GNGTS 2021 - Atti del 39° Convegno Nazionale

GNGTS 2021 S essione 2.1 200 only few modes need be considered. While the physical picture of wave-guide mode theory is less easy to visualize compared with ray-phats it provides a better explanation of certain features of VLF propagation which are not readily explicable on ray-phat/wave-hop theory. VLF and LF radio transmitters in common use radiate a vertically polarized field. The ionosphere and terrestrial magnetic field may introduce a horizontally polarized component, and theoretical calculations suggest that at night in low latitudes there may be considerable horizontal polarization (Foley et al., 1973). Horizontal electrical dipole excitation of vertical E field at the ground for the 19.8 kHz radiation beneath a hypothetical daytime ionosphere and beneath a highly anisotropic ionosphere has been numerically investigated as a function of ground conductivity and source elevation (Pappert, 1970). Results show that for source elevations comparable to or greater than a wavelength, horizontal and vertical dipole excitations can became comparable. When one end or both ends of a propagation phat are at an elevated position, gains in signal can be achieved by using transmitting and receiving antenna orientations other than vertical (Pappert and Bickel, 1970). This property is of particular importance for air-to-air VLF and LF communications or carrier wave monitoring such as obtained by the Central Italy Electromagnetic Network (CIEN). In fact, the CIEN antennas are horizontal wires elevated many meters above the ground (Fidani, 2011), which are connected to wide band amplifiers able to cover the entire SLF/ELF/VLF/LF band from 3 Hz to 300 kHz. CIEN is a continuous recording system of electromagnetic fields actually operative up to LF band in the four stations of Fermo, Chieti, Camerino and Perugia. The Fermo and Camerino stations recording electric fields up to 50 kHz, the Chieti station recording electric fields up to 100 kHz, and the Perugia station located in the “A. Bina” Seismic Observatory has been recently updated to record electric fields up to 200 kHz. VLF and LF spectrograms created in Perugia are plotted in Fig. 1 and 2, respectively. Fig. 2 - The spectrogram recorded at Perugia station of the “A. Bina” Seismic Observatory for LF band.