Localized Time Rotation of the Electric Field Near the Boundary of the Reactive Near Field of a Dipolar Antenna

It is well known that in the reactive near electromagnetic field of a dipolar antenna such as a 1.4- m biconical antenna the electromagnetic field is predominantly electric in nature and that in some immunity measurements such as MIL- 461, RS 103, the DUT is located in this region. That the field is predominantly electric in nature is considered acceptable if the anticipated coupling mechanism into the DUT is electric. It is less widely noted that near the boundary of the reactive near field (the induction zone) and outside of the H-plane of the dipole, the near electric field vector undergoes time rotation. For the geometry of MIL-461 RS 103 with a DUT located 1 meter from a 1.4-m biconical dipole we show that in the vicinity of 100 MHz, in some portions of the uniform field area, the electric field appears to be pseudo-circularly polarized with two orthogonal equalmagnitude components in phase quadrature. A simple analytical model is used to confirm the electric field rotation for an isolated dipole. A numerical simulation is employed to compute the field of a more complex and practical 1.4-m biconical antenna and also to model the effects of ground. Preliminary experimental results confirm the simulation. Clearly, such a field would affect a DUT differently from the way a linearly-polarized electric field would and also very differently from the way a true circularly-polarized plane wave would.