Radio propagation antennas (monopoles) are required. Since the ground is not a perfect electrical conductor, ground waves are attenuated as they follow the Earth’s surface. Attenuation is proportional to frequency, so ground waves are the main mode of propagation at lower frequencies, in the MF, LF and VLF bands. Ground waves are used by radio broadcasting stations in the MF and LF bands, and for time signals and radio navigation systems. At even lower frequencies, in the VLF to ELF bands, an Earth-ionosphere waveguide mechanism allows even longer range transmission. These frequencies are used for secure military communications. They can also penetrate
Radio atmospheric a sferic signal observed simultaneously at several stations. Presumption of such measurements is the concentration on one individual impulse. If one measures simultaneously several pulses, interference takes place with a beat frequency equal to the inversal average sequence time of the pulses. The signal-to-noise ratio determines the [sensitivity|sensibility] of telecommunication systems (e.g., radio receivers). An analog signal must clearly exceed the noise amplitude in order to become detectable. Atmospheric noise is one of the most important sources for the limitation of the detection of radio signals. The steady electric discharging currents in a lightning channel cause a series of incoherent