There are many sources of very low frequency (VLF—3–30 kHz) and extremely low frequency (ELF—3–3000 Hz) radiation in the Earth‐ionosphere waveguide (e.g., lightning and ELF/VLF communication transmitters). At distances of thousands of kilometers from these sources, the vertical component of the ELF/VLF AC magnetic fields is expected to be very weak and several orders of magnitude lower than the horizontal magnetic components. However, measurements in Israel show a relatively strong vertical magnetic component in both the ELF and VLF bands, at the same order of magnitude as the horizontal components. Our measurements suggest that the real Earth‐ionosphere waveguide might often be very different from the theoretical waveguide used in model calculations. In addition, our results imply that using only the horizontal components for direction finding or the absolute magnetic field strength may result in errors, since often a significant fraction of the magnetic field energy hides in the vertical component.

[1] Very low frequency (VLF) (3-30 kHz) and extremely low frequency (ELF) (3-3000 Hz) electromagnetic transient signals and noise are generated by various natural and anthropogenic processes. On a global basis by far the most significant source is ELF/VLF radiation from lightning propagating in the Earth-ionosphere waveguide. This atmospheric "noise," originating essentially from lightning discharges, is the main source of interference for VLF/LF telecommunications. One of the statistical measures that is used to define the properties of low-frequency radio noise is the voltage deviation V d , which is a measure of the impulsiveness of the noise that is widely used to characterize radio noise, particularly in the International Radio Consultative Committee reports. In this paper we present atmospheric noise statistics based on VLF measurements at different temporal resolution (from minutes to seasonal variations). For the first time we present analysis of the statistical parameters of V d from continuous broadband VLF measurements for a period extending more than 1 year. Our analysis shows that the long-term observed V d characteristics can be reasonably estimated as the sum of two Gaussians distribution functions, while the hourly and seasonal distributions of V d values can be fitted using a single Gaussian distribution with different mean and variance values.

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Recently, unattended near ground wireless sensor networks are attracting increasing attention. The number of sensors needed to cover a certain area is of prime importance and therefore an accurate channel model is needed, especially to evaluate the maximum effective distance between adjacent terminals. In this paper, results of a measurements campaign performed in two environments, flat open yard and sandy dunes with bushes, are presented. Narrowband (NB) and wideband (WB) measurements at 916 MHz were conducted. The measured received power was compared with two ground wave prediction models. Additionally, we found that change of ground type leads to significant changes in both path loss and antenna gain. Based on the NB measurements we found a good match between predicted and measured received power values, and the predicted received power reflects the physical path profile. Based on the WB measurements we found that over flat open yard the channel can be classified as flat fading channel, while over sandy dunes with bushes the channel is rather frequency-selective with a coherence bandwidth of ~20MHz. Moreover, the temporal variations of the transfer function are negligible even though there was moderate wind during the measurements.Index Terms-wireless sensor networks, ground wave propagation, channel measurements. irregular terrain.