Very low frequency the bottom of the ionosphere called the D layer at 60 to 90 km (37 to 56 miles) altitude, which reflects VLF radio waves. The conductive ionosphere and the conductive Earth form a horizontal "duct" a few VLF wavelengths high, which acts as a waveguide confining the waves so they don't escape into space. The waves travel in a zigzag path around the Earth, reflected alternately by the Earth and the ionosphere, in TM (transverse magnetic) mode. VLF waves have very low path attenuation, 2-3 dB per 1000 km, with little of the "fading" experienced at higher frequencies, This is
Radio star and beam out coherent bursts of microwaves. The Sun, the nearest star to Earth, is known to emit radio waves, though it is virtually the only regular star that has been detected in the radio spectrum, because it is so close. It is not considered a radio star because it is not a strong radio source. Some studies have found that main-sequence stars may extremely rarely emit radio waves. A 2009 survey found a maximum of 112 candidate radio stars cross-matching the FIRST and NVSS surveys, but estimated that 108 ± 13 of the samples are from "contamination" from background