GB2329071A

GB2329071A - Radio frequency absorber system

Info

Publication number: GB2329071A
Application number: GB9718770A
Authority: GB
Inventors: David Graham Cass
Original assignee: Individual
Current assignee: Individual
Priority date: 1997-09-05
Filing date: 1997-09-05
Publication date: 1999-03-10
Also published as: GB9718770D0

Abstract

A system for the absorption of radio frequency electromagnetic waves is intended for use as absorptive cladding in the construction of anechoic chambers and similar applications. A layer of wave impeding material [2] is placed between the antenna mechanism [3] and the external ground wall [1]. The antenna mechanism [3], which is typically bonded to a substrate [4], is designed in such a way as to absorb any incident electromagnetic radiation. The purpose of the wave impeding material [2] is to enable the antenna mechanism [3] to operate in close relative proximity to the ground wall [1]. The characteristics of the wave impeding material [2] and the antenna mechanism [3] may be altered dynamically or statically to modify the performance of the system. Active elements [6,9] driven by appropriate amplifier components [7,8] may be included to enhance specific performance criteria. The antenna elements may be nested to improve the system bandwidth. Several layers using materials of different impedence and antennae can be used; material impedance within a layer may also vary.

Description

Radio Frequency Absorber System This invention is a system for the absorption of radio frequency electromagnetic waves.

Background In applications which require the measurement of low level radio transmissions, it is essential to exclude external interference. A typical method of excluding external interference is to construct a "shielded room", which is typically a sealed metal box.

However, a shielded room is not appropriate for some measurements, where wall reflections would adversely affect the results. In such cases, an absorber material is required to deaden the reflections. A screened room clad with absorber material is referred to as an anechoic chamber. To be fully effective, all of the walls and ceiling should be clad with absorber material.

Commercial absorber materials fall into two general categories, resistive and magnetically permeable. These two mechanisms are substantially different in their mode of operation.

However, the common objective with all absorber methods is that the energy in the radiated electromagnetic waves is converted into heat.

Resistive absorbers are typically in the form of a pyramidal cone manufactured from a carbon loaded plastic foam. Abrupt changes in impedance can cause an approaching wave to be reflected. The objective of the cone is to provide a gradually changing impedance, whose resistive portion serves to dissipate the energy of an incident wave. In order to be efficient, the cones are required to be at least one quarter wavelength long. At lower frequencies, the dimensions can become exceedingly restrictive.

Permeable absorbers are usually found in the form of tiles mounted onto a support frame.

These absorbers rely on the magnetic properties of the approaching wave in order to dissipate the energy. They are typically of a high intrinsic resistance and therefore do not absorb significant quantities of power in the resistive mode.

An electromagnetic wave passing through a magnetically permeable material will excite the magnetic dipoles at a molecular level. The constant realignment of magnetic dipoles requires energy, the source of which is the incident wave. In this way energy is extracted from the incident electromagnetic wave causing it to be dissipated.

Although the permeable tiles overcome the problem of size presented by the resistive cones, the permeable absorbers have a limited range of frequency absorption compared to their resistive counterparts. A further problem of the permeable absorbers is the manufacturing process, which requires precise control over the production conditions. The manufacturing process is therefore difficult to implement and this can result in high rejection rates.

Antennas can also be used to absorb radio waves, although a conventional antenna would be severely limited when used purely as an absorber. Absorptive antennas are not generally considered to be practical for broadband absorption purposes.

The objective of this invention is to provide an alternative absorber, without either the size limitations of the resistive cones or the manufacturing difficulty of the ferrite tiles.

The invention described below utilises elements of all of the aforementioned techniques in a complex arrangement of multi-dimensional absorptive antenna systems incorporating permeable and resistive elements.

Glossary Radiation source - the radiation source shall be defined as the location from which an incident electromagnetic wave is deemed to have originated Wave impeding material - a wave impeding material is defined as any substance or compound which exhibits a substantial velocity reduction factor for incident electromagnetic waves propagating through it.

Velocity reduction factor - for a given material, the velocity reduction factor shall be defined as the square root of the arithmetic product of the relative permeability and the relative permittivity.

Antenna mechanism - an antenna mechanism will be defined as any structure, assembly, substance or compound which exhibits the property of converting electromagnetic waves to electrical signals.

