antenna can be reduced compared to that of a conventional centre fed antenna as centre feeds do not need to be accommodated at the centre of the spiral. Further, the complexity of the feeding arrangement, in order to prevent mutual coupling between the feeding ports for a centre fed antenna is obviated, and simple microstrip line or coaxial cable feed connections can be used instead. The reduced overall area of the antenna can provide extra space for longer spiral arms for the same area as a centre fed antenna or reduced area for the same length of spiral arm radiating element .

The antennae 100, 200 described above are particularly suitable for use in a personal communications mobile terminal for use in the big low earth orbit (big-LEO) satellite communications system utilizing the uplink and downlink bands of 1.61-1.6265GHz and 2.4835-2.3GHz respectively. The antennae generate a substantially circularly polarised electromagnetic radiation field pattern extending over an angular region of approximately 60° either side of the central axis perpendicular to the plane of the antenna spiral at 1.6GHz and substantially 30° at 2.5GHz . Tilting the feed elements away from perpendicular to the ground plane improves the degree of circular polarisation of the radiation field pattern. The multi-filar antenna structure improves the purity of the polarisation for the dual bands used for the up and downlink in the satellite telecommunications system. A single radiating spiral element does not provide the required polarisation purity and an array of such single spiral arm radiating elements is too large to use in a mobile communications terminal.

Claims

CLAIMS :

1. A multi-filar spiral antenna for a telecommunications device, comprising: at least two spiral arm radiating elements each having an outer end and an inner end; and a feed connected to each outer end, in which when an electrical signal is supplied to each respective feed with a respective phase step of substantially 360° divided by the number of spiral arms an electromagnetic radiation field pattern for a telecommunications system is generated.

2. An antenna as claimed in claim 1, in which the antenna is a printed circuit.

3. An antenna as claimed in claim 2, and including a phasing circuit provided as part of the printed circuit.

4. An antenna as claimed in claim 2, and including an impedance matching circuit as part of the printed circuit.

5. An antenna as claimed in claim 1, in which the feeds extend away from a ground plane of the antenna.

6. An antenna as claimed in claim 5, in which the feeds extend at an angle away from perpendicular to the ground plane so as to improve the circular polarization of the electromagnetic radiation field pattern.

7. An antenna as claimed in claim 6, in which the angle is between substantially 0° and 60°.

8. An antenna as claimed in claim 1, in which the inner ends are free.

9. An antenna as claimed in claim 1, in which the inner ends are connected together.

10. An antenna as claimed in claim 1, in which there are four spiral arm radiating elements .

11. An antenna as claimed in claim 1, in which the telecommunications system is a satellite telecommunications system.