Near Vertical Incidence Skywave Propagation Research Articles

Performance Limiters of Near-Vertical-Incidence Skywave Propagation: A Scientific Approach

Near-vertical-incidence skywave (NVIS) propagation is defined as providing continuous coverage from nearly 0 km (just beyond the line of sight) to a couple hundred kilometers from the transmitter with no skip or dead zones. NVIS communications are especially effective during disaster-relief operations when infrastructure is severely damaged. The ability to accurately determine the performance limiters of NVIS propagation can help in the planning of high-frequency (HF) (3-30 MHz) emergency communication links. In the literature, widely varying radial distances (from as few as 50 to up to 160 or even 320 km) for the coverage attainable by NVIS propagation have been reported. It is very difficult to plan an NVIS link for homeland security or disaster relief when the published guidelines vary to such a degree. In this study, a scientific approach was utilized to determine the NVIS performance limiters for varying solar conditions, times of day, and geophysical locations.

Impact of a Solar X-Flare on NVIS Propagation: Daytime characteristic wave refraction and nighttime scattering

An experiment to investigate several characteristics of near vertical incidence skywave (NVIS) propagation (polarization, characteristic waves and their isolation, elevation angles, and radio noise) over a distance of 105 km and at 7 MHz was performed in The Netherlands. Daytime characteristic wave propagation with almost perfectly circular polarization and high isolation values had been expected from our earlier work and could now be demonstrated. Simultaneous elevation angle measurements showed pure single-hop F2-layer reflection and no ground wave component. Nighttime scattering with loss of polarization was observed when the critical frequency (fxF2) dropped below the operating frequency. The Happy Hour propagation, shown in our previous work [2], was confirmed with new measurements. The occurrence of a solar X-ray flare during the measurements provided a unique opportunity to compare the observations in a quiet ionosphere with those during a short wave fadeout (SWF). During the SWF, the signal strength of the ordinary wave dropped 35 dB and that of the extraordinary wave 45 dB. This lasted 15 min; recovery to normal signal levels took 35 min. The radio noise level, which was already low in the quiet rural measurement location, dropped another 12 dB during the SWF, with the remainder being generated locally. This proves that in a quiet rural location the major part of the radio noise arrives via the ionosphere.

Flexible Low-Cost SDR Platform for HF Communications: Near vertical incidence skywave preliminary results

A new low-cost reconfigurable software-defined radio (SDR) platform for high-frequency (HF) applications is presented in this article. The platform can work in the entire HF band (3-30 MHz), with bandwidths up to 36 kHz. We present the preliminary results working in a near vertical incidence skywave (NVIS) communication, where only the frequency range 3- 12 MHz is needed. The modem has been designed so that the modulations to be used, time frames, and scheduling can be completely reprogrammable in both narrow-band and wide-band modes. This is a strong advantage, because tests can be redesigned and reprogrammed to evaluate channel performance, which is not only important for research purposes but also for its final application in emergency communications in the NVIS propagation case of study. The transmission can be adapted to the best propagation frequency at every moment by means of a previous sounding analysis.

Measuring the Isolation of the Circularly Polarized Characteristic Waves in NVIS Propagation [Measurements Corner

Separate excitation of the characteristic waves in the ionosphere results in two orthogonal propagation channels on the same frequency, which may be used in diversity and multiple-input, multiple-output (MIMO) systems. In this article, a method to measure the isolation between these paths is proposed and demonstrated in a near vertical incidence skywave (NVIS) experiment at a frequency of 7 MHz over a 105-km distance. Characteristic wave isolation exceeding 25 dB is measured during Happy Hour: the interval when the propagation path just opens or closes and only the extraordinary wave propagates.