146 



SITING AND COVERAGE OF GROUND RADARS 



Fire control and searchlight radars normally 

 operate at high angles so that they also are mainly 

 concerned with the direct wave. The GCI and other 

 low-sited radars have their reflection areas within a 

 mile of the antenna so that earth curvature may be 

 ignored, which means a considerable simplification. 

 The case which requires the most careful considera- 

 tion is early warning, VHF, high-sited radar which 

 is dependent on the reflected wave for much of its 

 performance. A careful analysis of all factors involved 

 is therefore usually required. Prepared diagrams for 

 various heights and wavelengths must be considered 

 carefully before being used, as local terrain features 

 may radically alter the lobe pattern. 



The accuracy and detail desired and the type of 

 site influence the amount of calculation involved. 

 With a low-sited VHF radar only a few lobes are 

 formed so that the shape and location of the lobes 

 is of interest. With a high-sited VHF radar the lobes 

 are numerous and the gaps are small so that there 

 is little likelihood of losing a target in a null area or 

 of being able to associate an echo with a particular 

 lobe. In this case the envelope of the lobes is of 

 particular interest. 



The high-power microwave radars are best suited 

 for vital areas with high traffic density. However, 

 for most purposes the basic long-range, early warning 



radars used by the ground forces operate in the VHF 

 band. They are normally sited high, that is several 

 hundred feet and up, in order to secure low lobe 

 angles and numerous lobes. The need for good rein- 

 forcement and tactical considerations lead to the 

 use of the sea as a reflecting surface where feasible. 

 The general high-sited radar problem will be ana- 

 lyzed in detail, and the use of approximate, simplified 

 methods of calculation will be described where 

 applicable. 



The Vertical Coverage Diagram 



The object of test flights and field intensity 

 calculations is the construction of the vertical cover- 

 age diagram. A typical diagram for a long-range, 

 early warning, VHF radar is shown in Figure 46. 

 The contours or lobes on this diagram represent the 

 locus of all points in space along a particular azimuth 

 where an incoming plane of standard type, usually 

 a twin engine medium bomber, will produce a mini- 

 mum detectable signal. A minimum detectable signal 

 is ordinarily taken to be one that has a signal-to-noise 

 ratio of unity. This may also be expressed in other 

 terms such as field intensity or voltage at the 

 receiver terminals. For other types of planes, or a 

 number of planes, or different aspects of the same 

 plane, the lobe pattern has a different size. 



30,000 



Figure 46. Vertical lobe diagram. 



