§ 76. Mr. Onslowasked the Postmaster General what he assumes the radio radius of the earth to be in United Kingdom latitudes and at a frequency of 7,000 MHz, in considering applications for new microwave links, and why.
§ Mr. MasonThe value of "radio radius" assumed depends upon the circumstances but for high quality microwave radio-relay links is taken as 0.7 times the actual radius of the earth.
In planning a microwave radio-relay link it is necessary to ensure that the aerial at one station on the route has a clear "line-of-sight" path to that at the next station. Although radio waves are normally considered to travel in straight lines, a certain amount of bending does take place within the earth's atmosphere. Under normal atmospheric conditions this bending of the path of radio waves between two points on the earth's surface can be allowed for in planning calculations by assuming that the earth's radius is four-thirds of its actual radius. However, under certain atmospheric conditions the radio-wave paths are bent in such a way as to make the earth appear to have a smaller radius than it actually has and the value of 0.7 times the actual radius is used in planning to cater for the occurrence of this condition. If this 124W were not done, the signal would be lost and the service interrupted during these abnormal atmospheric conditions.
The frequency of 7,000 MHz falls within one of the bands used by the Post Office for trunk telephone circuits and television relays. Other microwave bands are available to meet applications for private systems, the choice of band depending on the type of system and particularly the bandwidth required. The Post Office is not concerned with the performance of such systems and individual users might be prepared to accept a lower standard of service and assume a higher value of "radio radius" for planning purposes.