Hindawi Publishing Corporation Research Letters in Physics Volume 2009, Article ID 216373, 4 pages doi:10.1155/2009/216373 Research Letter Waveguide Parameters of 19.8 kHz Signal Propagating over a Long Path Sushil Kumar School of Engineering and Physics, The University of the South Pacific, Private Mail Bag, Suva, Fiji Correspondence should be addressed to Sushil Kumar, kumar su@usp.ac.fj Received 15 April 2009; Accepted 21 June 2009 Recommended by Faramarz Farahi The amplitude and phase of 19.8 kHz signal from navigational transmitter located in North West Cape, Australia, recorded at Suva, Fiji, have been utilized to determine the waveguide mode parameters. The propagation path is mixed over land and sea having Transmitter-Receiver Great Circle Path distance 6.7Mm. The experimental values of the parameters were found to be consistent with the theoretical values calculated using the mode theory of VLF wave propagation in the waveguide. Copyright © 2009 Sushil Kumar. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 1. Introduction Extremely Low Frequency (ELF: 3–3000Hz) and Very Low Frequency (VLF: 3–30 kHz) signals propagate to great dis- tances in the waveguide bounded by the Earth’s surface (ground or sea) and the lower ionosphere (∼60–140 km), consisting of the D and E-regions. This atmospheric waveg- uide is known as the Earth-Ionosphere Waveguide (EIWG). The D and E-regions are too high for balloons and too low for the satellite measurements. Radio sounding does not work, particularly at night, since electron densities in this region are low to reflect high frequency radio waves with ionosondes and incoherent radars. Therefore, the lower ionosphere remains the least studied region of the Earth’s atmosphere [1]. The conductivity of the D-region increases exponentially with height which may also vary with latitude and longitude along the propagation path. At ELF/VLF, the Earth’s surface and the lower ionosphere act as good electrical conductors. Electromagnetic waves reflect when incident upon conducting boundaries and are guided along the conducting structures. The EIWG offers very little attenuation hence the guided ELF-VLF signals can be received literally around the world [2] and have long been used for long-distance communication, positioning and timings. Communications with submarines immersed in the conducting sea neces- sitated the use of VLF waves and later ELF waves, with their comparatively large skin depths in sea water [3]. Yokoyama and Tanimura [4] first observed diurnal vari- ation of the amplitude of VLF (17.7 kHz and 22.9 kHz) signals propagated over long distances (>5000 km). Diur- nal variations of VLF transmitter signals show amplitude minima associated with phase steps during sunrise and sunset transitions between transmitter and receiver [4–6] which could not be explained by single mode propagation theory. Wait [2] suggested that multiple modes are needed to explain VLF propagation in the EIWG. Crombie [5] proposed a model based on two modes being present in the nighttime portion of the propagation path and only one mode in the daytime portion of the propagation path in the EIWG. Clilverd et al. [6] have presented some interesting long-term (1990–1995) studies of VLF propa- gation over a long North-South path from Cutler (USA), Marine (NAA transmitter, 24kHz) to Faraday, Antarctica. They found the timings of the minima in the received signal strength to be remarkably consistent from year to year. They also found that the timings of minima were consistent with modal conversion taking place as the day- night boundary (terminator) crossed the propagation path, at specific, consistent locations. The stability and repro- ducibility of the received amplitude and phase of VLF navigational transmitter signals make VLF propagation a useful tool for long-range communication and navigation system.