WO2006136843A1

WO2006136843A1 - Improvements in or relating to antennas

Info

Publication number: WO2006136843A1
Application number: PCT/GB2006/002309
Authority: WO
Inventors: Michael Pywell; Paul Newham
Original assignee: Bae Systems Plc
Priority date: 2005-06-23
Filing date: 2006-06-22
Publication date: 2006-12-28

Abstract

Described herein is a spiral antenna arrangement in which each arm (8) of the antenna arrangement has a peripheral surface with a substantially longer width than the width of the arm at a particular point.

Description

IMPROVEMENTS IN OR RELATING TO ANTENNAS

The present invention relates to antennas and is more particularly but not exclusively concerned with spiraf antennas.

In many antenna applications, for example, in direction finding or emitter location systems, an antenna with a broad bandwidth is required. Frequency- independent antennas are often used in such applications, and many of these antennas have a radiating element with spiral geometry that enables the antenna to transmit and receive signals over a wide range of frequencies.

Conventional spiral antennas are bi-directional and radio frequency (RF) energy is radiated equally both forwards and rearwards. The rearwardly radiated energy tends to be reflected from a rear ground-plane and interferes , with the forwardly directed energy resulting in a severe loss in gain. To mitigate this problem_., a lossy cavity is often positioned to one side of the antenna element to form a cavity-backed spiral antenna. The cavity is filled with radiation absorbing material to absorb rearwardly-radiating energy over the central octave band of operation of the antenna. The depth of the cavity and the radiation absorbing material is generally designed to reflect back in phase at the upper and lower octaves thereby enhancing the gain of the antenna in these regions. However, the antenna rarely has an efficiency better than 50% because of the absorption of half the available RF energy.

A spiral-mode microstrip (SMM) antenna is described in US-A-5313216 that has been used to increase the efficiency of spiral antennas. The SMM antenna does not use a cavity and hence all the incident RF energy is detected. The construction of this type of antenna comprises a ground-plane in close proximity to the spiral arms, the spacing being only a few millimetres and filled with a low-loss foam dielectric material. SMM antennas have a gain that is typically 3dB greater than that of equivalent cavity-backed spiral antennas. Moreover, as SMM antennas are not cavity-backed, they are capable of being conformed, that is, mounted substantially flush with an exterior surface of a platform, for example, a high performance aircraft. This increases the sensitivity of the antenna out to wider angles. Ail other characteristics of the SMM antenna are similar to those of cavity-backed spiral antennas.

A sinu-sinuous antenna is disclosed in GB-A-2345798. Such an antenna has been used to reduce spiral antenna size for both cavity-backed and SMM antennas. This is achieved by superimposing sinusoidal modulation on a sinuous spiral track. This effectively increases the electrical path length and means that the one-wavelength circumference in the active region now occurs at a smaller diameter. The size of the antenna is therefore reduced for a required minimum frequency of operation. However, current antennas, cavity-backed spiral antennas, sinu-sinuous antennas and SMM antennas, have shortfalls that affect the performance limitations of the systems in which they form an integral part. These limitations ^' may adversely affect platform survivability and/or effectiveness. Specific shortfalls affecting such antennas include: • low gain - typically OdBiI (a gain of unity matched to linear polarisation) but ranging from -3OdBi to 2dBi according to antenna frequency range. (Gain flatness with frequency can exceed ±4dB.)

• voltage standing wave ratio (VSWR) ranges typically from 2.0:1 to 6.5:1 with frequency (for maximum power transfer, VSWR is ideally 1:1).

• beamwidth is below 180° (ideal value) and varies with frequency (half-power (3dB) beamwidths of 50° to 140° are typical (with decreasing frequency), with up to 170° being achievable for narrow band antennas (0.7GHz to 2.0GHz) and 1OdB beamwidths have nominal angles around 125°)

• beam pattern squint of the order of 15° can be seen at some frequencies ('squint-free' is considered to be approximately 0.25°)

• frequency range is sub-optimal with the result that a number of spiral antennas are required to cover adequately the frequency band of interest with acceptable gain ■ • the quality of circular polarisation (defined as the axia! ratio of elliptical polarisation) is suboptimal and is easily degraded especially at the lower frequencies as the diameter of the spiral is reduced

• radiation patterns have many, and often significant, nulls, pits and/or dips (the exact nature of the gain reduction (from the average) at any given off-boresight direction is difficult to predict as it is usually a result of installation effects rather than the antenna per se)

Ideally, the radiation pattern at any frequency should be hemispherical in shape with the hemisphere being orthogonal to the plane of the antenna face. Any degradation from the ideal equates to a decrease in capability and performance and prevents precise location of RF emitters, for example, using such antennas.

