Determining the electromagnetic constants of ground and analyzing HF propagation with the aid of a modern interferometric direction finder

Abstract

Although high frequency (HF, 3 to 30 MHz) radio propagation (both skywave and ground wave) has been studied for more than a century and is a mature science at this point in time, some answers remain elusive and must be further investigated. The advent of modern, cost effective computers and especially Digital Signal Processing (DSP) techniques and hardware as implemented in an interferometric HF direction finder (DF) have opened exciting new opportunities to study and gain new insights into classic propagation phenomena that are due to the electromagnetic properties of an imperfect earth. These include the propagation loss and the tilt of a ground wave signal. An interferometric DF can also now add a new dimension to the study of the propagation of skywave HF signals through the ionosphere. Until recently it was challenging to analyse, in near real time, skywave ionospheric propagation modes between a transmitter and a receiver that monitors the signal. Received signal strength used to be one of the few available indicators of propagation conditions. During the course of this study, techniques were developed that utilises the elevation angle as measured by an interferometric DF and the output of a ray-tracing algorithm to study and identify propagation effects that the HF signal was subjected to. It is now possible to identify phenomena such as multi-hop and multi-layer propagation. The ionospheric layer(s) that refracted the received signal as well as the Optimum Working Frequency (OWF) for the circuit can now also be identified. The most stable region of the ionosphere for propagation over a specified circuit can also be determined with this new approach, thus helping to improve the reliability of HF communications. Accurate electromagnetic ground constants are required for applications such as the modelling of ground wave propagation of radio signals and determining the radiation characteristics of antennas above a real, imperfect earth. Accurate electromagnetic ground constants are thus paramount in any HF link planning or propagation study, but are not always available for a specific frequency and geographical location. A RF technique, Time Domain Reflectometry (TDR), implemented in modern, cost effective consumer equipment is used to determine the soil moisture content. With the aid of a previously published ground model, an easy to use method was developed to determine the electromagnetic ground parameters (conductivity and relative dielectric constant) at any radio frequency using the measured soil moisture content. This method offers significant advantages in terms of simplicity, speed and cost when compared with current techniques. It has been verified over the 2 to 30 MHz frequency range, but should be applicable up to 200 MHz, the upper frequency limit of the ground model used. An innovative application of an interferometric DF is to determine the forward wave tilt of a ground wave signal in the proximity of the DF antenna array. This application offers significant advantages in terms of simplicity and speed when compared to current manual techniques to determine wave tilt. The soil moisture content can be determined from the wave tilt and in conjunction with the ground model described above, an easy to use method was developed to determine the electromagnetic ground parameters with greater accuracy than is possible from the ITU publications. Having accurate ground electromagnetic constants in hand is very beneficial for the modelling of both ground wave and skywave propagation for the geographical location of the DF. Single Site Location (SSL) direction finding is a technique used to determine the origin (position of the transmitter) of a long-distance HF signal with the aid of a single, interferometric DF, an ionospheric model and a ray-tracing system. The elevation angle of the intercepted signal is used to calculate a ground range by modelling the path that the signal travelled through the ionosphere. With the measured direction (azimuth angle) and the calculated ground range, it is possible to determine the origin of the HF transmission. Simply being able to intercept (monitor) a signal and determining the wave angle (azimuth and elevation) with the aid of an interferometric DF does not guarantee the quality of the calculated SSL ground range. The reigning propagation conditions for the communications circuit must also be taken into consideration for optimum results. However, computing the performance of a SSL system is a complex problem with many variables to be considered. To overcome this difficulty, a quality factor, which only depends on the standard propagation metrics for HF communication links, was derived. Measured results are presented to demonstrate that this quality factor is a useful indicator of SSL performance.