Near Vertical Incidence Skywave (NVIS) communication uses the ionosphere as a reflector to cover a continuous area with a radius of at least 150 km around the transmitter, on frequencies typically between 3 and 10 MHz. In developing countries, in areas lacking any other telecommunication infrastructure, it is used on a daily basis for voice and data communication. It may also be used in ad-hoc emergency (disaster) communication in other regions. This paper proposes optimum heights above ground for horizontal dipole antennas for NVIS, based on simulations and empirical data. First, the relationship between elevation angle and skip distance is obtained using ionospheric ray tracing. The high elevation angles found by simulation are confirmed by elevation angle measurements using a professional radio direction finder. The measurements also show the dominance of NVIS over ground wave propagation starting at a short distance. For these elevation angles, the optimum receive and transmit antenna heights above ground are derived using antenna simulations. A distinction is made between optimum transmit signal strength and optimum received signal-to-noise ratio (SNR). These optima are verified experimentally, demonstrating a novel evaluation method that can be used in the presence of the fading typical for ionospheric propagation. For farmland soil ( 20 mS/m, "r 17) the optimum height above ground for the transmit antenna is 0.180.22. If the antenna is lowered to 0.02 a transmit signal loss of 12 dB occurs. This corresponds with the theory. The receive antenna height, however, while appearing uncritical in the simulations, showed a clear optimum at 0.16 and a 27 dB SNR deterioration when lowered to 0.02.
Witvliet, B. A., van Maanen, E., Petersen, G. J., Westenberg, A. J., Bentum, M. J., Slump, C. H., & Schiphorst, R. (2015). Near vertical incidence skywave propagation: Elevation angles and optimum antenna height for horizontal dipole antennas. IEEE antennas and propagation magazine, 57(1), 129-146. https://doi.org/10.1109/MAP.2015.2397071