Near Vertical Incidence Skywave: Interaction of antenna and propagation mechanism

摘要

In areas where no telecommunication infrastructure exists, or when that infrastructure is destroyed by a natural disaster, Near Vertical Incidence Skywave (NVIS) radio wave propagation may provide a lifeline to the outside world. To exploit NVIS propagation, radio waves are transmitted straight up, where, at heights between 80 and 350 km, the ionosphere will bend these waves back towards the earth. The frequency dependent character of the radiowave propagation requires that operating frequencies are chosen considering ionospheric parameters. Typical frequencies are between 3 and 10 MHz. Due to the great reflection height a large continuous area around the transmitter, exceeding 400 x 400 km, will be covered. As the downward waves arrive at steep angles, large objects such as buildings and mountain ridges cannot block the NVIS radio path. The independence of a network operator enables quick deployment, and the antennas and radio equipment are relatively easily to build and maintain, even in countries with a lower technological standard. These aspects make NVIS radio communication especially suited for disaster relief operations and tele-education and tele-medicine in poor and/or remote regions. Research into the use of NVIS propagation for point-to-point links and broadcasting is spread over several decades and encompasses a large number of subjects. In this thesis, specific blank spots in the NVIS research field are identified and targeted, to augment and connect existing research, with a focus on antennas and propagation. The following research questions were formulated: 1. How does the NVIS propagation mechanism function, and what parameters of this mechanism are important for NVIS telecommunication system optimization? 2. How can we optimize the NVIS antenna to (a) produce the strongest signal across the coverage area, and (b) to realize the greatest signal-to-noise ratio (SNR) on reception of signals from that coverage area? 3. How important is the interaction between NVIS antenna and propagation mechanism? Emphasis of the research is on empirical verification of antenna performance and propagation phenomena, and several novel measurement methods are introduced for this purpose. The measurements are performed in The Netherlands (52°N, 6°E), and are considered representative for mid-latitudes in the Northern hemisphere. Investigations into the NVIS propagation mechanism shows that elevation angles, polarization, fading and noise are the most important parameters to consider in NVIS telecommunication system optimization. The relationship between elevation angle and coverage distance is established as a function of frequency and sunspot number, and confirmed by measurement. The dominance of NVIS over groundwave propagation is shown to start at short distances (20 km at 7 MHz). Measurements show that NVIS propagation is efficient: one 100 Watt transmitter will cover a 400 x 400 km area with 35 to 55 dB SNR. Nighttime propagation over 110 km distance is observed above the critical frequency of the ionosphere, showing signal fluctuation similar to scattering and unlike ground wave propagation. The importance of characteristic wave propagation in NVIS has been demonstrated by measurement, showing nearly perfectly circular polarization of downward waves and high isolation (>25 dB) between both characteristic waves. An antenna with only 0.5 x 0.5 λ footprint is designed that provides separate reception of both characteristic waves. When applied for characteristic wave diversity reception, a reduction of 8 to 11 dB of the necessary transmit power can be realized. Investigations show that NVIS transmit and receive antenna optimizations require a different approach, and result in different optima. Receive antenna optimization requires knowledge of the propagation of electromagnetic ambient noise (radio noise), considering both angular distribution and polarization. Initial experiments indicate that the angular distribution is not uniform. A novel method to evaluate the performance of radio noise measurement antennas is described. For in-situ antenna performance comparison a new method is designed using live NVIS propagation. With this method, the optimum transmit antenna height of a horizontal dipole used as transmit antenna is determined, ranging from 0.18 to 0.22 λ for most soil types. For a receive antenna this is around 0.16 λ, but that optimum is less pronounced. Contrary to popular believe, low dipole antennas are poor performers: a dipole antenna at a height of 0.02 λ is 11 to 12 dB less effective than the optimum on transmission and 2 to 6 dB less effective on reception. However, such a low dipole antenna will still outperform a car whip antenna by 12 dB. Significant interaction between the NVIS antenna and the NVIS propagation mechanism is shown, and optimization considering antenna and propagation as a hybrid system is likely to yield better results than isolated optimization of the antenna alone.