MadSci Network: Physics |
At VHF the actual absorption is very low, but there can be substantial refraction, even amounting to total reflection, depending on the angle. Electromagnetic propagation in ionized media such as the ionosphere results in modification of the phase that is caused by changes in the path length traversed by the wave. As occurs at optical frequencies, deviation of the ray path results as the wave passes through media with different refractive indices. In the ionosphere, the refractive index n is found as a function of several parameters, the most important being the 'free' electron density. The function describing the refractive index within the ionosphere is known as the Appleton-Hartree equation, a complicated expression in its full form. Neglecting absorption and geomagnetic effects, a simple approximation is Eq 1, where n is the index of refraction, w is the angular freq, wn the plasma freq. The angular plasma freq is given by Eq 2, where N is the electron density, e is the electron charge, m is the electron mass, and e0 is the permittivity of free space. Effects of the medium include polarization rotation (Faraday rotation), refraction, and scintillations due to non-uniformity within the ionosphere. The structure of the ionosphere is complex and time variable, with several distinct layers. Verious attempts to model it computationally have met with limited success. The IONCAP model, embedded in several commercial packages, currently represents the state of the art in ionospheric propagation forecasting. These models require a recent history of solar flux measurements, the time, path coordinates, and frequency for the forecast. One of the parameters produced is the MUF (Maximum Useable Frequency) for the path, that is the highest frequency that a wave tangent to the Earth surface will be reflected. The MUF rarely exceeds 100 MHz, so the main gross at 127 MHz effects are refraction and rotation.
Try the links in the MadSci Library for more information on Physics.