130 TECHNICAL SURVEY 
Ficure 5 Rays in an elevated duct. In this, another 
common form of duct, the amount of bending may be 
approximately normal both below and above the duct. 
The rays oscillate between the upper and lower bound- 
aries; maximum ranges in or near the duct may be even 
greater than with a ground-based duct. 
increase in effective ranges. If the angle of elevatior 
of the aircraft is greater than 1 degree, the effects 
become inappreciable and failure to detect the target 
cannot be attributed to excessive refraction. 
If the duct does not. include the radar within its 
boundaries, as, for example, when a duct forms below 
a high-sited radar, the effective ranges on surface 
craft may be either increased or de¢reased. Similar 
reasoning may be applied in the case of -airborne 
VHF radio communication. Usually there is no very 
pronounced effect upon the signal strength when 
VHF communication is carried on between two 
aircraft, both flying above the duct. 
Interference between the direct rays and the rays 
reflected from the ground—resulting in the well- 
known lobe pattern of the coverage diagram—has 
not been mentioned. Under standard conditions the 
position of the lobes depends only on the wavelength 
used and the height of the radar above the ground. 
When a duct is present the lowest part of the coverage 
diagram may be strongly distorted. 
Coverage depends upon a variety of factors of 
which the most important are these: height of the 
top and base of the duct, amount of refraction in 
the duct, position of the transmitter relative to the 
duct, frequency (or wavelength) of the radar equip- 
ment, and height of the transmitter above ground. 
A coverage diagram for standard conditions is 
shown in Figure 6, diagram 1, with height strongly 
exaggerated. Only the lowest three lobes are shown, 
and the higher lobes appear compressed as compared 
to the lowest lobe. In diagrams 2, 3, 4, 5 the lower 
part of the same diagram is drawn as it appears 
under various conditions of guided propagation. The 
bottom part of the ‘‘standard’”’ main lobe is shown 
by a broken line. The lines which separate the “blind 
zones” from the “detection zones” represent the 
range at which a medium bomber would just become 
visible to this particular radar set. 
The diagrams clearty indicate the great extension 
of ranges in the duct and also the moderate change 
in ranges—sometimes an extension, sometimes a 
reduction—above the duct. Another feature of some 
ALTITUDE IN FEE. 
7000 
6000-— 
2 
GROUND BASED DUCT) _ 
° 
d ! 2000 
ELEVATED puct > 1000 
TRANSMITTER HEIGHT-100 FEET 
FREQUENCY- 200 MC 
NORMAL LIMITING COVERAGE . 
RANGE IN NAUTICAL MILES 50 
Figure 6. Standard and nonstandard coverage diagrams. 
of these diagrams is the appearance of ‘‘skip-ranges.”’ 
A plane flying at an altitude of 500 ft, for instance, 
would be detected early under the conditions shown 
in diagrams 4 and 5. As the plane approaches, the 
echo will disappear from the scope and reappear only 
at a range less than 20 miles. Similar conditions will 
prevail for ground clutter. In diagram 3 there would 
be ground clutter close in and also from beyond 33 
miles but not from the space between. For conditions 
shown in diagram 5, there-would be echoes from very 
remote ground targets but not frem targets at inter- 
mediate ranges. 
A change in echo strength from day to day is not 
necessarily caused by the weather but might simply 
be caused by a variation in performance of the set. 
Cases have occurred where there was extensive trap- 
