Earth–ionosphere waveguide Earth and the ionosphere. The round-the-world nature of the waveguide produces resonances, like a cavity, which are at ~7 Hz. Radio propagation within the ionosphere depends on frequency, angle of incidence, time of day, season, Earth's magnetic field, and solar activity. At vertical incidence, waves with frequencies larger than the electron plasma frequency (f) of the F-layer maximum (1) f = 9 (N) kHz (N in cm is the electron density) can propagate through the ionosphere nearly undisturbed. Waves with frequencies smaller than f are reflected within the ionospheric D-, E-, and F-layers. f is of the order of 8–15
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