Developing a broadband circuit model for the Cutler VLF antenna

To investigate intermodulation problems related to two-frequency operation with the two identical elements comprising the Cutler VLF (15-30 kHz) antenna, a broadband (15-90 kHz) equivalent circuit was developed. The antenna is depicted by a simplified wire model, consisting of two 6-panel top-loading sections of /spl sim/1 km in radius supported by 13 towers at an elevation of /spl sim/300 m. Two /spl sim/850-m coaxial transmission lines lead from a centrally located transmitter building to tuning systems located near the centers of each element. Although designed for use at a single frequency, current operational requirements make it important to consider increasing the information rate by operating the two antenna elements at two different frequencies. The purpose of this study was to provide an accurate two-port circuit model of the two antennas as part of a PSpice simulation of the entire transmitting system. Since the transmitters and the saturable reactors used for dynamic tuning involve substantial non-linearities, intermodulation products and harmonics at frequencies ranging up to the third harmonic were of concern. This dictated that an equivalent circuit was required that could accurately simulate terminal behavior over a frequency range of 15-90 kHz, i.e., the VLF range and its third harmonics.