established, path attenuation is 27.10.4 V.h.f. and u.h.f.: 30MHz to 1GHz high due to the scattering process, and is typically 175dB for a 40MHz, 1000km path (Cannon, 1987). This is approximately 50dB Officially, u.h.f. extends to 3GHz. Lower frequencies are affected greater than the free space attenuation of the same distance. by anomalous propagation conditions. Line of sight communica- tions, typically 80km (meteor scatter on 50MHz can provide ser- vices up to 2000km). Used for multi-channel point to point Propagation for satellite communications, FM sound and TV broadcasting and mobile com- munications. communications 27.10.5 Microwave: 1GHz to 30GHz Most communications satellites are placed in geo-stationary orbits 36000km above the equator, therefore transmitting and receiving Unaffected by ionospheric conditions, but some attenuation at earth stations can fix their antenna positions with only minor adjust- higher frequencies due to rain. Line of sight communications, typi- ments being required for small shifts in satellite position or changes cally 50km (tropospheric scatter can provide services up to 500km). in atmospheric propagation conditions. Such orbits also have the Satellite services provide world wide coverage. Wide band multi- advantage of providing potential coverage of almost one third of the channel communication available. Extensively used for terrestrial earth's surface, but the disadvantage of high free space loss com- point to point and satellite communications, also radar. pared with lower non stationary 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 27.10.6 Millimetric: 30GHz to 100GHz receiver noise performance. The free space loss can be calculated using Equation 27.7 and distance will have to take into account both Unaffected by ionospheric conditions but moderate to severe atte- the difference in latitude and longitude of the position of the satellite nuation due t