Active element - an active element will be defined as any structure, assembly, substance or compound, which when actively driven by a electrical power or signal source will generate electromagnetic fields.

Active impedance matching network - an active impedance matching network will be construed as an impedance matching network containing active components which would require the application of electrical power in order to function.

Normal plane - the normal plane shall be defined as the two dimensional extrapolation of a surface which would be perpendicular to the nominal direction of an approaching electromagnetic wave.

Description This invention is a system for the absorption of radio frequency electromagnetic waves. The simplest form of this system consists of the following: 1. One or more layers of wave impeding material, capable of significantly reducing the speed of propagation of electromagnetic waves which pass through it.

2. An antenna mechanism capable of absorbing electromagnetic radiation which is incident upon it.

3. Zero or more active elements.

The performance of a ground coupled antenna is dependent upon the distance between the antenna and the ground. By artificially increasing the distance between the antenna mechanism and a grounded wall, the antenna can be used to collect any incident electromagnetic radiation. One function of the wave impeding material is to increase the effective distance between the antenna and any external ground planes. It may also have absorptive properties which can be utilised to compound the overall effectiveness of the system. Once collected, the incident radiated power can be terminated into a matched load, thereby converting the energy to heat.

Multiple layer systems could be arranged such that any wave passing through the first layer will impinge onto the layer below. This arrangement could be organised to act like a multiple stage filter mechanism. A technique of this type could be used to increase absorption efficiency or to increase the frequency range of operation.

The wave impeding material may be constructed to provide a gradated performance with respect to the depth of material in a direction perpendicular to the normal plane. This would have the effect of presenting a gradually changing impedance to an incident wave, which could minimise the reflective tendencies of the material.

The antenna mechanism may consist of an array of antennas to give spatial coverage. If the array also included nested antennas which would be resonant at different frequencies, then the range of frequency absorption could be increased. By using multiple elements with ninety degree rotation, it is possible to absorb waves of different plane polarisation.

Where an array of nested antennas is used, null points could develop at certain frequencies related to the apparent distance from a ground plane. This arises from interference with the antenna image behind the ground plane and results in acute lobes and nulls in the polar response. Variation in the composition or dimensions of the wave impeding material can be used to distort the polar response to minimise the effect of the antenna image. Such variations would directly affect the directional response of the system.

According to accepted theory, a horizontally polarised dipole antenna at a distance of L2 from a ground plane will display a null response to perpendicular incident radiation. A gradient of AJ4 in the effective ground plane proximity over the length of the dipole would distort the polar diagram, thereby allowing absorption of radiation over a wider angle of incidence. More complex variations in the effective ground plane gradient can maximise the acceptance angle.

Modifying the spatial characteristics of the wave impeding material in a plane parallel to the normal plane could provide effective ground plane proximity changes. This could be utilised to modify the polar response of the system.

An array of individual absorber elements as described above could be utilised to provide absorption over a large surface area.

The general structure of the absorber system in a simple form is depicted in FIG1. The wave impeding material [2] is placed close to the wall [1] which is also a reference ground plane.

The antenna mechanism [3] is situated at a distance from the wall determined by the antenna characteristics and the properties of the wave impeding material.[2].

An active version of the absorber system is depicted in FIG3. In this version of the system, active elements are utilised to provide phase cancellation and active impedance matching of the incident electromagnetic waves. An anterior active element [6] is driven by an active phase shift amplifier [7] to provide cancellation of the incident electromagnetic waves. A posterior active element [9] driven by an active impedance matching network [8] with an appropriate phase shift, has the effect of modifying the influence of the reference ground plane [1]. Either or both of the active elements may be used independently with respect to an individual antenna mechanism. Spacing of the active elements from an antenna mechanism is determined by the antenna coupling factors and the desired phase shift. The system may still contain wave impeding material [2] placed close to the reference ground plane [1].

By utilising active electronic components in the matching network of the antenna mechanism, a dynamically variable system could be produced. This would permit external manipulation or adjustment of the absorber characteristics. When linked to an appropriate control system, this technique could allow synthesis of specific absorption patterns.

Various combinations of the aforementioned techniques may be utilised to achieve specific characteristics for the absorber system.

