Very low frequency

About: Very low frequency is a research topic. Over the lifetime, 1540 publications have been published within this topic receiving 24233 citations. The topic is also known as: VLF.

Specific features of variations in the characteristics of VLF signals when the lunar shadow propagated along the path during the solar eclipse of March 29, 2006

Abstract: The effects of the solar eclipse of March 29, 2006, in the signals of ULF radio stations, in the intensity of regular radio noise at frequencies of 0.3–10 kHz, and in the number of atmospherics received in Yakutsk mostly from the west have been considered. The observations were performed using a multichannel parallel analyzer-recorder (11 channels in the frequency band 0.47–8.7 kHz), one-point lightning direction and range finder (0.3–100 kHz), narrow-sector radio noise direction-finder (0.3–10 kHz), recorder of signals from VLF radio stations, and broadband radio noise recorder (0.3–100 kHz). A GPS clock was used to synchronize a recorder of signals from VLF radio stations. The effect was observed in radio signals, radio noise, and number of atmospherics from the direction 270° ± 20° counted off clockwise from the north during the last stage of the eclipse (∼ 1100–1200 UT), when the lunar shadow approached the line of the nighttime terminator and obscured part of the signal propagation path. The effect was observed as an enhancement of the received signals by a factor of ∼1.2, a factor of ∼1.4 increase in the number of atmospherics, and a change in the radio station phase values.

Magnetic Field Penetration Into a Metal Enclosure Using an ELF/VLF Loop Antenna

Abstract: The ability of extremely and very low frequency (ELF/VLF, 0–30 kHz) radio waves to penetrate conductive media is well established. Magnetic field penetration into a thin but highly conductive box using an ELF/VLF loop antenna transmitter is investigated. The work is relevant for electromagnetic shielding of ELF/VLF sensors, defect detection, inductive power transfer, and through container imaging. Analytical solutions are reviewed for related shielding problems, however, determining the penetration through a realistic shield geometry and finite sized near-field source requires a numerical approach. Surface integral equation (SIE) methods are well suited for shield modeling due to the low surface area to volume ratio of the shield. Method of moment techniques have been successfully applied to solving SIEs in the past, however, enforcing algorithm stability at low frequencies is known to require considerable effort. To alleviate the low-frequency concerns, a high-order locally corrected Nystrom (LCN) scheme is utilized to solve an SIE based on an augmented Muller formulation of Maxwell's equations. To validate the LCN simulations, an experiment is conducted using a loop antenna inside a 1.2 m aluminum cube of 2.7 mm thickness with an external ELF/VLF loop transmitter. Experimental results are shown to match within 3 dB of the LCN code predictions.

VLF/LF High-Voltage Design and Testing

Abstract: : The U.S. Navy has had a requirement for long-distance communications from the beginning and it is natural that their interest in high-power wireless transmitting systems dates back to the beginning of radio. Until only recently, the Navy was still operating some of the very-low-frequency and lowfrequency (VLF/LF) antennas that were constructed not long after the beginning of radio. For example, the LF transmitting antenna at Chollas Heights in San Diego, torn down in 1997, dated back to 1915. Three of the towers in the VLF antenna at Annapolis are identical to towers in the original VLF antenna constructed in 1917. They were added to the original antenna between 1934 and 1937 as part of an upgrade. The Annapolis antenna was modified in 1969, keeping those three towers. The VLF station was closed in late 1997 and the antenna has been demolished. The design of high-power VLF/LF antennas has evolved over the years, but the design of the modern antennas still leans heavily on the early antenna designs. The existing VLF/LF system is the only visible remnant of the Navys original radio system constructed at the beginning of this century, and it is appropriate that this report begin with a brief review of the history of the Navys involvement in radio communications.

Effect of humidity on the breakdown characteristics of air in uniform field for the very low frequency (VLF) band

Abstract: The paper reports about the role of humidity and frequency on the electrical breakdown of air in uniform field gaps. Experiments were conducted on Rogowski-profile electrodes with gap lengths ranging from 5 to 53 mm at power frequency (60 Hz) and frequencies in the range of 18-50 kHz, corresponding to the VLF/LF bands used for long-range communication. The results show that breakdown voltage at VLF decreases with humidity, opposite to that observed at 60 Hz. Breakdown mechanisms for explaining this important phenomenon are proposed. A correction factor to calculate breakdown voltage as a function of humidity for VLF is presented.