Propagation of VLF radiation in the earth-ionosphere waveguide excited by an airborne dual trailing wire antenna

Field strength variations produced by an orbiting aircraft dual trailing wire VLF transmitting antenna are investigated. The towplane is assumed to be executing a circular orbit at a constant altitude and speed. A steady-state mechanical model is adopted for determination of the shape of the dual trailing wire antenna. The exact current distribution on this antenna is calculated using the Numerical Electromagnetics Code (NEC) which is based on a method of moments solution of the Electric Field Integral Equation (EFIE). A propagation code developed at the Naval Ocean Systems Center (NOSC) called TWIRE has been modified to be used in conjunction with NEC. This modified version of TWIRE has been called TWIRENEC. The TWIRENEC code uses the current distribution information provided by NEC to determine the dipole moments for a segmented antenna. The wire segmentation geometry and corresponding dipole moments are then used to calculate the electric field strength as a function of distance and azimuth in the earth-ionosphere waveguide. The waveguide can be considered as either horizontally homogeneous or inhomogeneous. It is demonstrated that the periodic variations in field intensity resulting from an orbiting transmitter are a function of receiver position. These periodic variations can range from a small fraction of a dB to several dB depending upon the location of the receiver with respect to the transmitter. A point dipole approximation of the dual trailing wire antenna is suggested for use in the study of VLF radiation excited by an orbiting antenna in the presence of wind shear. The point dipole approximation is applied to estimate the field strength variations caused by a yo-yo oscillation of the transmitting antenna as it orbits. These yo-yo oscillations are characterized in terms of the change in verticality of the point dipole which occurs over one complete orbit.