There is also the possibility of: degradation in frequency and polarisation detection capability; increasingly wide spatial 'blind zones' with distance away from the platform caused by the extension of very narrow nulls, pits and/or dips in the antenna radiation pattern.

It can readily be appreciated that there is still a need for size reduction of antennas while providing antennas with acceptable gain and bandwidth.

In accordance with one aspect of the present invention, there is provided an antenna comprising: a dielectric substrate; and at least one pair of interwound spiral antenna arms carried on the substrate, each antenna arm having dimensions of width and of height, at least one antenna arm having a surface distal from the dielectric substrate with, at at least one point along the arm, a peripheral surface distance across the arm substantially longer than the width of the arm at that point.

The width of the arm is understood to be the greatest distance, parallel to the substrate, between the two sides of the arm. The height of the arm is taken to be the greatest distance, perpendicular to the substrate, between the substrate and the surface distal from the substrate. Such an antenna requires . selective shaping of the antenna arms.

. Etching or other manufacturing processes used to form the antenna arms may cause rounding of the antenna arms but will not, on their own, provide an antenna arm surface distal from the substrate having a peripheral . surface distance across the arm substantially longer than the width of the arm at that point.

Selective shaping of the cross-section of antenna arms leads to an improved-performance antenna. Such antennas can be used to increase the minimum detection range of RF emitters, improve RF emitter location accuracy , compared to systems with conventional spiral antennas, and similarly improve direction finding accuracy.

Preferably, at least one antenna arm has a side surface with, at at least one point along the arm, a peripheral surface distance over the height of the arm substantially longer than the height of the arm at that point. Removal of sharp edges from the antenna arm track gives a more uniform current density across the track and this leads to an increase in gain of the antenna.

The antenna arm surface distal from the substrate may, at at least one point along the arm, be generally convex or concave. The provision of an upper surface which is generally convex or concave provides an upper surface with a peripheral surface distance across the arm substantially longer than the width of the arm at that point. The surface area per unit length of the antenna arm is increased, leading to an increase in gain of the antenna. Ideally the distal surface is substantially hemispherical. Optionally, the width of at least one antenna arm is not constant and may increase with radial distance.

Increasing the width of an antenna arm increases the surface area per unit length of the arm, and thereby increases the gain of the antenna.

A non-metallic support may be interposed between the substrate and at least part of an antenna arm. The antenna arm tracks may be formed with minimal contact with the substrate. A non-metallic support between the antenna arm and the substrate on either side of the line of contact of the antenna arm with the substrate provides support for the arm track without interfering with the antenna characteristics.

Dielectric loading of the substrate can be used to change the characteristics of an antenna.

For a better understanding of the present invention, reference will now be made by way of example only, to the accompanying drawings, in which: Figure 1 illustrates a typical Archimedean spiral antenna;

Figure 2 is a schematic view of a section through an antenna arm of the antenna of Figure 1 ;

Figure 3 is a schematic view of a section through an embodiment of an antenna arm in accordance with the present invention; Figures 4a to 4e are cross-sectional views of antenna arms in accordance with the present invention;

Figure 5 shows an antenna arm, in accordance with the present invention, supported on a base by a non-metallic support material; and

Figure 6 is a cross-sectional view of a composite antenna in accordance with the present invention.

Referring initially to Figure 1 , a two-arm spiral antenna 2 of Archimedean configuration is shown. The antenna 2 comprises spiral copper tracks of the antenna arms 4 formed on a dielectric base 6.

Antenna arms 4 have a rectangular cross-section as illustrated schematically in Figure 2.

Figure 3 illustrates a cross-section through the arms 8 of an antenna (not shown in full) in accordance with the present invention. The antenna arms 8 are formed by coating a dielectric base 6 with a copper layer. The copper layer is then etched to form tracks 10 of copper supported on the dielectric base 6. Surface deposition techniques are used to deposit a further layer 12 of copper

^■ . on top of the etched track 10. This further layer 12 has an upper surface with a peripheral surface distance across the arm substantially longer than the width of the arm at that point. For example, the track 10 may have a width of 2mm and

5 a height of 0.1mm and the further layer 12 may have a 1 mm radius formed on top of the track 10. The antenna arm surface area per unit length is therefore increased enabling the antenna to capture more energy from impinging electromagnetic wavefronts.

