  In this paper we confine our attention to the analysis of amplitude and phase
data acquired by monitoring VLF/LF radio signals emitted by four European
transmitters during a seven-year period (2008-2014). All the data were recorded
at a Belgrade site (44.85$^{0}$ N, 20.38$^{0}$ E) by the Stanford University
ELF/VLF receiver AWESOME. Propagation of VLF/LF radio signal takes place in the
Earth-ionosphere waveguide and strongly depends on ionization level of the
D-region, which means that it is mainly controlled by solar conditions. Some
results of amplitude and phase variations on GQD/22.10 kHz, DHO/23.40 kHz,
ICV/20.27 kHz and NSC/45.90 kHz radio signals measurements at short distances
($D < 2$ Mm) over Central Europe and their interpretation are summarized in
this paper. Attention is restricted to regular diurnal, seasonal and solar
variations including sunrise and sunset effects on propagation characteristics
of four VLF/LF radio signals. We study VLF/LF propagation over short path as a
superposition of different number of discrete modes which depends on the
variations of the path parameters. Although the solar X-ray flare effects on
propagation of VLF/LF radio signals are well recognized on all paths,
similarities and differences between them are defined under existing conditions
over the paths. Statistical results show that the size of amplitude and phase
perturbations on VLF/LF radio signal is in correlation with the intensity of
X-ray flux. We present the calculations of electron density enhancements in the
D-region caused by different classes of solar X-ray flares during the period of
ascending phase and maximum of the solar cycle 24.

Earth and Planetary Astrophysics;Space Physics