GB2539279A - Frequency selective surface for reducing antenna coupling - Google Patents

Abstract

A compact radome for an antenna with a frequency selective surface (FSS) 10 circuit consisting of a meanderline structure 3 on a substrate 2, which reflects electromagnetic radiation at pre-determined frequencies and thereby reduces mutual coupling effects from adjacent antennas. The FSS may be made from a flexible polymeric sheet material, for example mylar polymer, and the meanderline structure may be a slot, or formed from a conductive material, for example copper (Cu), Aluminium (Al), or gold (Au). The serpentine structure of the FSS may be formed from, for example, rectangular, triangular, helical or fractal shapes, and may be designed so that the FSS is tuned to operate at long wavelength megahertz (MHz) frequencies, including microwave.

Description

Frequency Selective Surface for Reducing Antenna Coupling

Technical Field of the Invention

This invention relates to a radome comprising a frequency selective surface, for use with an antenna element, which is employed to reduce unwanted mutual coupling effects caused by adjacent antenna elements by filtering electromagnetic radiation at pre-determined frequencies.

Background to the Invention

Antenna systems that are positioned in close (electrical) proximity to other antenna systems are liable to experience undesirable mutual coupling effects when transmitting and receiving electromagnetic energy. In this situation some of the energy which should have been radiated away or received by a first antenna system may be unintentionally coupled in by an adjacent antenna system. These coupling effects can cause a reduction in or be a limiting factor in the efficiency of the two (or more) systems. This is particularly acute when the associated antennas are omnidirectional in nature (i.e. they distribute and receive electromagnetic energy at all angles around in the azimuth plane). It is important therefore to minimise mutual coupling (losses) in order to maximise the efficiency and performance of each individual system.

In the antennas field, previous research has configured ways of combatting and overcoming these problems in the form of various 'diversity' techniques: Spatial Diversity -Antenna to antenna coupling is reduced by placing a large enough electrical distance (separation) between adjacent antennas such that individual signals from/to each antenna are uncorrelated. General guidelines here normally use a half wavelength (A/2) separation distance as a starting point.

ii. Polarisation Diversity -Adjacent antennas can be configured with differing polarisation characteristics with a view to minimising antenna to antenna coupling. The theory of this is founded upon the fact that signals transmitted on two orthogonal polarisations (particularly) in the mobile radio environment exhibit uncorrelated fading statistics.

iii. Angle or Pattern Diversity -Antennas equipped with multiple feeds which can produce narrow beams or directional type antennas which can direct energy away in slightly different directions can apply angle or pattern diversity. It has been observed that energy that can be scattered in slightly different directions exhibits uncorrelated behaviour.

However, a requirement often exists for antenna systems to be located in areas in which physical space is restricted. This therefore prevents the use of spatial diversity, particularly so when the antennas operate at frequencies corresponding to ever larger wavelengths.

Furthermore, not all systems can be operated in alternate polarisations. For co-existing systems that must maintain vertical polarisation, polarisation diversity cannot be applied for this obvious reason.

Similarly, systems that are required to operate with omnidirectional characteristics cannot apply angle or pattern diversity as the omnidirectional nature of the radiation means that electromagnetic energy will be radiated / received from angles in which neighbouring systems will be located, resulting in inevitable mutual coupling.

Frequency Selective Surfaces (FSS) are known to reflect, transmit or absorb electromagnetic radiation based on frequency, and have been used previously in various applications. In particular, it is known to use FSS as a means of applying different filtering techniques on electromagnetic waves, and more particularly as a filtering means to protect a single antenna from unwanted distant radiation (wherein the field element is weak) and consequential interference effects. Specific applications for-ES have ranged from supressing radar returns on aircraft, to providing filtering techniques for single antennas on missiles. FSS have also found use with maximising the transmission of electromagnetic, waves -for example, coupling maximum amounts of electromagnetic waves through walls. Furthermore, FSS structures have also found use as circular polarisers.