The deposition of the further layer 12 also provides antenna arms. 8 with 0 an outer surface which has no sharp edges. Sharp edges and corners lead to higher current density. Antenna arms without such sharp edges and corners have a more uniform current density across the track, and therefore resistive loss is reduced and the gain of the antenna increased.

It will be appreciated that, in some situations, high current density may 5 not be a problem. In these circumstances, the upper surface of the antenna arm may have a cross-section, taken perpendicular to the base substrate, of an "inverted V" as shown in Figure 4a, rather than the semicircular cross-section shown in Figure 3. Alternatively, a concave upper surface may be formed, as shown in Figure 4b. 0 Any transfer of high current density to the sharp corners of the antenna arms in contact with the dielectric base might reduce the benefits of the additional upper part of the antenna. This can be reduced by selective shaping of the lower face of the track in contact with the base as shown in Figure 4c. The ideal cross-section is circular as shown in Figure 4d. This arrangement has minimal contact with the dielectric base, but is dependent upon physical limitations on the manufacturing processes and the requirement for physical robustness of the antenna in its particular environment. In this case, the current would be distributed uniformly around the circumference, with much iower resistive loss than an antenna arm with a rectangular cross-section. This lower resistive loss would lead to an antenna having much higher gain. Figure 5 shows a non-metallic support material 20 deposited on the dielectric base 6 under part of the lower antenna arm surface of Figure 4d.

Minimal contact between the circular cross-section antenna arm 22 and the base 6 is maintained along the base line 24 of the antenna arm 22. The non- metallic material 20, deposited on either side of the line of contact of the antenna arm with the substrate, provides support for the copper track of the

^■ antenna arm 22, without affecting the antenna characteristics.

In another embodiment of the present invention (not shown), the width of the antenna arms may be increased continuously along their length. This leads to a reduced total number of turns and, hence, to a reduced total electrical path length to the periphery of the spiral. The reduction in total electrical path length reduces the severe gain loss at the lowest frequencies of operation where sensitivity is confined to the periphery of the antenna.

A composite antenna 30, in accordance with the present invention, is shown in Figure 6. The antenna 30 comprises two pairs of inter-wound spiral antenna arms 4 as indicated generally by 32, 34. Each antenna arm 4 has a convex upper surface and is supported on a substrate 36. An insert of absorbing material 38, such as dielectric material, is provided in the substrate 36 in the centre of the two pairs 32, 34 of interwound antenna arms. The insert 38 eliminates currents that would otherwise be reflected from the outer edge of the spiral and thereby disrupt the desired pattern and polarisation of the antenna. Dielectric loading can therefore be used to change antenna characteristics, such as impedance and bandwidth. The size of antennas can be reduced by this loading, as the insert causes the electric dipoles to be formed over a shorter distance.

The surface area per unit length of the antenna arms may be increased by providing an antenna arm with a combination of different cross-sectional areas perpendicular to the substrate. For example, the cross-section of an antenna arm may be rectangular at one point along the arm and convex at another. There may be gradual shaping of the arm to blend the two sections together. Moreover, the width of an antenna arm may vary non-continuously along its length in accordance with particular application requirements.

Claims

1. An antenna comprising: a dielectric substrate, and at least one pair of interwound spiral antenna arms carried on the substrate, each antenna arm having dimensions of width and of height, at least one antenna arm having a surface distal from the dielectric substrate with, at at least one point along the arm, a peripheral surface distance across the arm substantially longer than the width of the arm at that point.

2. An antenna according to Claim 1 , wherein at least one antenna arm has a side surface with, at at least one point along the arm, a peripheral surface distance over the height of the arm substantially longer than the height of the arm at that point.

3. An antenna according to Claim 1 or 2, wherein the antenna arm surface distal from the substrate is, at at least one point along the arm, substantially convex.

4. An antenna according to claim 1 or 2, wherein the antenna arm surface distal from the substrate is, at at least one point along the arm, substantially concave.

5. An antenna according to claim 3 or 4, wherein the distal surface is substantially hemispherical.

6. An antenna according to any preceding claim, wherein the width of at least one antenna arm is not constant

7. An antenna according to Claim 6, wherein the width of at least one antenna arm increases with radial distance.

8. An antenna according to any preceding claim, wherein a non-metallic support is interposed between the substrate and at least part of an antenna arm.

9. An antenna according to any preceding claim wherein the antenna is an Archimedean antenna.

10. An antenna according to Claims 1 to 8, wherein the antenna is an equiangular antenna.