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Radio wave propagation and the frequency spectrum
Published in Laurie Tetley, David Calcutt, Electronic Navigation Systems, 2007
VLF radio signals propagate using a combination of both ground and space waves. They require vast amounts of power at the transmitter to overcome earth surface attenuation and can be guided over great distances between the lower edge of the ionosphere and the ground. Because VLF possesses a very long wavelength, huge antenna systems are required. As an example, at 10 kHz the wavelength is 30 km. An efficient antenna, often quoted as ‘a half-wavelength antenna’, needs to be 15 km long and it is only possible to construct one on land, usually slung between mountain peaks.
Highway plant
Published in Malcolm Copson, Peter Kendrick, Steve Beresford, Roadwork, 2019
Malcolm Copson, Peter Kendrick, Steve Beresford
(b)Radio mode: This enables the tool to detect long conductors even if they do not carry any energy of their own. These conductors include live, but not loaded, power cables, main telephone cables and some continuous metal pipes. The signals originate from distant VLF radio transmitters and these signals are re-radiated by the buried conductors in sufficient strength to be detected by the tool.
Radio transmission
Published in Michael Talbot-Smith, Sound Assistance, 1999
Figure 4.1 shows the elements of Table 4.1. Two important facts about the spectrum are: The visible spectrum is actually a very narrow band of frequencies: the ratio of the highest (violet light) to the lowest (deep red) is less than 2:1, with yellow coming near the middle. When dealing with light it’s usual to use wavelengths rather than frequencies and the most convenient unit is the nanometre, nm. 1 nm = 10’9m or 1/1000 mm.Radio waves are divided up, for convenience, into sections. These are: VLF (very low frequency): The wavelengths here are extremely long and range from 10000 m to 100000 m.LF (low frequency): From 1000 m to 10000 m. This covers the broadcast Long Wave band.MF (medium frequency): Changing our units, this band is centred on about 1 MHz and covers the ‘medium wave’ band on a domestic radio.HF (high frequency): Around 10 MHz and into the ‘short wave’ region.VHF (very high frequency): Around 100 MHz. In the UK vhf radio is from about 88 MHz to 110 MHz.UHF (ultra high frequency): Around 1000 MHz (1 gigahertz). Television broadcast frequencies come into this range.SHF and EHF (super-and extra-high frequencies): These go up to 100 GHz and beyond. (Microwave cookers operate in the SHF/EHF frequencies, using radio waves of about 10 cm.)
Lightning fatalities and its correlation with social variables in Northwestern Mexico
Published in Geomatics, Natural Hazards and Risk, 2023
Grisel Alejandra Gutiérrez-Anguamea, Carlos Manuel Minjarez-Sosa, Xochitl Guadalupe Torres-Carrillo, Guadalupe Esteban Vázquez-Becerra
The data analyzed in this study are CG strokes, mortalities officially reported with a cause of death by CG strokes, social vulnerability, and the landscape of the region studied. The basis of this research is the CG strokes that are contained in records from January 1, 2015, to December 31, 2019, from the lightning location system (LLS) called Global Lightning Detection (GLD360) Network. GLD360 was launched by Vaisala, Inc. in 2009 and uses a combination of the direction-finding and time-of-arrival techniques for lightning geolocation. This LLS uses broadband sensors that detect the VLF range (VLF; 3-30 kHz) worldwide (Said et al. 2010, 2013, 2017) recording total lightning; however, the system can distinguish between CG strokes and IC pulses in this frequency range. Said and Murphy (2016) validated the GLD360’s relative detection efficiency (DE) at approximately 75-85% of all CG flashes compared to data collected by the National Lightning Detection Network (NLDN), they demonstrated only a 40-50% relative IC pulse DE and a median location accuracy ranging from 2.4 km to 1.8 km in CG flashes over the United States. GLD360’s sensors can detect storms and lightning strokes (CG and IC) that take place anywhere in the world, both on the continental surface and over the ocean, allowing for a geospatial analysis with this dataset in Northwestern Mexico.
Pre-Evaluation method of the experiential architecture based on multidimensional physiological perception
Published in Journal of Asian Architecture and Building Engineering, 2023
Wanyu Pei, Xiangmin Guo, Tiantian Lo
PPG is mainly used to measure the user’s heart rate variability (HRV), which can give objective indicators of sympathetic (SNS) and parasympathetic nerve activity (PNS) balance in the autonomic nervous system (Electrophysiology 1996). Descriptive values (mean ± sd) of HRV in time, including i) the mean of Heart Rate (Mean HR); ii) standard deviation regular RR interval (SDNN) and iii) root mean square of successive differences squared (RMSSD). In the frequency domain variables, the following was considered of the spectral components: i) very low-frequency component (VLF); ii) low-frequency component (LF); iii) high-frequency component (HF), and iv) LF/HF ratio.
An alternative amelioration of ME-based DZT-PML for truncating FDTD problems
Published in Electromagnetics, 2021
Naixing Feng, Xue-Shi Li, Yuxian Zhang, Yanming Sun
As known to all, the signal in shorter-wavelength radio waves will be weakened to be neglected when propagating farther and farther, and easily prohibited by the rock layer. To solve out this problem, the VLF radio wave is employed because of its advantages of long-distance propagation and strong-power penetration so that it can propagate and span over hundreds of feet in water and earth, and thousands of miles in the air. Therefore, to validate the absorption accuracy of the proposed DZT-ME-PML, we present 3D VLF subsurface sensing cases for the geophysics exploration.