However, use of FSS has, in general, been associated with very high frequencies i.e. several to tens of GHz. The reason for this has been partly application driven, but also driven by fundamental design principles. It is well known that the dimensions of the FSS structures determine the wavelength which is acted on by the FSS. Therefore, FSS structures need to be appropriately sized with respect to wavelength in order to enact the intended operation at the desired frequencies of interest. Unfortunately, simply scaling up known FSS structures to provide filtering in the MHz region could result in the physical size of the FSS becoming larger than the physical space one has available for the application.

Furthermore, the use of FSS as a means of mitigating antenna to antenna (mutual) coupling, where the field element is strong, particularly coupling at larger wavelengths, has not been adequately addressed.

It is therefore an aim of the invention to provide an improved means of mitigating antenna to antenna (mutual) coupling, particularly coupling at larger wavelengths.

Summary of the Invention

According to an embodiment of the invention, there is provided A radome for use with an antenna element in order to reduce unwanted mutual coupling effects caused by adjacent antenna elements, the radome comprising a frequency selective surface (FSS) circuit wherein the FSS circuit comprises a substrate provided with at least one meanderline structure which is configured to reflect electromagnetic radiation at predetermined frequencies.

The term "radome" is used here to refer to any protective covering, casing or enclosure designed to house a microwave antenna and to be substantially transparent to electromagnetic signals at the operating frequencies of the housed antenna. The FSS substrate may form the structure of the radome. Alternatively, the FSS substrate may be a flexible sheet applicable by way of retro-fit to an existing radome structure.

The deployment of a meanderline structure FSS in close proximity to an antenna allows electromagnetic energy at pre-defined frequencies of interest to be filtered before any surface current is induced on the antennas surface. The detailed geometry of the meanderline structure determines the particular frequency response of the surface, however the use of meanderline structures enables the filtering or reflection of radiation at larger wavelengths than can be achieved using traditional repetitive surface FSS previously reported. The reason for this is simply that the meanderline structures allow for an increase in the electrical length of the conductive FSS shapes that will provide operation at larger wavelengths without becoming prohibitively large in size.

The meanderline structure FSS provides a high performance filtering capability suitable for reducing undesirable coupling effects associated with the activity of close proximity antenna systems. As a direct consequence the performance of the protected antenna system is enhanced.

Furthermore, the meanderline structure will be easier to manufacture and design than, for example, a very high order hillbert curve.

The meanderline FSS has been found to be design tunable throughout the MHz frequency range, and optimisable in terms of the bandstop and bandpass performance The substrate of the FSS is transparent to electromagnetic radiation and preferably comprises a suitable durable but flexible sheet material, for example a polymeric material such as mylar polymer. If the FSS substrate is manufactured from a flexible sheet it can be wrapped around an existing radome structure and secured by any known technique such as taping or gluing in place.

The FSS circuit may be provided in a band pass or band stop configuration. For band stop filtering requirements, the FSS has one or more printed meanderline structures embedded on the substrate. In this case the meanderline structures can be manufactured from any conductive material such as Cu, Al, Au, or other highly conductive metals or alloys. For band pass filtering requirements, the meanderline structure can be applied as a slot, cut out from the FSS substrate.

In order to filter out electromagnetic energy at increasingly large wavelengths, the meanderline structures are formed using a meanderline design philosophy this is the mechanism by which the printed structures or slots can be used to filter out electromagnetic energy at increasingly large wavelengths without becoming prohibitively large in size. This meanderline philosophy may be formed by various means including, but not limited to, rectangular shapes, triangular shapes, helical shapes and varying fractal shapes.

The trace thickness of the conductive meanderline structures (band stop) or slots (bandpass) can be tuned to optimise exact frequencies of interest. In terms of the bandstop mechanism, a smaller trace thickness has been found to equate to a lower resonant frequency and this response is also sensitive to be able to maximise the overall level of bandstop filtering performance. For the bandpass variant, a smaller slot thickness has been found to equate to a lower frequency in which the bandpass centre frequency occurs.

