In his seminal work, Extremely Low Frequency Propagation Measurements Along a 4,900-km Path, Lawrence Ginsberg measured the amplitude of a Continuous Wave signal transmitted from North Carolina at receiving sites located in New York State, Labrador, and Iceland (Ginsberg, 1974). Ginsberg designed his experiment to obtain the maximum amount of all-daylight path propagation data possible at that time of the year.
The transmitters emitted large amounts of power into antennas supported by power poles similar to electric power lines. His work influenced Project Sanguine in 1969 where the United States Navy experimented with ELF systems near Clam Lake, Wisconsin (Coe, 1969).
This experiment follows in Ginsberg’s footsteps but with variations that take into account the advances in technology nearly 50 years later.
The main disadvantages of the LF Band are the high cost and practical difficulties associated with the construction of radiators having dimensions appreciable with respect to the wavelength (Martin & Carter, 1961). A half-wave dipole for 136.75 kHz, for example, is over a kilometer long. The size of such an antenna dictates that the largest practical antennas for the LF Band are electrically short as a result. Because an electrically short antenna is smaller than the wavelength of the wave it radiates, antenna efficiency also drops. The final result is that an electrically short antenna only radiates a tiny fraction of the input power supplied by a transmitter.
To compensate for loss of antenna efficiency, both primary users and experimental users in the LF Band employ techniques such as driving high transmitter power output (TPO) into an inefficient antenna. It is not uncommon to find experimental stations where TPO is in excess of 100 watts while Effective Radiated Power is near 5 watts. The purpose of these techniques is to achieve long distance communication during a variety of atmospheric conditions.
This experiment will take the opposite approach; driving low TPO into an antenna. Our objective is to determine how far a signal will propagate when we drive low TPO into an antenna such as the horizontal electric dipole, spiral top-loaded antenna, and the helical antenna. We will examine signal strength at various locations within a 50 kilometer zone surrounding the fixed transmitter location, accumulate data, and statistically analyze the data to determine practical methods to communicate using ground-wave techniques.
As we continue to overcome the practical difficulties of the transmission system outlined above, our attention now turns to the information carried by the signal. Transmitter waveforms during the Ginsburg era overcame the signal-to-noise ratio with lower baud rates and higher transmitter power. Modern experimenters, however, employ waveforms specifically designed to probe potential propagation paths with low-power transmission. In this experiment, we will employ two waveforms, a Ginsberg-era mode called Amplitude-Shift Keying, and a modern frequency-shift keying technique called WPSR-15.
We will discuss these waveforms in the article.