Currently there is considerable interest in developing a method to measure the radiated EM emissions and EM susceptibility of electronic equipment. Rectangular coaxial TEM cells developed at NBS for this purpose have been found to be quite satisfactory in establishing standard test field environment for such measurements. One must have a method to obtain the free space radiation characteristics of a test object from measurements taken inside such a cell. To do this we may consider a typical equipment under test (EUT) to consist of an electrically small box housing low frequency AC circuits. The current carrying conductors in each AC circuit may be replaced by equivalent electric/ magnetic dipoles with appropriate phases, and all such dipoles may be combined into a system consisting of three orthogonal electric and three orthogonal magnetic dipoles each excited with arbitrary phase.

A variational expression for the scattering matrix of a double-step discontinuity in a two-mode coaxial transmission line is obtained and generalized to the case of a multimode coaxial transmission fine. The result is used to analyze the transmission characteristics of a two-mode coaxial TEM cell.

An electrically small radiating source of arbi­ trary shape may, to a first order, be modeled by an equivalent dipole system consisting of three orthogonal electric dipoles and three orthogonal magnetic dipoles each excited with arbitrary amplitude and phase. In an earlier report [1,2] we described a measurement proce­ dure for determining the individual electric/magnetic dipole moments and the cross-components of such an equivalent dipole radiating system by tests inside a TEM cell (figure 1). This method enables us to deter­ mine the free space emission characteristics of an equipment under test (EUT). If the size of the radiating source is so small that E may be assumed to be constant over the volume n occupied by the source, the coefficients a and b n n are simply given in terms of the electric and/or mag­ netic dipoles characterizing the source. If E^ is

Various approximations to the exact formal integral representations of the near-field and the impedance are considered. A new approximate form is obtained which appears to be valid even when the dipole is near a poorly conducting earth. Numerical comparisons with the exact integral formula verify some of the conjectures. The result shows that the ground has a significant influence on the input impedance of the dipole, particularly for low heights.