would depend upon the transmission line openings required for the uniform illumination of the DUT. Ideally DUT should be pertual the propagating plane wave field vecors within the line, so as to have faithful simulation of the desired waveshape. This is theoretically impossible to achieve but, in a practical facility, if the dimensions of the perturbation - like the object under test are such as not significantly distort the field vectors, the simulation is still considered valid. A general rule of the thumb is that an acceptable low-distortion illumination of DUT within the test volume is achieved if, the DUT height and width are less than one-third the corresponding lines openings. The DUT should occupy a length not exceeding two-three line. In addition the DUT should not be heavily loaded with high permittivity and / or high permeability material as to significantly distortion the line's TEM fields. Having decided on the dimensions of the parallel plate region based on the device size, the characteristic impedance of the line is determined from the Unit Cell approach [6] or from the expression given for the strip line geometry [7]. For our BWTL simulate with dimensions of 300 mm (L) x 150 mm (W) x 50 mm (H) a characteristic impedance of 75 ohms was obtained for an infinite ground plane. However, the measured characteristic impedance of the line is 98 ohms, which is due to finiteness of the ground plane in the practical facility. The line was terminated at the receiving end to copper sulphate load of 98 ohms value, thereby, constituting reflectionless termination.

Substituting the values of the constants in (1) and equating them to the variables expressed in (2), we get

For the matched load of , the circuit parameters that are worked out are: and . (Corresponding circuit parameters for the IEC waveform are 153 nH and 272 pF respectively).

Having determined the circuit parameters leading to generation of the desired waveshape for the simulation, near distortion free plane wave propagation of this waveshape is described now. It is known that an infinite parallel plate region of perfect conductors in vacuum support uniform plane wave simulator due to finiteness of dimensions and conductivity of materials used in its fabrication. But the deviation from an ideal situation can be minimised by fulfilling the following conditions [8]. (i) The displacement current within the conductor employed for the guide is negligible compared to the conduction current it carries. (ii) the intrinsic impedance of the dielectric employed between the guide conductors is much greater than the skin effect surface resistivity of the conductors. (iii) The guide openings are much less than the smallest wavelength being propagated along the guides. This condition ensures negligible radiation loss from the guide openings and minimizes the likelihood of exciting the TE, TM or higher order modes of wave

propagation which are essentially non-uniform plane wave types.

The three essential conditions mentioned above which are vital for the plane wave simulation of the EMP fields within the transmission line simulator test volume are mathematically represented as follows:

In the above expressions, s is the conductivity of the guide conductor, w is the radian frequency of the signal being propagated within the guide. e is the dielectric permittivity of the guide medium between the conductors, Rs is the skin effect surface resistivity of the guide conductors, h is the intrinsic impedance of the dielectric, a is the vertical opening between the guide conductors and l is the smallest wave length present in the signal being propagated between the guide conductors.

III. CONSTRUCTION

The physical construction of an NEMP facility, besides the design guide lines stated above, is dictated by factors such as availability of components and materials, portability and compactness of the equipment and above all, on the intended use, i.e., whether an R&D facility for study purposes or, a production unit to test the components/systems compliance to given specifications.

The geometrical schematic of the bounded wave transmissions line simulator, designed for in-house purposes to study the effects of NEMP simulation on the electronic sub-systems, is shown in Fig.2.