11.5.5 Advantages And Disadvantages Of Sky-Wave Communications
Sky-wave communications have the following advantages:
- over-the-horizon communications are possible;
- medium distance communications (<1,000 km) are available using man-portable and mobile equipment;
- both net communications and point-to-point links can be established;
- relatively low-cost antennas and terminal equipment are required;
- the transmission medium (the ionosphere) is difficult to interrupt; and
- sky-wave communications are versatile in that the same communications system can support voice and data at reasonable speeds down to low-speed Morse code.
Sky-wave communications have the following disadvantages:
- limited available spectrum leading to a limited number of available channels;
- channels are available worldwide with a large number of users possible, leading to significant co-channel interference;
- limited bandwidth is available on each channel leading to low data rates;
- the transmission medium (the ionosphere) is difficult to predict accurately and contains noise and long-range interference, so that channel use varies with time;
- since use of the lower frequencies is desirable, the long wavelengths at sky-wave communications mean that large antennas are required, leading to significant restrictions for mobile systems; and
- management is required for efficient use of the ionosphere.