VLF Field Strength Variations From an Airborne Trailing-Wire Antenna


  • Francis J. Kelly


The periodic variations in field strength and phase received from an inclined, elevated VLF transmitting antenna as the antenna is moved in a circular path (orbit) are calculated. This is a simplified model of a trailing-wire transmitting antenna carried by an orbiting aircraft. The earth-ionosphere waveguide propagation model with an isotropic homogeneous ionosphere is used. For an inclined orbiting antenna, the magnitude and sense (whether increasing or decreasing at a given time) of the variations are found to depend on the range of the receiver from the transmitter. For an antenna whose inclination angle oscillates with its orbital position, the orbit-produced variations are both range-dependent and bearing-dependent. Either one or two relative maximum values of field strength occur per orbit. Additional complications introduced by an anisotropic ionosphere are also considered.