44 OCEAN ELECTRONIC NAVIGATIONAL AIDS 



Panning or spreading of the beam in the vertical plane is desired to elimi- 

 nate any necessity for mechanical stabilization of the antenna, that is, to 

 retain energy on the surface as the ship rolls. This in turn considerably 

 reduces the vertical dimension of the antenna. The wide beam in the 

 vertical plane will result in some loss of azimuth resolution ahead as the ship 

 rolls, and abeam as the ship pitches, which again brings out the futility of 

 decreasing the horizontal beam width beyond about 1° or 2°. If the PPI is 

 not stabilized in azimuth (true bearing presentation) there will be an ap- 

 preciable decrease in bearing resolution and a smearing of the picture as 

 the ship yaws. Another limitation, though relatively unimportant with 

 antenna beam widths above 2°, is the consideration of how closely the PPI 

 scan can be made to follow the antenna. 



It is extremely difiBcult to design a reflector that will direct the radiated 

 energy in a pencil beam. There are generally some side lobes of radiation. 

 The lobes in the horizontal plane should be sufBciently small to be relatively 

 unimportant as compared to the main lobe. In most cases, if these side lobes 

 are from 25 to 35 decibels down, no difficulty will be encountered. No harm 

 results from side lobes in the vertical plane other than wasted energy di- 

 rected skyward. 



In addition to the above, both range and bearing resolution will be limited 

 to the size of the spot of light on the scope caused by the electron beam. 

 Because of this spot, which is not pin point and is the same on all range scales, 

 the range and range scale at which the desired resolution is expected should 

 be stated. For example, in figures 4-2 and 4-3, the resolution is illustrated 

 at 1 mile on the 2-mile scale. 



Because of the many factors entering into resolution it is generally ex- 

 pressed as the result to be expected providing the equipment has been designed 

 properly for these factors. 



COVERAGE. — Because of the fundamental nature of the electromagnetic 

 waves employed in Radar the coverage of a surface Radar will be improved 

 by increasing the antenna height. Increase in frequency will improve resolu- 

 tion. At the present time, however, there is a practical limit to which the 

 frequency may be increased. The attenuation of electromagnetic waves by 

 the atmosphere is a function of frequency and increases at an amazing rate 

 at frequencies corresponding to wavelengths below 3 centimeters. Likewise 

 at the higher frequencies such conditions as rain, snow, and fog, appreciably 

 reduce coverage. Over-all consideration indicates the most desirable wave- 

 length insofar as reliable coverage is concerned is in the area embracing 3 

 to 10 centimeters. 



In general, a Radar is limited in coverage to about 15 percent beyond the 

 visible horizon, and has a minimum practical range limit of approximately 

 100 yards. (See fig. 4-4.) 



The maximum range is increased by increasing the height of the antenna 

 and in a like manner a higher object will be observed at a greater range. It 

 is not unusual for a good Radar to pick up objects as far away as 100 miles 

 providing they are above the visible horizon. Atmospheric conditions play 

 an important part in range coverage at distances greater than 10 or 15 miles. 

 However, we are primarily interested in vessels located in the area between 

 the minimtun range of the equipment and the horizon. The subject of the 

 vertical beamwidth has a bearing on range coverage. The beam must be suffi- 

 ciently wide in the vertical to illuminate with electromagnetic energy all 

 targets from the minimum range to the maximum in order that all ships will 

 be indicated. 



Power output has a considerable bearing on range coverage. It is well 

 known that the reflected energy from the target to the antenna is an inverse 

 fourth power function, increasing to a much higher power inverse function 

 at a distance somewhat short of the horizon. Hence small increases in power 

 do not mean much in increased efficiencies. At the same time, the realized 

 power is considerably affected by the losses in the transmission line and 

 antenna, the antenna gain, the receiver gain, and other factors. A large 

 amount of power is essential to insure the indication of all above-water 

 objects in the vicinity of the Radar. The real target illumination will also 

 be a function of the pulse rate and speed of antenna rotation as the Radar is 

 pulsed at the same time that the antenna rotates. Electromagnetic waves 

 cannot pierce conducting surfaces of any practical thickness. Therefore, 

 masts, stacks, and other obstructions will give shading effects, and objects 

 located in the shade of these obstructions will not be indicated. The desir- 