Further optimisation of each meanderline structure in terms of the overall width of each (unit) cell can yield a lower resonant frequency for a wider meanderline (cell) width and this response is also sensitive to be able to maximise the overall level of bandstop filtering performance.

In order to provide effective filtering for omnidirectional antennas a plurality of meanderline structures can be provided around the radome. For example, the FSS may comprise a cylindrically wrapped substrate sheet bearing a plurality of meanderline structures at-ranged in a periodic or non-periodic pattern depending upon the filtering performance that is desired.

The meanderline structures may be printed or cut in a vertical orientation to filter out vertically polarised electromagnetic radiation, or horizontally to filter out horizontally polarised electromagnetic radiation. Equally, if both polarisations are required to be filtered, multiple meanderline structures can be combined orthogonally or any other (angular) combination. This may consist of multiple structures on the same sheet or a combination of different layers as appropriate.

Also provided is a method of reducing unwanted mutual coupling effects between adjacent antenna elements comprising, configuring a frequency selective surface (FSS) circuit by providing at least one meanderline structure on a substrate and locating the FSS circuit between the adjacent antenna elements as part of a radome structure, such that it reflects electromagnetic radiation at pre-determined frequencies.

Brief Description of the Drawings

Embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which: Fig. 1 shows a unit cell FSS suitable for use in accordance with the invention; Fig. 2 shows a FSS comprising multiple structures suitable for use in accordance with the invention.

The drawings are purely illustrative and are not to scale. Same or similar reference signs denote same or similar features.

Detailed Description

A first embodiment of the invention will now be described with reference to Fig.1 which shows a unit cell FSS 1 in a band stop configuration, suitable for use with a radome (not shown) in accordance with the invention. In this example substrate 2 is manufactured from a mylar polymer sheet material having a thickness of 0.5 mm selected to provide flexibility in being able to adapt the FSS to any surface configuration desired yet is robust enough not to become easily damaged. A rectangular meanderline structure 3 made from copper has been printed onto the substrate 2. Arrows U, V, W generally indicate that in this arrangement vertically polarised electromagnetic radiation aligned to axis V, of a pre-defined frequency will be reflected by the meanderline structure 3.

Fig. 2 shows an alternative embodiment in which a FSS 10 comprising multiple structures in a band stop configuration is formed into a cylindrical shape suitable for application to the outer surface of a radome (not shown) in accordance with the invention. In this example substrate 2 is manufactured from a similar mylar polymer material having a thickness of 0.5 mm,. A periodic pattern of rectangular meanderline structures 3 made from copper has been printed onto the substrate 2. The substrate 2 has then been formed into a cylindrical shape which can be positioned around a radome and secured in place using suitable adhesive. With regards to the total number of meanderline structures printed on the FSS substrate, this number is optmisable considering the overall diameter that the sheet has to fit over (i.e. diameter of an existing radome that the FSS sheet will cover).

Further embodiments falling within the scope of the appended claims will also be apparent to the skilled person.

Claims (8)

1. CLAIMS1. A radome for use with an antenna element in order to reduce unwanted mutual coupling effects caused by adjacent antenna elements, the radome comprising a frequency selective surface (FSS) circuit wherein the FSS circuit comprises a substrate provided with at least one meanderline structure which is configured to reflect electromagnetic radiation at pre-determined frequencies.
2. 2. A radome according to claim 1 wherein the FSS is design tunable throughout the MHz frequency range.
3. 3. A radome according to claim 1 or claim 2 wherein the substrate of the FSS comprises a flexible polymeric sheet material.
4. 4. A radome according to any preceding claim wherein the meanderline structure is formed from a conductive material.
5. 5. A radome according to any one of claims 1 to 3 wherein the meanderline structure comprises a slot, cut out from the FSS substrate.
6. 6. A radome aaccording to any preceding claim wherein the FSS is provided with a plurality of meanderline structures located around the radome.
7. 7. A radome aaccording to any preceding claim wherein multiple meanderline structures are combined in an angular combination.
8. 8. A method of reducing unwanted mutual coupling effects between adjacent antenna elements utilising the radome of any preceding claim.