Peter R. Bannister

Reception of ELF signals at antipodal distances

Measurements of 82-Hz radio signals from a Russian ELF transmitter located on the Kola Peninsula are described. The measurements were made at several locations around the world, including Dunedin, New Zealand, and Arrival Heights, Antarctica, which are close to the antipodal point for the transmitter. This is the first time man-made ELF signals have been observed over such long distances, and their clear reception makes possible a comparison of the theoretically predicted and measured amplitudes near the antipode. The agreement is excellent.

Modal phenomena in the natural electromagnetic spectrum below 5 kHz

This paper presents new measurements of the magnetic field component of naturally produced electromagnetic radiation in the ELF/VLF range. The measurements are compared to calculations based on modal propagation theory. The nocturnal spectrum below 5 kHz often contains a sharp increase in spectral level at the cutoff frequency of the first mode (∼1700 Hz) and a distinct variation of the spectral behavior at the cutoff frequency of the second mode (∼3400 Hz), where the indicated cutoff frequencies were calculated for perfectly conducting Earth and ionosphere, with the ionosphere 88 km above the Earth. These features are attributed to an enhancement in the first (quasi-transverse electric (QTE1)) and second (QTE2) propagating modes at night, which makes their level comparable to the basic (transverse electromagnetic) mode. Another spectral feature, which also prevails during the night, is semiperiodic fluctuations of the spectrum between the two cutoff frequencies, with a short period at the low end of the band and a gradual increase of the period as frequency increases. A similar semiperiodic fluctuation is apparent above the cutoff frequency of the second mode up to the limit of the measured band at 5 kHz. These semiperiodic fluctuations of the spectrum are related to the modal nature of the electromagnetic wave propagation. We show that computations of the spectrum of the ELF/VLF radiation emitted by a vertical lightning in the Earth-ionosphere waveguide gives spectra that compare well with the measurements.

Further examples of seasonal variations of ELF radio propagation parameters

In this paper we use experimentally determined values of effective attenuation rate, excitation factor, and relative phase velocity, along with the theoretical expressions derived by C. and P. Greifinger, to establish the seasonal variation of representative ionospheric conductivity parameters. These parameters include the reflection heights h0 and h1 (or hE), inverse scale height β, and reference height H. The basis for this analysis is provided by the 1990–1992 76-Hz field strength measurements taken at four land-based ELF monitoring sites established by the U.S. Navy. The source for these measurements was the U.S. Navys dual-antenna transmitting system (WTF/MTF). The main conclusion of this paper is that the summertime and January nighttime attenuation rates are substantially lower than during other times of the year. This nighttime attenuation rate decrease appears to be mainly due to an increase in the inverse scale height β, rather than to an increase in the reflection heights h0 and hE.

Wisconsin Test Facility Transmitting Antenna Pattern and Steering Measurements

During August and September 1971, the New London Laboratory, Naval Underwater Systems Center, performed pattern and steering measurements on the Wisconsin Test Facility (WTF) antennas. The pattern measurements were made at 13 locations in Minnesota and Wisconsin (covering approximately 120° of arc), while the far-field steering tests were taken in Maine and North Carolina. To be certain that the receiving sites were acceptable, several components of the received magnetic field were measured at each site and plotted on a normalized cosine curve. The 45- and 75-Hz pattern measurements were made at a range of approximately 300 km, while the steering measurements were performed at 1.7 Mm. The principal results from the pattern and steering measurements were 1) the east-west (EW) antenna pattern is skewed clockwise; 2) the north-south (NS) antenna pattern is skewed counterclockwise; and 3) the effective conductivity under the EW antenna is greater than the effective conductivity under the NS antenna.

Nighttime Variations of Extremely Low Frequency (ELF) Signal Strengths in Connecticut

A limited amount of extremely low frequency (ELF) horizontal magnetic field strength measurements have been taken in Connecticut during the past 3 years. The transmission source for these 1.6 Mm range measurements was the U. S. Navy ELF Wisconsin Test Facility. The principal result obtained from these measurements is that there are considerable variations in the ELF nighttime propagation parameters.

Results of the Wisconsin Test Facility Buried Versus Elevated North-South Antennas Test

In March and April 1973, the 76-Hz magnetic field strength, produced by the Wisconsin Test Facility elevated and (newly installed) buried north-south antennas, was measured in Wisconsin and Texas. The principal result obtained is that there is no measurable difference in performance between the two antennas.