GB2159667A - Anechoic chambers - Google Patents

Abstract

Anechoic chambers for use in testing antennas. The anechoic chamber is so made as to have an inner wall surface formed as a contour generated by at least partial rotation of a first (10) and at least one further ellipse (12) about the major axis (20) of said first ellipse. Prior to rotation the ellipses lie in a plane with a focus (16) of each of them coincident and their axes divergent. R.A.M. is provided within the anechoid chamber at the focus of the first ellipse spaced from the coincident focii location and along the paths traced by the other focii of the other ellipses and/or on the inner surface of the anechoic chamber. The arrangement provides a quiet zone volume located between the focii of said first ellipse and centred on the major axis of that ellipse to which volume no radio waves emanating from a source antenna at the coincident focii location and diverging from the major axis of the first ellipse are received. An antenna under test may be located in this quiet zone volume.

Description

SPECIFICATION Anechoic chambers The invention concerns radio anechoic chambers.

Anechoic chambers are widely used in the testing of antennas, particularly those operating in the microwave spectrum above approximately 1 GHz. Such chambers provide a volume in which the radio frequency (R.F.) field conditions are controlled by absorbing reflected radio waves, thus providing as nearly as possible an idealised plane wave form of illumination for an antenna under test. A properly designed and constructed anechoic chamber should resemble, as far as possible, a "free-space" environment for antenna testing.

To achieve this desirable result both the shape of the chamber and the material located within it which absorbs unwanted radio waves (R.A.M.) must be carefully considered. Again both the shape of the chambers and the choice of R.A.M. used in the chambers are important factors in determining the performance at defined prequencies and antenna sizes. The actual geometry and overall dimensions of the different chambers, that is to say chambers for testing different antennas may vary considerably.

A wide range of R.A.M. material is readily available but in general the materials which are available are costly and they are not perfect absorbers - they do actually reflect small amounts of incident energy from a source antenna (R.A.M. reflectivities are typically - 30dB to - 40dB). Careful geometrical design of anechoic chambers can minimise the effect of such reflections within a quietzone volume within which a test antenna may be located.

From the foregoing comments it will be seen that the cost of designing, building and commissioning a large anechoic chamber is great.

A form of anechoic chamber is now proposed which, it is believed, will give a significantly improved performance for a given volume of R.A.M; and which will provide a chamber in which the volume of reflected rays in the "quiet-zone" will be reduced substantially below the levels obtainable with chamber designs currently available. Furthermore the proposals now made permit a substantial reduction in the volume of R.A.M. used in chambers for the testing of specific antennas with a significant cost reduction.

One aspect of the present invention provides an anechoic chamber the inner wall surface of which is formed as a contour generated by at least the partial rotation of a first and at least one further ellipse about the major axis of said first ellipse, prior to rotation the ellipses lying in a plane with one focus of each of them coincident and their axes divergent.

Preferably the anechoic chamber is formed by rotation through 360 about said major axis of said first ellipse.

Radio absorbing material (R.A.M.) is provided within the anechoic chamber. The R.A.M. may be provided at the focus of said first ellipse spaced from said coincident focus location and along the or each path traced by the focus of the or each further ellipse. Alternatively, or additionally, the wall of the anechoic chamber may be formed of radio absorbing material, or have radio absorbing material located thereon. With advantage we envisage that if R.A.M. is placed on the inner surface of the wall of the anechoic chamber it would have a pyramidal form-the points of the pyrimidical parts of the R.A.M. lying on said contour.

Alternatively the inner wall surface of the anechoic chamber may be highly reflective being of metal or a metallised finish on a nonmetal e.g. thermoplastics material wall. If the anechoic chamber is formed of thermoplastics material we prefer that it be formed of moulded aromatic thermoplastic polymer materials such as P.E.S. reinforced with an appropriate fibre reinforcement.

Various embodiments of the invention will now be described with reference to the accompanying drawings in which: Figure 1 illustrates diagrammatically an outline elliptical contour from which an anechoic chamber embodying the invention may be generated, Figure 2 is a sectional side elevation through an anechoic chamber embodying the present invention, Figure 3 is a sectional view through another anechoic chamber embodying the present invention, and Figure 4 is a sectional view through a further form of anechoic chamber embodying the present invention.

Figure 1 shows in full line 10 part of a contour (continued in dotted line) formed by laying an ellipse 1 2 on top of an ellipse 14 with one focus of each of the two ellipses coincident as shown at 1 6 whilst the axes 1 8 and 20 of the two ellipses are displaced through an angle + as shown, such that the other focii 22 and 24 of the two ellipses 1 2 and 14 respectively are displaced one from the other. It will be seen that rotation of contour 10 about 'axis' 20 will generate a volume having generally the outline shown in Figure 2. In revolving the contour 10 about the axis 20 the positions of the coincident focii 1 6 in the focus 24 remain unchanged whilst the focus 22 traces a circie about the axis 20.

An anechoic chamber the inner wall surface of which is formed in the way described above is illustrated in Figure 2. The surface 30 of the anechoic chamber shown in Figure 2 is reflective to radio waves, that is to say it is a metal (smooth and continuous or of fine mesh). surface, or a metallised surface formed of a suitable rigid material substrate. Within the chamber, at the coincident focii location 32, a source antenna 34 is mounted. At the other focus 36 of the main elliptical part of the anechoic chamber a volume of radar/radio absorbent material (R.A.M.) e.g. such as carbon loaded Eurethane foam is located.

