The present invention relates to an apparatus and method for use in electromagnetic compatibility testing. Increasingly, there are problems caused by the unintended interaction between electronic instruments . Electromagnetic radiation "leaks" from one piece of apparatus, and may affect the operation of another. In many places, including the European Union, there are regulations concerning both the "leakiness" (that is, the generation of unwanted electromagnetic radiation) of devices, and the susceptibility of devices to external radiation. However, both the enforcement of such regulations, and the ability of manufacturers and others to comply with them, depend on the availability of systems for monitoring the behaviour of apparatus . The present invention concerns a system for use in determining the susceptibility of apparatus to electromagnetic radiation. The system may be used in ensuring that apparatus complies with the latest EU directive on electromagnetic compatibility. (Currently the relevant regulation relating to susceptibility is EN50082. ) Background Art Basically, to determine the susceptibility of a piece of apparatus to electromagnetic radiation, it is necessary to generate a radiation field around the apparatus . Plainly this should be done without leaking

great amounts of radiation externally of the test rig. Furthermore the radiation field should desirably be of known and controllable strength at the test location. This can be achieved by locating a test site and a radiating antenna within an anechoic chamber. However, this is extremely expensive. A more practical approach was provided by M.L. Crawford, IEEE Trans . Electromagn.Co pat. , EMC-16, 189-195 (1974) . This makes use of a square-section cell with an outer conductive shell 10 surrounding a central conductor plate or septum 12 (see Fig. 1) . The cell is, in effect, a length of giant rectangular-section co-axial transmission line, so big that equipment to be tested can be placed inside the outer conductor 10. In use, power from a broad-band amplifier is supplied to the cell at one end and absorbed in a terminating load resistor at the other end. Equipment to be tested is located within the cell, on one side of the septum 12, in a region of approximately uniform field. The field essentially consists of transverse electromagnetic waves. At the ends of the cell, the four faces of the outer case and the septum are tapered down to the dimensions of an ordinary co-axial line, for connection to the rest of the equipment.

The cell itself is symmetrical, but this symmetry is lost once the apparatus to be tested is introduced. This leads to distortions in the field produced within the cell, which can lead to various problems, e.g. associated with a change in the characteristic impedance, preventing

the cell from remaining matched to the amplifier and load. An attempt has been made to reduce this problem by displacing the septum from the centre of the cell (M Crawford and J L Workman, IEEE International Symposium on EMC, 204-210 (1978)) . A cell of this type is commercially available under the name G-STRIP, from Comtest Limited (Wokingham, GB) . This employs a cubic aluminium box (e.g. with sides 2.0m long) as the outer case, with a septum 50mm from the bottom side. Several other variants of Crawford's original cell have been proposed. For example, X.D. Cai and G.I. Costache (IEEE Trans. Electromagn.Cσmpat .,EMC-36, 391-404 (1994) ) have described a cell having two septums at right-angles, with a switch so that either one or the other septum can be powered. Thus apparatus can be subjected to fields in two orthogonal directions without having to move it. Summary of the Invention

According to the present invention in a first aspect, there is provided a cell having an elongate hollow conductive body and two conductive septums within the body, the septums being symmetrically disposed within the body, and extending in parallel with one another. Generally the body is of substantially rectangular section in its central region (where apparatus to be tested will be located) . The septums are disposed, on each side of a central plane of the body. A support for apparatus to be tested will generally be provided, such

that the apparatus can be centrally located within the body.

Preferably the corner regions of the substantially rectangular body are bevelled. This can reduce the field concentrations in these regions. Preferably the end regions of the cell are tapered, being substantially pyramidal .

It is convenient for the cell to be adapted for use with its axis vertical. Thus there will generally be a non-conductive support for apparatus to be tested which is located within the body, between the septums, perpendicular to the axis.

In a second aspect the invention provides test apparatus comprising a cell according to the first aspect and electrical circuitry connected thereto and adapted to apply AC signals to both septums simultaneously but out of phase, so that substantially transverse electromagnetic radiation is generated in differential mode. In a third aspect, the invention provides a method of measuring the susceptibility of apparatus to electromagnetic radiation, wherein the apparatus is located in a cell according to the first aspect; and transverse electromagnetic radiation is generated in the cell by means of both septums simultaneously, in differential mode.

The symmetrical two-septum design allows the cell to be operated so that a plane of zero potential

exists between the septums. This allows cables from the test apparatus to be positioned along a plane of zero potential. This makes the tests more easily repeatable. Furthermore, by placing the test object mid-way between the septums, the symmetry of the field is maintained.

The change in characteristic impedance of the line due to placing the test object within the cell is one half that of the single septum cell, so that the amplitude of the reflection caused by the test object is reduced. An embodiment of the invention will now be described in more detail with reference to Figs. 2 and 3 of the accompanying drawings. Brief Description of the Drawings In the accompanying drawings : Fig. 1 is a section through a prior art cell;

Fig. 2 is a perspective view of a cell embodying the present invention; and

Fig. 3 is a schematic perspective view showing the septums and the external electrical connections. Description of the Preferred Embodiment

As shown in Fig. 2, the cell has a hollow body 20 having a main portion 21 of substantially rectangular section, with bevelled corners 22. At each end there is a substantially pyramidal end portion 24, where the dimensions of the cell are smoothly reduced. One face of the rectangular portion is formed as a door 26 for allowing access to the interior, for inserting and removing apparatus to be tested. Electromagnetic seals

are provided around the edges of the door. (Suitable components are available from Radiofrequency Investigations Limited.) At the vertices, there are lugs 28 for mounting the cell to a rack (four at the corners of the door frame, and four at the back, symmetrically disposed with reference to the door.) The body is electrically grounded.

Inside the body 20, there are two septums 30. They are mutually identical. Each has a main planar plate portion 32 which extends within the main rectangular- section portion 21 of the body. At end each, there is an angled tapering portion 34, which extends within one of the pyramidal end portions 24 of the body, adjacent one of the faces thereof. The main rectangular portions 32 are of constant width, with constant separation between the two septums. In the end regions, the plate widths and separations are reduced in such a way as to maintain constant characteristic impedance along the length of the cell. At one end of the cell, each septum plate is grounded via a non-inductive resistor 36 having a resistance equal to the characteristic impedance as measured between the septum and the adjacent cell wall. A broad band amplifier 38 is connected to the other ends of both septums via a balun 40. The amplifier input is connected to a signal generator 42. In this way an alternating voltage (differential mode) is applied between the plates. This generates a region of uniform, substantially transverse mode, electromagnetic field

between the plates. The equipment to be tested is placed mid-way between the septums in this region of uniform field, on a non-conductive support 44.

The body and septums are formed of a conductive metal, suitably aluminium. The septums are supported by insulating spacers, e.g. of nylon.

In use, a field of 10 V/m to within 3 db limits over the frequency range 26Mhz to 1GHz can be produced within a target volume of 275 X 510 X675 mm, allowing the requirements of the various standards to be fulfilled. The power required for this is of the order of 2 Watts into a cell where the length of the centre section 21 is 750mm and the rectangular section is 750 X 750mm. (We actually use a 10W amplifier.) The characteristic impedance between a septum and the adjacent cell wall is 25 Ohms. Of course, the cell can be produced in different sizes.