Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R29/00—Arrangements for measuring or indicating electric quantities not covered by groups G01R19/00 - G01R27/00
  • G01R29/08—Measuring electromagnetic field characteristics
  • G01R29/10—Radiation diagrams of antennas
  • G01R29/105—Radiation diagrams of antennas using anechoic chambers; Chambers or open field sites used therefor

Definitions

• 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.
• R.F. radio frequency
• a properly designed and constructed anechoic chamber should resemble, as far as possible, a "free-space" environment for antenna testing.
• both the shape of the chamber and the material located within it which absorbs unwanted radio waves 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.
• 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 quiet-zone volume within which a test antenna may be located.
• 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.
• the anechoic chamber is formed by rotation through 360° about said major axis of said first ellipse.
• Radio absorbing material 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.
• the wall of the anechoic chamber may be formed of radio absorbing material, or have radio absorbing material located thereon.
• 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.
• 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.
• 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 am anechoic chamber embodying the present invention
• FIG. 3 is a sectional view through another anechoic chamber embodying the present invention.
• 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 12 on top of an ellipse 14 with one focus of each of the two ellipses coincident as shown at 16 whilst the axes 18 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
• FIG. 2 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.
• a source antenna 34 is mounted within the chamber.
• a volume of radar/radio absorbent material e.g. such as carbon loaded Eurethane foam is located at the other focus 36 of the main elliptical part of the anechoic chamber.
• R.A.M. radar/radio absorbent material
• 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.
• 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 ⁇ between the axes of the ellipses forming the chamber (lines 18 and 20 Figure 1) has a minimum value equal to Tan -1 where
• X is the length of the minor axis of the main ellipse part-
• Y the length of the major axis of the main ellipse part of the chamber
• 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.
• FIG 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.
• 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.
• 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 110 - 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,
• 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.
• R.A.M. is further provided . at 130 that is to say on the inner surface of the main ellipse part of the anechoic chamber.
• any radio waves not passing along the axis joining the focii 120, 122 - that 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.

Landscapes

• Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• General Physics & Mathematics (AREA)
• Aerials With Secondary Devices (AREA)
• Shielding Devices Or Components To Electric Or Magnetic Fields (AREA)

Applications Claiming Priority (2)

Publications (1)

Family

ID=10561755

Family Applications (1)

Country Status (5)

Families Citing this family (5)

Family Cites Families (4)

• 1985
  • 1985-05-31 JP JP60502446A patent/JPS61502278A/ja active Pending
  • 1985-05-31 AU AU44076/85A patent/AU4407685A/en not_active Abandoned
  • 1985-05-31 WO PCT/GB1985/000231 patent/WO1985005692A1/en not_active Ceased
  • 1985-05-31 GB GB08513767A patent/GB2159667B/en not_active Expired
  • 1985-05-31 EP EP19850902599 patent/EP0216768A1/de not_active Withdrawn

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events