ensity of ionization; however, frequencies in the 30- to 100-MHz range are most effective, with 50 MHz being the optimum frequency. Hori- zontal polarization is generally preferred. When a link is established, path attenuation is high because of the scattering process and is typically 175 dB for a 40-MHz, 1000-km path (Cannon, 1987). This is approximately 50 dB greater than the free-space attenuation of the same distance. Conclusion Propagation for Satellite Communications Most communications satellites are placed in geostationary orbits 36,000 km above the equator; therefore, transmitting and receiving Earth stations can fix their antenna positions with only minor required adjustments for small shifts in satellite position or changes in atmospheric propagation conditions. Such orbits also have the advantage of providing potential coverage of almost one-third of the Earth's surface, but the disadvantage of high free-space loss compared with lower nonstationary orbits. The systems planner will need to calculate the link budget, taking into account such factors as the satellite EIRP (Equivalent Isotropically Radiated Power) and receiver noise performance. The free-space loss can be calculated using equation 8.7, and distance will have to take into account the difference in both latitude and longitude of the position of the satellite on the Earth's surface from that of the transmitting or receiving station. If the distance and great circle bearing are calculated using equations 8.10 and 8.11, the elevation and distance of the satellite can also be calculated. In addition to the free-space loss, the loss due to atmospheric attenuation must be taken into account, and this will depend on precipitation conditions in the earth station area. Typical values at 11 GHz would be 1.0 dB for an "average year," increasing to about 1.5 dB for the worst month. The actual figures to be used should be calculated from local meteorological data and the attenuation curves given in Figure 8.12. Calculation of satellite paths is not ofte