  Very Low Frequency, VLF and Low Frequency, LF signal perturbations were
examined to study ionospheric disturbances induced by solar X-ray flares. The
aim was to understand processes in propagation VLF/LF radio signals over short
paths, and to estimate specific characteristics of each short path. The
receiver at Belgrade station continuously monitor the amplitude and phase of
coherent and subionospherically propagating LF signal operated in Sicily, NSC
at 45.90, kHz and VLF signal operated in Isola di Tavolara ICV at 20.27 kHz,
with great circle distances of 953 km and 976 km, respectively. Geographical
locations of transmitters and receiver site result that these short paths have
many similarity. The main difference is in transmitter frequencies. In period
from 2008 to February 2014 were selected around 200 events for further
examination. In all selected examples amplitude and phase on VLF and LF signals
were perturbed by occurrence of solar X-ray flares. This six years period
covers minimum and maximum of solar activity. Also, simultaneous measurement of
amplitude and phase on the VLF/LF signals during occurrence of solar flare were
employed for constructing electron density profile versus altitude and also in
a function of time over middle Europe.