Embodiment A preferred embodiment of the invention will now be described by way of example only with reference to the accompanying drawings.

The general structure of the absorber system is depicted in FIG1. The wave impeding material [2] is placed close to the wall [1] which is also a reference ground plane. The antenna mechanism [3] is situated at a distance from the wall determined by the antenna characteristics and the properties of the wave impeding material.[2].

The antenna mechanism [3] consists of an array of nested antennas, operating at substantially different frequencies. In this case, the antenna consists of thin metal sheet bonded to a support substrate [4]. The direction of the approaching electromagnetic wave [5] is indicated in relation to the system.

One possible pattern for a broadband antenna mechanism is depicted in FIG2. This pattern is etched into the surface of a metal clad substrate to enable it to be used as a multiple antenna.

Quadrature symmetry in the pattern permits the absorption of waves of any polarisation. Each segment of the antenna pattern should contain a specific matching network which terminates and therefore absorbs any incident radiated power. The impedance matching components are electrically connected to the antenna segments at appropriate termination points.

The wave impeding material is manufactured by mixing granulated ferromagnetic compounds with plastic and ceramic binding materials. The resulting mixture may then be compressed into an appropriate mould and allowed to set. The precise formulation of the mixture will need to be optimised for the specific frequency range intended and to provide characteristic gradients of the desired proportions.

Claims

1. A radio frequency absorber system for the absorption of incidept radio frequency electromagnetic waves and consisting of.

(a) One or more layers of wave impeding material of finite depth.

(b) One or more antenna mechanisms capable of absorbing incident electromagnetic radiation.

(c) Zero or more active elements,

2. An absorber system substantially as described in claim (1), wherein the wave impeding material is constructed from a ferromagnetic substance

3. An absorber system as described in claim (1), wherein the wave impeding material is constructed from an aggregate compound consisting of one or more permeable and/or permittive materials bound by an adhesive medium.

4. An absorber system as described in claim (1), wherein one dr more layers of the wave impeding material is placed between the antenna mechanism and the radiation source.

5. An absorber system as described in claim (1), wherein one or more layers of the wave impeding material is placed between the antenna mechanism and any ground reference plane which may be present.

6. An absorber system substantially as described in claim (1), wherein the antenna system consists of an array of elements arranged in such a way as to provide absorptive properties in one or more polarisation mode of the incident wave.

7. An absorber system substantially as described in claim (1), wherein the antenna mechanism consists of a nested array of elements arranged in such a way as to provide absorptive properties over an extended frequency range.

8. An absorber system substantially as described in claim (1), wherein the characteristics of the wave impeding material are intentionally varied perpendicular to the normal plane, in order to alter the directional response ofthe system.

9. An absorber system as described in claim (1), wherein the characteristics ofthe wave impeding material are intentionally varied with depth, perpendicular to the normal plane.

10. An absorber system substantially as described in claim (1), wherein the characteristics ofthe wave impeding material are intentionally varied in the plane of the normal plane, in order to alter the directional response of the system.

11. An absorber system substantially as described in claim (1), wherein the system consists of multiple layers of wave impeding material and antenna mechanisms to provide additional absorptive properties.

12. An absorber system substantially as described in claim (1), wherein the system consists of multiple layers of wave impeding material and antenna mechanisms to provide absorptive properties over an extended frequency range.

13. An absorber system substantially as described in claim (1), wherein the antenna mechanism contains a self terminating network.

14. An absorber system substantially as described in claim (1), wherein the frequency response characteristics ofthe system can be altered dynamically with respect to time.

15. An absorber system substantially as described in claim (1), wherein the degree of absorption of the system can be altered dynamically with respect to time.

16. An absorber system substantially as described in claim (1), wherein the polar response characteristics ofthe system can be altered dynamically with respect to time.

17. A multi dimensional array of absorber systems as described in claims (1-16).

18. An absorber system substantially as described in claims (1-17), wherein anterior active elements are present on one or more ofthe antenna mechanisms.

19. An absorber system substantially as described in claims (1-17), wherein posterior active elements are present on one or more ofthe antenna mechanisms.

20. An absorber system substantially as described in claims (1-17), wherein the active elements on multiple antenna mechanisms can perform both anterior and posterior functions.