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.

Practical Application of an Ultra-High-Frequency Radio-Relay Circuit

Abstract: The utilization of an ultra-high-frequency radio circuit for the transmission of telegraph, teletype printer, and facsimile signals is described. The operating procedure is stressed throughout, particularly with regard to equipment maintenance tests and the methods employed to determine the conditions of the circuit such as degree of modulation, signal-to-noise ratio, etc. Considerations are presented with respect to the most efficient division of the total modulation band into the communication channels, as well as the signal-to-noise ratios required for the different types of service. It is found that fading, static, and weather conditions at these frequencies are of little importance as to their effect on the economic use of the circuit. However, diathermy machines and similar sources of disturbance are troublesome and their effect must be minimized. Experience during the past year and a half indicates that the dependability of the ultra-high-frequency radio circuit is of a high order.

Special purpose pulsar telescope for the detection of cosmic gravitational waves

Abstract: Pulsars can be used to search for stochastic backgrounds of gravitational waves of cosmological origin within the very low frequency band (VLF), 10(-7) to 10(-9) Hz. We propose to construct a special 50 m radio telescope. Regular timing measurements of about 10 strong millisecond pulsars will perhaps allow the detection of gravitational waves within VLF or at least will give a more stringent upper limit.

Experience with high potential testing hydro generator multi turn stator coils using 60 Hz AC, DC, and VLF (0.1 Hz)

Abstract: There are many different methods of employing a high potential test on a stator winding. Three such methods that this paper will explore with reference to one another are the AC (50-60 Hz), DC, and very low frequency (VLF) (0.1 Hz). Some users choose the AC high potential test knowing that this test best simulates the voltage stress on the winding while in service. Other users prefer the DC high potential test largely due to ease in performing the test. However, the DC voltage does not stress the stator coils the same way as when they are in service and may result in overly pessimistic results due to the influence of surface contaminants in the end windings. Finally, the VLF test, due to recent advances in technology, is becoming more practical for use in field conditions. However, the present standard governing the test is almost 40 years old and there is significant interest in what VLF voltage level best correlates with the AC and DC high potential tests. This paper reports preliminary test results on three generator windings that were destructively tested using the AC, DC, and VLF methods as part of an ongoing effort to provide a database upon which to set the appropriate VLF hipot level for modern synthetic resin-based stator insulation systems.

VLF loop array antenna

Abstract: A directional loop array antenna for very low frequency (VLF) reception. array comprises four closely spaced loop antennas forming a unidirectional reception pattern with a main beam of less than 43° between half power points. The array combines two double loop coaxial antennas dispersed with their axis parallel along 45° lines. The signal from one double loop array is delayed sufficiently such that a unidirectional reception pattern is formed.

VLF, ELF and ULF research using the Tromsø heating facility

Abstract: : A review is presented of results from the Tromso Heating facility in the areas of wave generation from ULF (mHz) to VLF (kHz) frequencies, as well as VLF wave propagation under heated ionospheres. Results from similar facilities in the USSR and USA will not be included. High power high frequency (HF) radio waves heat the electrons in the lower ionosphere through non-deviative absorption on the time-scale of microseconds resulting in the enhancement of the electron-neutral collision frequency. The heated region, typically 25 km diameter at 80 km altitude, can act as a perturbation on the upper wall of the Earth-ionosphere waveguide, thereby affecting the propagation of VLF waves. On the other hand, by amplitude modulating the heating wave the heated region can act as an oscillating current source, if there is an external driving electric field, which can itself radiate waves at the modulation frequency into the waveguide or into the magnetosphere. The auroral zone is ideally situated for experiments of the second type because of the frequent presence of large (tens of mV/m) electric fields. For experiments of the first type, the auroral zone is not quite so ideal since large natural ionospheric variations can mask the artificially produced ones. We shall review results from both types of experiment performed using the heating facility of the Max-Planck-Institut fur Aeronomie, a description of which can be found in Stubbe and Kopka (1979) and Stubbe et al. (1982a, 1985). This facility will be operated by the EISCAT scientific association from 1993 onwards.