Along the circle 38 formed by rotation of the free focus of the smaller ellipse an anulus 40 of R.A.M. material is located. This particular arrangement generates a quiet volume as shown at 42. In this quiet volume a test antenna 44 may be located. Any R.F. waves coming from the source antenna 34 directly along the main axis of the chamber are received by the test antenna 44. Any R.F.

waves (e.g. as shown at A, B or C) diverging from that axis will strike the inner surface of the anechoic chamber and be reflected either to the volume of R.A.M. 36 or onto the ring 40 of absorber material. Waves passing directly to the ring 40 of absorber material will, of course, be absorbed by it.

The particular shape of the metallic, highly reflective chamber wall geometry may be infinitely variable through selection of the basic parameters of the elliptic sections from which the volume of the chamber is generated viz: the length of the major axes and their ellipticity.

A constraint in designing anechoic chambers embodying the invention is that the angle f between the axes of the ellipses forming the chamber (lines 1 8 and 20 Figure 1) has a minimum value equal to Tan-' X/Y where X is the length of the minor axis of the main ellipse part-, and Y the length of the major axis of the main ellipse part of the chamber. Rays emitted by the source antenna and diverging from the major axis of the main ellipse part of the chamber by an angle greater than the angle + will be reflected onto the absorber material 38 and not onto the absorber material 34. In this way the position and size of the 'quiet-zone' volume is determined by rays reflected from the inner surface of the main part of the chamber onto the absorber material 34.

In Figure 2 the test antenna 44 is shown to be located within a spherical volume centred on the anechoic chamber main axis. All the radio waves from the source antenna off the main axis of the chamber, for example rays reflected from the chamber wall, pass outside this volume or quiet zone. The position and size of the quiet zone is controllable primarily by controlling the size of the chamber.

Figure 3 illustrates an alternaive form of anechoic chamber, and the same reference numerals are used in this Figure to denote parts common to both this Figure and Figure 2.

In the arrangement of Figure 3 the anechoic chamber surface is coated with pyramidical R.A.M. material 50 the points 52 of which lie on the multi-ellipsoid surface of the chamber.

In view of the primary absorbing qualities of the walls this arrangement provides better performance than might be expected from the reflective wall type described above. However whilst giving better preformance the arrangement is considerably more expensive to produce than that described with reference to Figure 2 in view of the greatly increased volume of R.A.M. required to coat the inner surface of the chamber.

Modifications may be made to the arrangements described above for example by providing that the inner wall surface of the anechoic chamber is generated by rotating a first and two or more further ellipses overlying one another about the major axis of the first ellipse. In this way a number of focii (in the form of rings) can be provided within the anechoic chamber to which reflected rays are directed. In such an arrangement some or more of the ellipses may be located such that their axes lie at an angle which is obtuse to the major axis joining the focii of the main ellipse. In this way back lobes of the source antenna may be directed to absorber rings located behind that antenna.

Figure 4 illustrates such an arrangement in which it can be seen that the contour of the inner surface of the chamber wall has been generated by rotating four ellipses 100, 102, 104 and 106 about the major axis of the ellipse 100. A volume of R.A.M. is provided at a 11 0-the free focus of the ellipse 100 and rings of R.A.M. are provided as shown at 112, 114 and 116 on the paths traced by the moving focii of the ellipses 102, 104 and 106 as they are rotated. The quiet zone 120 is formed in which an antenna under test may be placed to receive rays from the source antenna located at 122 -the location of the coincident focii of the several ellipses. With the particular arrangement shown in Figure 4 it will be noted that R.A.M. is further provided at 1 30 that is to say on the inner surface of the main ellipse part of the anechoic chamber.

With the particular arrangement shown in Figure 4 any radio waves not passing along the axis joining the focii 120, 122that is to say not passing directly to the test antenna-will be absorbed by the R.A.M. which is provided within the anechoic chamber.

It will be appreciated that the present invention provides a simple form of anechoic chamber which may be readily made to any required size.

Any suitable radio/radar absorbing material have a reflectivity of - 40dB or less may be used and the position and size of the quiet zone volume may be determined for a particular chamber by consideration of the optical geometry of rays emitted by the source antenna.

Finally it will be noted that various other modifications may be made to the arrangements described in this Application without departing from the scope of the present invention.

Claims (9)

1. An anechoic chamber the inner wall surface of which is formed as a contour generated by at least partial rotation of a first and at least one further ellipse about the major axis of said first ellipse, before rotation the ellipses lying in a plane with one focus each of them coincident and their axes divergent.

2. An anechoic chamber as claimed in Claim 1, and which is formed by rotation through 360 about the major axis of said first ellipse.

3. An anechoic chamber as claimed in Claim 1 or Claim 2, wherein radio/radar absorbing material (R.A.M.) is provided at the focus of the first ellipse spaced from the coincident focii location and along the paths traced by the focus of the or each other ellipse in rotation.

4. An anechoic chamber as claimed in any one of the preceding Claims, wherein R.A.M.

is provided on the inner surface of the wall of anechoic chamber.

5. A chamber as claimed in Claim 4, wherein said R.A.M. provided on the inner wall surface of the anechoic chamber is pyrimidal, the innermost points of the pyrimidical coating surface lying on said contour.

6. An anechoic chamber as claimed in any one of Claims 1 to 3, wherein the inner surface of the anechoic chamber is relective to radio waves.

7. An anechoic chamber as claimed in Claim 6, wherein said anechoic chamber wall is formed of metal.

8. An anechoic chamber as claimed in Claim 6, wherein said anechoic chamber wall is formed of a plastics material which has been metallised.

9. An anechoic chamber in accordance with Claim 1, and as described herein with reference to Figure 2 or Figure 3 or Figure 4 of the accompanying drawings.